Pneumonia is an infection of the lung parenchyma characterized by inflammation and consolidation of the alveolar spaces, typically caused by bacterial, viral, or fungal pathogens.

Exam domain	One-glance essentials
Definition / diagnosis	Acute infection of pulmonary parenchyma (alveoli ± interstitium) with new infiltrate on CXR plus clinical features of infection (fever, cough ± sputum ± dyspnoea) ^[1]^[2] Classified by context (CAP, HAP, VAP, aspiration, immunocompromised), anatomy (lobar, bronchopneumonia, interstitial), and organism (typical, atypical, viral, fungal, mycobacterial) ^[1]^[2] CAP = acquired outside hospital (or <48 h after admission); HAP = ≥48 h after admission; VAP = ≥48 h after intubation — classification dictates empirical antibiotic choice ^[3]^[2]
Epidemiology / risks	Extremes of age (<5 y, ≥65 y), smoking, COPD/bronchiectasis, DM, CKD/CLD, immunosuppression, splenectomy, and institutionalisation are major risk factors ^[1]^[2] *Always take TOCC history (Travel, Occupation, Contact, Clustering)* — critical in Hong Kong given endemic TB, avian flu risk, and outbreak history (SARS, COVID-19) ^[1]^[4] Neurological impairment (stroke, dementia, impaired consciousness) → aspiration risk; NGT feeding does not reduce aspiration pneumonia risk ^[5]^[6] Post-operative pneumonia risk increased by smoking, pre-existing lung disease, prolonged anaesthesia → pre-op smoking cessation and chest physiotherapy reduce risk ^[7]^[8]
Core mechanism	Pathogen reaches alveoli (inhalation > aspiration > haematogenous) → alveolar macrophage overwhelmed → neutrophilic inflammation → exudate fills alveoli → consolidation + V/Q mismatch → hypoxaemia ^[1]^[2] CAP: *S. pneumoniae* is #1 cause; HAP/VAP: Gram-negatives + MRSA; immunocompromised: PJP, Aspergillus, CMV, TB ^[1]^[9] Atypical organisms (Mycoplasma, Legionella, Chlamydophila) → interstitial inflammation; *β-lactams cannot kill Mycoplasma (no cell wall) → must use macrolides* ^[10]^[11]
Clinical picture	Fever/rigors, productive cough (rusty = pneumococcal; currant-jelly = Klebsiella), pleuritic chest pain, dyspnoea; lower lobe → referred upper abdominal pain via phrenic nerve ^[1]^[2] Consolidation signs: dull percussion, bronchial breathing, ↑vocal resonance/tactile fremitus, coarse crackles; ↓air entry over effusion ^[1]^[12] Atypical pneumonia: prominent systemic symptoms (headache, myalgia, GI), dry cough, normal WBC; Legionella → hyponatraemia + diarrhoea + confusion ^[2]^[13] Red flags: RR ≥30, BP <90/60, confusion, SpO₂ <92% — suggest severe pneumonia requiring urgent escalation and ICU consideration ^[1]^[4]
Investigations	*CXR is gold-standard* (sensitivity ~70%): lobar consolidation = typical bacterial; bilateral interstitial = viral/atypical/PJP; cavitation = TB/S. aureus/Klebsiella; if CXR negative but suspicious → repeat or CT ^[12]^[14] CURB-65 severity score: Confusion, Urea >7, RR ≥30, BP <90/60, age ≥65; score 0–1 = home; 2 = admit; ≥3 = consider ICU ^[1]^[2] Bloods: CBC, RFT (urea for severity + dosing), CRP, ABG if SpO₂ <93% (expect Type 1 RF; rising PaCO₂ is ominous); 2 sets blood cultures if severe ^[1]^[15] Microbiology: sputum Gram stain + C/ST, NPA viral PCR, urinary antigens (Legionella serogroup 1, pneumococcal), Mycoplasma PCR/serology, AFB if TB suspected; BAL for PJP ^[16]^[10]
Management	*Four pillars: (1) assess severity + monitor, (2) send microbiology, (3) start empirical antibiotics, (4) supportive care* (O₂ target SpO₂ ≥92%, IV fluids, analgesia) ^[4]^[1] HK empirical CAP therapy: outpatient high-dose amoxicillin or amoxicillin–clavulanate ± doxycycline; hospital/anti-pseudomonal regimens depend on severity, bronchiectasis/prior isolates and local IMPACT guidance ^[17] For mild outpatient CAP, 5 days is usually effective if improving; extend for instability, severe/complicated infection or specific pathogens; review at 48–72 h and de-escalate to microbiology ^[17] Non-response at 48–72 h: re-evaluate for wrong organism (TB, resistant), complication (empyema/abscess), or non-infective mimic (lung cancer, PE, COP); escalate with CT/bronchoscopy ^[18]^[19]
Complications	*Respiratory failure: Type 1 (V/Q mismatch) → Type 2 (exhaustion) → ARDS; septicaemia with multi-organ failure* (shock + DIC + AKI) ^[20]^[21] *Empyema: pus in pleural space → drain + prolonged Abx 4–6 weeks; complicated parapneumonic effusion (pH <7.2, glucose <2.2, LDH >1000) also requires drainage ^[20]^[21] Lung abscess: cavity with air-fluid level (acute angle, spherical on CXR); treat with prolonged Abx 4–6 weeks — usually do NOT drain percutaneously* (unlike empyema) ^[20]^[21] *HypoNa from SIADH* (especially Legionella); *acute MI* (demand ischaemia + plaque instability); *new AF* (hypoxia + inflammation); recurrent same-lobe → bronchoscopy for obstruction ^[20]^[13]
Prevention / follow-up	Pneumococcal vaccination (PCV13/PPSV23) for elderly, immunocompromised, chronic disease; annual influenza vaccination reduces pneumonia incidence and mortality ^[1]^[2] Smoking cessation is the single most important modifiable risk factor; pre-operative cessation + incentive spirometry + chest physiotherapy reduce post-op pneumonia ^[7]^[8] Aspiration precautions: elevate HOB, swallowing assessment post-stroke, oral hygiene; PJP prophylaxis (co-trimoxazole) in immunocompromised patients (CD4 <200) ^[22]^[5]
Exam traps	*CXR sensitivity is only ~70%* — a negative CXR does NOT exclude early pneumonia; give empirical antibiotics if clinically suspicious and repeat CXR or CT ^[12]^[14] *Always consider TB in Hong Kong* when pneumonia fails standard Abx: subacute onset, upper lobe cavitation, night sweats, weight loss → send sputum AFB ×3 + Xpert MTB/RIF ^[1]^[23] Empyema = must drain; lung abscess = usually do NOT drain — a classic exam confusion; CXR distinction: empyema = obtuse angle/lenticular vs abscess = acute angle/spherical ^[20]^[21] Elderly/immunocompromised may present afebrile with confusion or functional decline only — low threshold for CXR; NGT feeding does NOT prevent aspiration pneumonia ^[6]^[5]

References

^[1] Lecture slides: GC 052. Fever and purulent sputum.pdf ^[2] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.146-151 – Pneumonia microbiology and DDx) ^[3] Lecture slides: GC 106. Practical issues in antibotic use [Notes].pdf ^[4] Lecture slides: GC 052. Fever and purulent sputum.pdf (p.46 – Learning points) ^[5] AOS material: AOS - Geriatrics.pdf (p.25 – NGT feeding does not reduce aspiration pneumonia risk) ^[6] AOS material: AOS - Geriatrics.pdf (p.25 – Old age home resident with pneumonia scenario) ^[7] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p.386-387 – Post-operative pneumonia, prevention of pulmonary complications) ^[8] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p.387 – Prevention of post-operative respiratory complications) ^[9] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf ^[10] AOS material: AOS - Microbio.pdf (p.4 – Mycoplasma PCR and macrolide treatment MCQ) ^[11] AOS material: AOS - Microbio.pdf (p.4 – Mycoplasma treatment with macrolides) ^[12] Lecture slides: Respiratory- Introduction to Resp investigations (CXR, CT thorax and lung function tests).pdf ^[13] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.21 – SIADH causes including chest infection) ^[14] Lecture slides: Respiratory- Introduction to Resp investigations (CXR, CT thorax and lung function tests).pdf (p.35) ^[15] Lecture slides: Data Interpretation - Respiratory - by Dr DCL Lam (Rotation 2).ppt.pdf ^[16] Lecture slides: GC 101. Diagnosis of infections.pdf ^[17] Hong Kong guidance: Centre for Health Protection. Antibiotic Guidance Notes — Community-Acquired Pneumonia (2026); IMPACT 6th edition ^[18] Senior notes: Ryan Ho Respiratory.pdf (p.65 – Complications of CAP, causes of unresolving pneumonia) ^[19] Senior notes: Ryan Ho Respiratory.pdf (p.63 – DDx table, CXR findings, unresolving pneumonia) ^[20] Lecture slides: GC 052. Fever and purulent sputum.pdf (p.29-30 – Complications of pneumonia; p.46 – Learning points) ^[21] Senior notes: Maksim Medicine Notes.pdf (p.188-189 – Complications, lung abscess vs empyema table) ^[22] Lecture slides: GC 102. Fever after chemotherapy infections in immunocompromised hosts [Handout].pdf ^[23] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p.33 – Clinical presentation of TB)

Pneumonia

Pneumonia is an acute infection of the pulmonary parenchyma — specifically the alveoli and surrounding interstitial tissue — resulting in inflammation, exudation of fluid and inflammatory cells into the airspaces, and impaired gas exchange ^[1]^[2].

The term derives from Greek: pneumon (πνεύμων) = lung, -ia = condition of. So pneumonia literally means "condition of the lung."

Strictly speaking, pneumonia or pneumonitis represents any inflammatory condition involving the lungs including the visceral pleura, airways, alveoli, and vascular structures ^[2]. In clinical practice, however, "pneumonia" almost always refers to infectious pneumonia caused by bacteria, viruses, fungi, or other microorganisms.

Lower respiratory tract infection (LRTI) is sometimes used interchangeably to include bronchitis, bronchiolitis, and pneumonia, or any combination of the three ^[2]. However, true pneumonia is distinguished from simple bronchitis by the presence of new radiographic infiltrates on chest imaging plus clinical features of acute infection.

Key Distinction

Pneumonia = infection of the lung parenchyma (alveoli ± interstitium) with new infiltrate on imaging. Simple bronchitis = infection limited to airways without parenchymal involvement. This distinction matters because pneumonia requires antibiotics while acute bronchitis (usually viral) often does not.

2. Epidemiology

Pneumonia is the leading infectious cause of death worldwide and the single largest cause of death in children under 5 years of age globally.
Among adults, community-acquired pneumonia (CAP) is the most common infectious cause of hospitalization in developed countries.
Incidence rises sharply at the extremes of age: < 5 years and > 65 years.

Pneumonia is a leading cause of death in Hong Kong, consistently ranking among the top 3 causes of mortality (alongside malignant neoplasms and cardiovascular disease) ^[3].
Hong Kong's ageing population, dense urban living, and subtropical climate (favouring certain pathogens like Burkholderia pseudomallei during the rainy season) create a particular epidemiological landscape.
Important emerging agents in the Hong Kong/Asia-Pacific context include MERS, avian influenza (H5N1, H7N9), SARS-CoV, SARS-CoV-2, and seasonal influenza ^[1].
Tuberculosis remains endemic in Hong Kong (~4,000 cases/year), and must always be considered in the differential of non-resolving pneumonia or chronic lower respiratory tract symptoms ^[4].

Bacterial pneumonia (especially S. pneumoniae) peaks in winter months.
Influenza-associated pneumonia follows seasonal influenza epidemics (typically January–March and July–August in Hong Kong's bimodal influenza season).
Legionella outbreaks may occur year-round, often linked to cooling towers or contaminated water systems.

3. Risk Factors

Understanding risk factors is critical because they influence both the likelihood of developing pneumonia and the likely causative organisms.

Category	Risk Factors	Why? (Mechanism)
Age	Extremes of age (< 5y, > 65y)	Immature immune system in children; immunosenescence, reduced mucociliary clearance, and reduced cough reflex in elderly
Smoking	Active or passive	Damages ciliated epithelium → impaired mucociliary escalator; ↑mucus production; impairs alveolar macrophage function; causes chronic inflammation
Chronic lung disease	COPD, bronchiectasis, asthma, ILD	Structural damage + impaired local defences; COPD patients have colonized airways
Immunocompromise	HIV/AIDS, post-chemotherapy, post-transplant, corticosteroids, biologics (anti-TNF)	Reduced humoral and/or cell-mediated immunity → susceptible to opportunistic organisms (Pneumocystis jirovecii, CMV, fungi)
Diabetes mellitus		Impaired neutrophil chemotaxis and phagocytosis; microvascular disease reduces tissue perfusion
Chronic organ disease	CKD, CLD (liver cirrhosis), CHF	CKD → uraemic immunosuppression; CLD → reticuloendothelial dysfunction and reduced opsonization ^[5]; CHF → pulmonary congestion creates a favourable medium
Neurological impairment	Stroke, dementia, Parkinson's, reduced consciousness	Impaired swallowing reflex and cough reflex → aspiration
Alcohol use		Impairs cough reflex, mucociliary clearance, and neutrophil function; ↑aspiration risk
Malnutrition		Impaired cellular immunity and mucosal barrier integrity
Splenectomy / functional asplenia	Post-splenectomy, sickle cell disease	Loss of splenic filtration of encapsulated organisms → overwhelming post-splenectomy infection (OPSI), most commonly S. pneumoniae ^[6]

Factor	Relevance
Travel, Occupation, Contact, Clustering (TOCC) ^[1]	Critical in outbreak settings (e.g., avian flu, MERS, COVID-19); occupational exposure (e.g., construction → silicosis → TB risk)
Institutionalization (nursing homes, old age homes)	↑MDR organisms, aspiration risk; healthcare-associated pneumonia
Hospital admission / ventilation	Hospital-acquired and ventilator-associated pneumonia with resistant organisms
Animal exposure	Psittacosis (C. psittaci) from birds; Q fever (Coxiella burnetii) from farm animals
Water system exposure	Legionella from cooling towers, water heaters, spas

High Yield – TOCC History

Always take a TOCC history (Travel, Occupation, Contact, Clustering) in any patient with pneumonia ^[1]. This is especially important in Hong Kong given its position as an international travel hub and history of respiratory disease outbreaks (SARS 2003, COVID-19 2020). A missed TOCC can delay outbreak recognition and put contacts at risk.

4. Anatomy and Function: Normal Lung Defences

To understand why pneumonia develops, you need to understand the layered defence system that normally prevents lung infection. Pneumonia occurs when these defences are overwhelmed or impaired.

Nasal filtration: Nasal hairs and turbinates trap large particles (> 10 μm) before they reach the lower airways.
Mucociliary escalator: The tracheobronchial epithelium is lined with ciliated pseudostratified columnar epithelium covered by a mucus layer. Inhaled particles and microbes are trapped in mucus and propelled cephalad (towards the pharynx) by coordinated ciliary beating at ~1,000 beats/min. This is the single most important mechanical defence.
- Why does smoking cause pneumonia? → Cigarette smoke paralyzes cilia (ciliotoxicity) and causes metaplasia of ciliated epithelium to squamous epithelium → loss of the mucociliary escalator.
Cough reflex: A powerful secondary clearance mechanism triggered by irritant receptors in the airways. Cough generates velocities up to 500 mph to expel secretions.
Epiglottic closure: Prevents aspiration during swallowing.

Alveolar macrophages: The primary resident immune cells in the alveoli. They phagocytose and kill inhaled organisms. They also recruit neutrophils via cytokine release (IL-1, IL-8, TNF-α) when overwhelmed.
Surfactant proteins A and D: Collectins (innate immune proteins) that opsonize bacteria and facilitate macrophage phagocytosis.
Secretory IgA: Present in airway secretions; prevents microbial adherence to epithelium.
Complement system: Activated by bacterial surfaces, leading to opsonization (C3b), chemotaxis (C3a, C5a), and membrane attack complex formation.
Neutrophil recruitment: When the initial macrophage response is insufficient, pro-inflammatory cytokines recruit neutrophils from the bloodstream. This massive neutrophil influx is what causes the characteristic consolidation seen in bacterial pneumonia — the alveoli fill with neutrophils, fibrin, and exudate.

Cell-mediated immunity (T cells): Critical for defence against intracellular organisms (TB, Legionella, Pneumocystis, viral pathogens).
Humoral immunity (B cells, antibodies): Important for opsonization and clearance of extracellular encapsulated organisms (S. pneumoniae, H. influenzae, K. pneumoniae).

Clinical correlation: Patients with HIV (↓CD4 T cells) are susceptible to Pneumocystis jirovecii and TB because these require cell-mediated immunity. Patients post-splenectomy (↓opsonization of encapsulated organisms) are susceptible to S. pneumoniae because they need intact humoral/splenic function ^[6].

Understanding how organisms reach the lung parenchyma explains the pattern and type of pneumonia:

Direct inhalation of droplets ^[2]: The most common route. Aerosolized organisms (e.g., from coughing, sneezing, or environmental sources like cooling towers for Legionella) are inhaled and deposited in the lower airways. Particles < 5 μm can reach the alveoli.
Aspiration of secretions ^[2]: Micro-aspiration of oropharyngeal secretions colonized with bacteria occurs even in healthy people during sleep. In patients with impaired consciousness (stroke, alcohol, sedation) or dysphagia, macro-aspiration occurs → aspiration pneumonia.
Haematogenous spread ^[2]: Organisms from a distant focus (e.g., infective endocarditis, IV drug use, infected IV lines) seed the lungs via the bloodstream. Typically produces bilateral, multifocal, peripheral ("septic emboli") pattern.
Direct spread from contiguous focus ^[2]: Rarely, infection spreads from adjacent structures (e.g., liver abscess through diaphragm, mediastinitis, oesophageal perforation).

6. Aetiology

The clinical context is highly indicative of the causative organism ^[1]. This is the most clinically useful classification:

Classification	Definition	Key Organisms
Community-acquired pneumonia (CAP) ^[1]^[2]	Acute infection of pulmonary parenchyma acquired in the community (i.e., onset before or within 48h of hospital admission)	S. pneumoniae (most common), H. influenzae, M. pneumoniae, C. pneumoniae, Legionella, influenza, SARS-CoV-2
Hospital-acquired pneumonia (HAP) ^[1]^[2]	Onset ≥ 48h after hospital admission, not incubating at time of admission	Gram-negatives (P. aeruginosa, Acinetobacter, Enterobacteriaceae), S. aureus (incl. MRSA)
Ventilator-associated pneumonia (VAP) ^[1]	Onset ≥ 48h after endotracheal intubation	Similar to HAP; P. aeruginosa, Acinetobacter, MRSA
Healthcare-associated pneumonia (HCAP) ^[2]	Acquired in healthcare facilities (nursing homes, haemodialysis centres, outpatient clinics) within past 3 months	Higher risk of MDR pathogens ^[2]
Aspiration pneumonia ^[1]	Pneumonia following aspiration of oropharyngeal or gastric contents	Anaerobes (Bacteroides, Fusobacterium, Peptostreptococcus), mixed flora; often polymicrobial
Pneumonia in immunocompromised ^[1]	Broad category depending on type of immunodeficiency	Pneumocystis jirovecii (HIV, post-chemo), CMV, fungi (Aspergillus, Cryptococcus), Nocardia, mycobacteria ^[1]^[7]

High Yield – CAP vs HAP

The distinction between CAP and HAP/VAP fundamentally changes your empirical antibiotic choice. CAP typically involves community organisms susceptible to standard antibiotics, while HAP/VAP organisms are often multi-drug resistant and require broader coverage.

Aetiology is usually associated with age group (but NOT clinical or radiological picture) ^[1]:

Age Group	Common Pathogens
Neonates	GBS (Group B Streptococcus), Listeria monocytogenes, E. coli (all from maternal genital tract) ^[1]
Infants and young children	S. pneumoniae, H. influenzae, S. aureus ^[1]; viral causes very common (RSV, parainfluenza, influenza, adenovirus, human metapneumovirus)
Children > 5 years	S. pneumoniae, M. pneumoniae, C. pneumoniae ^[1]
2–4 months (afebrile pneumonia of infancy)	C. trachomatis, M. hominis, U. urealyticum, CMV → staccato cough, conjunctivitis ^[2]

Setting	Most Common Organisms	Notes
CAP – Typical	S. pneumoniae (most common overall), H. influenzae, Moraxella catarrhalis, S. aureus, K. pneumoniae	K. pneumoniae more common in diabetics, alcoholics, and Southeast Asian populations
CAP – Atypical	M. pneumoniae, C. pneumoniae, Legionella pneumophila	"Walking pneumonia" ^[1]
CAP – Viral	Influenza A/B, SARS-CoV-2, RSV, parainfluenza, adenovirus	Viral causes more common in children ^[1]
Aspiration	Anaerobes + mixed oral flora	Common in elderly with dysphagia, post-stroke, nursing home residents
HAP/VAP	P. aeruginosa, Acinetobacter baumannii, MRSA, Enterobacteriaceae (including ESBL-producers)	Require anti-pseudomonal cover
Immunocompromised	Pneumocystis jirovecii (CD4 < 200), Aspergillus (neutropenic), CMV (post-transplant), TB, NTM	P. jirovecii in patients with HIV or severe immunocompromised state (e.g., post-chemo) ^[1]^[7]

Other organisms relevant to Hong Kong:

Burkholderia pseudomallei (melioidosis): endemic in SE Asia including southern China/HK; associated with diabetes, CKD; causes necrotizing pneumonia especially during rainy season ^[1]^[8]
Mycobacterium tuberculosis: always consider in Hong Kong; TB may rarely present acutely mimicking bacterial pneumonia ^[4]
Emerging agents: MERS, avian influenza ^[1]

Atypical pneumonia syndrome ('walking pneumonia') ^[1]:

Cause: usually refers to M. pneumoniae, C. pneumoniae, and L. pneumophila ^[1]
Characterized by:
- Symptomatology: less severe, prominent systemic complaints ^[1]
- Investigations: relatively normal WBC counts ^[1]
This term is now considered less relevant because clinical picture does not correlate well with organisms involved ^[1]

However, the concept remains useful for understanding the spectrum of presentation:

Feature	"Typical" Pneumonia	"Atypical" Pneumonia
Onset	Abrupt, acute	Insidious, gradual
Fever	High, with rigors	Low-grade
Cough	Productive with purulent sputum	Dry, non-productive initially
Systemic symptoms	Less prominent relative to respiratory	Prominent systemic complaints (headache, myalgia, malaise) ^[1]
CXR	Lobar consolidation	Diffuse/patchy infiltrates (often bilateral)
WBC	Leukocytosis with neutrophilia	Relatively normal WBC counts ^[1]
Organisms	S. pneumoniae, H. influenzae, S. aureus	Mycoplasma, Chlamydophila, Legionella

Exam Trap – Atypical ≠ Mild

Don't equate "atypical" with "benign." Legionella pneumonia can cause severe pneumonia with multi-organ failure (liver, kidney involvement, hyponatraemia from SIADH). The term "atypical" refers to the pathogen type and clinical presentation pattern, not severity.

7. Pathophysiology

The pathophysiology of pneumonia can be understood as a sequence of events:

The pathological progression of untreated lobar pneumonia (classically S. pneumoniae) follows four stages:

Stage	Timing	Pathology	Clinical Correlation
1. Congestion	Day 1–2	Vascular engorgement, serous exudate in alveoli, many bacteria	Fever, malaise; CXR may be normal or show subtle haze
2. Red hepatization	Day 2–4	Alveoli filled with RBCs, neutrophils, and fibrin → lung resembles liver (hence "hepatization")	Rusty sputum (altered blood mixed with purulent sputum) ^[9]^[10]; consolidation signs on exam
3. Grey hepatization	Day 4–8	RBCs lysed, dense fibrinopurulent exudate, macrophages accumulate	Continued consolidation; organism destruction beginning
4. Resolution	Day 8+	Enzymatic digestion of exudate by macrophages; restoration of normal architecture	Improving clinically; cough with expectoration of residual exudate

Why "rusty sputum"? Characteristically, S. pneumoniae gives rusty coloured sputum ^[1] because red blood cells extravasate into the alveoli during the red hepatization phase and their haemoglobin is oxidized as it mixes with the inflammatory exudate, creating a rust colour. K. pneumoniae gives currant-jelly sputum ^[1] — this is thicker, more viscous, and blood-tinged because Klebsiella causes necrotizing inflammation with more extensive tissue destruction and haemorrhage.

Streptococcus pneumoniae (Pneumococcus)

Polysaccharide capsule is the major virulence factor → resists phagocytosis → requires opsonization by complement and antibodies
This is why asplenic patients are at extreme risk ^[6] — the spleen is the primary site for clearance of encapsulated organisms opsonized by antibodies
Triggers intense neutrophilic response → lobar consolidation pattern

Staphylococcus aureus

Produces toxins (Panton-Valentine leukocidin/PVL) that destroy leukocytes → necrotizing pneumonia
Tends to cause cavitating/abscess-forming pneumonia
Common after influenza infection (damaged respiratory epithelium allows S. aureus to invade)

Klebsiella pneumoniae

Thick polysaccharide capsule → bulging fissure sign on CXR (so much exudate it pushes lobar boundaries)
Necrotizing with cavitation → currant-jelly sputum
More common in alcoholics, diabetics, and elderly

Mycoplasma pneumoniae

Lacks a cell wall → β-lactam antibiotics are ineffective (no target for penicillins/cephalosporins)
Attaches to respiratory epithelium via P1 adhesin protein → triggers immune-mediated damage rather than direct tissue destruction
Explains the disproportionate systemic symptoms (immune activation) with relatively mild respiratory findings
Can cause extrapulmonary manifestations: cold agglutinins (IgM autoantibodies against RBC I-antigen → haemolytic anaemia), erythema multiforme, myocarditis, encephalitis

Legionella pneumophila

Intracellular pathogen that survives within alveolar macrophages
Requires cell-mediated immunity for clearance → why immunosuppressed patients are vulnerable
Causes SIADH → hyponatraemia ^[11] — mechanism: hypothalamic osmoreceptor reset + direct ADH release triggered by pulmonary inflammation
GI symptoms (diarrhoea, nausea) are characteristic because Legionella also infects intestinal macrophages

Pneumocystis jirovecii (PJP/PCP)

Fungal organism (previously classified as protozoan)
Requires intact CD4+ T cell immunity for clearance → presents when CD4 < 200 cells/μL
Causes diffuse bilateral ground-glass opacities on CT; elevated LDH; desaturation with exercise

Why does pneumonia cause hypoxaemia?

Ventilation-perfusion (V/Q) mismatch: Consolidated alveoli are perfused but not ventilated (intrapulmonary shunt). Blood passes through the lung without being oxygenated → ↓PaO₂.
Intrapulmonary shunting: In complete consolidation, no ventilation reaches the affected alveoli at all → true shunt (does not improve with supplemental O₂ alone, unlike V/Q mismatch).
Diffusion impairment: Inflammatory exudate thickens the alveolar-capillary membrane → impaired O₂ diffusion (minor contribution compared to shunting).
Compensatory hyperventilation: The body responds to hypoxaemia by ↑respiratory rate → CO₂ is blown off → respiratory alkalosis is common early in pneumonia. If the patient tires or develops ARDS, CO₂ retention (respiratory acidosis) may follow.

Fever: Bacterial products (endotoxins, exotoxins) and host cytokines (IL-1, IL-6, TNF-α) → act on hypothalamus → ↑prostaglandin E2 → reset thermostat upward
Sepsis cascade: If organisms enter the bloodstream (bacteraemia in up to 25% of fulminant cases ^[5]), systemic inflammatory response can lead to septic shock and multi-organ dysfunction
SIADH and hyponatraemia: Particularly with atypical pneumonia (especially Legionella), COVID-19, and SARS ^[11] — pulmonary inflammation triggers non-osmotic ADH release
Cardiac complications: Acute MI and atrial fibrillation can be precipitated by pneumonia ^[8] — mechanisms include hypoxaemia, systemic inflammation increasing myocardial oxygen demand, and catecholamine surge
- Pneumonia is listed as a predisposing/trigger factor for AMI ^[12]

8. Classification Systems

CAP, HAP, VAP, HCAP, aspiration pneumonia, pneumonia in immunocompromised ^[1]^[2]

Pattern	Description	Common Organisms	CXR Appearance
Lobar pneumonia	Homogeneous consolidation of an entire lobe	S. pneumoniae, K. pneumoniae	Dense homogeneous opacification confined to one lobe with air bronchograms
Bronchopneumonia	Patchy consolidation centred on airways, often bilateral	S. aureus, H. influenzae, Gram-negatives	Patchy, multifocal opacities, often bilateral and basal
Interstitial pneumonia	Inflammation of alveolar walls and interstitium	Viruses, Mycoplasma, Pneumocystis	Reticular/reticulonodular pattern, ground-glass opacities
Lung abscess	Cavitary lesion with necrosis	Anaerobes, S. aureus, K. pneumoniae, TB	Thick-walled cavity ± air-fluid level
Necrotizing pneumonia	Extensive destruction with multiple small cavities	S. aureus (PVL+), K. pneumoniae, B. pseudomallei	Multiple small cavities within consolidation

Typical bacterial: S. pneumoniae, H. influenzae, S. aureus, aerobic Gram-negatives, anaerobes ^[2]
Atypical: M. pneumoniae, C. pneumoniae, C. psittaci, L. pneumophila ^[1]^[2]
Viral: Influenza, RSV, parainfluenza, SARS-CoV-2, adenovirus, measles ^[1]
Fungal: Pneumocystis jirovecii, Aspergillus, Cryptococcus, endemic fungi
Mycobacterial: M. tuberculosis, NTM (non-tuberculous mycobacteria)

9. Clinical Features

9.1 Symptoms

Patients usually present acutely ^[1]:

Symptom	Pathophysiological Basis
Fever, chills, rigors ^[1]	Endotoxins/exotoxins and pro-inflammatory cytokines (IL-1, IL-6, TNF-α) act on the anterior hypothalamus → ↑PGE2 → thermostat reset upward. Rigors = skeletal muscle contraction to generate heat during temperature resetting phase. Rigors are particularly associated with bacteraemia (organisms in the bloodstream triggering a massive cytokine surge).
Malaise, headache ^[1]	Non-specific effects of circulating cytokines on the CNS — these act as "sickness behaviour" mediators
↓Appetite ^[1]	Cytokine-mediated (TNF-α, also called "cachexin," suppresses appetite centres in the hypothalamus)
Restlessness ^[1]	Hypoxaemia → sympathetic activation; also cytokine-mediated cerebral effects
Night sweats	Diaphoresis as temperature cycles (fever breaks → hypothalamic setpoint returns to normal → body dissipates excess heat via sweating)
Weight loss (if prolonged)	Catabolic state driven by TNF-α and IL-6; ↓appetite

Clinical Pearl – Rigors

True rigors (uncontrollable shaking) suggest bacteraemia. If your pneumonia patient has rigors, think: blood cultures BEFORE antibiotics, and be alert for developing sepsis.

Symptom	Pathophysiological Basis
Cough: short, painful, dry (initial) → productive with mucopurulent sputum ± haemoptysis (later) ^[1]	Initially dry: irritation of airway receptors by inflammation before significant exudate accumulates. Becomes productive: as inflammatory exudate (neutrophils, fibrin, debris) fills the alveoli and is expectorated. Mucopurulent: yellow-green colour is due to myeloperoxidase (a green enzyme) in degranulating neutrophils.
Pleuritic chest pain ^[1]	Occurs when the visceral pleura is involved ^[1]. The lung parenchyma itself has no somatic pain fibres. When inflammation extends to the parietal pleura (via the adjacent visceral pleura), the intercostal nerves are stimulated → sharp, well-localized pain that worsens with deep inspiration, coughing, or movement.
Dyspnoea ^[1]	V/Q mismatch and intrapulmonary shunting → hypoxaemia → stimulation of peripheral chemoreceptors (carotid body) → sensation of breathlessness. Also: ↓lung compliance from consolidation and inflammation → increased work of breathing.
Haemoptysis	Necrosis of lung tissue and/or rupture of inflamed capillaries in the alveolar walls → blood in sputum. Sputum characteristics are important diagnostic clues ^[9]^[10]:

Sputum characteristics and their diagnostic significance ^[10]:

Sputum Appearance	Suggests
Bloody mucopurulent	Bronchiectasis, lung abscess ^[10]
Blood-streaked	TB, lung cancer ^[10]
Pink frothy	Pulmonary oedema ^[10]
"Rusty"	Altered blood in purulent sputum from bacterial consolidation (classically pneumococcal) ^[1]^[10]
Fresh blood or clots	Signify significant amount of blood ^[10]
Currant-jelly	Klebsiella pneumoniae ^[1]

Symptom	Pathophysiological Basis
Upper abdominal tenderness ^[1]	If lower lobe pneumonia or associated hepatitis ^[1]. Lower lobe pneumonia can cause diaphragmatic pleurisy → referred pain to the abdomen (shared innervation of diaphragmatic pleura and abdominal wall via phrenic nerve C3-C5 and lower intercostal nerves). This can mimic an acute abdomen!
Confusion/altered mental status	Especially in elderly patients — may be the ONLY presenting feature. Mechanism: hypoxaemia, sepsis-related cerebral hypoperfusion, direct cytokine effects on the brain
GI symptoms (nausea, vomiting, diarrhoea)	Non-specific cytokine effects; particularly prominent with Legionella (intestinal macrophage involvement)
Myalgias	Cytokine-mediated (IL-6, TNF-α cause skeletal muscle proteolysis); prominent in atypical and viral pneumonia

Exam Pearl – Lower Lobe Pneumonia Mimicking Acute Abdomen

Upper abdominal tenderness may occur with lower lobe pneumonia ^[1]. This is a classic exam scenario: a child or elderly patient presents with "abdominal pain" and fever → don't forget to listen to the chest and check CXR! The phrenic nerve (C3-5) innervates the diaphragmatic pleura, and inflammation here causes referred pain to the shoulder tip (C3-5 dermatome) or upper abdomen.

9.2 Signs

Signs vary and depend on clinical severity ^[1]:

Sign	Pathophysiological Basis
Fever ^[1]	Cytokine-mediated thermoregulatory set-point elevation (as above)
↓BP (hypotension) ^[1]	Sepsis → vasodilation from nitric oxide and cytokines → distributive shock. Also: dehydration from fever and poor oral intake → hypovolaemic component
↑HR (tachycardia) ^[1]	Compensatory response to hypotension (baroreceptor reflex), fever (HR ↑ ~10 bpm per 1°C rise), hypoxaemia (sympathetic activation), and pain
↑RR (tachypnoea) ^[1]	Hypoxaemia → peripheral chemoreceptor stimulation; ↓lung compliance → ↑work of breathing; metabolic acidosis from sepsis → respiratory compensation
± ↓SpO₂ ^[1]	V/Q mismatch and intrapulmonary shunting → reduced arterial oxygen saturation

Appearing unwell, toxic: reflects severity of systemic inflammatory response
Use of accessory muscles: sternocleidomastoid, scalenes, intercostals → indicates significant respiratory distress
Central cyanosis: visible when deoxyhaemoglobin > 5 g/dL → indicates severe hypoxaemia (PaO₂ usually < 60 mmHg)
Dehydration: dry mucous membranes, reduced skin turgor → from fever, sweating, poor oral intake
Herpes labialis: reactivation of HSV-1 due to febrile illness — classically associated with pneumococcal pneumonia (non-specific but a useful clinical association)

Sign	Pathophysiological Basis
↓Air entry ^[1]	Consolidated lung does not expand normally → reduced air movement through the affected area
Coarse crackles (usually present) ^[1]	Air bubbling through inflammatory exudate (fluid) in the alveoli during inspiration. Crackles are "coarse" (low-pitched, loud) because the exudate in pneumonia is thick and proteinaceous, unlike the fine crackles of pulmonary oedema or fibrosis.
Percussion dullness (consolidation sign) ^[1]	Normal lung is resonant because it is air-filled. Consolidated lung (filled with exudate instead of air) transmits sound poorly → dull percussion note (similar to percussing a solid organ like the liver)
Bronchial breath sounds (consolidation sign) ^[1]	Normally, breath sounds are "vesicular" (soft, low-pitched, inspiration > expiration) because air in normal lung tissue filters out high-frequency sounds. In consolidation, the solid, exudate-filled lung conducts high-frequency sounds directly from the large airways to the chest wall → breath sounds become "bronchial" (loud, high-pitched, with equal inspiration and expiration, and a gap between them)
↑Vocal resonance (consolidation sign) ^[1]	Same principle: consolidated lung transmits voice sounds more efficiently than air-filled lung. Patient says "99" → sound is louder and clearer over consolidated area. Whispering pectoriloquy (whispered "1-2-3" heard clearly through stethoscope) is the most specific sign of consolidation.
↑Tactile vocal fremitus	Palpable vibration increased over consolidated lung — solid medium transmits vibrations better than air

Understanding Consolidation Signs from First Principles

Think of it this way: air is a poor conductor of sound, solid is a good conductor. Normal lung is air-filled → muffles sounds. Consolidated lung is solid (filled with exudate) → transmits sounds efficiently. This explains ALL consolidation signs:

Bronchial breath sounds (high-frequency sounds transmitted)
↑Vocal resonance (voice transmitted more clearly)
↑Tactile vocal fremitus (vibrations transmitted better)
Dull percussion (solid = dull, air = resonant)

Contrast this with pleural effusion, which also causes dullness but has ↓breath sounds, ↓vocal resonance, and ↓tactile fremitus because fluid between the lung and chest wall absorbs and blocks sound transmission.

This is a high-yield comparison for clinical exams:

Feature	Consolidation (Pneumonia)	Pleural Effusion
Percussion	Dull	Stony dull
Breath sounds	Bronchial	↓ or absent
Vocal resonance	↑	↓ (except aegophony at upper border)
Tactile fremitus	↑	↓
Trachea	Central (unless massive collapse)	Deviated AWAY from effusion (if large)
Mediastinal shift	No	Yes (if large; pushed away)

In elderly patients, pneumonia may present atypically (this is a different "atypical" from atypical organisms — here it means atypical clinical presentation):

Confusion may be the only or predominant feature (delirium)
Falls — due to generalized weakness and hypotension
Absence of fever — blunted febrile response due to immunosenescence
Functional decline — "not themselves," reduced mobility, incontinence

A 94-year-old woman with dementia, stroke, CAD, and CKD was transferred from her old age home for fever and productive cough. Temperature 38.5°C, SpO₂ 95% on 2L nasal cannula, crepitus in right lower zone, WBC 22.0 × 10⁹, CXR showed RLL infiltrate ^[13]. This is a classic geriatric pneumonia scenario — old, comorbid, institutionalized, with aspiration risk (bedbound, non-communicable).

Certain extrapulmonary findings can point to specific organisms:

Finding	Organism Suggested	Mechanism
Hyponatraemia	Legionella ^[11], also other atypical, COVID	SIADH triggered by pulmonary inflammation
Diarrhoea	Legionella	GI macrophage involvement
Bullous myringitis (blisters on tympanic membrane)	Mycoplasma	Unclear; possibly immune-mediated
Erythema multiforme	Mycoplasma	Immune complex-mediated skin reaction
Cold agglutinins / haemolytic anaemia	Mycoplasma	IgM autoantibodies against RBC I-antigen
Relative bradycardia (pulse-temperature dissociation)	Legionella, psittacosis	Endotoxin-mediated effect on cardiac conduction
Hepatitis / ↑LFT	Legionella, Q fever	Hepatic involvement by intracellular organisms
Skin lesions (ecthyma gangrenosum)	Pseudomonas aeruginosa	Vascular invasion by organism in bacteraemia

10. Important Lecture Slide Concepts

From the GC 052 Fever and Purulent Sputum lecture ^[14]:

Key learning points include: recognizing that fever with purulent sputum is the classic presentation of bacterial pneumonia, understanding the approach to empirical antibiotic selection based on local microbiology and antibiotic sensitivity patterns, and the importance of obtaining appropriate microbiological specimens before starting antibiotics.

From the General Clerkship Teaching Clinic – Haemoptysis (Prof MSM Ip, 25 October 2024) ^[10]:

Learning Points on Community-acquired pneumonia:

Symptoms and signs of pneumonia

How to assess severity of pneumonia

Relevant microbiological investigations

Local microbiology and antibiotic susceptibility (or resistance)

Choose the appropriate antibiotic coverage for the likely pathogen/s (or "group" of microbes) in each case of pneumonia based on:

Host factors: clinical features, background factors e.g., travel, any chronic lung disease such as COPD, bronchiectasis

Microbial epidemiology and recent trends of infection in the locality

Local antibiotic sensitivity pattern

Relevant supportive treatment

Monitor – for treatment response and subsequent complications

From Prof Ip's Haemoptysis slides ^[10]:

Important history to elicit: Details of the haemoptysis – amount and characteristics; other associated clinical symptoms – respiratory and other systems, constitutional; demographics, comorbidities & predisposing factors to various underlying conditions: e.g., age, smoking, recent travel or contacts, diabetes, anticoagulants or anti-platelet agents, past history of tuberculosis, occupational exposure…

From Respiratory – Introduction to Resp Investigations ^[15]:

Key points relevant to pneumonia:

CXR is the primary imaging modality for diagnosing pneumonia
CT thorax has higher sensitivity but is not routinely needed for straightforward CAP
Air bronchograms on CXR = air-filled bronchi visible within consolidated lung parenchyma (the surrounding alveoli are opacified with exudate while the bronchi remain patent) — highly suggestive of consolidation

From GC 102 – Fever after Chemotherapy: Infections in Immunocompromised Hosts ^[7]:

Key concepts: Immunocompromised patients present with atypical organisms; the degree and type of immunosuppression determines the likely pathogen spectrum. Neutropenic patients are susceptible to bacteria and fungi (Aspergillus, Candida); T-cell deficient patients to Pneumocystis, CMV, mycobacteria; hypogammaglobulinaemic patients to encapsulated organisms.

From GC 101 – Diagnosis of Infections ^[16]:

Key principles: Obtain appropriate specimens before starting antibiotics; blood cultures (at least 2 sets from different sites), sputum culture (morning specimen, deep cough), and consider atypical pathogen testing (urinary antigens for Legionella and Pneumococcus, serology for Mycoplasma).

From GC 021 – Upper Respiratory Tract Infections ^[17]:

Important to distinguish URTI (which is usually viral and self-limiting) from LRTI/pneumonia (which often requires antibiotics). URTI symptoms include rhinorrhoea, sore throat, sneezing — these do NOT indicate pneumonia. Pneumonia should be suspected when there is fever with productive cough, pleuritic chest pain, dyspnoea, and/or systemic toxicity.

From GC 106 – Practical Issues in Antibiotic Use ^[18]:

Principles of empirical antibiotic selection: consider the most likely organism based on clinical syndrome and local epidemiology; choose the narrowest effective spectrum; de-escalate when culture results are available; complete the appropriate duration of therapy; monitor for adverse effects and treatment response.

From GC 105 – Medically Important Microbes: What Every Doctor Should Know ^[19]:

Key organisms to know for pneumonia: S. pneumoniae (Gram-positive diplococcus, lancet-shaped, optochin-sensitive, bile-soluble); H. influenzae (Gram-negative coccobacillus); S. aureus (Gram-positive cocci in clusters, coagulase-positive); K. pneumoniae (Gram-negative rod, mucoid colonies); P. aeruginosa (Gram-negative rod, oxidase-positive, produces pyocyanin/green pigment); Legionella (Gram-negative rod, requires buffered charcoal yeast extract agar); Mycoplasma (no cell wall → Gram-stain negative).

From GC 077 – Pleural Effusion in a Chronic Smoker ^[20]:

Parapneumonic effusion is a complication of pneumonia. Simple parapneumonic effusion resolves with antibiotics. Complicated parapneumonic effusion (pH < 7.2, glucose < 2.2 mmol/L, LDH > 1000 IU/L) and empyema (frank pus in pleural space) require drainage. This will be covered in detail in the Complications section.

From Data Interpretation – Respiratory ^[21]:

Interpreting ABG in pneumonia: expect Type 1 respiratory failure (hypoxaemia with normal or low PaCO₂) due to V/Q mismatch with compensatory hyperventilation. If PaCO₂ rises (Type 2 respiratory failure), this indicates exhaustion/impending respiratory arrest – urgent escalation needed.

From GC 040 – Cough and Wheezing: Asthma and Allergic Lung Diseases ^[22]:

Important differential: not all fever + wheeze = pneumonia. Infective exacerbation of asthma can mimic pneumonia. However, new focal consolidation on CXR distinguishes pneumonia from asthma exacerbation.

From GC 083 – Shortness of Breath in a Construction Site Worker ^[23]:

Occupational exposure (e.g., silicosis) predisposes to TB and NTM infection. Always ask about occupational history when assessing pneumonia in Hong Kong.

When a patient presents with suspected pneumonia, your clinical approach before moving to differential diagnosis and investigations should be structured as follows:

Recognize the clinical syndrome: Fever + cough (initially dry → productive) + dyspnoea ± pleuritic chest pain ± systemic toxicity
Assess severity: Vital signs (temperature, HR, BP, RR, SpO₂), mental status, work of breathing → guides disposition (outpatient vs ward vs ICU)
Determine the clinical context: CAP vs HAP vs immunocompromised → guides empirical antibiotic choice
Take a thorough history including TOCC: Travel, Occupation, Contact, Clustering → may reveal exposure to specific organisms
Identify risk factors: Age, smoking, comorbidities, immunosuppression, aspiration risk, recent antibiotics
Perform examination looking for: Signs of consolidation vs effusion, respiratory distress, signs of sepsis, extrapulmonary clues

High Yield Summary

Definition: Pneumonia = acute infection of pulmonary parenchyma (alveoli ± interstitium) with new infiltrate on imaging plus clinical features of infection.

Epidemiology: Leading infectious cause of death worldwide; top 3 cause of death in HK. Extremes of age (< 5y, > 65y) at highest risk.

Key Risk Factors: Extremes of age, smoking, COPD/bronchiectasis, immunocompromise, DM, CKD/CLD, neurological impairment (aspiration risk), splenectomy, institutionalization.

Routes of Infection: Inhalation (most common), aspiration, haematogenous, direct spread.

Classification: By context (CAP, HAP, VAP, HCAP, aspiration, immunocompromised); by anatomy (lobar, bronchopneumonia, interstitial); by organism (typical, atypical, viral, fungal, mycobacterial).

Key Organisms by Context: CAP → S. pneumoniae #1; HAP/VAP → Gram-negatives + MRSA; Immunocompromised → PJP, Aspergillus, CMV, TB. HK-specific: B. pseudomallei, TB endemic, avian flu.

Symptoms: Fever/chills/rigors (cytokine-mediated), cough (dry→productive with mucopurulent sputum ± haemoptysis), pleuritic chest pain (visceral pleura involvement), dyspnoea (V/Q mismatch), upper abdominal pain (lower lobe→referred via phrenic nerve).

Signs: Fever, tachycardia, tachypnoea, hypotension, ↓SpO₂. Chest: ↓air entry, coarse crackles, consolidation signs (dull percussion, bronchial breathing, ↑vocal resonance/tactile fremitus).

Sputum Clues: Rusty = pneumococcal; currant-jelly = Klebsiella; bloody mucopurulent = bronchiectasis/abscess; blood-streaked = TB/cancer; pink frothy = pulmonary oedema.

Atypical Pneumonia: Mycoplasma, Chlamydophila, Legionella → less severe respiratory symptoms, prominent systemic complaints, normal WBC. Term now considered less relevant clinically.

Always take TOCC history. Always consider TB in Hong Kong.

Active Recall - Pneumonia (Definition, Epidemiology, Risk Factors, Aetiology, Pathophysiology, Clinical Features)

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.164 – Pneumonia section) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.154 – Pneumonia overview and classification) ^[3] Centre for Health Protection, Hong Kong SAR (general epidemiological data) ^[4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p.33 – Clinical presentation of TB) ^[5] Senior notes: Block A - A jaundiced and incoherent patient_ liver failure.pdf (p.12 – Infections in liver failure) ^[6] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (p.20 – OPSI and post-splenectomy vaccination) ^[7] Lecture slides: GC 102. Fever after chemotherapy infections in immunocompromised hosts [Handout].pdf ^[8] Senior notes: Ryan Ho Respiratory.pdf (p.65 – CAP complications and unresolving pneumonia) ^[9] Senior notes: learning_points_output.txt (Respiratory - Four Cases of Haemoptysis learning points) ^[10] Lecture slides: General clerkship Teaching Clinic - Haemoptysis_Prof MSM Ip_25 October 2024.pdf (p.16, p.47) ^[11] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.21 – SIADH causes including chest infection) ^[12] Senior notes: Block A - Sudden severe chest pain_ acute myocardial infarction; aortic dissection.pdf (p.5 – Predisposing factors for AMI including pneumonia) ^[13] AOS material: AOS - Geriatrics.pdf (p.25 – Old age home resident with pneumonia scenario) ^[14] Lecture slides: GC 052. Fever and purulent sputum.pdf ^[15] Lecture slides: Respiratory- Introduction to Resp investigations (CXR, CT thorax and lung function tests).pdf ^[16] Lecture slides: GC 101. Diagnosis of infections.pdf ^[17] Lecture slides: GC 021. Upper respiratory tract infections.pdf ^[18] Lecture slides: GC 106. Practical issues in antibotic use [Notes].pdf ^[19] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf ^[20] Lecture slides: GC 077. Pleural effusion in a chronic smoker.pdf ^[21] Lecture slides: Data Interpretation - Respiratory - by Dr DCL Lam (Rotation 2).ppt.pdf ^[22] Lecture slides: GC 040. Cough and wheezing_asthma and allergic lung diseases.pdf ^[23] Lecture slides: GC 083. Shortness of breath in a construction site worker.pdf

Differential Diagnosis of Pneumonia

The differential diagnosis of pneumonia matters at two levels: (1) conditions that mimic pneumonia (i.e., the patient has fever + cough + CXR abnormality but the actual diagnosis is something else), and (2) once pneumonia is confirmed, determining the causative organism to guide targeted therapy. This section focuses primarily on the first — what else could this be?

Pneumonia's cardinal features — fever, cough, dyspnoea, and a new radiographic infiltrate — are individually non-specific. Many conditions share one or more of these features:

Fever → any infection, malignancy, autoimmune disease
Cough → URTI, asthma, COPD exacerbation, heart failure, GERD, lung cancer
Dyspnoea → cardiac failure, PE, pneumothorax, anaemia, metabolic acidosis
CXR opacity → atelectasis, pulmonary oedema, lung cancer, pulmonary haemorrhage, organising pneumonia

The key clinical skill is integrating the tempo of illness, associated features, risk factors, and response to treatment to distinguish pneumonia from its mimics and, within pneumonia, to identify the likely pathogen.

2. Major Differential Diagnoses — Organised by Category

The differentials can be systematically categorised as infective, exacerbation of chronic lung disease, and non-infective ^[8]:

Condition	Key Distinguishing Features	Why It Mimics Pneumonia
Acute bronchitis ^[8]	URTI followed by prolonged cough ± wheezing/SOB, no signs of consolidation on CXR, less systemic upset ^[8]	Shares cough and fever with pneumonia. Crucially, there is no new infiltrate on CXR — this is the defining distinction. Acute bronchitis is usually viral and self-limiting; antibiotics are NOT indicated.
URTI / Influenza / COVID-19 ^[8]	No consolidation on CXR ^[8]; predominantly upper airway symptoms (rhinorrhoea, sore throat, sneezing); influenza may cause myalgias, headache, and high fever out of proportion to respiratory signs	Overlapping systemic symptoms (fever, malaise) but lung parenchyma is not involved. However, influenza and COVID-19 can progress to true viral pneumonia or predispose to secondary bacterial pneumonia.
Tuberculosis ^[8]	History of exposure, upper lobe predilection with cavitary lesion, constitutional symptoms, subacute onset ^[8]; blood-streaked sputum suggests TB ^[9]^[10]; risk factors include diabetes mellitus, CKD, immunosuppression (HIV, anti-TNF), institutionalization ^[9]	TB can present acutely mimicking bacterial pneumonia, though it rarely presents as an acute pneumonia-like picture — usually a much more indolent presentation ^[4]. In Hong Kong where TB is endemic, this must always be on the differential, especially for unresolving pneumonia.
Bronchiolitis (paediatric) ^[2]^[24]	Occurs in children < 2 years; preceding coryzal symptoms → wheeze and crackles; RSV most common cause; clinical diagnosis ^[1]	In infants, can be confused with pneumonia. Key distinction: bronchiolitis causes diffuse wheeze and hyperinflation rather than focal consolidation.
Lemierre syndrome ^[2]^[24]	Considered in adolescents and young adults whose illness began with pharyngitis ^[2]^[24]; Fusobacterium sp. causes jugular vein suppurative thrombophlebitis → bacteraemia → metastatic spread to lungs and mediastinum ^[2]^[24]	Presents with septic pulmonary emboli mimicking multifocal pneumonia. Clue: recent severe pharyngitis + neck pain/swelling + multiple peripheral lung nodules/cavities on CT.
Lung abscess	Subacute presentation with foul-smelling sputum (anaerobes), fever, weight loss; CXR shows thick-walled cavity with air-fluid level; risk factors include aspiration, poor dentition, alcoholism	Can coexist with or develop from pneumonia. Distinguished by cavitation and the particular chronicity/sputum character.

High Yield – TB vs Pneumonia in Hong Kong

TB must always be considered in Hong Kong when pneumonia does not respond to standard antibiotics ^[4]^[8]. Red flags for TB: subacute onset (weeks), upper lobe involvement, cavitation, night sweats, weight loss, contact history, and risk factors (DM, CKD, HIV, anti-TNF therapy, institutionalization) ^[9]. However, TB can occasionally present acutely — have a low threshold for sputum AFB smear/culture and Xpert MTB/RIF.

Condition	Key Distinguishing Features	Why It Mimics Pneumonia
AE-COPD ^[8]	History of smoking, wheezing with acute worsening in SOB, sputum ^[8]; chronic history of progressive dyspnoea; spirometry showing fixed obstruction; CXR: hyperinflation ± bullae, no new focal consolidation	Both present with cough, sputum, and dyspnoea. However, AE-COPD typically shows diffuse changes rather than focal consolidation. Note that pneumonia can be the TRIGGER for AE-COPD — the two can coexist.
Asthma exacerbation ^[8]	Worsening in cough, wheezing and SOB ^[8]; history of atopy, variable symptoms, trigger exposure; good response to bronchodilators; no consolidation on CXR	Wheeze + cough + dyspnoea overlap with pneumonia. Distinguish by episodic nature, trigger history, reversibility, and absence of focal CXR changes. Not all fever + wheeze = pneumonia; new focal consolidation on CXR distinguishes pneumonia from asthma exacerbation ^[22].
AE-bronchiectasis ^[8]	History of bronchiectasis, cough, sputum, SOB ^[8]; chronic copious sputum production; CT: dilated airways (signet ring sign), tram tracks	Acute exacerbation presents with increased sputum volume/purulence ± haemoptysis, mimicking new pneumonia. HRCT shows pre-existing structural changes distinguishing it from de novo pneumonia.

Condition	Key Distinguishing Features	Why It Mimics Pneumonia
Acute decompensated heart failure (ADHF) ^[8]	Orthopnoea/PND, peripheral oedema, CXR bat-wing opacity ^[8]; history of cardiac disease; elevated BNP/NT-proBNP; bilateral basal crackles but also Kerley B lines, upper lobe venous diversion, cardiomegaly on CXR ^[25]	Both cause cough, dyspnoea, and bilateral crackles. BNP can be used to differentiate cardiac from respiratory causes of SOB ^[26] — elevated BNP strongly favours cardiac origin. CXR pattern also differs: ADHF → central 'bat's wing' distribution, bilateral, symmetrical ^[25]; pneumonia → typically focal/asymmetric. However, pneumonia can precipitate ADHF in patients with pre-existing heart disease.
Pulmonary embolism (PE) ^[8]^[27]	Risk factors for VTE, dyspnoea, pleuritic chest pain, haemoptysis, tachycardia, DVT symptoms ^[8]; acute onset pleuritic chest pain + dyspnoea + haemoptysis ^[27]; Wells score assessment; CTPA is diagnostic	PE with pulmonary infarction can cause fever, pleuritic chest pain, haemoptysis, and a wedge-shaped peripheral opacity on CXR (Hampton's hump) — closely mimicking pneumonia. Look for unilateral leg swelling (DVT), risk factors (immobility, surgery, OCP, malignancy), and ECG changes (sinus tachycardia, S1Q3T3, right heart strain) ^[27].
Atelectasis ^[8]	Can mimic pneumonia on CXR ^[8]; no fever or systemic upset in simple atelectasis; volume loss signs (mediastinal shift towards opacity, elevated hemidiaphragm)	Collapsed lung appears opaque on CXR, resembling consolidation. Key distinction: atelectasis causes volume loss (mediastinal shift TOWARDS the opacity), whereas consolidation/effusion causes displacement AWAY or no shift. No air bronchograms in atelectasis (airways also collapsed).
Cryptogenic organising pneumonia (COP) ^[8]	Can closely mimic CAP; subacute onset of pneumonia not responding to multiple courses of antibiotic therapy ^[8]; migratory infiltrates; steroid-responsive	COP is a form of interstitial lung disease that presents with consolidative opacities on CXR, often bilateral and peripheral, mimicking multifocal pneumonia. The hallmark clue is failure to respond to antibiotics with persistence or migration of infiltrates. Biopsy shows Masson bodies (intra-alveolar buds of granulation tissue).
Acute eosinophilic pneumonia ^[8]	Peripheral and BAL eosinophilia, bilateral diffuse mixed GGO/reticular opacities ^[8]; often in young patients with recent smoking initiation or drug exposure; rapid response to steroids	Presents acutely with fever, cough, dyspnoea — identical to severe CAP. CXR shows bilateral diffuse infiltrates. The clue is peripheral eosinophilia (not always present initially; BAL eosinophilia is more sensitive).
Hypersensitivity pneumonitis ^[8]	Positive exposure history, diffuse micronodules with GGO ^[8]; occupational/environmental antigen exposure (bird fancier, farmer); recurrent episodes with exposure	Acute HP presents with fever, cough, and dyspnoea 4–8 hours after antigen exposure, with bilateral GGO on HRCT. Looks like atypical pneumonia but has a temporal relationship to antigen exposure and responds to antigen avoidance/steroids, not antibiotics.
Lung cancer	Especially post-obstructive pneumonia: tumour obstructs a bronchus → distal collapse and infection; suspect when pneumonia recurs in the same lobe, or in a chronic smoker with haemoptysis and weight loss ^[9]^[10]; clubbing, cervical/supraclavicular lymphadenopathy, Horner's syndrome, SVC obstruction may be present ^[10]	Can present as "pneumonia" that doesn't resolve. Always consider malignancy in a smoker > 50 years with non-resolving consolidation. Bronchoscopy is both diagnostic and therapeutic.
Pulmonary vasculitis / alveolar haemorrhage	Haemoptysis with bilateral infiltrates; associated with anti-GBM disease (Goodpasture's), ANCA-associated vasculitis (GPA, MPA); may have renal involvement (pulmonary-renal syndrome)	Bilateral alveolar infiltrates with haemoptysis mimic bilateral pneumonia. Look for dropping haemoglobin without overt external bleeding, raised DLCO (blood in alveoli absorbs CO), and serological markers (ANCA, anti-GBM).
Drug-induced pneumonitis	History of offending drug (methotrexate, amiodarone, nitrofurantoin, bleomycin); bilateral interstitial or alveolar infiltrates; improvement on drug withdrawal	Presents with cough, dyspnoea, and CXR changes. Fever may or may not be present. Drug history is the critical clue.

High Yield – Unresolving Pneumonia: The 'Triple Think'

When pneumonia fails to respond to appropriate antibiotics within 48–72 hours, think in three directions ^[8]:

Wrong organism: Is this TB, a resistant organism, or a fungal infection?
Complicated pneumonia: Has empyema or lung abscess developed?
Not pneumonia at all: Is this actually lung cancer, COP, PE with infarction, eosinophilic pneumonia, or vasculitis?

Always re-examine, repeat CXR, and escalate investigations (CT thorax, bronchoscopy) for non-resolving cases.

Feature	Pneumonia	ADHF	PE	Asthma/COPD Exacerbation	TB	Lung Cancer
Onset	Acute (days)	Acute/subacute	Acute (sudden)	Acute on chronic	Subacute (weeks-months)	Chronic/insidious
Fever	Yes (typically high)	Usually absent	Low-grade possible	Low-grade if infective trigger	Low-grade, afternoon	Possible (if post-obstructive)
Cough	Productive, purulent	Pink frothy sputum	Dry ± haemoptysis	Usually dry or with mucoid sputum	Productive ± haemoptysis	Persistent ± haemoptysis
Chest pain	Pleuritic	Absent or cardiac	Pleuritic	Chest tightness	Dull ache	Dull, persistent
CXR	Focal consolidation	Bilateral bat-wing, cardiomegaly	Wedge opacity, clear lung fields	Hyperinflation	Upper lobe, cavitation	Mass ± collapse ± effusion
WBC	↑↑ Neutrophilia	Normal/mildly ↑	Normal/mildly ↑	Normal/mildly ↑	Normal/mildly ↑	Normal or ↑
BNP	Normal (unless cardiac comorbidity)	Elevated ^[26]	May be ↑ (RV strain)	Normal	Normal	Normal
Key Ix	Sputum culture, blood cultures	Echo, BNP	CTPA, D-dimer	PFT, bronchodilator response	AFB smear/culture, GeneXpert	Bronchoscopy, CT biopsy

This is one of the most frequently tested distinctions in clinical exams and a daily problem on the wards:

BNP (B-type natriuretic peptide) can be used to differentiate cardiac from respiratory causes of shortness of breath ^[26]:

Only a cardiac cause of SOB would result in elevated BNP ^[26]
Respiratory causes would not affect this ^[26]
However, BNP is not 100% specific for heart failure — there are non-cardiac causes of raised BNP including lung disease, renal disease, anaemia ^[26]
Therefore, interpret with clinical context ^[26]

Practical approach:

History: Orthopnoea and PND strongly favour heart failure; fever with rigors strongly favours pneumonia
Examination: S3 gallop, elevated JVP, bilateral pitting oedema → heart failure; focal crackles with bronchial breathing → pneumonia
CXR pattern: Central bat-wing with Kerley B lines and upper lobe diversion → heart failure; focal lobar consolidation with air bronchograms → pneumonia
BNP: < 100 pg/mL makes heart failure very unlikely; > 500 pg/mL makes it very likely; 100–500 is a grey zone requiring clinical correlation
Response to diuretics vs antibiotics: Rapid improvement with diuretics → heart failure; improvement over 48–72 hours with antibiotics → pneumonia

Exam Pearl – Both Can Coexist

Pneumonia can CAUSE acute decompensation of heart failure (via increased myocardial oxygen demand, tachycardia, and systemic inflammation). Conversely, heart failure with pulmonary congestion creates a favourable environment for bacterial growth. In elderly patients, both commonly coexist — so finding evidence for one does not exclude the other. Treat both when present.

CXR is always done on clinical suspicion of pneumonia ^[8]. The radiographic pattern helps narrow differentials:

CXR Pattern	Differential Diagnoses
Focal lobar consolidation with air bronchograms	Bacterial pneumonia (S. pneumoniae, K. pneumoniae), post-obstructive pneumonia (lung cancer), COP
Patchy bilateral infiltrates	Bronchopneumonia (S. aureus, Gram-negatives), viral pneumonia, PJP, pulmonary oedema, alveolar haemorrhage
Upper lobe consolidation ± cavitation	TB, K. pneumoniae, fungal infection, lung cancer
Bilateral diffuse interstitial infiltrates	Viral or atypical pneumonia as well as Pneumocystis jirovecii pneumonia ^[2]^[24]; hypersensitivity pneumonitis, drug-induced pneumonitis
Wedge-shaped peripheral opacity	PE with infarction (Hampton's hump)
Bilateral central bat-wing opacity	Cardiogenic pulmonary oedema ^[25]
Consolidation with bulging fissure	K. pneumoniae (large volume exudate pushes lobar boundary)
Multiple round opacities / cavities	Septic emboli (endocarditis), metastases, granulomatosis with polyangiitis, Lemierre syndrome

The silhouette sign helps localize the lesion ^[2]^[24]:

Left heart border is clearly seen = lesion in LLL; left heart border is NOT clearly seen = lesion in LUL ^[2]^[24]
Right heart border is clearly seen = lesion in RLL; right heart border is NOT clearly seen = lesion in RML ^[2]^[24]

From GC 141 – A Child with Cough ^[28]:

The approach to acute cough in children involves asking structured questions:

Is this an acute URI? → Coryzal symptoms, fever, sore throat → URI

Is this a croup syndrome? → Stridor, barking/croupy cough, hoarseness ± fever → Viral croup, recurrent spasmodic croup, bacterial tracheitis

Is this a lower respiratory tract illness? → Tachypnoea (> 60 for < 2m, > 50 for 2–12m, > 40 for > 1y), respiratory distress with increased work of breathing, chest signs (crepitations or wheeze/rhonchi), fever → Acute bronchiolitis, pneumonia (viral, bacterial), asthma

Is this an allergic/atopic illness? → Seasonal and diurnal variation, association with rhinitis → Post-nasal drip, reactive airway/asthma

In children, the key differential is between bronchiolitis (< 2 years, wheeze-predominant, diffuse, viral) and pneumonia (focal crackles/consolidation, higher fever, more systemic toxicity). Tachypnoea is the single most sensitive sign of pneumonia in children — age-specific thresholds must be used ^[28].

Here are common exam-style clinical scenarios with the most relevant differentials:

Scenario	Think About
Elderly nursing home resident with fever, confusion, CXR infiltrate	Aspiration pneumonia, CAP, UTI with incidental CXR finding, ADHF
Young adult with dry cough, myalgias, bilateral infiltrates, normal WBC	Atypical pneumonia (Mycoplasma, Chlamydophila) ^[8], viral pneumonia, COVID-19
Smoker > 50y with non-resolving pneumonia in same lobe	Post-obstructive pneumonia secondary to lung cancer ^[9]^[10] — needs bronchoscopy
Post-chemotherapy neutropenic patient with fever and bilateral GGO	PJP, invasive aspergillosis, CMV pneumonitis ^[7]
Farmer with recurrent episodes of fever + cough after barn exposure	Hypersensitivity pneumonitis (farmer's lung) ^[8]
Young patient with recent pharyngitis → septic-appearing with multiple lung nodules	Lemierre syndrome ^[2]^[24]
Sudden pleuritic chest pain + dyspnoea + unilateral leg swelling	PE ^[8]^[27] — use Wells score, D-dimer, CTPA
Migratory infiltrates not responding to multiple antibiotic courses	Cryptogenic organising pneumonia (COP) ^[8] — needs biopsy, responds to steroids

From Respiratory Four Cases of Haemoptysis (Prof Mary Ip) ^[9]:

When a patient presents with CAP, the key management steps on admission are: (1) Assess severity of condition and monitor; (2) Send specimens for microbiology – to include common causes of community-acquired pneumonia; (3) Start treatment with coverage of most likely microbial organisms; (4) Provide supportive treatment – e.g., hypotension, respiratory failure. ^[9]^[10]

From Prof Ip's physical examination teaching ^[10]:

Important physical signs to elicit in haemoptysis include: clubbing (lung cancer, bronchiectasis), cervical and supraclavicular lymph nodes (malignancy vs tuberculosis), Horner's syndrome and SVC obstruction (lung cancer), deep vein thrombosis (malignancy or pulmonary embolism). These signs help differentiate pneumonia from its serious mimics.

High Yield Summary – Differential Diagnosis of Pneumonia

The major differentials of pneumonia fall into three categories ^[8]:

Infective mimics: Acute bronchitis (no CXR consolidation), URTI/influenza/COVID (no consolidation), TB (subacute, upper lobe, cavitation, endemic in HK), bronchiolitis (paediatric, < 2y), Lemierre syndrome (post-pharyngitis in young adults)
Exacerbation of chronic lung disease: AE-COPD (diffuse, smoker), asthma exacerbation (wheeze, triggers, reversible), AE-bronchiectasis (pre-existing structural disease)
Non-infective mimics: ADHF (orthopnoea, PND, bat-wing CXR, elevated BNP), PE (pleuritic pain, DVT, wedge opacity), atelectasis (volume loss), COP (non-resolving, migratory, steroid-responsive), eosinophilic pneumonia (BAL eosinophilia), lung cancer (non-resolving, same lobe, smoker), hypersensitivity pneumonitis (exposure history), vasculitis (haemoptysis, renal involvement)

Key distinguishing tools: CXR pattern, BNP, D-dimer/CTPA, sputum AFB, response to antibiotics, bronchoscopy for non-resolving cases.

Always consider TB in Hong Kong. Always re-evaluate if pneumonia fails to respond within 48–72 hours.

Active Recall - Differential Diagnosis of Pneumonia

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.163-164 – Pneumonia section) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.154, p.161, p.166) ^[4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p.33 – Clinical presentation of TB) ^[7] Lecture slides: GC 102. Fever after chemotherapy infections in immunocompromised hosts [Handout].pdf ^[8] Senior notes: Ryan Ho Respiratory.pdf (p.63 – DDx table, CXR findings, unresolving pneumonia) ^[9] Senior notes: learning_points_output.txt (Respiratory – Four Cases of Haemoptysis) ^[10] Lecture slides: General clerkship Teaching Clinic - Haemoptysis_Prof MSM Ip_25 October 2024.pdf (p.14, p.16, p.47, p.48) ^[22] Lecture slides: GC 040. Cough and wheezing_asthma and allergic lung diseases.pdf ^[24] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.146-151 – Pneumonia microbiology and DDx) ^[25] Senior notes: Ryan Ho Fundamentals.pdf (p.238-239 – CXR patterns, pulmonary oedema vs other causes) ^[26] Senior notes: Block A - Shortness of breath on exertion_ heart failure.pdf (p.11 – BNP to differentiate cardiac vs respiratory SOB) ^[27] Senior notes: Ryan Ho Haemtology.pdf (p.131 – VTE clinical features and differentials) ^[28] Lecture slides: GC 141. A child with cough acute and chronic cough in children.pdf (p.15)

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities

1. Diagnostic Criteria for Pneumonia

There is no single universally accepted set of "diagnostic criteria" for pneumonia in the way that Duke criteria define endocarditis or Jones criteria define rheumatic fever. Instead, the diagnosis of pneumonia is made by integrating clinical features + radiographic findings.

Diagnosis is usually based on clinical signs and symptoms together with compatible CXR findings ^[1]^[8].

A diagnosis of pneumonia requires both:

Clinical features consistent with acute lower respiratory tract infection:
- New-onset cough (± sputum production) AND/OR fever AND/OR dyspnoea
- ± pleuritic chest pain, tachypnoea, tachycardia
- ± signs of consolidation on auscultation (crackles, bronchial breathing, ↑vocal resonance)
Radiographic evidence of new pulmonary infiltrate:
- New consolidation, air bronchograms, or infiltrates on CXR or CT thorax

Why do we need BOTH? Clinical features alone cannot distinguish pneumonia from acute bronchitis, URTI, or exacerbation of chronic lung disease. The CXR is what confirms parenchymal involvement. Conversely, a CXR opacity alone could be atelectasis, tumour, or pulmonary oedema — the clinical context confirms infection.

Pneumonia = syndrome of fever/hypothermia, chills, SOB, cough, chest pain, sputum production, etc. PLUS compatible consolidation on X-ray imaging ^[29].

High Yield – CXR is Gold Standard for Diagnosis

CXR is considered the gold-standard for CAP diagnosis ^[1]^[29]. If clinically suspicious but CXR is negative, give empirical antibiotics and consider CT/MRI ^[1] — initial CXR sensitivity is approximately 70%, so a negative CXR does not exclude early pneumonia. Repeat CXR or proceed to CT if clinical suspicion remains high.

1.2 Severity Assessment Criteria

Once the diagnosis of pneumonia is established, you must assess severity — this determines the setting of care (outpatient vs ward vs ICU) and the empirical antibiotic regimen.

CURB-65 is the most widely used severity scoring system for CAP ^[29]:

Parameter	Criterion	Points	Pathophysiological Rationale
C – Confusion	AMTS ≤ 8/10 (or new-onset confusion)	1	Cerebral hypoperfusion (sepsis) or hypoxaemia-induced encephalopathy
U – Urea	> 7 mmol/L	1	Reflects pre-renal AKI from sepsis-related renal hypoperfusion; also a marker of dehydration
R – Respiratory rate	≥ 30 breaths/min	1	Severe V/Q mismatch requiring marked compensatory hyperventilation
B – Blood pressure	SBP < 90 mmHg or DBP ≤ 60 mmHg	1	Haemodynamic compromise — vasodilatory shock from sepsis
65 – Age	≥ 65 years	1	Reduced physiological reserve, immunosenescence, comorbidities

Interpretation ^[29]:

Score	30-day Mortality	Disposition
0–1	Low (1.5%)	Home treatment
2	Intermediate (9.2%)	Hospital short-stay treatment (consider hospital admission)
≥ 3	High (22%)	Assess for ICU admission

Why CURB-65 works: Each component represents a different organ system being affected by severe pneumonia — brain (confusion), kidney (urea), lungs (RR), cardiovascular (BP), and overall reserve (age). The more organs involved, the sicker the patient.

ATS criteria for severe CAP: 1 major OR ≥ 3 minor ^[29]:

	Criteria
Major	Invasive mechanical ventilation; septic shock requiring vasopressors
Minor	Confusion, urea > 7, RR ≥ 30, BP < 90/60, multilobar involvement, PaO₂/FiO₂ < 250

Meeting any major criterion = ICU admission. ≥ 3 minor criteria also warrants ICU consideration.

Uses 20 variables (demographics, comorbidities, vital signs, lab values) to stratify into Classes I–V
More complex than CURB-65 but more granular
Less commonly used at the bedside in HK practice; mentioned for completeness ^[29]

Exam Focus – CURB-65

CURB-65 is the severity score you are most likely to be tested on. Remember the mnemonic: Confusion, Urea > 7, Respiratory rate ≥ 30, Blood pressure < 90/60, age ≥ 65. Score 0–1 = home; 2 = consider admission; ≥ 3 = consider ICU.

The following algorithm outlines the stepwise approach when pneumonia is suspected:

Key teaching points from the algorithm:

CXR is ALWAYS done on clinical suspicion of pneumonia ^[1]^[8]

If clinically suspicious but CXR negative, give empirical antibiotics and consider CT/MRI ^[1]

Severity assessment (CURB-65) determines the care setting and investigation intensity

Microbiological investigations are not usually necessary for outpatients; usually only for hospitalized patients ^[1]^[8]

Non-response at 48–72 hours mandates re-evaluation

3. Investigation Modalities

Investigations serve four purposes:

Confirm the diagnosis (CXR)
Identify the causative organism (microbiology)
Assess severity and complications (blood tests, ABG)
Exclude differentials (D-dimer, BNP, CT)

3.1 Imaging

CXR: ALWAYS done on clinical suspicion of pneumonia ^[1]^[8].

CXR is the gold standard for CAP diagnosis with initial sensitivity ~70% ^[29]. A repeat CXR should be considered if the first is negative but clinical suspicion remains high.

Key CXR patterns and their interpretation ^[1]^[2]^[8]^[24]:

Pattern	Description	Typical Organisms
Lobar pneumonia: consolidation with air bronchograms ^[1]^[8]	Homogeneous opacification in a lobar pattern ^[2]^[24]; air bronchograms = "black airways against white background" because alveolar space is filled with fluid and appears white while airways retain air and appear black ^[2]^[24]	S. pneumoniae, K. pneumoniae
Bronchopneumonia: patchy infiltrates, segmental shadowing ^[1]^[8]	Multiple small nodular or reticulonodular opacities ^[2]^[24]; bilateral, often basal predominance	S. aureus, H. influenzae, Gram-negatives
Bilateral diffuse interstitial infiltrates	Usually indicates viral or atypical pneumonia as well as Pneumocystis jirovecii pneumonia ^[2]^[24]	Viruses, Mycoplasma, PJP
Alveolar infiltrates	Usually indicates bacterial pneumonia ^[2]^[24]	Typical bacteria
Cavitation	Thick-walled cavity ± air-fluid level	Anaerobes, S. aureus, K. pneumoniae, TB
Bulging fissure	Lobar consolidation with the interlobar fissure pushed outward	K. pneumoniae (copious exudate)
Complications: parapneumonic effusion, abscess, empyema ^[1]^[8]	Meniscus sign (effusion), cavity with air-fluid level (abscess)	Various

The silhouette sign helps localise the lesion ^[2]^[24]:

Left heart border is clearly seen → lesion in LLL; left heart border is NOT clearly seen → lesion in LUL (lingula) ^[2]^[24]
Right heart border is clearly seen → lesion in RLL; right heart border is NOT clearly seen → lesion in RML ^[2]^[24]

From first principles: The silhouette sign occurs because when 2 structures of similar densities are side by side, their borders become undefinable on plain CXR ^[2]^[24]. Normally heart and lung have different densities. ONLY in situations of consolidation and lung collapse does the lung density become similar to the heart ^[2]^[24]. So if a consolidation is in a lobe that borders the heart, it obliterates that heart border.

Paediatric considerations ^[2]^[24]:

PA view should be obtained in children age > 4 to minimize the cardiac shadow ^[2]^[24]
Lateral view should be obtained to demonstrate infiltrates behind the dome of diaphragm and cardiac shadow ^[2]^[24]
Lateral decubitus view with the affected side down should be obtained to identify pleural effusion ^[2]^[24]

Higher sensitivity than CXR but not routinely needed for straightforward CAP ^[15]
Indicated when:
- CXR is equivocal or negative but clinical suspicion is high
- Non-resolving pneumonia (to exclude underlying malignancy, abscess, or alternative diagnosis)
- Suspected complications (empyema, lung abscess)
- Immunocompromised patients (to characterize pattern: GGO for PJP, halo sign for aspergillosis)
HRCT (high-resolution CT): thin 1mm sections without contrast; best for parenchymal, interstitial and airway lesions ^[25]; particularly useful for interstitial pneumonia, PJP, hypersensitivity pneumonitis
Spiral CT with contrast: best for finding small mass lesions (e.g., post-obstructive tumour) ^[25]

Increasingly used as a bedside tool
Can identify consolidation (tissue-like pattern, "hepatization"), pleural effusion (anechoic fluid above diaphragm), and air bronchograms (hyperechoic lines within consolidated lung)
Useful for guiding thoracentesis in parapneumonic effusion
Sensitivity comparable to CXR for consolidation; higher sensitivity for small effusions

Blood investigations serve as markers of severity (marked with * below) and help assess organ function ^[1]^[8]:

Test	Expected Findings in Pneumonia	Clinical Significance
CBC with WBC differential	↑/↓WBC, neutrophilia* ^[1]^[8]. Typical bacterial pneumonia → leukocytosis with neutrophilia. Atypical/viral → relatively normal WBC ± lymphopenia	↑ or ↓WBC is a severity marker ^[1]^[8]. Very high WBC ( > 20 × 10⁹/L) suggests severe infection; ↓WBC (leukopenia) is ominous → indicates overwhelmed marrow or severe sepsis
RFT	↑Urea, hypoNa** ^[1]^[8]	Urea > 7 mmol/L is part of CURB-65 and reflects renal hypoperfusion. HypoNa is a severity marker ^[1]^[8] → may indicate SIADH (especially with Legionella, atypical pneumonia) ^[11]. Also needed for renal dosing of antibiotics ^[1]^[8]
LFT	Liver involvement (in basal pneumonia), ↓Alb* ^[1]^[8]	Basal pneumonia can irritate the diaphragm/liver → reactive hepatitis. ↓Albumin is a severity/prognostic marker reflecting chronic disease or acute phase response
ESR/CRP	↑↑ ^[1]^[8]	Acute phase reactants; CRP > 100 mg/L strongly suggests bacterial infection. Serial CRP useful for monitoring treatment response. Procalcitonin (PCT) is more specific for bacterial infection and can help distinguish from viral aetiology
ABG	Type 1 respiratory failure (↓PaO₂, normal/↓PaCO₂) in most cases	If clinically severe or SaO₂ < 93% ^[1]^[8]. Type 1 failure = V/Q mismatch with compensatory hyperventilation. If PaCO₂ rises (Type 2), indicates respiratory muscle exhaustion → impending respiratory arrest → urgent escalation
Blood culture	Positive in 7–16% of CAP ^[1]; higher yield in severe pneumonia	At least 2 sets from different sites before antibiotics ^[16]. Positive blood culture provides definitive microbiological diagnosis and is critical for HAP/bacteraemic pneumonia
Lactate	↑ in sepsis	Reflects tissue hypoperfusion; lactate > 2 mmol/L associated with sepsis; > 4 mmol/L indicates severe sepsis/septic shock
Cardiac enzymes / BNP	May be elevated if pneumonia triggers cardiac event	Pneumonia can trigger AMI ^[12]; BNP helps differentiate cardiac from respiratory causes of dyspnoea ^[26]
Clotting profile	PT/INR ± D-dimer	Baseline for sepsis-associated DIC; D-dimer may be elevated in pneumonia (not specific) but useful if PE is in the differential

High Yield – Key Severity Markers in Pneumonia

The severity markers to remember from blood tests (all relate to CURB-65 or ATS criteria):

WBC (↑ or ↓)
Urea > 7 (U in CURB-65)
HypoNa (SIADH, severity marker)
↓Albumin (poor prognostic marker)
ABG showing hypoxaemia (PaO₂/FiO₂ < 250 is ATS minor criterion)

3.3 Microbiological Investigations

Microbiological investigations are not usually necessary for mild outpatient CAP; usually only indicated for hospitalized patients ^[1]^[8].

The purpose is to identify the causative organism so antibiotics can be narrowed (de-escalated) from empirical broad-spectrum to targeted therapy.

Test	Details	Interpretation
Gram stain, culture and sensitivity ^[1]^[8]	Deep cough specimen; ideally before antibiotics; adequate sample = > 25 PMN and < 10 squamous epithelial cells per low-power field	Gram stain gives rapid preliminary information (e.g., Gram-positive diplococci → S. pneumoniae; Gram-negative coccobacilli → H. influenzae). Culture provides definitive identification and antibiotic susceptibility. MALDI-TOF (matrix-assisted laser desorption/ionization – time of flight) has largely replaced conventional biochemical identification ^[29]
AFB smear and TB culture	3 early morning sputum specimens for acid-fast bacilli smear (Ziehl-Neelsen stain) and mycobacterial culture (Lowenstein-Jensen medium or BACTEC liquid culture)	Always consider TB in Hong Kong ^[4]. AFB smear is rapid (same day) but sensitivity only ~50–60%; culture takes 2–8 weeks but is gold standard. GeneXpert MTB/RIF (nucleic acid amplification test) provides results in 2 hours with sensitivity approaching culture
Sputum for Mycoplasma PCR	Polymerase chain reaction	Rapid, specific detection of M. pneumoniae nucleic acid

Nasopharyngeal aspirate: rapid test for influenza, PCR for viruses, M. pneumoniae, C. pneumoniae ^[1]:

Test	Details
Respiratory virus rapid antigen detection test (RADT)	Rapid (30 min) detection of influenza A/B, RSV; lower sensitivity than PCR
Respiratory virus PCR panel	Seasonal influenza, RSV, parainfluenza, adenovirus, SARS-CoV-2, atypical organisms (Mycoplasma) ^[29]

Blood serology for Mycoplasma, Chlamydia, Legionella, viral causes ^[1]:

Test	Details	Limitations
Mycoplasma IgM	Rises early in infection (within 1 week)	Single titre can be false positive; paired sera (acute + convalescent 2–4 weeks apart) showing 4-fold rise is more reliable
Cold agglutinins	IgM against RBC I-antigen; associated with Mycoplasma	Non-specific; found in ~50% of Mycoplasma cases
Legionella serology	IgG antibodies	Takes 2–6 weeks to seroconvert; retrospective diagnosis only

Urine antigen test for Legionella and S. pneumoniae ^[1]:

Test	Sensitivity	Specificity	Notes
Legionella urinary antigen	~70–80%	~99%	Only detects L. pneumophila serogroup 1 (accounts for ~70% of Legionella disease) ^[1]. Positive from day 3 to several weeks after infection. Not affected by antibiotic treatment. Rapid (takes only a few hours). Has good sensitivity and specificity ^[1]
Pneumococcal urinary antigen	~70–80%	~90%	Detects S. pneumoniae C-polysaccharide; can remain positive weeks after infection; may be false positive in children with nasopharyngeal colonisation

Exam Pearl – Legionella Urinary Antigen

The Legionella urinary antigen only detects serogroup 1, which accounts for approximately 70% of Legionella infections ^[1]. A negative test does NOT exclude Legionella disease from other serogroups. If clinical suspicion is high (hyponatraemia, diarrhoea, elevated LFTs, relative bradycardia, exposure to water systems), send sputum culture on buffered charcoal yeast extract (BCYE) agar as well.

If parapneumonic effusion is present (> 1 cm thick on lateral decubitus film ^[29]):

Test	Significance
Gram stain and C/ST	Identify causative organism in complicated effusion/empyema
pH	< 7.2 → complicated parapneumonic effusion requiring drainage
Glucose	< 2.2 mmol/L → complicated effusion
LDH	> 1000 IU/L → complicated effusion
WBC and differential	Neutrophil predominance → bacterial infection; lymphocyte predominance → TB or malignancy
Protein (Light's criteria)	Exudate if: fluid/serum protein > 0.5, fluid/serum LDH > 0.6, or fluid LDH > 2/3 upper limit of normal serum LDH ^[30]
Cytology	If malignancy suspected
ADA (adenosine deaminase)	↑ in tuberculous pleurisy ^[4]

Consider more invasive specimens (BAL, transbronchial biopsy, thoracoscopic or open lung biopsy) if pneumonia does not resolve ^[1]:

Investigation	Indication	Key Role
Bronchoscopy with bronchoalveolar lavage (BAL)	Non-resolving pneumonia; immunocompromised patients; suspected PJP	BAL is the ideal specimen for diagnosis of Pneumocystis jirovecii pneumonia ^[2]^[24] — staining with methenamine silver or immunofluorescence. Also useful for cytology, mycobacterial cultures, and fungal cultures
Protected specimen brush (PSB)	HAP/VAP to avoid upper airway contamination	Quantitative cultures ( > 10³ CFU/mL significant)
CT-guided percutaneous lung biopsy	Peripheral mass or consolidation not accessible by bronchoscopy	Tissue for histology (to exclude malignancy or COP) and microbiology
Thoracoscopic/open lung biopsy	Last resort for diagnosis in non-resolving cases	Provides the largest tissue sample for definitive histopathological diagnosis

PJP presents with acute onset of fever, cough, SOB, and associated type 1 respiratory failure ^[31]:

Investigation	Finding
CXR	Diffuse, bilateral interstitial infiltrates ^[31]; bilateral ground glass opacities ^[31]
HRCT	Bilateral ground glass opacities, often with apical sparing
Serum LDH	Elevated (non-specific but sensitive marker; normal LDH has high negative predictive value)
ABG	Type 1 respiratory failure; desaturation with exercise is characteristic
BAL	Microscopy with staining of bronchoalveolar lavage fluid for Pneumocystis jirovecii ^[31] — gold standard diagnostic method
β-D-glucan	Elevated (component of fungal cell wall); sensitive but not specific for PJP

Treatment of PJP: high-dose Septrin (Trimethoprim/Sulfamethoxazole) ^[31]. Prevention in high-risk patients: Septrin prophylaxis ^[31].

	Outpatient (CURB-65 0–1)	Ward (CURB-65 2)	ICU (CURB-65 ≥ 3 or ATS criteria)
CXR	✓	✓	✓
Basic bloods (CBC, RFT, LFT, CRP)	Optional	✓	✓
ABG	Not routine	If SpO₂ < 93%	✓
Sputum Gram stain + C/ST	Not routine	✓	✓
Blood cultures	Not routine	✓ (1 set)	✓ (2 sets)
Urinary antigens	Not routine	Consider	✓
NPA/swab for viruses	Consider (if influenza season)	✓	✓
Sputum AFB	If TB suspected	If TB suspected	✓
Pleural fluid analysis	N/A	If effusion present	If effusion present
CT thorax	Not routine	If non-resolving	Consider early
Bronchoscopy/BAL	Not routine	If non-resolving / I/C	If non-resolving / I/C

From GC 052 – Fever and Purulent Sputum ^[14]:

Learning points on pneumonia include: diagnostic approach and investigations, common pathogens in different settings, empirical antibiotic treatment, and complications.

From Respiratory – Introduction to Resp Investigations (CXR, CT thorax and lung function tests) ^[15]^[21]:

Key data interpretation competencies include: interpretation of investigation results for community-acquired pneumonia, interpretation of arterial blood gases (especially different settings of type 2 respiratory failure), interpretation of pleural fluid analyses.

From Respiratory Four Cases of Haemoptysis (Prof Mary Ip) ^[9]^[10]:

Important investigations to perform include: chest X-ray; blood tests – overall assessment, usually not diagnostic for the underlying condition; sputum tests as relevant to suspected diagnosis; computed tomography of lungs (HRCT vs spiral/volumetric, with or without contrast); bronchoscopy – airway inspection, samples for microbiology and cytology, samples for histology (alternative for tissue: percutaneous lung biopsy); sampling of other tissues e.g., LN, pleural effusion; SpO₂ and arterial blood gases p.r.n; others as relevant to suspected diagnosis.

From Prof Ip's Teaching Clinic on CAP ^[10]:

Key management steps on admission: (1) Assess severity of condition and monitor; (2) Send specimens for microbiology – to include common causes of community-acquired pneumonia; (3) Start treatment with coverage of most likely microbial organisms; (4) Provide supportive treatment.

From AOS Microbiology ^[32]:

A 25-year-old man is diagnosed to have acute pneumonia. Mycoplasma pneumoniae is detected in sputum using polymerase chain reaction. Which class of antibiotics is suitable? Answer: Macrolides. This underscores that PCR is a valid diagnostic method for Mycoplasma and that macrolides are the treatment of choice (because Mycoplasma lacks a cell wall, rendering β-lactams ineffective).

Interpretation of arterial blood gases, especially different settings of type 2 respiratory failure is a key competency ^[15]^[21]:

ABG Pattern	What It Means	Clinical Implication
↓PaO₂, ↓/normal PaCO₂ (Type 1 respiratory failure)	V/Q mismatch; patient compensating by hyperventilating → blowing off CO₂	Most common pattern in pneumonia; give supplemental O₂, monitor
↓PaO₂, ↑PaCO₂ (Type 2 respiratory failure)	Ventilatory failure — patient is tiring, cannot maintain minute ventilation	Ominous sign → consider non-invasive ventilation (BiPAP) or intubation
Respiratory alkalosis (↑pH, ↓PaCO₂)	Compensatory hyperventilation driven by hypoxaemia	Common early finding; usually self-corrects with O₂ therapy
Metabolic acidosis (↓pH, ↓HCO₃⁻, ↑lactate)	Sepsis → tissue hypoperfusion → anaerobic metabolism → lactic acid production	Indicates severe sepsis; needs aggressive resuscitation
A-a gradient	Calculate: A-a = PAO₂ − PaO₂; normal = age/4 + 4 mmHg	Elevated A-a gradient confirms intrapulmonary pathology (V/Q mismatch or shunt) as cause of hypoxaemia, as opposed to hypoventilation alone (which has normal A-a gradient)

From first principles: In early pneumonia, the body detects hypoxaemia via peripheral chemoreceptors → drives hyperventilation → CO₂ is blown off (PaCO₂ drops) → respiratory alkalosis. If disease progresses and the patient cannot maintain the increased respiratory effort (fatigue, overwhelming consolidation, ARDS), CO₂ starts to rise → Type 2 failure → this is a late, dangerous sign requiring urgent ventilatory support.

High Yield Summary – Diagnosis and Investigations

Diagnostic criteria: Pneumonia = clinical features of LRTI (fever + cough ± sputum ± dyspnoea) + new infiltrate on CXR. No single diagnostic criterion; integration of clinical + radiological findings.

Gold standard: CXR (sensitivity ~70%; repeat or CT if negative but suspicious).

Severity assessment: CURB-65 (Confusion, Urea > 7, RR ≥ 30, BP < 90/60, age ≥ 65). Score 0–1 = home; 2 = admit; ≥ 3 = consider ICU. ATS criteria: 1 major (mechanical ventilation, septic shock) or ≥ 3 minor = ICU.

Key investigations:

CXR: Always. Lobar consolidation (typical bacterial), bilateral interstitial (viral/atypical/PJP), cavitation (TB, S. aureus, Klebsiella), effusion (complication).
Bloods: CBC (WBC, neutrophilia), RFT (urea – severity + renal dosing), LFT, CRP/ESR, ABG (if SpO₂ < 93%), blood cultures (2 sets if severe).
Microbiology: Sputum Gram stain + C/ST, NPA for viral PCR, urinary antigens (Legionella serogroup 1, pneumococcal), serology (Mycoplasma), AFB (if TB suspected).
Invasive: BAL (ideal for PJP), bronchoscopy (non-resolving), CT-guided biopsy (peripheral lesion).
Pleural fluid: If effusion present — pH, glucose, LDH, Gram stain, C/ST, cytology.

Silhouette sign: Consolidation adjacent to heart border obliterates it; helps localise lobe.

ABG: Expect Type 1 respiratory failure; rising PaCO₂ (Type 2) is ominous.

Active Recall - Diagnostic Criteria, Algorithm and Investigations for Pneumonia

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.165 – Diagnosis of CAP, investigations, and management) ^[2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.154, p.166 – CXR interpretation, silhouette sign, BAL) ^[4] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf (p.30 – TB immunological diagnosis) ^[8] Senior notes: Ryan Ho Respiratory.pdf (p.63 – Diagnosis, DDx, investigations for pneumonia) ^[9] Senior notes: learning_points_output.txt (Respiratory – Four Cases of Haemoptysis learning points) ^[10] Lecture slides: General clerkship Teaching Clinic - Haemoptysis_Prof MSM Ip_25 October 2024.pdf (p.14, p.16) ^[11] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.21 – SIADH causes) ^[12] Senior notes: Block A - Sudden severe chest pain_ acute myocardial infarction; aortic dissection.pdf (p.5) ^[14] Lecture slides: GC 052. Fever and purulent sputum.pdf (p.46 – Learning points) ^[15] Lecture slides: Respiratory- Introduction to Resp investigations (CXR, CT thorax and lung function tests).pdf (p.35) ^[16] Lecture slides: GC 101. Diagnosis of infections.pdf ^[21] Lecture slides: Data Interpretation - Respiratory - by Dr DCL Lam (Rotation 2).ppt.pdf (p.35) ^[24] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.151 – CXR patterns, silhouette sign) ^[25] Senior notes: Ryan Ho Fundamentals.pdf (p.238-239 – Spiral CT vs HRCT, pulmonary oedema CXR) ^[26] Senior notes: Block A - Shortness of breath on exertion_ heart failure.pdf (p.11 – BNP utility) ^[29] Senior notes: Maksim Medicine Notes.pdf (p.187 – Pneumonia definitions, CURB-65, ATS criteria, investigations) ^[30] Senior notes: Learning_Points_All_Lectures.txt (Light's criteria learning point) ^[31] Senior notes: Block A - Renal Replacement Therapies.pdf (p.37-38 – PJP presentation, CXR, diagnosis, treatment) ^[32] AOS material: AOS - Microbio.pdf (p.4 – Mycoplasma PCR and macrolide treatment MCQ) ^[9] Lecture slides: Respiratory Four cases of Haemoptysis.pdf (p.45 – Important investigations to perform)

Management of Pneumonia

The management of pneumonia rests on four pillars, elegantly summarized in the GC Teaching Clinic on CAP (Prof MSM Ip) ^[10]:

Key management steps on admission to the ward:

Assess severity of condition and monitor

Send specimens for microbiology – to include common causes of community-acquired pneumonia

Start treatment with coverage of most likely microbial organisms

Provide supportive treatment – e.g., hypotension, respiratory failure

These four steps should be performed simultaneously, not sequentially. Do not delay antibiotics while waiting for culture results.

3. Supportive Management

General therapy forms the foundation of pneumonia management ^[1]^[8]:

Oxygen therapy for all patients with respiratory failure ^[1]:

Aim: keep PaO₂ ≥ 60 mmHg or SaO₂ ≥ 92% ^[1]
Use high concentration ( > 35%) O₂ ^[1]
CPAP or mechanical ventilation if remain hypoxic ^[1]

O₂ Delivery Method	Flow Rate	FiO₂ Achieved	When to Use
Nasal cannula	1–6 L/min	24–44%	Mild hypoxaemia; comfortable, allows eating/talking
Simple face mask	6–10 L/min	35–60%	Moderate hypoxaemia
Venturi mask	Variable	Precise FiO₂ (24–60%)	When precise FiO₂ control needed (e.g., COPD with CO₂ retention risk)
Non-rebreather mask	10–15 L/min	60–90%	Severe hypoxaemia
High-flow nasal cannula	Up to 60 L/min	21–100%	Refractory hypoxaemia; provides some PEEP
Non-invasive ventilation — CPAP/BiPAP	—	Adjustable	Persistent hypoxaemia despite high-flow O₂; Type 2 respiratory failure (BiPAP); avoids intubation
Invasive mechanical ventilation	—	Adjustable	Respiratory failure unresponsive to NIV; ATS major criterion for severe CAP

Why is oxygen important? Pneumonia causes V/Q mismatch → hypoxaemia. Supplemental O₂ increases the alveolar partial pressure of oxygen in the non-consolidated alveoli, partially compensating for the shunt through consolidated lung. However, in true shunt (completely consolidated lung with no ventilation), even 100% O₂ cannot fully correct hypoxaemia — the blood passing through consolidated lung never contacts the supplemental O₂.

Caution – O₂ in COPD Patients with Pneumonia

In patients with COPD who develop pneumonia, use controlled oxygen therapy (target SpO₂ 88–92%, use Venturi mask for precise FiO₂) to avoid suppressing the hypoxic drive. These patients chronically retain CO₂ and rely on hypoxic drive for respiratory stimulation. Excessive O₂ → loss of hypoxic drive → hypoventilation → CO₂ narcosis → respiratory arrest. Monitor with serial ABGs.

Fluid rehydration (IV if oral intake not possible) ^[1]:

Patients with pneumonia are often dehydrated due to:
- Fever (insensible losses ↑ by ~200 mL/day per °C above normal)
- Tachypnoea (increased respiratory water loss)
- Reduced oral intake (malaise, anorexia)
- Vomiting (especially in children)
Oral rehydration preferred if patient can tolerate
IV normal saline or Ringer's lactate if unable to take orally or if haemodynamically compromised
Avoid fluid overload — particularly in elderly patients with cardiac comorbidities, as excess fluid worsens pulmonary oedema

Analgesia for pleuritic pain ^[1]:

Usually only require simple analgesics (e.g., paracetamol, NSAIDs) ^[1]
- Paracetamol: also serves as antipyretic; 1g QDS (max 4g/day in adults)
- NSAIDs (e.g., ibuprofen): ↓prostaglandin-mediated inflammation and pain; caution in renal impairment, GI bleeding risk, and asthma
Opiates sometimes required but caution in patients with poor respiratory function ^[1]
- Why caution? Opiates depress the central respiratory drive (act on medullary respiratory centre) → ↓respiratory rate and tidal volume → risk of CO₂ retention and respiratory arrest in a patient already struggling to breathe

Paracetamol is first-line
Treating fever also reduces metabolic demand (each 1°C rise increases metabolic rate by ~10–13%), which is beneficial in patients with limited cardiorespiratory reserve

Vital signs: Temperature, HR, BP, RR, SpO₂ — at least 4–6 hourly (more frequently in ICU)
Fluid balance chart: Input vs output, target euvolaemia
Serial CRP/WBC: Falling CRP suggests response to treatment; persistent or rising CRP at 48–72h should prompt re-evaluation
Serial ABG: If initial hypoxaemia or if deteriorating clinically
Clinical assessment: Mental status (confusion may worsen in severe sepsis), work of breathing

Monitor for treatment response and subsequent complications ^[10].

4. Empirical Antibiotic Therapy

This is the core of pneumonia management. The key principles are:

Microbiological diagnosis is retrospective in nature (culture takes time) ^[1]^[8]:

Need to administer appropriate antibiotics timely within 6–8 hours ^[1]^[8]. Early, appropriate antibiotics reduce mortality — delays beyond 4–8 hours are associated with significantly worse outcomes.

Initial management based on ^[1]^[8]:

Clinical setting: severity, clinical context (CAP vs HAP vs aspiration)
Host factors: comorbidity, immune status
Risk of resistance: previous antibiotic use

Choice of antibiotic should be based on ^[8]:

Clinical factors: severity, clinical context, management setting (outpatient vs inpatient)
Patient factors: immune status, comorbidity (e.g., DM ↑ risk of melioidosis), allergy, factors affecting pharmacokinetics (e.g., renal/liver impairment, RRT, drug interactions, overweight/underweight)
Bacterial factor: likely causative organism based on TOCC, clinical presentation (atypical vs typical, radiological appearance), risk of resistance (recent antibiotic use and local resistance data)

General principle ^[1]^[8]:

In ALL cases of CAP, cover Streptococcus pneumoniae
In SEVERE cases of CAP, cover Legionella pneumophila

Why always cover S. pneumoniae? Because it is the single most common cause of CAP across all age groups. Even when the presentation looks "atypical," S. pneumoniae cannot be reliably excluded by clinical features alone.

Why cover Legionella in severe CAP? Legionella causes disproportionately severe pneumonia (ICU admission rate ~25–50%). Failure to cover it in severe CAP is associated with higher mortality. β-lactam antibiotics do NOT cover Legionella (intracellular organism that lives within macrophages — β-lactams cannot penetrate cells well). Adding a macrolide or fluoroquinolone provides intracellular coverage.

Before presenting the regimen, let's understand why each drug class is chosen:

Drug Class	Examples	Spectrum / Target	Mechanism of Action	Why Used in Pneumonia
Penicillin + β-lactamase inhibitor	Amoxicillin-clavulanate (Augmentin) ^[1]^[8]	Covers S. pneumoniae, H. influenzae (incl. β-lactamase producers), M. catarrhalis, some anaerobes, some Gram-negatives	β-lactam ring binds PBPs → inhibits peptidoglycan cross-linking → cell wall deficient → lysis. Clavulanate inhibits β-lactamases that would otherwise hydrolyse amoxicillin	First-line for typical CAP organisms. The clavulanate component extends coverage to β-lactamase-producing H. influenzae and M. catarrhalis
3rd-gen cephalosporin	Ceftriaxone (Rocephin), cefotaxime ^[1]	Broad Gram-positive and Gram-negative coverage; covers S. pneumoniae, H. influenzae, many Enterobacteriaceae	Same mechanism as penicillins (PBP inhibition) but resistant to many β-lactamases due to different R-group	Alternative for hospitalized patients; IV formulation; long half-life (ceftriaxone allows once-daily dosing)
Anti-pseudomonal penicillin	Piperacillin-tazobactam (Tazocin) ^[1]^[8]	Extended Gram-negative coverage including Pseudomonas aeruginosa + anaerobes	Piperacillin = extended-spectrum penicillin; tazobactam = β-lactamase inhibitor	Required for severe CAP, HAP, and patients with chronic lung disease such as bronchiectasis ^[1] because colonized airways harbour Pseudomonas
4th-gen cephalosporin	Cefepime ^[1]	Similar to ceftriaxone but better anti-pseudomonal activity	Similar to 3rd-gen but resistant to AmpC β-lactamases	Alternative anti-pseudomonal agent for severe CAP
Macrolide	Clarithromycin (Klacid), azithromycin ^[1]^[8]	Covers atypical organisms: M. pneumoniae, C. pneumoniae, L. pneumophila + good Gram-positive cover	Binds 50S ribosomal subunit → inhibits translocation → bacteriostatic. Concentrates intracellularly → excellent for intracellular pathogens	Added to β-lactam to cover atypical pathogens. Macrolides are the answer for Mycoplasma ^[32] because Mycoplasma lacks a cell wall → β-lactams are ineffective
Tetracycline	Doxycycline ^[1]^[8]	Similar atypical coverage; also covers Chlamydia, Rickettsia	Binds 30S ribosomal subunit → inhibits aminoacyl-tRNA binding → bacteriostatic	Alternative to macrolide for atypical coverage
Respiratory fluoroquinolone	Levofloxacin, moxifloxacin	Broad Gram-positive, Gram-negative, AND atypical coverage (all-in-one)	Inhibits DNA gyrase (Gram-neg) and topoisomerase IV (Gram-pos) → prevents DNA replication	Monotherapy option in some guidelines; in HK, generally reserved for penicillin-allergic patients or specific situations to prevent resistance

Why β-Lactams Cannot Kill Mycoplasma – First Principles

β-lactam antibiotics (penicillins, cephalosporins, carbapenems) work by inhibiting bacterial cell wall synthesis — specifically, they block the transpeptidase enzymes (PBPs) that cross-link peptidoglycan chains. Mycoplasma pneumoniae has NO cell wall — it is the smallest free-living organism. With no target to bind, β-lactams simply cannot work. This is why macrolides are the appropriate treatment ^[32] — they target protein synthesis (ribosomal 50S subunit), which Mycoplasma does possess.

Local guidelines on empirical treatment ^[1]^[8]:

Setting	Regimen	Rationale
Outpatient	PO amoxicillin-clavulanate (Augmentin) ± macrolide or doxycycline ^[1]^[8]	Oral therapy sufficient for mild CAP; Augmentin covers typical organisms; macrolide/doxycycline added if atypical suspected or per local practice
Hospitalized, mild-moderate	PO/IV amoxicillin-clavulanate (Augmentin) ± macrolide or doxycycline ^[1]^[8]; Alternatives: IV ceftriaxone (Rocephin) or cefotaxime ± macrolide or doxycycline ^[1]	IV route preferred if oral intake compromised; ceftriaxone is convenient (once daily); macrolide added for atypical coverage
Hospitalized, with chronic lung disease (e.g., bronchiectasis)	Antipseudomonal antibiotics: IV piperacillin-tazobactam (Tazocin) or cefepime ± macrolide or doxycycline ^[1]^[8]	Patients with chronic lung disease such as bronchiectasis need antipseudomonal coverage ^[1] because colonised airways harbour Pseudomonas aeruginosa
Hospitalized, severe	IV piperacillin-tazobactam (Tazocin) or ceftriaxone or cefepime ± macrolide or doxycycline ^[1]^[8]	Broad-spectrum coverage essential; macrolide for Legionella coverage (always cover Legionella in severe CAP)

Severe CAP defined as 1 out of 3 major criteria OR 2 in 6 minor criteria ^[1]:

Major: need of mechanical ventilation, septic shock, acute renal failure
Minor: multilobar involvement, mental confusion, RR > 30/min, PaO₂/FiO₂ < 250, BP < 90/60, urea > 7

Duration: approximately 7–10 days ^[1]^[8].

Current trend (2024–2026): Studies (e.g., CAP-START trial) suggest 5 days of antibiotics may be sufficient for uncomplicated CAP if the patient has been clinically stable for ≥ 48 hours. However, the HA guideline of 7–10 days remains the standard local practice.

Patients started on IV antibiotics can be switched to oral when ALL of the following are met:

Temperature < 37.8°C for ≥ 24 hours
Improving clinically (↓cough, ↓sputum, ↓dyspnoea)
WBC trending down
Able to tolerate oral medications
Functioning GI tract (no vomiting, no ileus)

Why switch? IV antibiotics are not inherently "stronger" — many oral antibiotics achieve equivalent bioavailability (e.g., fluoroquinolones have ~100% oral bioavailability). Early switch reduces IV line-associated complications (infection, thrombophlebitis), reduces hospital stay, and saves cost.

Choose appropriate antibiotic coverage for the likely pathogen/s based on local microbiology and antibiotic susceptibility (or resistance) ^[10]:

Organism	Targeted Antibiotic	Duration	Notes
S. pneumoniae (penicillin-sensitive)	IV/PO amoxicillin or penicillin G	5–7 days	De-escalate from broad-spectrum
S. pneumoniae (penicillin-intermediate/resistant)	IV ceftriaxone or respiratory fluoroquinolone	7–10 days	Penicillin resistance rising in Hong Kong (~20–30% intermediate)
H. influenzae (β-lactamase negative)	PO amoxicillin	5–7 days
H. influenzae (β-lactamase positive)	PO augmentin or IV ceftriaxone	5–7 days	~30% β-lactamase positive in HK
S. aureus (MSSA)	IV cloxacillin/flucloxacillin	14–21 days	Longer course needed; high virulence
S. aureus (MRSA)	IV vancomycin or linezolid	14–21 days	ICU patients, known MRSA colonization
K. pneumoniae	IV ceftriaxone or carbapenem (if ESBL)	10–14 days	ESBL-producing Klebsiella requires carbapenem (meropenem)
M. pneumoniae	Macrolide (azithromycin/clarithromycin) ^[32] or doxycycline	5–7 days (azithromycin) or 7–14 days (others)	Macrolides are the treatment of choice ^[32]; macrolide resistance rising in Asia — doxycycline or fluoroquinolone as alternatives
Legionella pneumophila	Macrolide (azithromycin preferred) or fluoroquinolone (levofloxacin)	14–21 days	Longer course needed; intracellular organism
Pseudomonas aeruginosa	IV piperacillin-tazobactam (Tazocin) or cefepime or meropenem	10–14 days	Often requires combination therapy (add aminoglycoside or ciprofloxacin)
Pneumocystis jirovecii	High-dose co-trimoxazole (Septrin) ^[31]	21 days	Add prednisolone if PaO₂ < 70 mmHg (reduces mortality by dampening immune-mediated alveolitis)
Anaerobes (aspiration pneumonia)	IV Augmentin or clindamycin ± metronidazole	14–21+ days	Prolonged course if lung abscess

Hospital-acquired and ventilator-associated pneumonia require broader empirical coverage:

Setting	Empirical Regimen	Rationale
HAP (non-severe, no MDR risk factors)	IV Augmentin or IV ceftriaxone	Similar to moderate CAP but with lower threshold for escalation
HAP (severe or MDR risk factors)	Anti-pseudomonal β-lactam (Tazocin, cefepime, or meropenem) + anti-MRSA agent (vancomycin or linezolid) ± aminoglycoside	MDR organisms (P. aeruginosa, Acinetobacter, MRSA, ESBL-producers) are common in hospital settings
VAP	Similar to severe HAP; guided by local antibiogram	Ventilated patients have biofilm-associated infections; longer courses (10–14 days) often needed

Risk factors for MDR pathogens ^[1]^[8]:

Recent or frequent antibiotic use

Recent or frequent hospitalization

OAH (old age home) resident or institutionalization ^[8]

Immunocompromised state

4.7 Special Situations

Cover anaerobes (from oropharyngeal flora): IV Augmentin (has anaerobic coverage via clavulanate) or clindamycin
Positioning: Head elevation 30–45° to prevent further aspiration
Speech therapy assessment for swallowing safety
Address underlying cause: stroke rehabilitation, consciousness level, NGT feeding consideration
- Note: NGT feeding does NOT reduce risk of aspiration pneumonia ^[13] — this is a common misconception. NG tubes themselves can predispose to aspiration (by holding open the lower oesophageal sphincter).

Oseltamivir (Tamiflu) 75 mg BD × 5 days — neuraminidase inhibitor; most effective if started within 48 hours of symptom onset, but should still be given in severe influenza even if > 48 hours
Add empirical antibiotics for secondary bacterial pneumonia if suspected (fever with purulent sputum after initial improvement)

This is a medical emergency ^[33]:

Empirical antibiotics ASAP after immediate septic workup (ideally < 1–2 hours after presentation) ^[33]
Principle: timely administration of broad-spectrum antibiotics which should be bactericidal (neutropenia) with minimal toxicity ^[33]
For high-risk patients (ANC ≤ 0.1 for > 7 days, shock, pneumonia) ^[33]: IV anti-pseudomonal β-lactam monotherapy (piperacillin-tazobactam or meropenem or cefepime)
Consider empirical antifungal (e.g., voriconazole for Aspergillus) if persistent fever > 4–7 days on broad-spectrum antibiotics
Consider PJP prophylaxis/treatment if indicated

Choose the appropriate antibiotic coverage for the likely pathogen/s in each case of pneumonia based on: host factors, microbial epidemiology and recent trends of infection in the locality, local antibiotic sensitivity pattern ^[10]:

Concept	Explanation
Empirical → Targeted	Once culture and sensitivity results are available (typically 48–72 hours), narrow the antibiotic spectrum to the identified organism. This reduces antimicrobial resistance, adverse effects, and cost.
Broad → Narrow	Start broad (cover most likely organisms), narrow once you know what you're treating. "Hit hard, then step down."
IV → PO switch	Switch to oral when clinically improving, afebrile ≥ 24h, tolerating PO
Duration	~7–10 days ^[1]^[8] for uncomplicated CAP. Shorter courses (5 days) may be appropriate if stable for ≥ 48h. Longer courses for Legionella (14–21d), Staphylococcal (14–21d), lung abscess (4–6 weeks), PJP (21d)

Resolution ^[8]:

Clinical: SOB and systemic upset resolve in 2–3 days but cough may take 2 weeks
Radiological: resolution generally delayed (10 weeks if severe, 3–4 weeks if mild-moderate)

Generally recommend to follow up in 6 weeks' time to ensure complete resolution ^[8]:

Repeat CXR at 6 weeks, especially if persistent symptoms or suspect underlying malignancy ^[8]
In smokers > 50 years, a non-resolving infiltrate should prompt CT thorax and bronchoscopy to exclude post-obstructive malignancy

When pneumonia does not respond to appropriate antibiotics within 48–72 hours ^[8]:

Category	Causes
Host factors leading to delayed resolution	COPD and chronic lung disease with impaired cough and secretion clearance; recurrent aspiration due to depressed consciousness or alcohol use; immunocompromised state ^[8]
Pathogen factors	Resistant organisms: Pseudomonas aeruginosa, MRSA, Acinetobacter baumannii; unusual pathogens: Mycobacterium tuberculosis, Burkholderia pseudomallei, Chlamydia psittaci with avian exposure, Nocardia in immunocompromised hosts, fungal pathogens e.g. Aspergillus in immunocompromised hosts ^[8]
Complicated pneumonia	Empyema; lung abscess especially with anaerobes and aspiration pneumonia ^[8]
Non-infectious causes	Lung cancer, COP, PE, eosinophilic pneumonia, vasculitis ^[8]

8. Prevention

Vaccine	Target	Who Should Receive	Schedule
Pneumococcal conjugate vaccine (PCV13/PCV15/PCV20)	S. pneumoniae (13/15/20 serotypes)	All children as part of routine immunization; adults ≥ 65; adults with chronic diseases (DM, CKD, CLD, COPD, asplenia, immunocompromise)	PCV20 single dose (or PCV15 followed by PPSV23 one year later)
Pneumococcal polysaccharide vaccine (PPSV23)	S. pneumoniae (23 serotypes)	Adults ≥ 65; high-risk groups	After PCV15 (if PCV20 not used); booster after 5 years in high-risk
Influenza vaccine	Influenza A and B	Annual; all persons ≥ 6 months, especially elderly, chronic disease, HCW	Annual (new formulation each year matching circulating strains)
COVID-19 vaccine	SARS-CoV-2	Per local guidelines; updated boosters as available	Per CHP recommendations

Vaccination is particularly critical for post-splenectomy patients: pneumococcus, H. influenzae type B, meningococcus, and influenza ^[6] — these patients are at extreme risk for overwhelming infection by encapsulated organisms.

Smoking cessation: Single most important modifiable risk factor. Reduces risk of pneumonia, COPD exacerbation, and lung cancer.
Aspiration precautions: Head elevation, swallowing assessment (speech therapy referral), oral care in intubated/institutionalized patients, appropriate diet texture modification
Infection control: Hand hygiene, droplet precautions for respiratory infections, isolation for TB/influenza/COVID
Antimicrobial stewardship: Judicious antibiotic use to prevent resistance — this is where GC 106 – Practical Issues in Antibiotic Use ^[18] principles apply:

Principles of empirical antibiotic selection: consider the most likely organism based on clinical syndrome and local epidemiology; choose the narrowest effective spectrum; de-escalate when culture results are available; complete the appropriate duration of therapy; monitor for adverse effects and treatment response. ^[18]

Transplant recipients given Septrin prophylaxis to prevent PJP ^[31]:

Indications: HIV with CD4 < 200, post-organ transplant (typically first 6–12 months), post-bone marrow transplant, patients on high-dose steroids or certain chemotherapy regimens
Regimen: Co-trimoxazole (TMP-SMX) 480 mg daily or 960 mg three times per week
Alternative (if sulfa allergy): Dapsone, atovaquone, or inhaled pentamidine

Drug	Contraindication / Caution	Why
Amoxicillin-clavulanate	Penicillin allergy (anaphylaxis); hepatic cholestasis (previous clavulanate-induced)	Cross-reactivity with cephalosporins ~1–2% (low); clavulanate can cause cholestatic hepatitis
Ceftriaxone	Neonates receiving IV calcium (risk of fatal ceftriaxone-calcium precipitates); severe penicillin allergy (use with caution — cross-reactivity < 1% for 3rd-gen)	Ceftriaxone binds calcium → insoluble precipitates in lungs and kidneys
Macrolides	QT prolongation risk (azithromycin, clarithromycin); clarithromycin inhibits CYP3A4 → major drug interactions (statins, warfarin, cyclosporine)	Macrolides block hERG potassium channels → ↑QTc → risk of Torsades de Pointes. Clarithromycin is a potent CYP3A4 inhibitor
Doxycycline	Children < 8 years (permanent tooth discolouration); pregnancy (teratogenic — tooth/bone effects)	Tetracyclines chelate calcium in developing teeth and bones
Aminoglycosides	Renal impairment (nephrotoxic); hearing impairment (ototoxic)	Accumulates in proximal tubular cells and cochlear hair cells → dose-dependent toxicity; requires TDM (therapeutic drug monitoring)
Vancomycin	Renal impairment (nephrotoxic)	Requires TDM; risk of "red man syndrome" if infused too rapidly (histamine release → flushing, hypotension — NOT a true allergy)
Fluoroquinolones	Tendon rupture (especially with steroids); QT prolongation; seizure risk; children/adolescents (cartilage damage in animal studies); aortic dissection risk	FDA black box warning; tendon damage thought to be due to metalloproteinase activation in tendon matrix
Co-trimoxazole (Septrin)	Folate deficiency; G6PD deficiency (haemolysis); severe renal impairment; sulfonamide allergy; pregnancy (1st trimester — teratogenic)	Sulfamethoxazole inhibits dihydropteroate synthase; trimethoprim inhibits DHFR → both block folate synthesis → can worsen folate deficiency. Sulfonamide component is an oxidizing agent → haemolysis in G6PD deficiency

	Outpatient	Ward	ICU
CURB-65	0–1	2	≥ 3
O₂	Not routine	Target SpO₂ ≥ 92%	High-flow ± NIV ± intubation
Fluids	Oral	IV if needed	IV resuscitation ± vasopressors
Microbiology	Not routine	Sputum, blood cultures, NPA, urinary Ag	Full septic workup
Empirical Abx	PO Augmentin ± macrolide/doxy	PO/IV Augmentin or IV ceftriaxone ± macrolide/doxy; Tazocin if chronic lung disease	IV Tazocin or ceftriaxone or cefepime ± macrolide/doxy
Duration	5–7 days	7–10 days	7–14+ days (organism-dependent)
Follow-up	48–72h review	Daily assessment	Continuous monitoring

High Yield Summary – Management of Pneumonia

Four pillars ^[10]: (1) Assess severity + monitor, (2) Send microbiology, (3) Start empirical antibiotics, (4) Supportive treatment.

Supportive: O₂ therapy (target SpO₂ ≥ 92%; PaO₂ ≥ 60 mmHg), IV fluids, analgesia (paracetamol/NSAIDs; opiates with caution), management of complications.

Severity-driven antibiotic selection (HA guidelines):

Outpatient (CURB-65 0–1): PO Augmentin ± macrolide/doxycycline
Ward, mild-mod (CURB-65 2): PO/IV Augmentin or IV ceftriaxone ± macrolide/doxycycline
Chronic lung disease: IV Tazocin or cefepime ± macrolide/doxycycline (anti-pseudomonal)
Severe (CURB-65 ≥ 3): IV Tazocin or ceftriaxone or cefepime ± macrolide/doxycycline

Key principles: Always cover S. pneumoniae; cover Legionella in severe CAP (add macrolide); cover Pseudomonas in chronic lung disease (use Tazocin/cefepime). β-lactams cannot kill Mycoplasma (no cell wall) → use macrolides. Duration 7–10 days. IV→PO switch when improving, afebrile ≥ 24h, tolerating PO.

De-escalation: Start broad empirically → narrow based on culture + sensitivity.

Follow-up: CXR at 6 weeks; clinical resolution faster than radiological.

Prevention: Pneumococcal + influenza vaccination; smoking cessation; aspiration precautions; PJP prophylaxis in immunocompromised.

Non-response at 48–72h: Re-evaluate for wrong organism, complication (empyema/abscess), or non-infective cause.

Active Recall - Management of Pneumonia

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.165-166 – Management of CAP, empirical antibiotics, local guidelines) ^[6] Senior notes: Block A - Splenomegaly_ common causes of splenomegaly; myeloproliferative diseases.pdf (p.20 – Post-splenectomy vaccination) ^[8] Senior notes: Ryan Ho Respiratory.pdf (p.63-65 – Empirical antibiotics, prognostic factors, complications, unresolving pneumonia) ^[10] Lecture slides: General clerkship Teaching Clinic - Haemoptysis_Prof MSM Ip_25 October 2024.pdf (p.14, p.16 – Key management steps on admission) ^[13] AOS material: AOS - Geriatrics.pdf (p.25 – NGT feeding does not reduce aspiration pneumonia risk) ^[18] Lecture slides: GC 106. Practical issues in antibotic use [Notes].pdf ^[31] Senior notes: Block A - Renal Replacement Therapies.pdf (p.37-38 – PJP treatment with Septrin, prophylaxis) ^[32] AOS material: AOS - Microbio.pdf (p.4 – Mycoplasma treatment with macrolides) ^[33] Senior notes: Ryan Ho Haemtology.pdf (p.71 – Neutropenic fever management principles)

Complications of Pneumonia

Complications of pneumonia arise when the infection is not contained within the lung parenchyma, when the host response becomes excessive, or when the systemic inflammatory cascade damages distant organs. Understanding complications is critical because they change the management (drainage, prolonged antibiotics, ICU care) and worsen prognosis.

From GC 052 – Fever and Purulent Sputum (slides 29–30) ^[14], the key complications to know are:

Complications of pneumonia:

Respiratory failure

Septicaemia with multi-organ failure

Lung abscess — air-fluid level, cavity; could be polymicrobial; needs prolonged (4–6 weeks) antibiotics; no need for percutaneous drainage in most cases

Parapneumonic effusion

Empyema thoracis — pus in pleural cavity (gross appearance, Gram's staining or culture); needs prolonged (4–6 weeks) antibiotics; pleural drainage (e.g., intercostal tube) is needed

Electrolyte abnormalities e.g., hypoNa due to SIADH

Acute myocardial infarction

Cardiac arrhythmia (AF)

These match exactly with the senior notes ^[1]^[8]^[29]:

Complications: respiratory failure; lung abscess formation; spread → septicaemia with multi-organ failure, parapneumonic effusion ± empyema thoracis; systemic effects → electrolyte abnormalities (e.g., hypoNa due to SIADH), cardiac complications (acute MI, cardiac arrhythmia esp AF) ^[8].

Let's go through each complication in detail.

Respiratory failure ^[1]^[8]^[14] is the most immediate and life-threatening complication.

Pathophysiology: Extensive consolidation → large areas of lung filled with exudate → ventilation lost in those areas → severe V/Q mismatch and intrapulmonary shunting → hypoxaemia. If enough lung is affected, or if the patient tires from the increased work of breathing, ventilatory failure (Type 2) supervenes with CO₂ retention.

Type	ABG Pattern	Mechanism in Pneumonia	Management
Type 1 (hypoxaemic)	↓PaO₂, normal/↓PaCO₂	V/Q mismatch from consolidated alveoli; patient compensates by hyperventilating	Supplemental O₂ (target SpO₂ ≥ 92%), high-flow nasal cannula
Type 2 (hypercapnic)	↓PaO₂, ↑PaCO₂	Respiratory muscle fatigue → cannot maintain minute ventilation; may also occur in COPD overlap	Non-invasive ventilation (BiPAP) or intubation with mechanical ventilation

Why does Type 2 failure develop? The patient initially hyperventilates to compensate for hypoxaemia. But hyperventilation is energetically expensive — the respiratory muscles (diaphragm, intercostals) consume more oxygen and produce more CO₂. Eventually, the muscles fatigue. At this point, tidal volume drops, respiratory rate may paradoxically slow, PaCO₂ rises, and pH falls. This is a pre-arrest state requiring urgent ventilatory support.

ARDS (Acute Respiratory Distress Syndrome): The most severe form of respiratory failure in pneumonia. Defined by: acute onset within 1 week, bilateral opacities on CXR not fully explained by effusion/atelectasis/nodules, PaO₂/FiO₂ < 300 (mild), < 200 (moderate), < 100 (severe). Mechanism: diffuse alveolar damage from overwhelming inflammatory response → protein-rich pulmonary oedema → severely impaired gas exchange. Requires lung-protective ventilation (low tidal volume 6 mL/kg, PEEP, prone positioning).

Parapneumonic effusion ^[1]^[8]^[14]^[34]^[29]:

Definition: A pleural effusion that develops adjacent to a pneumonia. It is one of the most common complications of bacterial pneumonia (occurs in ~40% of hospitalized pneumonia patients).

Pathophysiology — A three-stage continuum:

Stage	Name	Pathology	Pleural Fluid Characteristics	Management
1	Simple (exudative)	Inflammation of visceral pleura → increased capillary permeability → sterile exudative fluid accumulates in pleural space	Clear, pH > 7.2, glucose > 2.2 mmol/L, LDH < 1000 IU/L, no organisms on Gram stain/culture	Antibiotics alone — effusion resolves as pneumonia responds
2	Complicated (fibrinopurulent)	Bacteria invade pleural space → neutrophil influx → bacterial metabolism produces acid and consumes glucose → fibrin deposition creates loculations	Turbid, pH < 7.2, glucose < 2.2 mmol/L, LDH > 1000 IU/L, ± positive Gram stain/culture	Antibiotics + drainage (chest tube/intercostal drain). Intrapleural fibrinolytics (tPA + DNase) if loculated ^[29]
3	Empyema (organizing)	Frank pus in pleural cavity; thick fibrinous peel forms over visceral and visceral pleura (cortex) trapping the lung	Frank pus (gross appearance), positive Gram stain or culture ^[14]	Prolonged antibiotics (4–6 weeks) + pleural drainage (intercostal tube) ^[14]^[34]. May require surgical decortication if lung trapped

Why does the pleural fluid pH fall? Bacteria in the pleural space metabolize glucose anaerobically → produce lactic acid and CO₂ → pH drops. Neutrophil degranulation also releases acidic lysosomal contents. A pH < 7.2 indicates that the effusion will NOT resolve with antibiotics alone and requires drainage — the low pH signifies bacterial invasion of the pleural space.

Why is LDH elevated? LDH is released from dead neutrophils and damaged pleural/lung cells. LDH > 1000 IU/L reflects massive cell death and is another marker of complicated effusion.

High Yield – When to Drain a Parapneumonic Effusion

Drain if ANY of the following are present:

Frank pus (empyema by definition)
Positive Gram stain or culture of pleural fluid
pH < 7.2
Glucose < 2.2 mmol/L
LDH > 1000 IU/L
Loculated effusion

Simple parapneumonic effusions (pH > 7.2, glucose > 2.2, sterile) resolve with antibiotics alone and do NOT need drainage.

Lung abscess vs Empyema — A common exam comparison ^[29]:

Feature	Empyema	Lung Abscess
Definition	Collection of pus within the pleural cavity ^[29]	Localised collection of pus that forms a thick-walled cavity within the lung ^[29]
Risk factors	Poor dentition, aspiration risk, immunocompromised (DM, HIV, malignancy) ^[29]	Same
Microbiology	GP: Strep milleri group, S. aureus; GN: Klebsiella (esp in DM); anaerobes ^[29]	Same
CXR	Obtuse angle with chest wall; lenticular shape (much larger on one projection than another) ^[29]	Acute angle with chest wall; spherical shape (similar size in different projections); air-fluid level ^[29]
CT	Split pleura sign (fibrin coating both parietal and visceral pleura) ^[29]	Thick irregular walls ^[29]
Management	Prolonged antibiotics (weeks); large-bore chest drain ± intrapleural tPA + DNase; surgical decortication/thoracoplasty/drainage ^[29]	Prolonged antibiotics (months): Augmentin/clindamycin; drainage (postural drain, image-guided); surgical resection if failed medical treatment ^[29]

Lung abscess ^[1]^[8]^[14]^[34]:

Air-fluid level, cavity; could be polymicrobial; needs prolonged (4–6 weeks) antibiotics; no need for percutaneous drainage in most cases ^[14].

Definition: Accumulation of inflammatory cells accompanied by tissue destruction or necrosis that produces one or more cavities in the lung ^[34].

Pathophysiology: Virulent organisms or aspiration of large amounts of oropharyngeal secretions → intense localised infection → necrosis of lung parenchyma → liquefaction → cavity formation. If the cavity communicates with a bronchus, the liquefied contents are partially expectorated (producing foul-smelling sputum) and air enters the cavity → the characteristic air-fluid level on CXR.

Key organisms: Staphylococcus aureus and anaerobic flora from the upper respiratory tract are most commonly involved ^[34]. Also Klebsiella (especially in diabetics), Streptococcus milleri group.

Clinical features: Insidious onset of fever, night sweats, weight loss, cough with copious foul-smelling (putrid) sputum (the foul smell comes from anaerobic metabolism). The most common complication of lung abscess is intracavitary haemorrhage which can cause haemoptysis or spillage of abscess contents with extrapulmonary spread of infection ^[34].

Management ^[14]^[29]:

Prolonged antibiotics (4–6 weeks): IV Augmentin or clindamycin (excellent anaerobic and Gram-positive coverage + good lung penetration); switch to oral when improving
No need for percutaneous drainage in most cases ^[14] — most lung abscesses respond to antibiotics alone and drain spontaneously via the bronchial tree (postural drainage helps)
Image-guided percutaneous drainage: reserved for large abscesses ( > 6 cm) not responding to antibiotics or unable to drain bronchially
Surgical resection (lobectomy): last resort if failed prolonged medical therapy, massive haemoptysis, or suspected underlying malignancy

Septicaemia with multi-organ failure ^[1]^[8]^[14]:

Pathophysiology: Bacteria (or their products — endotoxin from Gram-negatives, exotoxins from Gram-positives) enter the bloodstream → activate a massive, dysregulated systemic inflammatory response → cytokine storm (TNF-α, IL-1, IL-6) → widespread endothelial damage → vasodilation, capillary leak, microthrombi formation (DIC) → organ hypoperfusion and failure.

Organ systems affected (as per Sepsis-3 / SOFA criteria):

Organ System	Manifestation	Mechanism
Lungs	ARDS	Diffuse alveolar damage from inflammatory mediators
Cardiovascular	Septic shock (vasodilatory), myocardial depression	Nitric oxide-mediated vasodilation; cytokine-induced myocardial stunning
Kidneys	Acute kidney injury (acute tubular necrosis)	Renal hypoperfusion + direct inflammatory injury to renal tubules
Liver	"Shock liver" (↑ALT/AST), jaundice	Hepatic ischaemia from hypoperfusion; cholestasis from sepsis
Haematological	DIC	Tissue factor release → widespread thrombin generation → consumption coagulopathy
CNS	Septic encephalopathy	Cerebral hypoperfusion + blood-brain barrier disruption + cytokine effects

Management: Source control (antibiotics for pneumonia), IV fluid resuscitation (30 mL/kg crystalloid bolus within first 3 hours), vasopressors (noradrenaline first-line) if MAP < 65 mmHg despite fluids, organ support (mechanical ventilation for ARDS, RRT for AKI), lactate-guided resuscitation.

Electrolyte abnormalities, e.g., hypoNa due to SIADH ^[8]^[11]^[14]:

Pathophysiology: Pulmonary inflammation → non-osmotic release of ADH (antidiuretic hormone) from the posterior pituitary → ADH acts on collecting duct V2 receptors → ↑aquaporin-2 insertion → ↑water reabsorption → dilutional hyponatraemia with euvolaemia.

Respiratory causes of SIADH include: chest infection (atypical pneumonia, COVID, SARS), CA lung, positive pressure breathing ^[11]. Patients with SARS commonly came in with bad CXR and very low sodium ^[11].

The SIADH from pneumonia is usually mild and self-correcting as the pneumonia resolves. However, severe hyponatraemia (Na < 120 mmol/L) can cause:

Non-specific: malaise, lethargy, headache ^[11]
More serious: confusion, convulsions, coma ^[11]

Management: Treat the underlying pneumonia (the SIADH resolves as infection resolves). If severe/symptomatic → fluid restriction (first-line), ± hypertonic saline (3% NaCl) if seizures or severe neurological compromise. Correct slowly (≤ 10 mmol/L per 24h) to avoid osmotic demyelination syndrome.

6. Cardiac Complications

Acute myocardial infarction and cardiac arrhythmia (AF) ^[8]^[14]:

Why does pneumonia cause AMI? Multiple mechanisms converge:

Increased myocardial oxygen demand: Fever, tachycardia, and the systemic inflammatory response increase cardiac workload → an already stenosed coronary artery cannot meet the increased demand → demand ischaemia (Type 2 MI)
Hypoxaemia: ↓arterial oxygen content → direct myocardial hypoxia
Catecholamine surge: Sympathetic activation from pain, fever, and hypoxaemia → coronary vasoconstriction in diseased vessels
Plaque instability: Systemic inflammation → activated macrophages and cytokines → destabilize atherosclerotic plaques → plaque rupture → thrombosis (Type 1 MI)
Procoagulant state: Acute infection → ↑fibrinogen, ↑platelets, ↑tissue factor → hypercoagulability

Pneumonia is listed as a predisposing/trigger factor for AMI ^[12]. This is why cardiac enzymes and ECG should be checked in any patient with severe pneumonia, especially those with pre-existing cardiovascular risk factors.

Why does pneumonia trigger AF?

Hypoxaemia: Atrial myocytes are sensitive to oxygen supply; hypoxia → altered atrial electrophysiology → re-entrant circuits
Systemic inflammation: Inflammatory cytokines directly affect atrial ion channels and gap junctions
Sympathetic activation: ↑catecholamines → ↑automaticity of pulmonary vein foci (the commonest source of AF triggers)
Atrial distension: Right heart strain from hypoxic pulmonary vasoconstriction → atrial stretch → triggers AF

Management: Rate control (beta-blocker or diltiazem), anticoagulation if AF persists, treat the underlying pneumonia (AF often resolves when the pneumonia resolves — "infection-triggered AF").

Necrotizing pneumonia ^[34]^[29]:

Definition: Pneumonia with development of necrosis within infected lung tissue ^[29] — characterised by multiple small cavities ( < 2 cm) within consolidated lung, distinguished from a single large lung abscess.

Follows pneumonia caused by virulent bacteria including Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus ^[34]. Also Klebsiella pneumoniae ^[29].

Pathophysiology: Particularly virulent organisms (e.g., PVL-producing S. aureus, mucoid K. pneumoniae) produce toxins and enzymes that cause direct tissue destruction → thrombosis of pulmonary vessels → ischaemic necrosis → multiple small cavities form within the consolidation.

Management: Prolonged IV antibiotics (4–6 weeks), surgical resection if refractory or complicated by bronchopleural fistula.

Spread from pneumonia can seed other sites ^[1]^[29]:

Metastatic Complication	Mechanism	Features
Meningitis	Bacteraemia → haematogenous seeding of meninges (especially S. pneumoniae)	Headache, neck stiffness, photophobia, altered consciousness
Infective endocarditis	Bacteraemia → seeding of damaged or prosthetic heart valves	New murmur, fever, embolic phenomena
Septic arthritis	Bacteraemia → joint seeding	Acute hot swollen joint (usually monoarticular)
Brain abscess	Haematogenous spread or direct extension (via infected sinuses in contiguous pneumonia)	Focal neurological deficits, headache, fever

Pneumococcal-associated HUS ^[34]:

Associated with Streptococcus pneumoniae infection; typically presents with pneumonia usually accompanied by empyema or effusion ^[34]
Triad: microangiopathic haemolytic anaemia + thrombocytopenia + AKI ^[34]
Pneumococcal-associated HUS patients are generally younger and have more severe initial disease with longer duration of oliguria and thrombocytopenia compared to Shiga-toxin producing E. coli HUS ^[34]
Extra-renal manifestations common: pancreatitis, cholecystitis, thrombosis, cardiac dysfunction, hearing loss ^[34]
Treatment is mainly supportive: RBC/platelet transfusion, fluid/electrolyte management, dialysis if indicated ^[34]

Mechanism: S. pneumoniae neuraminidase enzyme cleaves N-acetylneuraminic acid from RBC, platelet, and glomerular endothelial cell surfaces → exposes the Thomsen-Friedenreich (T) antigen → naturally occurring IgM anti-T antibodies bind → complement activation → RBC lysis, platelet consumption, and renal endothelial injury.

Recurrent pneumonia: defined as ≥ 2 episodes in a year or ≥ 3 episodes at any time with radiographic clearing in between ^[1].

This is not a complication per se but an important clinical scenario that demands investigation for an underlying cause:

Category	Underlying Causes
Structural disease ^[1]	Bronchial obstruction (tumour, foreign body), bronchiectasis, sequestration
Host factors ^[1]	Recurrent aspiration, immunocompromised state

Investigations ^[1]:

CT thorax and bronchoscopy for structural disease
Sputum for AFB and cytology
Blood for immunodeficiency (immunoglobulins, HIV testing, complement levels)

Clinical Pearl – Same Lobe = Obstruction

If pneumonia recurs in the same lobe, think obstruction: endobronchial tumour (lung cancer, carcinoid), foreign body, or extrinsic compression by lymphadenopathy. Bronchoscopy is mandatory. If pneumonia recurs in different lobes, think systemic host factor: immunodeficiency, recurrent aspiration, or bronchiectasis.

Pneumonia is itself a common post-operative complication, especially after thoracic and abdominal surgery ^[35]^[36]:

Risk factors for post-operative pneumonia:

Smoker status, pre-existing lung disease (COPD)
Thoracotomy (disrupts chest wall mechanics)
One-lung ventilation (atelectasis of the deflated lung)
Prolonged anaesthesia/immobilisation
Post-operative pain limiting cough and deep breathing
Aspiration from impaired consciousness or vocal cord palsy

Prevention ^[36]:

Pre-operative: encourage respiratory muscle exercise, smoking cessation
Intra-operative: prevent prolonged operation (prevent atelectasis by mucus accumulation), prevent large volume fluid infusion (prevent pulmonary oedema)
Post-operative: encourage respiratory muscle exercise (deep breathing exercises), encourage coughing (prevent sputum retention), chest physiotherapy, pain control (PCA, thoracic epidural), early mobilisation

Complication	Pathophysiology	Key Management Points
Respiratory failure ^[14]	V/Q mismatch → hypoxaemia; if exhaustion → hypercapnia	O₂, NIV, mechanical ventilation
Parapneumonic effusion ^[14]	Pleural inflammation → exudate; if infected → complicated/empyema	Simple: antibiotics alone. Complicated/empyema: drainage + prolonged Abx
Empyema ^[14]	Frank pus in pleural space	4–6 weeks antibiotics + intercostal drain ^[14]
Lung abscess ^[14]	Necrosis and cavitation within lung	4–6 weeks antibiotics; drainage rarely needed ^[14]
Septicaemia/MOF ^[14]	Bacteraemia → cytokine storm → organ failure	Source control, fluids, vasopressors, organ support
HypoNa/SIADH ^[14]	Non-osmotic ADH release from pulmonary inflammation	Fluid restriction; treat underlying infection
Acute MI ^[14]	↑O₂ demand + plaque instability + hypercoagulability	Monitor troponin/ECG; treat both pneumonia and ACS
AF ^[14]	Hypoxia + inflammation + sympathetic activation → atrial re-entry	Rate control; usually resolves with pneumonia treatment
Necrotizing pneumonia ^[34]	Virulent organisms → tissue destruction with multiple small cavities	Prolonged IV Abx; consider surgery
HUS ^[34]	Pneumococcal neuraminidase → T-antigen exposure → MAHA + thrombocytopaenia + AKI	Supportive (transfusion, dialysis)
Metastatic infection	Bacteraemia → seeding meninges, valves, joints	Targeted Abx based on organism

High Yield Summary – Complications of Pneumonia

From GC 052 (slides 29–30) — the must-know complications:

Respiratory failure — most immediate threat; Type 1 (V/Q mismatch) → Type 2 (exhaustion) → ARDS in worst cases
Septicaemia with multi-organ failure — bacteria enter bloodstream → cytokine storm → shock + DIC + AKI + ARDS
Lung abscess — cavity with air-fluid level; polymicrobial; prolonged Abx 4–6 weeks; no percutaneous drainage usually needed
Parapneumonic effusion — simple (resolves with Abx) vs complicated (pH < 7.2 → drain) vs empyema (pus → drain + prolonged Abx)
Empyema thoracis — pus in pleural cavity; 4–6 weeks Abx + intercostal drain
HypoNa from SIADH — especially Legionella, COVID, SARS; fluid restrict + treat cause
Acute MI — pneumonia triggers demand ischaemia and plaque instability
Cardiac arrhythmia (AF) — hypoxia + inflammation + sympathetic activation

Exam pearls: Empyema = drain; lung abscess = usually DON'T drain. Empyema CXR = obtuse angle, lenticular; abscess CXR = acute angle, spherical, air-fluid level. Recurrent same-lobe pneumonia = bronchoscopy for obstruction.

Active Recall - Complications of Pneumonia

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.167 – Complications, prognosis, recurrent pneumonia) ^[8] Senior notes: Ryan Ho Respiratory.pdf (p.65 – Complications of CAP, causes of unresolving pneumonia) ^[11] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.21 – SIADH causes including chest infection) ^[12] Senior notes: Block A - Sudden severe chest pain_ acute myocardial infarction; aortic dissection.pdf (p.5 – Pneumonia as AMI trigger) ^[14] Lecture slides: GC 052. Fever and purulent sputum.pdf (p.29-30 – Complications of pneumonia; p.46 – Learning points) ^[29] Senior notes: Maksim Medicine Notes.pdf (p.188-189 – Complications, lung abscess vs empyema table) ^[34] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.171 – Complications including HUS, necrotizing pneumonia, lung abscess) ^[35] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p.386-387 – Post-operative pneumonia, prevention of pulmonary complications) ^[36] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p.387 – Prevention of post-operative respiratory complications)