Respiratory failure is a condition in which the respiratory system fails to maintain adequate gas exchange, resulting in hypoxemia (Type I) or hypercapnia with hypoxemia (Type II).

Exam domain	One-glance essentials
Definition / diagnosis	*Respiratory failure is an ABG diagnosis: PaO₂ < 60 mmHg ± PaCO₂ > 50 mmHg on room air; it is a clinical state — always identify the underlying cause ^[1]^[2] Type 1 (hypoxaemic): ↓PaO₂ with normal/low PaCO₂ and elevated A-a gradient — lung parenchyma/vascular problem (V/Q mismatch, shunt, diffusion impairment) ^[1]^[3] Type 2 (hypercapnic): ↓PaO₂ AND ↑PaCO₂ — alveolar hypoventilation ± V/Q mismatch; normal A-a gradient if pure pump failure, elevated if concurrent lung disease ^[1]^[2] Acute vs chronic vs acute-on-chronic*: Acute = uncompensated pH + normal HCO₃⁻; chronic = renal compensation (↑HCO₃⁻, near-normal pH); acute-on-chronic = elevated baseline HCO₃⁻ with falling pH ^[1]^[4]
Epidemiology / risks	*COPD is the most common cause of chronic respiratory failure; smoking is the dominant aetiology* — highly prevalent in Hong Kong's ageing population ^[5]^[1] Acute Type 1 RF risk factors: pneumonia (including TB), PE, ARDS triggers (sepsis, aspiration, trauma), acute cardiogenic pulmonary oedema ^[1]^[6] Type 2 RF pump-failure risks: drug overdose (opioids, sedatives), neuromuscular disease (GBS, MG, MND), obesity hypoventilation syndrome, kyphoscoliosis ^[1]^[2] Type 1 → Type 2 progression occurs with respiratory muscle fatigue — an ominous sign of impending ventilatory collapse requiring urgent escalation ^[1]^[6]
Core mechanism	Type 1: V/Q mismatch (commonest, e.g. pneumonia), intrapulmonary shunt (ARDS — refractory to O₂), or diffusion impairment (ILD) → elevated A-a gradient ^[1]^[3] Type 2 with normal A-a gradient: pure pump failure — CNS depression, neuropathy, NMJ disease, myopathy, or chest wall deformity → global hypoventilation ^[1]^[2] *O₂-induced hypercapnia in COPD involves three mechanisms: Haldane effect (↑O₂ → Hb releases CO₂), release of HPV (worsens V/Q mismatch), and reduced hypoxic ventilatory drive (minor role) ^[1]^[5] ARDS*: diffuse alveolar damage → bilateral non-cardiogenic oedema → intrapulmonary shunt → refractory hypoxaemia unresponsive to supplemental O₂ alone ^[7]^[6]
Clinical picture	Respiratory distress signs: tachypnoea, accessory muscle use, tripod position, intercostal retractions, inability to speak in full sentences; cyanosis is a late sign ^[1]^[2] CO₂ retention signs: morning headache, drowsiness → confusion → coma (CO₂ narcosis), asterixis (CO₂ flap), bounding pulse, papilloedema ^[1]^[2] *In children, respiratory failure is the commonest pathway to cardiac arrest* — tachypnoea, grunting, nasal flaring, and head bobbing are key early signs ^[8]^[1] Cannot-miss emergencies: tension pneumothorax (absent breath sounds, tracheal deviation), massive PE (acute RV failure), silent chest in acute severe asthma, anaphylaxis ^[1]^[6]
Investigations	*ABG is the definitive first-line test: determines PaO₂, PaCO₂, pH, HCO₃⁻, and A-a gradient; SpO₂ alone does NOT detect hypercapnia — always get ABG in suspected Type 2 RF ^[1]^[3] A-a gradient* is the most important diagnostic fork: normal → pure pump failure (extrapulmonary); elevated → lung parenchyma/vascular disease; pH + HCO₃⁻ separates acute from chronic ^[1]^[4] CXR is first-line imaging to narrow the differential; BNP helps distinguish cardiac from respiratory causes; CTPA is gold standard for PE; co-oximetry required for CO/MetHb ^[3]^[1] Serial FVC is critical in neuromuscular disease (GBS/MG) — FVC < 20 mL/kg or declining trend → intubation; ARDS: Berlin criteria with PaO₂/FiO₂ ratio on PEEP ≥ 5 cmH₂O ^[1]^[7]
Management	O₂ therapy is first-line: target SpO₂ 94–98% (general) or *88–92% in COPD/CO₂ retainers; use Venturi mask 24–28%* for precise FiO₂; re-check ABG 30–60 min after starting O₂ ^[1]^[2] *NIV (BiPAP): strongest indication = AECOPD with pH < 7.35; also APO (CPAP), OHS, neuromuscular disease; contraindicated if GCS < 8, respiratory arrest, haemodynamic instability, facial trauma ^[9]^[10] Check ABG 1–2 h after starting NIV; if persistent acidosis (pH < 7.25), rising PaCO₂, or worsening GCS → do not delay intubation; ARDS requires lung-protective ventilation (Vt 6 mL/kg IBW, Pplat < 30, prone positioning) ^[10]^[6] Treat the underlying cause*: antibiotics (pneumonia), bronchodilators (COPD/asthma), anticoagulation/thrombolysis (PE), diuretics + CPAP (APO), naloxone (opioid OD), IVIg/plasmapheresis (GBS/MG) ^[1]^[6]
Complications	Hypoxaemia complications: arrhythmia, MI, AKI, seizures/encephalopathy, lactic acidosis, SIADH; hypercapnia: CO₂ narcosis (drowsiness → coma), raised ICP, respiratory acidosis with CV depression ^[1]^[2] *Cor pulmonale: chronic hypoxaemia → hypoxic vasoconstriction → pulmonary hypertension → RV failure; secondary erythrocytosis* increases viscosity and thrombotic risk ^[1]^[3] Treatment complications: O₂-induced hypercapnia (COPD); *NIV: facial skin necrosis, aspiration, hypotension; mechanical ventilation: VAP, barotrauma, volutrauma, diaphragm atrophy, ICU-acquired weakness ^[11]^[10] MODS* in severe RF/ARDS/sepsis: septic shock, AKI, DIC, hepatic failure; chronic RF vicious cycles: ↑WOB → cachexia → muscle weakness → worse ventilation → recurrent infections ^[12]^[13]
Prevention / follow-up	*LTOT improves survival in stable hypoxaemic COPD: confirm by ABG; PaO₂ ≤ 7.3 kPa, or 7.3–8.0 kPa with polycythaemia, oedema or pulmonary hypertension; prescribe ≥ 15 h/day* after safety assessment ^[14] Smoking cessation is the single most important intervention to slow COPD progression; *pulmonary rehabilitation* improves exercise tolerance, dyspnoea, and quality of life ^[5]^[15] VAP prevention bundle: head-of-bed elevation 30–45°, oral hygiene, sedation holidays, DVT prophylaxis, early mobilisation to prevent ICU-acquired weakness ^[6]^[16] Vaccination (influenza, pneumococcal, COVID-19) reduces infective exacerbations; lung-protective ventilation strategies prevent ventilator-induced lung injury in ARDS ^[17]^[6]
Exam traps	*Respiratory failure is a clinical state, not a final diagnosis — always identify the underlying cause; stopping at "respiratory failure" without aetiological diagnosis is a common exam error ^[1]^[2] Not all hypoxaemia is respiratory failure* (e.g. cyanotic CHD, severe anaemia, MetHb, CO poisoning); *not all RF is due to lung disease* (e.g. narcotic OD, GBS) — always think beyond the lungs ^[1]^[3] *Elevated baseline HCO₃⁻ is the key clue to acute-on-chronic Type 2 RF* — if HCO₃⁻ ~32 with low pH and high PaCO₂, this is NOT pure acute RF; kidneys were already compensating for chronic hypercapnia ^[1]^[4] *SpO₂ does not detect hypercapnia* — a COPD patient with SpO₂ 96% on high-flow O₂ may be in severe CO₂ narcosis; always ABG; in massive PE, avoid positive pressure ventilation (worsens RV failure) — use HFNC ^[1]^[6]

References

^[1] Lecture slides: Respiratory failure - 20221010.pdf ^[2] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf ^[3] Lecture slides: Data Interpretation - Respiratory - by Dr DCL Lam (Rotation 2).ppt.pdf; Respiratory- Introduction to Resp investigations (CXR, CT thorax and lung function tests).pdf ^[4] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf ^[5] Lecture slides: GC 041. Cough in a chronic smoker_COPD; smoking cessation.pdf ^[6] Lecture slides: GC 005 - The patient is critically ill - Intensive care medicine; Unstable vital organ function [Updated 2025].pdf ^[7] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, Section on ARDS (Berlin definition) ^[8] Lecture slides: GC 145. A critically ill child childhood medical emergencies.pdf ^[9] Lecture slides: Handbook of Internal Medicine 2024.pdf, p.421 (NIV evidence and contraindications) ^[10] Lecture slides: Handbook of Internal Medicine 2024.pdf, p.423 (NIV monitoring and complications) ^[11] Senior notes: Maksim Medicine Notes.pdf, p.284 (NIV complications) ^[12] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf, p.36 (MODS) ^[13] Senior notes: Maksim Medicine Notes.pdf, p.300 (COPD complications) ^[14] Guideline: NICE NG115. Chronic obstructive pulmonary disease in over 16s — long-term oxygen therapy ^[15] Senior notes: Maksim Medicine Notes.pdf, p.304 (pulmonary rehabilitation) ^[16] Senior notes: Ryan Ho Critical Care.pdf, p.6 ^[17] Senior notes: Maksim Medicine Notes.pdf, p.300 (COPD section)

Respiratory Failure

Respiratory failure is not a disease — it is a syndrome (a clinical state) in which the respiratory system fails in one or both of its two fundamental gas-exchange functions:

Oxygenation of arterial blood
Elimination of carbon dioxide (CO₂) from venous blood

It is defined by arterial blood gas (ABG) criteria ^[1]^[2]:

Respiratory failure is defined as PaO₂ < 60 mmHg (8 kPa) and/or PaCO₂ > 50 mmHg (6.7 kPa) on room air at sea level. ^[1]

Why these cut-offs?

PaO₂ < 60 mmHg: This sits on the steep part of the oxyhaemoglobin dissociation curve. Below this point, small further drops in PaO₂ cause precipitous falls in SaO₂ (oxygen saturation), meaning tissue oxygen delivery plummets rapidly. Above 60 mmHg, the curve is flat and saturation is relatively preserved — so clinically, 60 mmHg is the inflection point where danger begins.
PaCO₂ > 50 mmHg: Normal PaCO₂ is 35–45 mmHg. A PaCO₂ > 50 mmHg indicates the lungs are unable to adequately ventilate (blow off CO₂), leading to respiratory acidosis.

Key Distinction: Respiratory Failure vs. Respiratory Distress

Respiratory distress is the clinical appearance of a patient struggling to breathe (tachypnoea, accessory muscle use, diaphoresis). Respiratory failure is a blood-gas diagnosis. A patient can be in respiratory distress without failure (compensation is working), and conversely, a patient can be in respiratory failure without obvious distress (e.g., a sedated COPD patient quietly retaining CO₂). Always confirm with ABG.

Breaking Down the Term

Respiratory = relating to breathing (Latin: respirare = to breathe again)
Failure = inability to fulfil normal function

2. Epidemiology and Risk Factors

Respiratory failure is one of the most common reasons for ICU admission worldwide and in Hong Kong ^[3].
Acute respiratory failure is one of the commonest reasons for ICU admission and carries significant morbidity and mortality. ^[3]
In Hong Kong, the leading causes include:
- Acute exacerbation of COPD (AECOPD) — given high historic smoking prevalence among older males, COPD is extremely common in HK's ageing population ^[4].
- Pneumonia — community-acquired and hospital-acquired, including TB (tuberculosis remains endemic in HK).
- Heart failure with pulmonary oedema.
- ARDS secondary to sepsis, pneumonia, or pancreatitis.
- Asthma exacerbation — particularly in younger patients.
Mortality varies dramatically: uncomplicated Type 1 respiratory failure from pneumonia may have low mortality with appropriate treatment; ARDS carries 30–40% mortality overall, and severe ARDS up to 45–50%.

Category	Specific Risk Factors
Smoking	Most important and dominant risk factor for COPD ^[4], the leading cause of chronic Type 2 respiratory failure in HK
Age	Elderly patients have reduced respiratory reserve, weaker respiratory muscles, and more comorbidities
Pre-existing lung disease	COPD, asthma, bronchiectasis, interstitial lung disease (ILD), previous TB with lung destruction
Neuromuscular disease	Motor neurone disease (MND/ALS), myasthenia gravis, Guillain-Barré syndrome, muscular dystrophies
Chest wall abnormality	Severe kyphoscoliosis, flail chest, morbid obesity (obesity hypoventilation syndrome)
CNS depression	Drug overdose (opioids, benzodiazepines), brainstem stroke, raised ICP
Cardiac disease	Congestive heart failure, cardiogenic pulmonary oedema
Sepsis/Systemic illness	Sepsis is the leading cause of ARDS; pancreatitis, massive transfusion, trauma ^[5]
Occupational exposure	Occupational exposures to dusts or fumes ^[4] — construction workers (asbestosis, silicosis) ^[6]
Malnutrition	Respiratory muscle weakness → respiratory failure and chest infections ^[7]
Immunosuppression	HIV, post-transplant, biologics — predisposes to opportunistic infections causing respiratory failure

Hong Kong Focus

In Hong Kong, COPD from smoking is the dominant cause of chronic respiratory failure. Pneumonia (including TB), heart failure, and ARDS from sepsis are the leading causes of acute respiratory failure. Construction-related occupational lung disease (pneumoconiosis) is important and examinable ^[6].

3. Anatomy and Physiology Relevant to Respiratory Failure

To understand why the lungs fail, you need to understand how they normally work. Respiration involves a chain of events, and failure of any link causes respiratory failure.

The respiratory system involves a controller (brainstem respiratory centre), an effector (respiratory muscles and chest wall pump), and a gas-exchange unit (airways, alveoli, and pulmonary vasculature). ^[1]^[2]

Component	Normal Function	What Happens When It Fails
Brainstem respiratory centre (medulla: dorsal & ventral respiratory groups; pons: pneumotaxic centre)	Generates rhythmic drive to breathe, responds to chemoreceptors (central: CO₂/H⁺; peripheral: O₂, CO₂, H⁺)	Loss of respiratory drive → hypoventilation → Type 2 respiratory failure (e.g., opioid overdose, brainstem stroke)
Spinal cord (C3–C5)	Transmits motor signals to phrenic nerve (C3, 4, 5 "keeps the diaphragm alive")	High cervical cord injury → diaphragm paralysis → ventilatory failure
Peripheral nerves	Phrenic nerve → diaphragm; intercostal nerves → intercostal muscles	Guillain-Barré syndrome → ascending paralysis → respiratory failure
Neuromuscular junction	Acetylcholine-mediated transmission	Myasthenia gravis → fatigable weakness → ventilatory failure
Respiratory muscles	Diaphragm (main muscle of inspiration, accounts for ~70% of tidal breathing); intercostals; accessory muscles (SCM, scalenes)	Muscle weakness/fatigue → hypoventilation → Type 2 failure
Chest wall & pleura	Provides the bellows mechanism; negative intrapleural pressure keeps lungs expanded	Flail chest, tension pneumothorax, massive pleural effusion → mechanical restriction
Airways	Conduct air to alveoli; humidify, warm, filter	Obstruction (COPD, asthma, foreign body) → V/Q mismatch → Type 1 or Type 2 failure
Alveoli	Gas exchange across the alveolar-capillary membrane (~0.3 μm thick; ~70 m² surface area)	Consolidation (pneumonia), collapse (atelectasis), flooding (pulmonary oedema), destruction (emphysema) → impaired gas exchange
Pulmonary vasculature	Carries deoxygenated blood past alveoli for gas exchange	Pulmonary embolism → dead-space ventilation; pulmonary hypertension → V/Q mismatch

3.2 Key Physiological Concepts

Normal gas exchange requires matching of alveolar ventilation (V) to pulmonary capillary blood flow (Q).
Normal V/Q ratio ≈ 0.8 (alveolar ventilation ~4 L/min, cardiac output ~5 L/min).
V/Q mismatch is the most common mechanism of hypoxaemia in clinical practice ^[1]^[8]:
- Low V/Q (shunt-like): Perfused but poorly ventilated alveoli (e.g., pneumonia, atelectasis, asthma) → blood passes through without being adequately oxygenated.
- High V/Q (dead space): Ventilated but poorly perfused alveoli (e.g., pulmonary embolism) → wasted ventilation.

True shunt (V/Q = 0): Blood bypasses ventilated alveoli entirely.
- Intrapulmonary: Complete alveolar collapse, consolidation, ARDS.
- Intracardiac: Right-to-left cardiac shunts (e.g., Eisenmenger syndrome).
Key feature: Shunt does NOT respond to supplemental O₂. Why? Because the shunted blood never contacts the alveolus — no matter how high the FiO₂, if blood doesn't pass a ventilated alveolus, it can't pick up oxygen.

Thickening of the alveolar-capillary membrane (e.g., pulmonary fibrosis) or loss of surface area (emphysema) impairs oxygen transfer.
CO₂ is 20× more diffusible than O₂ → diffusion impairment causes hypoxaemia before hypercapnia.

Reduced minute ventilation (Minute ventilation = Tidal volume × Respiratory rate).
Both PaO₂ falls and PaCO₂ rises (because the alveolar gas equation dictates that if you ventilate less, alveolar CO₂ rises and O₂ falls proportionally).
A-a gradient is NORMAL in pure hypoventilation (the lung parenchyma itself is normal — the problem is the pump).

The A-a gradient is crucial for distinguishing the mechanism of hypoxaemia. ^[1]^[2]

\text{A-a gradient} = PA O_2 - Pa O_2

Where:

PA O_2 = FiO_2 \times (P_{atm} - P_{H_2O}) - \frac{PaCO_2}{R}

$FiO_2$ = fraction of inspired oxygen (0.21 on room air)
$P_{atm}$ = atmospheric pressure (760 mmHg at sea level)
$P_{H_2O}$ = water vapour pressure (47 mmHg at 37°C)
$R$ = respiratory quotient (0.8)
Normal A-a gradient ≈ (Age/4) + 4 mmHg, or roughly < 15 mmHg in young adults, < 25 mmHg in elderly

Mechanism	A-a Gradient	Response to O₂
Hypoventilation (pure)	Normal	Yes
V/Q mismatch	Elevated	Yes (partially to fully)
Diffusion impairment	Elevated	Yes
Shunt	Elevated	No (or minimal)

High Yield: A-a Gradient Interpretation

If PaO₂ is low and A-a gradient is normal → the problem is hypoventilation (pump failure — think CNS, nerves, muscles, chest wall). If A-a gradient is elevated → the problem is in the lung parenchyma or vasculature (V/Q mismatch, shunt, diffusion impairment).

4. Classification of Respiratory Failure

4.1 By Blood Gas Pattern: Type 1 vs Type 2

This is the most clinically important and exam-relevant classification ^[1]^[2]^[8]:

Feature	Type 1 (Hypoxaemic)	Type 2 (Hypercapnic / Ventilatory)
PaO₂	< 60 mmHg (< 8 kPa)	< 60 mmHg (< 8 kPa)
PaCO₂	Normal or low (≤ 50 mmHg)	> 50 mmHg (> 6.7 kPa)
Mechanism	V/Q mismatch, shunt, diffusion impairment	Alveolar hypoventilation ± V/Q mismatch
A-a gradient	Elevated	Normal (if pure hypoventilation) or elevated (if combined with lung disease)
Lung parenchyma	Abnormal	May be normal (pump failure) or abnormal (e.g., severe COPD)

Type 1 respiratory failure = oxygenation failure (low PaO₂, normal/low PaCO₂). Type 2 respiratory failure = ventilatory failure (low PaO₂ AND high PaCO₂). Type 2 indicates the patient cannot blow off CO₂ adequately. ^[1]^[2]

In Type 1 failure, the patient can still ventilate (the pump works). In fact, hypoxaemia stimulates the peripheral chemoreceptors (carotid bodies), causing hyperventilation — this blows off CO₂, so PaCO₂ is often low. CO₂ is also highly diffusible, so even with V/Q mismatch, compensatory hyperventilation of well-perfused alveoli can "wash out" excess CO₂. But this compensation does NOT work for O₂ because haemoglobin is already nearly fully saturated in well-ventilated alveoli — you can't load more O₂ to compensate for the poorly ventilated areas (the flat top of the oxyhaemoglobin dissociation curve).

In Type 2 failure, the ventilatory pump has failed — the patient simply cannot move enough air in and out of the alveoli to clear CO₂. ^[1] This can be due to:

CNS depression (the controller doesn't signal)
Neuromuscular disease (the effector can't execute)
Chest wall abnormality (the bellows is broken)
Severe airway obstruction where the work of breathing exceeds the capacity of the respiratory muscles (e.g., severe COPD)

Critical Concept: Type 1 → Type 2 Progression

Type 1 can progress to Type 2 if the underlying condition worsens or the patient fatigues. ^[1] For example, a patient with severe pneumonia (initially Type 1 — V/Q mismatch with compensatory hyperventilation) may develop respiratory muscle fatigue from the increased work of breathing. When the muscles fatigue, ventilation drops, PaCO₂ rises, and the patient transitions to Type 2. This is an ominous sign indicating impending respiratory arrest and need for mechanical ventilation.

Feature	Acute	Chronic
Onset	Minutes to hours/days	Weeks to months to years
pH	Abnormal (uncompensated) — respiratory acidosis (Type 2) or alkalosis from hyperventilation (Type 1)	Near-normal (compensated) — renal compensation over 3–5 days
Bicarbonate	Normal (acute — kidneys haven't had time to compensate)	Elevated (chronic — kidneys retain HCO₃⁻ to buffer the chronic respiratory acidosis)
Examples	Acute PE, pneumonia, asthma attack, drug overdose	COPD, obesity hypoventilation, neuromuscular disease, chest wall deformity
Clinical urgency	Immediate life-threatening emergency	May be stable but requires long-term management; acute-on-chronic decompensation is common

In chronic Type 2 respiratory failure (e.g., stable COPD), the kidney retains bicarbonate to compensate for the chronic respiratory acidosis. Therefore, ABG shows a near-normal pH with elevated PaCO₂ AND elevated HCO₃⁻. Acute-on-chronic respiratory failure occurs when a precipitant (e.g., infection) causes further deterioration, manifesting as a drop in pH. ^[1]^[2]^[9]

Acute-on-Chronic Respiratory Failure

This is the most common clinical scenario in HK practice — a COPD patient with chronic Type 2 failure (compensated) who develops an acute exacerbation (infection, heart failure, pneumothorax) and decompensates:

Chronic baseline: PaCO₂ ~55 mmHg, HCO₃⁻ ~32 mmol/L, pH ~7.36 (compensated)
Acute-on-chronic: PaCO₂ ~75 mmHg, HCO₃⁻ ~32 mmol/L (kidneys haven't had time to compensate further), pH ~7.22 (uncompensated acidosis)

The elevated baseline HCO₃⁻ is the clue that this is acute-on-chronic, not purely acute. ^[9]

Respiratory failure can also be classified by the affected component of the respiratory system: ^[1]

Type	Affected Component	Examples
Failure of the gas exchange unit	Alveoli, pulmonary vasculature	Pneumonia, ARDS, PE, ILD
Failure of the ventilatory pump	CNS, nerves, muscles, chest wall	Drug overdose, GBS, MND, kyphoscoliosis
Failure of the airways	Upper and lower airways	COPD, asthma, upper airway obstruction

5. Etiology (with Focus on Hong Kong)

The mnemonic "AAAPPPP" can help:

Cause	Mechanism	Notes
Pneumonia	Consolidation → intrapulmonary shunt + V/Q mismatch	Most common cause of acute Type 1 RF. Common in HK: community-acquired (Strep. pneumoniae, atypicals) and TB ^[10]
Pulmonary oedema (cardiogenic)	Alveolar flooding → shunt + diffusion impairment	Heart failure is extremely common in HK's ageing population
ARDS	Diffuse alveolar damage → non-cardiogenic pulmonary oedema → shunt	Secondary to sepsis, pneumonia, pancreatitis, trauma ^[5]
Pulmonary embolism	Dead space (high V/Q) + reflex bronchoconstriction → V/Q mismatch	Wells score/Geneva score for clinical probability; D-dimer to exclude; CTPA to confirm ^[11]
Asthma (acute severe)	Bronchospasm + mucous plugging → V/Q mismatch	Initially Type 1; severe cases → Type 2 (exhaustion)
Interstitial lung disease	Diffusion impairment + V/Q mismatch	Occupational lung disease relevant in HK (silicosis, asbestosis in construction workers) ^[6]
Pneumothorax	Alveolar collapse → shunt	Tension pneumothorax: life-threatening, mediastinal deviation + ↑resonance ^[12]
Atelectasis	Alveolar collapse → shunt	Post-operative, mucous plugging

Organized by the component of the respiratory chain that fails ^[1]^[2]:

Component	Cause	Notes
CNS (controller)	Drug overdose (opioids, benzodiazepines, alcohol)	Most common acute cause of central hypoventilation
	Brainstem stroke/lesion	Affects medullary respiratory centres
	Central sleep apnoea	Failure of automatic respiratory drive
	Encephalitis, raised ICP
Spinal cord	High cervical cord injury (above C3–C5)	Phrenic nerve origin: C3, 4, 5
Peripheral nerves	Guillain-Barré syndrome (GBS)	Ascending demyelinating polyradiculopathy; monitor FVC serially — if FVC < 20 mL/kg → intubate
	Critical illness polyneuropathy	ICU patients with prolonged ventilation
Neuromuscular junction	Myasthenia gravis (myasthenic crisis)	Anti-AChR antibodies; fatigable weakness
	Organophosphate poisoning	ACh excess → depolarization block
	Botulism	Blocks ACh release
Respiratory muscles	Motor neurone disease (MND/ALS)	Respiratory failure as common final pathway leading to death ^[13]
	Muscular dystrophy
	Respiratory muscle fatigue	Terminal event in any severe respiratory disease
	Malnutrition	Respiratory muscle weakness → resp failure and chest infections ^[7]
Chest wall	Flail chest (trauma)	Paradoxical chest wall movement
	Severe kyphoscoliosis	Restrictive ventilatory defect
	Obesity hypoventilation syndrome	BMI > 30 + chronic daytime hypercapnia (PaCO₂ > 45) without alternative cause
Pleura	Massive pleural effusion	Compresses lung parenchyma
	Tension pneumothorax	Compresses both lung and mediastinum
Airways (severe)	COPD (most common cause of chronic Type 2 RF)	V/Q mismatch (most common mechanism) + alveolar hypoventilation (in severe cases) ^[8]^[14]
	Severe acute asthma	Initially Type 1 → Type 2 when exhausted ("silent chest" is ominous)
	Upper airway obstruction	Foreign body, epiglottitis, angioedema, post-extubation stridor

High Yield: Most Common Causes in Hong Kong

Acute Type 1: Pneumonia (including TB), acute cardiogenic pulmonary oedema, ARDS
Acute Type 2: AECOPD, drug overdose, GBS
Chronic Type 2: COPD (most common by far), obesity hypoventilation, neuromuscular disease
COPD is the most common cause of chronic respiratory failure overall — smoking is the dominant aetiology ^[4]^[14]

5.3 Pathophysiology by Cause — Detailed

Mechanisms of respiratory failure in COPD ^[8]:

V/Q mismatch (most common mechanism): Airway obstruction distributes inspired air non-uniformly. Some alveoli are poorly ventilated but still perfused → low V/Q units → hypoxaemia.
Alveolar hypoventilation: In severe COPD, increased airway resistance and hyperinflation increase the work of breathing. Respiratory muscles fatigue. The diaphragm is flattened by hyperinflation and works at a mechanical disadvantage. Eventually, ventilation becomes inadequate → PaCO₂ rises.
Shunting: If complicated by pneumonia or CHF → complete alveolar filling → true shunt.

The two clinical phenotypes of COPD illustrate different patterns ^[8]:

Feature	"Blue Bloater" (Chronic Bronchitis)	"Pink Puffer" (Emphysema)
Primary pathology	Mucous gland hyperplasia, small airway fibrosis	Destruction of alveolar walls (protease-antiprotease imbalance)
Cyanosis	Prominent (tolerate hypoxaemia, reduced respiratory drive)	Absent (maintain oxygenation by increasing ventilation)
Dyspnoea	+	++
Hyperinflation	+	++
Cor pulmonale	++ (chronic hypoxaemia → pulmonary vasoconstriction → pulmonary HTN → RHF)	+
Inspiratory drive	Reduced	Present
Respiratory failure type	Early Type 2 (reduced drive + V/Q mismatch)	Late Type 2 (maintain ventilation until very late)

Infection → inflammatory exudate fills alveoli → consolidated alveoli are perfused but not ventilated → intrapulmonary shunt + V/Q mismatch → hypoxaemia.
Patient hyperventilates (intact respiratory drive) → PaCO₂ normal or low → Type 1.
If extensive or patient fatigues → Type 2.

ARDS ("Acute Respiratory Distress Syndrome") ^[5]:

A form of non-cardiogenic pulmonary oedema due to diffuse alveolar damage.
Inflammatory mediators damage the alveolar-capillary membrane → protein-rich fluid floods alveoli → severe intrapulmonary shunt → refractory hypoxaemia (characteristically does NOT respond well to supplemental O₂ — because it's shunt-mediated).
Berlin definition classifies severity by PaO₂/FiO₂ ratio ^[5]:
- Mild: 200 < PaO₂/FiO₂ ≤ 300
- Moderate: 100 < PaO₂/FiO₂ ≤ 200
- Severe: PaO₂/FiO₂ ≤ 100

Clot occludes pulmonary artery branch → ventilated alveoli are not perfused → dead space (high V/Q).
Reflex bronchoconstriction and redistribution of blood flow to non-occluded areas → V/Q mismatch.
Release of inflammatory mediators → increased alveolar-capillary permeability.
Net effect: hypoxaemia with hyperventilation → Type 1 failure.

The lungs themselves are normal — the problem is the "pump."
Weakness of diaphragm and intercostal muscles → reduced tidal volume → alveolar hypoventilation → PaCO₂ rises + PaO₂ falls.
A-a gradient is NORMAL (because the lung parenchyma is intact).
In GBS, serial FVC monitoring is critical — FVC < 20 mL/kg or declining FVC is an indication for intubation. ^[3]

6. Clinical Features

Symptom	Pathophysiological Basis	Notes
Dyspnoea (breathlessness)	Increased work of breathing (airway obstruction, stiff lungs, respiratory muscle weakness) + chemoreceptor stimulation by hypoxaemia/hypercapnia → perceived breathlessness	Most common presenting symptom. Severity assessed by NYHA or MRC dyspnoea scale. Ask about onset, progression, triggers, exercise tolerance ^[1]
Orthopnoea	Lying flat → redistribution of blood from lower extremities and splanchnic bed to pulmonary circulation → increased pulmonary venous pressure → pulmonary congestion. Also, diaphragm pushed up by abdominal contents	Suggests cardiac cause (LV failure) but also seen in bilateral diaphragm weakness, severe COPD, and abdominal distension
Paroxysmal nocturnal dyspnoea (PND)	Nocturnal reabsorption of peripheral oedema fluid → volume overload → pulmonary congestion, wakes patient from sleep gasping	More specific for cardiac cause than simple orthopnoea
Cough	Irritation of airways by secretions, inflammation, oedema, or tumour	Productive in pneumonia, COPD, bronchiectasis; pink frothy in pulmonary oedema; dry in ILD, PE
Sputum	Excessive mucus production (goblet cell hyperplasia in COPD); purulent sputum from bacterial infection; blood-tinged in PE, TB, cancer	Increased volume and purulence suggest infective exacerbation of COPD ^[14]
Haemoptysis	Erosion of blood vessels by tumour, TB cavity, bronchiectasis; alveolar haemorrhage in vasculitis (e.g., Goodpasture)	Always investigate — may indicate malignancy, TB, PE ^[10]
Chest pain	Pleuritic (sharp, worse with inspiration) in PE, pneumonia, pneumothorax; central tight pain in ACS; musculoskeletal in rib fractures	Character of pain guides differential diagnosis
Wheeze	Turbulent airflow through narrowed airways (bronchospasm, mucous plugging, dynamic airway compression)	Heard by patient as "whistling"; expiratory wheeze = lower airway obstruction (asthma, COPD); inspiratory stridor = upper airway obstruction ^[12]
Headache (morning)	CO₂ is a cerebral vasodilator → chronic hypercapnia causes cerebral vasodilation → headache, worse in morning (hypoventilation worse during sleep)	Clue to chronic Type 2 respiratory failure ^[1]
Drowsiness, confusion	Hypercapnia → CO₂ narcosis (central depressant effect of high CO₂); severe hypoxaemia → cerebral hypoxia	Progressive drowsiness in COPD patient = CO₂ retention → urgent ABG needed ^[1]
Ankle swelling	Cor pulmonale (right heart failure secondary to chronic hypoxaemia → pulmonary vasoconstriction → pulmonary hypertension → RV failure → peripheral oedema)	Suggests chronic respiratory failure with established pulmonary hypertension ^[8]

6.2 Signs

Organized by the ABCDE approach relevant to the critically ill patient ^[3]^[12]:

Sign	Pathophysiological Basis	Significance
Cyanosis (central)	Deoxyhaemoglobin > 5 g/dL in arterial blood → blue discolouration of lips, tongue, mucous membranes	Late and uncommon feature, suggests severe hypoxia (SaO₂ typically < 85%) or methaemoglobinaemia ^[12]. Note: anaemic patients may not become cyanosed despite severe hypoxia (not enough total Hb to produce 5g/dL of deoxy-Hb)
Peripheral cyanosis	Increased O₂ extraction in poorly perfused tissues → deoxyhaemoglobin in capillaries	Seen in fingers, toes; may be due to poor cardiac output or peripheral vasoconstriction, not necessarily respiratory failure
Altered mental status	Hypoxaemia → cerebral hypoxia; hypercapnia → CO₂ narcosis	Indicates severe hypoxia/hypercarbia OR other underlying causes ^[12]. GCS should be assessed
Diaphoresis	Extreme sympathetic stimulation due to hypoxia and respiratory distress	Indicates extreme sympathetic stimulation ^[12]
Asterixis (CO₂ flap)	Hypercapnia disrupts normal tonic neural control of posture → involuntary flapping tremor of dorsiflexed wrists	Specific for CO₂ retention / hepatic encephalopathy — ask patient to dorsiflex wrists and hold: positive if irregular flapping ^[1]
Plethoric facies	Secondary polycythaemia from chronic hypoxaemia (EPO-driven erythropoiesis)	Seen in chronic Type 2 RF (COPD)
Cachexia/muscle wasting	Advanced COPD, malignancy, MND	Indicates poor prognosis and respiratory muscle weakness

Sign	Pathophysiological Basis
Tachypnoea (↑ RR)	Compensatory increase in respiratory rate to maintain minute ventilation despite reduced tidal volume or increased CO₂/decreased O₂
Use of accessory muscles (SCM, scalenes, abdominals)	When the diaphragm and intercostals are insufficient, accessory muscles are recruited → fatigue of respiratory muscles → impending respiratory failure ^[12]
Pursed-lip breathing	Creates auto-PEEP (positive end-expiratory pressure) → splints airways open during expiration → prevents premature airway closure and air trapping → improves expiratory flow. Classic sign of COPD/emphysema ^[8]
Tripod position (sitting upright, leaning forward, arms braced)	Fixes the shoulder girdle → optimizes the mechanical advantage of accessory muscles (SCM, scalenes, pectorals) → indicates significant respiratory distress ^[12]
Intercostal/subcostal/suprasternal retractions	Generated by the highly negative intrapleural pressures required to overcome increased airway resistance or stiff lungs → indicates airway obstruction ^[12]
Nasal flaring	Reduces nasal airway resistance by widening the nares; sign of increased work of breathing
Paradoxical abdominal movement	Normally, abdomen moves out during inspiration (diaphragm descends, pushes abdominal contents down). If diaphragm is paralyzed or fatigued, it is sucked upward by negative intrathoracic pressure → abdomen moves inward during inspiration → ominous sign of diaphragmatic failure → imminent respiratory arrest
Inability to speak in full sentences	Severe dyspnoea; talks in words = severe; inability to speak = life-threatening (airway, breathing, or neurologically compromised) ^[3]^[12]

Warning: 'Silent Chest' in Asthma

In a patient with severe asthma who suddenly goes quiet with reduced wheeze — this is NOT improvement. It means airflow is so severely reduced that there isn't enough flow to generate a wheeze. This is a pre-arrest sign and an indication for immediate intubation.

Sign	Pathophysiological Basis	Associated Condition
Barrel chest	Chronic hyperinflation → AP diameter ↑ relative to lateral diameter	COPD (emphysema) ^[8]
Reduced chest expansion	Hyperinflation (trapped air limits further expansion); consolidation; pleural effusion; pain	COPD, pneumonia, effusion, rib fracture
Reduced cricoid-notch distance (< 3 finger-breadths)	Hyperinflation pushes the diaphragm down and the lungs up → trachea appears shortened above the sternal notch	COPD ^[8]
Tracheal tug	Forceful diaphragmatic contraction pulls the trachea downward during inspiration	COPD with severe hyperinflation ^[8]
Hoover's sign	Flattened diaphragm in hyperinflation → instead of pushing ribs out, it pulls the lower ribs inward during inspiration	COPD — inward movement of lower ribs during inspiration ^[8]
Loss of cardiac and liver dullness	Hyperinflated lungs interpose between the heart/liver and chest wall → percussion resonance replaces normal dullness	COPD ^[8]
Prolonged expiratory phase	Increased airway resistance → takes longer to expel air	Obstructive airways disease
Wheeze (expiratory)	Turbulent flow through narrowed intrathoracic airways during expiration (dynamic compression)	Asthma, COPD; lower airway obstruction ^[12]
Stridor (inspiratory)	Turbulent flow through narrowed extrathoracic (upper) airway	Upper airway obstruction ^[12]: foreign body, epiglottitis, laryngeal tumour, post-extubation oedema
Crackles (crepitations)	Early inspiratory: opening of collapsed small airways (COPD, bronchiectasis). Late inspiratory: opening of fluid-filled alveoli (pulmonary oedema, pneumonia, fibrosis). Coarse: secretions in larger airways	Alveolar congestion ^[12]; fibrosis; pneumonia
Bronchial breathing	Consolidation transmits large-airway sounds directly to the chest wall (normally filtered by air-filled alveoli)	Consolidation (pneumonia)
Decreased/absent air entry	No airflow (pneumothorax, massive effusion) or severe obstruction	Non-specific ^[12]; pneumothorax, effusion, severe obstruction
Increased resonance on percussion	Air-filled space (pneumothorax, hyperinflation)	Pneumothorax, emphysema
Stony dull percussion	Fluid in pleural space	Pleural effusion
Mediastinal deviation + ↑ resonance	Tension pneumothorax: progressive air accumulation in pleural space → pushes mediastinum to contralateral side	Tension pneumothorax — clinical diagnosis, do NOT wait for CXR ^[12]

Sign	Pathophysiological Basis	Significance
Tachycardia	Sympathetic activation from hypoxaemia, hypercapnia, distress	Non-specific but universal in acute respiratory failure
Raised JVP	Right heart failure (cor pulmonale) from pulmonary hypertension; or tension pneumothorax compressing the vena cava	Chronic RF: cor pulmonale. Acute RF: tension pneumothorax, massive PE
Pulsus paradoxus (exaggerated fall in SBP > 10 mmHg on inspiration)	Severe airway obstruction → large intrathoracic pressure swings → impaired venous return during inspiration → reduced LV filling → SBP drops	Severe asthma, cardiac tamponade
Loud P2 (loud pulmonary component of S2)	Pulmonary hypertension → forceful closure of the pulmonary valve	Chronic cor pulmonale
Right ventricular heave (parasternal heave)	RV hypertrophy from chronic pressure overload (pulmonary hypertension)	Cor pulmonale
Peripheral oedema	RV failure → elevated systemic venous pressure → transudation of fluid	Cor pulmonale
Hepatomegaly	RV failure → hepatic congestion (back-pressure in hepatic veins)	Cor pulmonale
Hypotension/shock	Massive PE (obstructive shock); tension pneumothorax (obstructive shock); severe sepsis with ARDS (distributive shock)	Life-threatening — requires immediate intervention

Sign	Suggests
Clubbing	Lung cancer, bronchiectasis, ILD, infective endocarditis, cyanotic heart disease
Tar staining of fingers	Smoking history → COPD risk
Cushingoid features	Chronic steroid use (severe asthma, COPD)
Muscle fasciculations + wasting	Motor neurone disease
Butterfly rash, joint deformity	SLE → diffuse alveolar haemorrhage, ILD, PE
Kyphoscoliosis	Chest wall restriction → Type 2 RF
Obesity (morbid)	Obesity hypoventilation syndrome

Clinical Pattern	Likely Cause	Key Features
Acute dyspnoea + fever + productive cough + crackles + consolidation	Pneumonia	Type 1 RF; elevated WCC; CXR consolidation
Acute dyspnoea + pleuritic chest pain + recent immobilization + tachycardia + clear lungs	Pulmonary embolism	Type 1 RF; elevated D-dimer; CTPA positive
Acute dyspnoea + bilateral crackles + S3 + raised JVP + peripheral oedema	Acute cardiogenic pulmonary oedema	Type 1 RF; elevated BNP; CXR: bilateral infiltrates, Kerley B lines, upper lobe diversion
Gradual ↑ dyspnoea + chronic productive cough in smoker + barrel chest + hyperinflation	COPD	Type 2 RF; obstructive spirometry; CXR hyperinflation
Acute worsening of chronic dyspnoea + increased sputum purulence in COPD patient	AECOPD	Acute-on-chronic Type 2 RF; treat infection + controlled O₂ + NIV
Acute dyspnoea + wheeze + atopy + episodic	Acute asthma	Type 1 → Type 2 if severe; reversible obstruction on spirometry
Bilateral infiltrates + severe hypoxaemia (low PaO₂/FiO₂) + non-cardiogenic + within 1 week of insult	ARDS	Berlin criteria; requires PEEP ^[5]
Progressive ascending weakness + areflexia + dyspnoea	Guillain-Barré syndrome	Type 2 RF; normal A-a gradient; monitor FVC
Fatigable weakness + ptosis + diplopia + dyspnoea	Myasthenic crisis	Type 2 RF; anti-AChR antibodies
Sudden severe dyspnoea + unilateral ↓ air entry + hyperresonance + mediastinal shift	Tension pneumothorax	Clinical diagnosis — needle decompression immediately ^[12]

7. Special Considerations

High Yield: The Danger of Uncontrolled O₂ in COPD

In patients with chronic CO₂ retention (e.g., severe COPD), administering high-flow oxygen can worsen hypercapnia. ^[1]^[9]

The mechanism is NOT simply "loss of hypoxic drive" — this is an oversimplification. The three mechanisms are:

Haldane effect: Oxygenated haemoglobin carries less CO₂ (Haldane effect). When you give O₂ and saturate Hb, it releases CO₂ into the blood → PaCO₂ rises.
Release of hypoxic pulmonary vasoconstriction (HPV): Hypoxia normally constricts vessels to poorly ventilated alveoli (HPV), diverting blood to better-ventilated areas. Giving O₂ relieves HPV → blood flows to poorly ventilated alveoli → worsened V/Q mismatch → CO₂ retention.
Reduced hypoxic ventilatory drive (minor contribution): Some COPD patients rely partly on hypoxic drive. Eliminating hypoxia reduces this drive → slightly less ventilation.

Therefore: Target SpO₂ 88–92% in known CO₂ retainers. Use controlled oxygen delivery (e.g., Venturi mask at 24–28% or low-flow nasal cannula 1–2 L/min). Always re-check ABG 30–60 minutes after starting O₂ therapy. ^[1]^[9]

The PaO₂/FiO₂ ratio (or P/F ratio) is used to:

Standardize the assessment of oxygenation across different FiO₂ levels.
Classify the severity of ARDS ^[5].

Normal P/F ratio: ~500 (PaO₂ ~100 mmHg on FiO₂ 0.21)
Acute lung injury / mild ARDS: 200–300
Moderate ARDS: 100–200
Severe ARDS: ≤ 100

In children, respiratory failure is the most common pathway to cardiac arrest (unlike adults, where primary cardiac events dominate). ^[15]

Key differences:

Children have higher metabolic rate → faster O₂ consumption → decompensate faster.
Smaller airways → more susceptible to obstruction (resistance ∝ 1/r⁴).
Compliant chest wall → more visible retractions, but also less effective in generating negative pressure.
Signs of impending respiratory failure in children: grunting (auto-PEEP), head bobbing, see-saw breathing (paradoxical abdominal movement), cyanosis.

High Yield Summary

Definition: Respiratory failure = PaO₂ < 60 mmHg ± PaCO₂ > 50 mmHg on room air (ABG diagnosis).
Type 1 (Hypoxaemic): Low PaO₂, normal/low PaCO₂. Mechanism: V/Q mismatch, shunt, diffusion impairment. Elevated A-a gradient. Common causes: pneumonia, PE, ARDS, acute cardiogenic pulmonary oedema.
Type 2 (Hypercapnic/Ventilatory): Low PaO₂ AND high PaCO₂. Mechanism: alveolar hypoventilation ± V/Q mismatch. Normal A-a gradient if pure pump failure. Most common cause: COPD. Other causes: drug overdose, GBS, MND, obesity hypoventilation.
Type 1 → Type 2 progression occurs with respiratory muscle fatigue — ominous sign.
Acute vs. Chronic: Acute = uncompensated pH. Chronic = renal compensation (elevated HCO₃⁻, near-normal pH). Acute-on-chronic = elevated baseline HCO₃⁻ with falling pH.
A-a gradient: Normal = pure hypoventilation (pump failure). Elevated = lung parenchyma/vascular problem.
Signs of respiratory distress: tachypnoea, accessory muscle use, tripod position, retractions, inability to speak, cyanosis (late).
CO₂ retention signs: morning headache, drowsiness, confusion, CO₂ flap (asterixis), bounding pulse.
COPD & O₂: Target SpO₂ 88–92% in CO₂ retainers. Mechanisms of O₂-induced hypercapnia: Haldane effect, release of HPV, reduced hypoxic drive.
ARDS: Berlin criteria. P/F ratio classifies severity. Refractory hypoxaemia (shunt-mediated).
Tension pneumothorax: Clinical diagnosis → immediate needle decompression. Do NOT wait for CXR.
In children: respiratory failure is the commonest pathway to cardiac arrest.

Active Recall - Respiratory Failure

Differential Diagnosis of Respiratory Failure

A patient presents with respiratory failure — confirmed on ABG as PaO₂ < 60 mmHg ± PaCO₂ > 50 mmHg. Now what? Respiratory failure is a syndrome, not a diagnosis. Your job is to find the underlying cause, because treatment is directed at the aetiology, not just at normalising gas tensions.

The approach to the differential diagnosis of respiratory failure is systematic: you first classify the type (Type 1 vs Type 2), then use the clinical context (history, examination, bedside tests) to narrow the differential.

High Yield: Respiratory Failure Is Not a Diagnosis

A common exam mistake is to stop at "respiratory failure" as the final diagnosis. Respiratory failure is a clinical state — you must always identify the underlying cause. ^[1] Think of it like "anaemia" — it's a finding, not a diagnosis. The examiner wants the aetiology.

The differential diagnosis depends on two key questions:

Is this Type 1 or Type 2? (ABG pattern)
Is this acute, chronic, or acute-on-chronic? (Time course, pH, bicarbonate)

Then, within each type, you narrow using the presenting symptom complex, the A-a gradient, and clinical clues.

The "Three Systems + Extras" Approach

The differential diagnosis of dyspnoea (the cardinal symptom of respiratory failure) should be organized by system ^[16]^[17]:

System	Differential Diagnoses	Why They Cause Respiratory Failure
Respiratory	Pneumonia, AECOPD, asthma, PE, pneumothorax, pleural effusion, ARDS, ILD, bronchiectasis, lung cancer, TB, upper airway obstruction	Direct impairment of gas exchange or ventilation
Cardiac	Heart failure (LVF/RVF), ACS, valvular disease, cardiomyopathy, arrhythmia, cardiac tamponade	Pulmonary oedema (backward failure) or low cardiac output → tissue hypoxia
Neuromuscular / CNS	Drug overdose, GBS, myasthenia gravis, MND, spinal cord injury, stroke	Pump failure → hypoventilation
Metabolic	Severe anaemia, metabolic acidosis (DKA, uraemia), thyrotoxicosis	Reduced O₂ carrying capacity; compensatory hyperventilation
Others	Sepsis/SIRS, anaphylaxis, poisoning (CO, cyanide, methaemoglobin), panic/anxiety, morbid obesity	Variable mechanisms

Differential diagnosis of shortness of breath includes: Heart failure, pleural effusion, ACS, PE, pneumonia, COPD, asthma, bronchiectasis, ACEI-induced cough, and anaemia. ^[17]

8.3 Differential Diagnosis Organized by Type of Respiratory Failure

The A-a gradient is elevated (problem is in the lung/vasculature). Subdivide by mechanism ^[18]:

Mechanism	Differential	Key Distinguishing Features
V/Q Mismatch	COPD	Chronic smoker, barrel chest, hyperinflation on CXR, obstructive spirometry ^[14]
	Asthma (acute)	Episodic wheeze, atopy, reversible obstruction, responds to SABA ^[17]
	Bronchiectasis	Chronic productive cough, purulent sputum, clubbing, CXR tramlines ^[19]
	Pulmonary embolism	Sudden onset dyspnoea, pleuritic chest pain, DVT signs, risk factors (immobility, malignancy, OCP). Wells/Geneva score → D-dimer → CTPA ^[11]^[20]
Shunt (V/Q = 0)	Pneumonia	Fever, productive cough, consolidation signs, CXR opacity ^[16]
	ARDS	Bilateral opacities, onset within 1 week of insult, non-cardiogenic, low P/F ratio on PEEP ^[5]
	Acute pulmonary oedema (cardiogenic)	PND, orthopnoea, S3, raised JVP, bilateral crackles, CXR: ABCDE ^[21]
	Atelectasis (lung collapse)	Post-operative, mucous plugging, reduced breath sounds over affected area
	Pneumothorax	Sudden onset, ↓breath sounds, ↑resonance, ± mediastinal shift ^[12]
	Alveolar haemorrhage	Haemoptysis, bilateral infiltrates, ↑DLCO, vasculitis (Goodpasture, GPA) ^[22]
Diffusion Impairment	Interstitial lung disease (ILD)	Progressive dyspnoea, dry cough, fine inspiratory crackles, clubbing, restrictive spirometry, HRCT honeycombing ^[19]
	Emphysema	Destruction of alveolar surface area → ↓DLCO; hyperinflation ^[8]
	Pneumocystis pneumonia (PCP)	Immunocompromised (HIV, transplant), fever, cough, SOB, Type 1 RF, bilateral interstitial infiltrates on CXR ^[23]

Subdivide by the component of the ventilatory pump that has failed ^[1]^[18]:

Component	Differential	Key Distinguishing Features	A-a Gradient
CNS (↓ drive)	Drug overdose (opioids, benzodiazepines)	Pinpoint pupils (opioids), ↓GCS, drug history/toxicology screen	Normal
	Brainstem stroke	Focal neurology, ↓consciousness, CT/MRI brain	Normal
	Central sleep apnoea	Witnessed apnoeas during sleep, ↓drive	Normal
	Severe hypothyroidism	Myxoedema, hypothermia, bradycardia	Normal
Spinal cord	High cervical cord injury	Trauma, quadriplegia, sensory level	Normal
Peripheral nerves	Guillain-Barré syndrome	Ascending weakness, areflexia, post-infectious; serial FVC monitoring: FVC < 20 mL/kg → intubate ^[3]	Normal
	Critical illness polyneuropathy	Prolonged ICU stay, difficulty weaning from ventilator	Normal
NMJ	Myasthenia gravis (myasthenic crisis)	Fatigable weakness, ptosis, diplopia, anti-AChR Ab	Normal
	Organophosphate poisoning	Cholinergic crisis: salivation, lacrimation, bradycardia	Normal
Muscles	Motor neurone disease (MND/ALS)	Mixed UMN + LMN signs, fasciculations, respiratory failure as common final pathway ^[13]	Normal
	Muscular dystrophy	Progressive weakness since childhood, ↑CK	Normal
	Malnutrition/cachexia	Muscle wasting, low BMI, respiratory muscle weakness ^[7]	Normal
Chest wall	Severe kyphoscoliosis	Visible deformity, restrictive spirometry	Normal
	Flail chest	Trauma, paradoxical chest wall segment	Normal
	Obesity hypoventilation syndrome	BMI > 30, chronic daytime hypercapnia	Normal
Airways (severe)	AECOPD	Most common cause of acute-on-chronic Type 2 RF; chronic smoker, ↑sputum purulence, ↓exercise tolerance ^[8]^[14]	Elevated
	Severe acute asthma	Initially Type 1 → "silent chest" = exhaustion → Type 2	Elevated
	Upper airway obstruction	Stridor, choking history, anaphylaxis/angioedema	Elevated

A-a Gradient: The Diagnostic Fork in Type 2 RF

In Type 2 respiratory failure, the A-a gradient separates pure pump failure (normal A-a gradient — lungs are fine, the problem is upstream) from lung disease (elevated A-a gradient — V/Q mismatch is also present, e.g., severe COPD). This distinction is clinically important and examinable.

This is a classic exam question and a daily ward-round challenge. Both cardiac and respiratory diseases cause dyspnoea and can cause respiratory failure (cardiogenic pulmonary oedema → Type 1 RF) ^[16]^[21].

Feature	Cardiac Cause	Respiratory Cause
Key symptoms	PND, orthopnoea (highly suggestive of cardiac) ^[16]; peripheral oedema, exertional dyspnoea	Cough, sputum, wheeze, haemoptysis
Examination	S3 gallop, raised JVP, displaced apex beat, bilateral crackles (fine, bibasal), peripheral oedema	Wheeze, prolonged expiratory phase, accessory muscle use, barrel chest, clubbing
CXR	ABCDE: Alveolar oedema/bat-wing opacities, Kerley B lines, Cardiomegaly, Dilated upper lobe vessels, pleural Effusion ^[21]	Hyperinflation, consolidation, mass, fibrotic change
BNP/NT-proBNP	Elevated. BNP < 100 rules out HF; > 400 suggests CHF. However, NT-proBNP is NOT 100% specific — can be elevated in PE, pulmonary HTN, renal failure ^[21]^[24]	Normal or mildly elevated
ECG	May show AF, LVH, ST changes, Q waves	May show P pulmonale, RVH, right axis deviation (cor pulmonale)
Echocardiography	Reduced EF (HFrEF), diastolic dysfunction (HFpEF), valvular lesion	Normal LV function; may show RV dilatation if cor pulmonale or PE
Spirometry	Usually normal (unless co-existing COPD)	Obstructive (COPD/asthma) or restrictive (ILD) pattern

BNP can help distinguish cardiac from respiratory causes of SOB. Only cardiac causes result in significantly elevated BNP. However, BNP is sensitive but not specific — other causes of cardiac stress (PE, pulmonary HTN, renal failure) can also elevate BNP. ^[24]

These are the "cannot-miss" diagnoses — conditions where delayed recognition leads to rapid death ^[20]:

Condition	Why It's Dangerous	Key Rapid Diagnostic Clue
Tension pneumothorax	Progressive mediastinal shift → ↓venous return → obstructive shock → cardiac arrest	Clinical diagnosis: tracheal deviation + ↑resonance + ↓breath sounds + hypotension → needle decompression, DO NOT wait for CXR ^[12]
Massive pulmonary embolism	RV outflow obstruction → obstructive shock	Sudden dyspnoea + hypotension + clear lungs + elevated JVP + ECG: S1Q3T3/sinus tachycardia → bedside echo (RV dilatation) → CTPA → heparin ± thrombolysis ^[11]^[20]
Acute severe asthma → near-fatal	Silent chest = pre-arrest; exhaustion → Type 2 RF	Inability to speak, RR > 25, HR > 110, PEF < 33% predicted, SpO₂ < 92%, normal/rising PaCO₂ in an asthmatic = danger ^[1]
ARDS	Refractory hypoxaemia; high mortality	Bilateral infiltrates + low P/F ratio + not explained by HF ^[5]
Anaphylaxis	Angioedema → upper airway obstruction	Exposure to allergen + urticaria + stridor/wheeze + hypotension → IM adrenaline
Epiglottitis	Complete upper airway obstruction	Drooling, stridor, "hot potato" voice, fever; do NOT examine throat — risk of laryngospasm
Cardiac tamponade	↓Diastolic filling → obstructive shock	Beck's triad: hypotension, muffled heart sounds, raised JVP + pulsus paradoxus

8.6 Differential Diagnosis in Special Populations

Immunocompromised patients are prone to opportunistic infections causing respiratory failure ^[23]:

Patient Population	Key Respiratory Differentials
HIV/AIDS (CD4 < 200)	Pneumocystis jirovecii pneumonia (PCP): bilateral interstitial infiltrates, elevated LDH, Type 1 RF. Also: TB, CMV pneumonitis, Kaposi sarcoma
Post-transplant (on immunosuppressants)	CMV, PCP, TB, BK virus; PCP presents with fever, cough, SOB, Type 1 RF, bilateral interstitial infiltrates ^[23]
Neutropenic (post-chemo)	Bacterial and fungal pneumonia (Aspergillus → invasive pulmonary aspergillosis: "halo sign" on CT)
On biologics (anti-TNF)	Reactivation TB

Timing	Common Differential	Mechanism
Immediate (0–24 h)	Atelectasis, aspiration pneumonia, residual anaesthetic effect (hypoventilation)	Mucous plugging, supine position, splinting from pain; CNS depression
Early (1–5 days)	Pneumonia, PE, ARDS, fluid overload / pulmonary oedema	Nosocomial infection; VTE from immobility; SIRS from surgery
Late ( > 5 days)	PE, hospital-acquired pneumonia, drug-related ILD	Prolonged immobility; resistant organisms

Respiratory failure is the most common pathway to cardiac arrest in children ^[25]. Key paediatric differentials include:

Age Group	Key Differentials
Neonates	Infant respiratory distress syndrome (surfactant deficiency), meconium aspiration, congenital heart disease, persistent pulmonary hypertension of the newborn
Infants (< 2 years)	Acute bronchiolitis (RSV most common), croup, foreign body aspiration, pertussis ^[25]
Older children	Asthma, pneumonia, croup, epiglottitis, foreign body

Not everything that looks like respiratory failure on ABG is due to a primary respiratory problem ^[18]:

Mimic	Explanation	How to Distinguish
Cyanotic congenital heart disease	Right-to-left intracardiac shunt → low PaO₂ → looks like Type 1 RF, but it's a cardiac problem	Echocardiography; hyperoxia test (PaO₂ does not rise > 150 mmHg with 100% O₂ → confirms shunt)
Severe anaemia	Tissue hypoxia without hypoxaemia (PaO₂ may be normal); SpO₂ and PaO₂ may be normal	Check Hb; PaO₂ is usually normal in isolated anaemia
Metabolic acidosis (e.g., DKA, uraemia)	Compensatory hyperventilation (Kussmaul breathing) → low PaCO₂, may have slightly low PaO₂ from demand	pH low, HCO₃⁻ low, anion gap elevated; primary process is metabolic, not respiratory
Methaemoglobinaemia	Oxidised Hb cannot carry O₂; SpO₂ classically reads ~85% regardless of PaO₂	Co-oximetry on ABG shows MetHb fraction; chocolate-brown blood that does not turn red with O₂
Carbon monoxide poisoning	CO binds Hb with 200× affinity of O₂; SpO₂ falsely normal (pulse oximeter cannot distinguish COHb from OxyHb); PaO₂ on ABG may be normal (measures dissolved O₂)	Co-oximetry on ABG shows elevated COHb; history of smoke/fire exposure, headache, cherry-red skin
Anxiety / panic attack	Hyperventilation → low PaCO₂, respiratory alkalosis; may have perioral tingling, carpopedal spasm	Diagnosis of exclusion; A-a gradient normal; PaO₂ normal or high

High Yield: Not All Cyanosis = Respiratory Failure

Not all blood gas abnormalities are due to respiratory failure (e.g., cyanotic congenital heart disease). Not all respiratory failure is due to diseases of the lung (e.g., narcotic overdose → decreased respiration). ^[18] Always think beyond the lungs.

When you see a patient with respiratory failure on the ward, run through this mental checklist:

Step	Action	Rationale
1	ABG: Classify Type 1 vs Type 2	Determines mechanism (oxygenation vs ventilation failure)
2	A-a gradient: Normal or elevated?	Separates pump failure from lung pathology
3	pH and HCO₃⁻: Acute, chronic, or acute-on-chronic?	Guides urgency and underlying chronicity
4	History: Onset, associated symptoms, PMH, medications, smoking, occupational exposure	Narrows the differential list
5	Examination: Respiratory signs, cardiac signs, neurological signs	Localises the problem
6	Bedside tests: SpO₂, CXR, ECG, BNP, FBC	First-line screening to confirm or exclude top differentials
7	Targeted investigations: CTPA, echo, spirometry, NCS/EMG, CT brain, toxicology	Based on working differential

High Yield Summary — Differential Diagnosis of Respiratory Failure

Type 1 RF (↓PaO₂, normal PaCO₂, ↑A-a gradient): Think lung parenchyma or vasculature — pneumonia, PE, ARDS, APO, ILD, pneumothorax.
Type 2 RF (↓PaO₂, ↑PaCO₂):
- Normal A-a gradient → pure pump failure: CNS (drug OD, stroke), nerves (GBS), NMJ (MG), muscles (MND, cachexia), chest wall (kyphoscoliosis, obesity).
- Elevated A-a gradient → pump failure PLUS lung disease: AECOPD (most common), severe asthma, upper airway obstruction.
Cannot-miss emergencies: tension pneumothorax, massive PE, acute severe asthma (silent chest), anaphylaxis, cardiac tamponade.
Cardiac vs respiratory: PND/orthopnoea favour cardiac; BNP helps distinguish but is not perfectly specific. Always get an ECG and CXR.
Mimics: Cyanotic CHD, severe anaemia, MetHb, CO poisoning, metabolic acidosis, anxiety — check ABG with co-oximetry, Hb, and clinical context.
Immunocompromised: Think PCP, CMV, TB, invasive aspergillosis.
Children: Respiratory failure is the commonest pathway to cardiac arrest; think bronchiolitis, croup, foreign body.

Active Recall - Differential Diagnosis of Respiratory Failure

References

^[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf ^[3] Lecture slides: GC 005 - The patient is critically ill - Intensive care medicine; Unstable vital organ function [Updated 2025].pdf ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, Section on ARDS (Berlin definition) ^[7] Senior notes: Ryan Ho Fluids and Nutrition.pdf, p.7 ^[8] Senior notes: Maksim Medicine Notes.pdf, p.300 (COPD section) ^[11] Senior notes: Block A - Chest Pain - Department of Medicine.pdf, p.2-3 ^[12] Senior notes: Ryan Ho Critical Care.pdf, p.6 ^[13] Senior notes: Ryan Ho Neurology.pdf, p.186 ^[14] Lecture slides: GC 041. Cough in a chronic smoker_COPD; smoking cessation.pdf ^[16] Senior notes: Block A - Three Cases of Dyspnea.pdf ^[17] Senior notes: Block A – Nephrology Data Interpretation.pdf, p.11 (DDx of SOB) ^[18] Senior notes: Ryan Ho Respiratory.pdf, p.29; Ryan Ho Fundamentals.pdf, p.230 ^[19] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, p.220 (COPD DDx) and p.238-280 (ILD) ^[20] Senior notes: Ryan Ho Cardiology.pdf, p.58 ^[21] Senior notes: Maksim Medicine Notes.pdf, p.18 (Heart failure) ^[22] Senior notes: Ryan Ho Urogenital.pdf, p.67 (Goodpasture) ^[23] Senior notes: Block A - Renal Replacement Therapies.pdf, p.37 (PCP) ^[24] Senior notes: Block A - Shortness of breath on exertion_ heart failure.pdf, p.11 ^[25] Lecture slides: GC 145. A critically ill child childhood medical emergencies.pdf

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities for Respiratory Failure

9.1 Diagnostic Criteria

Respiratory failure does not have "diagnostic criteria" in the way that, say, ARDS or heart failure does (with specific multi-component scoring systems). Instead, it is defined purely by arterial blood gas values ^[1]^[2]:

Respiratory failure is defined by ABG showing PaO₂ < 60 mmHg (8 kPa) ± PaCO₂ > 50 mmHg (6.7 kPa) on room air at sea level. ^[1]

However, simply meeting the ABG cut-off is insufficient — you must then:

Classify the type (Type 1 vs Type 2)
Determine the time course (acute, chronic, acute-on-chronic)
Identify the underlying cause (this is the actual "diagnosis")

Criterion	Type 1	Type 2
PaO₂	< 60 mmHg	< 60 mmHg
PaCO₂	≤ 50 mmHg (normal or low)	> 50 mmHg
Interpretation	Oxygenation failure; lungs cannot adequately oxygenate blood	Ventilatory failure; lungs cannot adequately blow off CO₂

Parameter	Acute	Chronic	Acute-on-Chronic
pH	Abnormal (< 7.35 in Type 2)	Near-normal (7.35–7.40) despite elevated PaCO₂	Falls again (< 7.35)
HCO₃⁻	Normal (22–26 mmol/L) — kidneys have not had time to compensate	Elevated ( > 26, often 30–38) — renal retention of HCO₃⁻ over 3–5 days	Elevated at baseline (proving chronicity), but pH drops because the acute insult adds more CO₂ than the already-compensated HCO₃⁻ can buffer
Clinical clue	Sudden onset, no prior history	Gradual onset, known COPD/neuromuscular disease, stable clinical state	Known chronic respiratory disease with sudden deterioration (e.g., infective exacerbation of COPD)

High Yield: How to Spot Acute-on-Chronic Type 2 RF

The elevated baseline HCO₃⁻ is the key clue. If you see PaCO₂ 75 mmHg + HCO₃⁻ 32 mmol/L + pH 7.22 → this is NOT pure acute Type 2 RF (where HCO₃⁻ would be normal ~24). The elevated HCO₃⁻ tells you the kidneys were already compensating for a pre-existing chronic hypercapnia — but a new insult has pushed PaCO₂ up further, overwhelming that compensation. This patient has acute-on-chronic Type 2 RF ^[1]^[2].

Since ARDS is a major and examinable cause of severe respiratory failure, its Berlin definition criteria are included here ^[5]:

Berlin Criterion	Description
Timing	Respiratory symptoms must have begun within 1 week of a known clinical insult OR new/worsening symptoms during the past week
Chest imaging	Bilateral opacities on CXR or CT not fully explained by pleural effusion, lobar collapse, or pulmonary nodules
Origin of oedema	Not fully explained by cardiac failure or fluid overload. Objective assessment (e.g., echocardiography) required to exclude hydrostatic pulmonary oedema if no risk factor for ARDS is present
Oxygenation (on PEEP/CPAP ≥ 5 cmH₂O)	Mild: PaO₂/FiO₂ 200–300. Moderate: PaO₂/FiO₂ 100–200. Severe: PaO₂/FiO₂ ≤ 100

The diagnostic approach to respiratory failure follows a logical sequence: Confirm → Classify → Compensate → Cause.

9.3 Investigations — Modalities, Key Findings, and Interpretation

The single most important investigation in respiratory failure. It is both the defining test and the classifying test ^[1]^[2]^[26].

Stepwise ABG interpretation ^[26]^[27]:

Step	What to Assess	How to Interpret
1. Oxygenation	PaO₂ and A-a gradient	PaO₂ < 60 mmHg = hypoxaemia = respiratory failure. Calculate A-a gradient to determine mechanism (normal = hypoventilation; elevated = lung pathology)
2. pH	pH < 7.35 = acidosis; > 7.45 = alkalosis	Determines urgency. Severely acidotic (pH < 7.20) → life-threatening
3. Primary disorder	PaCO₂: respiratory component. HCO₃⁻: metabolic component	High PaCO₂ + low pH = respiratory acidosis. Low PaCO₂ + high pH = respiratory alkalosis. Low HCO₃⁻ + low pH = metabolic acidosis
4. Compensation	Is the non-primary parameter moving in the expected direction?	Acute respiratory acidosis: HCO₃⁻ rises ~1 mmol/L per 10 mmHg rise in PaCO₂. Chronic: HCO₃⁻ rises ~3.5 mmol/L per 10 mmHg rise in PaCO₂ (renal compensation over days)
5. Anion gap	If metabolic acidosis: Na⁺ − (Cl⁻ + HCO₃⁻)	Normal AG = 8–14. Elevated AG = presence of unmeasured acids (lactic acid, ketoacids, uraemia, toxins) ^[28]

Interpretation of ABG, especially different settings of Type 2 respiratory failure, is a key learning point. ^[27]

Worked example — Acute-on-chronic Type 2 RF in COPD:

pH 7.25 (acidotic)
PaCO₂ 72 mmHg (elevated → respiratory acidosis)
HCO₃⁻ 30 mmol/L (elevated → this is the clue to chronicity — kidneys were already compensating)
PaO₂ 48 mmHg (hypoxaemic → respiratory failure)
A-a gradient: elevated (because COPD has V/Q mismatch — the lung parenchyma is abnormal)

Interpretation: Acute-on-chronic Type 2 respiratory failure. The elevated HCO₃⁻ proves chronic CO₂ retention; the acidotic pH proves an acute decompensation (likely infective exacerbation).

High Yield: ABG in Different Settings of Type 2 RF

Setting	pH	PaCO₂	HCO₃⁻	Interpretation
Acute Type 2	Low (< 7.35)	High	Normal (~24)	No time for renal compensation
Chronic Type 2 (compensated)	Near-normal (7.35–7.40)	High	Elevated (~30–38)	Renal compensation complete
Acute-on-chronic	Low	Higher than baseline	Elevated (pre-existing)	Prior compensation, but new insult

This is repeatedly examined in HKU respiratory data interpretation sessions. ^[27]

Feature	Detail
What it measures	Percentage of oxygenated haemoglobin using infrared light absorption. Non-invasive, continuous
Normal	≥ 95% on room air
Significance	SpO₂ < 92% on room air ≈ PaO₂ ~60 mmHg (the inflection point of the OxyHb dissociation curve). Below this, saturation drops precipitously
Limitations	Does NOT measure PaCO₂ → a patient can have normal SpO₂ on supplemental O₂ but be in dangerous Type 2 RF with rising CO₂. Also unreliable in: poor perfusion, severe anaemia, MetHb, COHb (reads falsely normal), nail polish, motion artefact
Clinical pearl	In COPD patients, target SpO₂ 88–92%. If SpO₂ is kept artificially high with uncontrolled O₂, PaCO₂ may rise silently. Always confirm with ABG ^[1]

Critical Error: Trusting SpO₂ Alone in COPD

A common and dangerous mistake is to see SpO₂ 94% in a COPD patient on 4L O₂ and assume they are "fine." They may be retaining CO₂ dangerously. SpO₂ tells you about oxygenation ONLY — ABG is essential to assess ventilation (PaCO₂) and acid-base status. ^[1]

CXR is the first-line imaging investigation in respiratory failure — it is rapid, cheap, portable (can be done at the bedside), and helps narrow the differential immediately. ^[29]

CXR Finding	Diagnosis Suggested	Pathophysiological Basis
Consolidation (opacification with air bronchograms)	Pneumonia	Alveoli filled with inflammatory exudate; air in bronchi visible against opacified lung ^[30]
Bilateral alveolar infiltrates ("white-out")	ARDS, bilateral pneumonia, alveolar haemorrhage	Diffuse alveolar damage → fluid flooding of alveoli bilaterally ^[5]
Bilateral perihilar "bat-wing" opacities	Cardiogenic pulmonary oedema	Fluid transudation from elevated pulmonary venous pressure, worst in perihilar zone
ABCDE of heart failure: Alveolar oedema, Kerley B lines, Cardiomegaly, Dilated upper lobe vessels, pleural Effusion	Heart failure ^[21]	Back-pressure from failing LV → pulmonary venous congestion → interstitial and then alveolar oedema
Hyperinflated chest, flattened diaphragm, long narrow heart shadow, hyperlucent lung fields	COPD / emphysema ^[8]	Air trapping → hyperinflation → diaphragm pushed down, ribs horizontalised, increased AP diameter
Absent lung markings on one side + visible visceral pleural line	Pneumothorax	Air in pleural space → collapsed lung retracts toward hilum
Mediastinal shift (trachea/heart deviated)	Tension pneumothorax (away), collapse (towards), massive effusion (away)	Pressure or volume effect displacing mediastinum
White-out of one hemithorax with meniscus sign	Massive pleural effusion	Fluid fills the pleural space, tracks up laterally (meniscus)
Wedge-shaped peripheral opacity (Hampton hump), focal oligaemia (Westermark sign), enlarged pulmonary artery	Pulmonary embolism ^[29]	Pulmonary infarction (Hampton); reduced perfusion distal to clot (Westermark). Note: CXR is often normal in PE
Reticulonodular shadows, honeycombing	Interstitial lung disease ^[19]	Fibrosis and cystic destruction of lung architecture
Upper lobe cavitation, fibrosis	Tuberculosis (post-primary)	Granulomatous inflammation with caseous necrosis and cavity formation
Mass / hilar enlargement	Lung malignancy	Tumour mass or lymphadenopathy
Bilateral interstitial infiltrates (ground-glass)	PCP (in immunocompromised) ^[23]	Inflammatory infiltration of interstitium by Pneumocystis

CXR in PE is often non-specific or normal (12–22%). Its main role is to rule out other causes such as pneumonia and pneumothorax ^[29].

ECG is done to assess for cardiac causes and complications of respiratory failure ^[8]^[29]:

ECG Finding	Significance
Sinus tachycardia	Most common finding in any acute respiratory failure — non-specific sympathetic response
S1Q3T3 pattern (deep S wave in Lead I, Q wave and inverted T in Lead III)	Classic (but not sensitive) for PE — suggests acute right heart strain ^[29]
T-wave inversion in V1–V4	RV strain pattern (PE, acute cor pulmonale)
New RBBB	Acute RV dilatation (PE)
Right axis deviation + P pulmonale (tall peaked P waves in II, III, aVF)	Cor pulmonale — chronic RV hypertrophy from chronic respiratory failure ^[8]
RV hypertrophy (tall R in V1, deep S in V5–V6)	Chronic cor pulmonale (COPD, ILD)
AF, atrial flutter	Can be triggered by hypoxia, PE, or cor pulmonale
ST elevation/depression	Consider concurrent ACS; or RV strain from massive PE
Low voltage QRS	Hyperinflation (COPD — air is a poor conductor)
Poor R wave progression	COPD or dilated cardiomyopathy

Test	Key Findings	Interpretation
CBC (Complete Blood Count)	↑WCC + neutrophilia: infection (pneumonia). Polycythaemia (↑Hb, ↑Hct): secondary to chronic hypoxaemia ^[8]. ↓Hb: anaemia contributing to tissue hypoxia. Eosinophilia: asthma/allergy	Polycythaemia = EPO-driven erythropoiesis from chronic hypoxaemia. Severe anaemia itself doesn't cause hypoxaemia on ABG (PaO₂ is normal — it's the O₂ carrying capacity that's reduced)
CRP / ESR	Elevated in infection and inflammation	Non-specific but helps support infective aetiology
Procalcitonin	More specific for bacterial infection than CRP	High specificity, lower sensitivity — absence cannot exclude infection
Urea and electrolytes (RFT)	↑Urea: dehydration, GI bleeding, CKD. ↑Creatinine: AKI/CKD. ↑K⁺: renal failure, acidosis	Renal failure can cause metabolic acidosis → compensatory hyperventilation. Also needed for drug dosing
BNP / NT-proBNP	BNP < 100 pg/mL: rules out HF. BNP > 400 pg/mL: suggests CHF ^[21]^[24]	Helps distinguish cardiac from respiratory causes of dyspnoea. Sensitive but not specific — also elevated in PE, pulmonary HTN, renal failure ^[24]
Cardiac troponin (hs-cTn)	Elevated in ACS; also mildly elevated in massive PE (RV strain), myocarditis	Prognostic in PE (elevated troponin = worse outcome). Must correlate with clinical context
D-dimer	Highly sensitive, not specific for PE/DVT. Elevated in infection, post-op, malignancy, DIC ^[29]	Useful for RULING OUT PE/DVT in low-to-intermediate probability patients (high NPV). If negative, PE is effectively excluded. If positive, need confirmatory imaging (CTPA) ^[29]
Lactate	> 2 mmol/L: tissue hypoperfusion / shock	Elevated lactate in respiratory failure suggests tissue-level oxygen debt — ominous sign
Sputum microscopy, culture, and sensitivity (C/ST)	Gram stain, ZN stain for AFB (TB), fungal culture	Sputum examination for infections. Sputum cytology if malignancy suspected ^[27]
Blood culture	Positive in bacteraemia (7–16% in CAP) ^[30]	Should be taken BEFORE starting antibiotics
TFT (Thyroid Function Tests)	↓T4, ↑TSH: hypothyroidism	Severe hypothyroidism → myxoedema coma with respiratory failure ^[31]
Toxicology screen	Positive for opioids, benzodiazepines, barbiturates	Drug overdose → Type 2 RF from CNS depression
Co-oximetry on ABG	Elevated COHb (CO poisoning) or MetHb (methaemoglobinaemia)	Must specifically request co-oximetry — standard pulse oximetry cannot distinguish these from OxyHb

Modality	When to Use	Key Findings
CTPA (CT Pulmonary Angiography)	Gold standard for diagnosing PE ^[29]	Filling defect within pulmonary arteries. Saddle embolism = clot at bifurcation. Also shows RV dilatation (RV:LV ratio > 1 = RV strain)
CT thorax with contrast	Suspected malignancy, ILD characterization, complicated pneumonia (abscess, empyema)	Mass, mediastinal lymphadenopathy, honeycombing (IPF), ground-glass (NSIP, PCP, early ARDS), cavitation (TB, abscess), bronchiectasis
HRCT thorax (High-Resolution CT)	Superior to CXR for detection and confirmation of ILD ^[19]	Basal and peripheral reticular opacities + honeycombing + traction bronchiectasis = classic for idiopathic pulmonary fibrosis ^[19]. Ground-glass = active inflammation/alveolitis (potentially treatable). Upper-zone predominance: silicosis, sarcoidosis
Limitations	Contrast-enhanced CT requires eGFR > 30 mL/min (risk of contrast-induced nephropathy). Radiation exposure	If eGFR too low → alternative is V/Q scan for PE. If patient is pregnant → consider LL venous duplex ultrasound first ^[29]

Feature	Detail
Indication	Used when CTPA is contraindicated (renal impairment, contrast allergy, pregnancy) or unavailable ^[29]
Principle	Inhaled radiotracer maps ventilation; injected radiotracer maps perfusion. Mismatch (ventilated but not perfused areas) = PE
Interpretation	Reported as probability: high, intermediate, or low probability for PE. High-probability scan + high clinical suspicion = treat as PE
Limitations	Intermediate results are common and unhelpful; less specific in patients with pre-existing lung disease (COPD)

Feature	Detail
Indication	Suspected cardiac cause of respiratory failure (heart failure, valvular disease); suspected massive PE with haemodynamic instability; assess for cor pulmonale
Key findings	Reduced LVEF → heart failure (HFrEF). Diastolic dysfunction → HFpEF. RV dilatation + RV dysfunction → acute PE or cor pulmonale. Valvular lesions (MR, MS, AR, AS). Pericardial effusion ± tamponade
In unstable PE	If haemodynamically unstable (sBP < 90 or drop ≥ 40) → bedside TTE for RV dilatation/clot-in-transit → empirical anticoagulation ± thrombolysis. Do NOT delay for CTPA ^[11]^[29]
In ARDS	Required to exclude hydrostatic (cardiogenic) pulmonary oedema when no clear risk factor for ARDS is present ^[5]

Feature	Detail
Indication	Chronic respiratory failure — characterize the underlying lung disease (obstructive vs restrictive)
Not done	In acute respiratory failure (patient too unwell to perform spirometry)
Obstructive pattern	FEV₁/FVC < 0.7 (or < LLN). Post-bronchodilator: COPD (irreversible or partially reversible); Asthma (reversible ≥ 12% and ≥ 200 mL increase in FEV₁) ^[8]
Restrictive pattern	FEV₁/FVC normal or high (≥ 0.7) but both FEV₁ and FVC are reduced. TLC reduced. Seen in ILD, chest wall disease, neuromuscular disease
DLCO (Diffusing capacity)	Reduced in emphysema (loss of alveolar surface area), ILD (thickened membrane). Increased in alveolar haemorrhage (Goodpasture — Hb in alveoli absorbs CO) ^[22]
Serial FVC	Critical in neuromuscular disease (GBS, MG): FVC < 20 mL/kg or declining FVC → indication for intubation ^[3]

Spirometry is mandatory for COPD diagnosis — PEFR cannot be used as replacement. Post-bronchodilator FEV₁/FVC < 0.7 ^[8].

Test	Indication	Key Findings
Gram stain and C/ST	Suspected bacterial pneumonia	Identify organism and antibiotic sensitivities
ZN stain (Ziehl-Neelsen) and TB culture	Suspected tuberculosis	Acid-fast bacilli (AFB) on ZN stain; culture takes weeks but is gold standard
Sputum cytology	Suspected lung malignancy	Malignant cells; may identify cell type
Induced sputum with silver stain	Suspected PCP	Pneumocystis cysts/trophozoites. Often requires BAL if sputum is negative

Investigation	Indication	Key Findings
Bronchoalveolar lavage (BAL)	Suspected PCP, alveolar haemorrhage, ILD characterization	Cellular analysis narrows DDx of ILD ^[19]. Progressively bloodier returns = alveolar haemorrhage. Silver stain for PCP
CT brain / MRI brain	Suspected CNS cause of Type 2 RF (brainstem stroke, raised ICP)	Infarct, haemorrhage, mass
NCS / EMG	Suspected neuromuscular cause (GBS, MND)	GBS: demyelinating pattern (↓conduction velocity, conduction block). MND: denervation with fasciculation, normal velocities ^[13]
Anti-AChR antibodies	Suspected myasthenia gravis	Positive in ~85% of generalized MG
Sleep study (polysomnography)	Suspected OHS or central/obstructive sleep apnoea	AHI (apnoea-hypopnoea index) elevated; desaturation events during sleep
Duplex US lower limbs	Suspected DVT (as part of VTE workup)	Non-compressible vein = DVT. Useful when CTPA is contraindicated (pregnancy, renal impairment) — finding DVT in the right clinical context supports PE diagnosis ^[29]
Cardiac catheterization	Suspected pulmonary hypertension requiring confirmation	Mean pulmonary artery pressure > 20 mmHg = pulmonary hypertension (2022 ESC definition)

The following algorithm summarizes the practical workflow when confronted with a patient in respiratory failure:

Exam Approach: The 'ABCDE' of Investigations in Respiratory Failure

When asked "What investigations would you order?" in an exam, structure your answer as:

ABG — define and classify the respiratory failure
Bedside — SpO₂, ECG, blood glucose
Blood tests — CBC, RFT, CRP, BNP, troponin, D-dimer, blood cultures, TFT, toxicology (as clinically indicated)
CXR — first-line imaging
Targeted — CTPA (PE), echo (cardiac), HRCT (ILD), serial FVC (neuromuscular), CT brain (CNS), BAL (PCP/haemorrhage), spirometry (obstructive vs restrictive — when stable)

This ensures you don't miss anything and shows the examiner a systematic approach.

Cause of RF	ABG Pattern	A-a Gradient	CXR	Key Confirmatory Test
Pneumonia	Type 1	Elevated	Consolidation, air bronchograms	Sputum C/ST, blood cultures, CRP
AECOPD	Acute-on-chronic Type 2	Elevated	Hyperinflation	Spirometry (when stable): FEV₁/FVC < 0.7
Acute severe asthma	Type 1 → Type 2	Elevated	Hyperinflation or normal	Peak flow < 33% predicted; reversible obstruction
PE	Type 1	Elevated	Often normal; Hampton hump, Westermark sign	CTPA (gold standard); D-dimer to rule out
ARDS	Severe Type 1	Elevated	Bilateral opacities	Berlin criteria; P/F ratio; echo to exclude cardiac
Cardiogenic APO	Type 1	Elevated	ABCDE pattern	BNP > 400; echo: reduced EF
Pneumothorax	Type 1	Elevated	Absent lung markings, visceral pleural line	Clinical diagnosis if tension; CXR confirms
ILD	Type 1	Elevated	Reticulonodular, honeycombing	HRCT; DLCO reduced; restrictive spirometry
Drug overdose	Type 2	Normal	Normal	Toxicology screen; GCS assessment
GBS	Type 2	Normal	Normal	Serial FVC < 20 mL/kg → intubate; NCS/EMG
MND	Type 2	Normal	Normal	EMG: denervation + fasciculation
Myasthenia crisis	Type 2	Normal	Normal	Anti-AChR Ab; Tensilon test; serial FVC
OHS	Type 2 (chronic)	Normal	± elevated diaphragm	BMI > 30; sleep study; TFT to exclude hypothyroidism
CO poisoning	PaO₂ may be normal; SpO₂ falsely normal	Normal	Normal	Co-oximetry: elevated COHb
MetHb	PaO₂ may be normal; SpO₂ ~85% plateau	Normal	Normal	Co-oximetry: elevated MetHb

High Yield Summary — Diagnosis of Respiratory Failure

Respiratory failure is an ABG diagnosis: PaO₂ < 60 mmHg ± PaCO₂ > 50 mmHg.
Type 1 vs Type 2 separates oxygenation from ventilation failure.
A-a gradient separates lung disease (elevated) from pump failure (normal) — the most important diagnostic fork.
pH + HCO₃⁻ separates acute from chronic from acute-on-chronic.
CXR is the first-line imaging — it immediately narrows the differential.
BNP distinguishes cardiac from respiratory causes (sensitive but not specific).
CTPA is the gold standard for PE; D-dimer is for ruling OUT PE in stable, low-to-moderate risk patients.
Serial FVC is critical in neuromuscular disease — FVC < 20 mL/kg → intubate.
ARDS requires Berlin criteria (timing, bilateral opacities, non-cardiogenic, PaO₂/FiO₂ ratio on PEEP).
SpO₂ does not measure PaCO₂ — always get ABG in suspected Type 2 RF.
Co-oximetry is required to diagnose CO poisoning and methaemoglobinaemia.
In COPD: target SpO₂ 88–92%; always re-check ABG 30–60 minutes after changing O₂ therapy.

Active Recall - Diagnostic Criteria, Algorithm, and Investigations in Respiratory Failure

References

^[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf ^[2] Lecture slides: Respiratory failure - 20221010.pdf ^[3] Lecture slides: GC 005 - The patient is critically ill - Intensive care medicine; Unstable vital organ function [Updated 2025].pdf ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, Section on ARDS (Berlin definition) ^[8] Senior notes: Maksim Medicine Notes.pdf, p.300 (COPD section) ^[11] Senior notes: Block A - Chest Pain - Department of Medicine.pdf, p.2-3 ^[13] Senior notes: Ryan Ho Neurology.pdf, p.186 ^[19] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, p.238 (ILD investigations) ^[21] Senior notes: Maksim Medicine Notes.pdf, p.18 (Heart failure) ^[22] Senior notes: Ryan Ho Urogenital.pdf, p.67 (Goodpasture) ^[23] Senior notes: Block A - Renal Replacement Therapies.pdf, p.37 (PCP) ^[24] Senior notes: Block A - Shortness of breath on exertion_ heart failure.pdf, p.11 ^[26] Senior notes: Learning_Points_All_Lectures.txt, Section 4 (Respiratory Medicine) ^[27] Lecture slides: Data Interpretation - Respiratory - by Dr DCL Lam (Rotation 2).ppt.pdf; Respiratory- Introduction to Resp investigations (CXR, CT thorax and lung function tests).pdf ^[28] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf, p.4 ^[29] Senior notes: Ryan Ho Respiratory.pdf, p.134-135 (PE investigations); Block A - Leg swelling and chest pain_ deep vein thrombosis; pulmonary embolism; Thrombophilia.pdf, p.23; Block A - Chest Pain - Department of Radiology.pdf ^[30] Senior notes: Adrian Lui Pediatrics Notes.pdf, p.165 (CAP diagnosis) ^[31] Senior notes: Block A - I am losing weight and sweating all the time_ causes of severe, weight loss; thyrotoxicosis; hypothyroidism.pdf, p.43

Management of Respiratory Failure

The management of respiratory failure follows a logical hierarchy — think of it as treating from the most urgent threat downwards ^[1]^[3]:

Save the patient — immediate life-saving interventions (ABC approach)
Correct the gas exchange abnormality — oxygen therapy, ventilatory support
Treat the underlying cause — the only way to definitively resolve the respiratory failure
Manage complications — organ support, prevention of deterioration
Long-term management — rehabilitation, prevention of recurrence

In acute heart failure, there is an urgent and intermediate phase of management. Urgent stage: save the patient first, assess for cardiogenic shock (inotropes/mechanical support) and respiratory failure (ventilatory support). Intermediate stage: find the underlying cause. ^[32] This principle applies identically to respiratory failure management.

10.3 Oxygen Therapy

Oxygen is the first-line treatment for hypoxaemia. The goal is to restore adequate tissue oxygenation — but the target and delivery method differ between Type 1 and Type 2 respiratory failure ^[1]^[33].

Device	Flow Rate	FiO₂ Delivered	Characteristics	Best For
Nasal cannula	1–6 L/min	~24–44%	Simple, well-tolerated, patient can eat/talk. FiO₂ is approximate and variable (depends on patient's minute ventilation)	Mild hypoxaemia; COPD: start with 1–2 L/min ^[33]
Venturi mask	2–15 L/min	24%, 28%, 31%, 35%, 40%, 60% (precise, fixed FiO₂)	Entrainment ports mix room air with O₂ at fixed ratios → delivers precise, known FiO₂	COPD / Type 2 RF — when precise FiO₂ control is needed. Start with 24–28% Venturi ^[33]
Simple face mask	5–10 L/min	~40–60%	Higher FiO₂ than nasal cannula but imprecise; minimum 5 L/min to prevent CO₂ rebreathing	Moderate hypoxaemia without CO₂ retention
Non-rebreather mask with reservoir	10–15 L/min	~60–90%	Reservoir bag fills with O₂; one-way valves prevent rebreathing	Acute severe hypoxaemia (Type 1 RF): target SpO₂ > 94% ^[3]
High-flow nasal cannula (HFNC)	Up to 60 L/min	21–100% (titratable)	Heated, humidified, high-flow oxygen; provides small amount of PEEP (~3–5 cmH₂O); washes out dead space	Moderate-severe Type 1 RF; bridge to NIV/intubation; post-extubation; preferred in acute PE with RV failure over positive pressure ventilation ^[11]

High Yield: Oxygen Targets

Patient	SpO₂ Target	Rationale
General acute illness	94–98%	Ensures adequate oxygenation without excess O₂ (which can cause absorption atelectasis and free radical damage)
COPD / known CO₂ retainer	88–92% ^[1]^[33]	Prevents O₂-induced hypercapnia (Haldane effect, release of HPV, reduced hypoxic drive)
Acute severe asthma	93–95% ^[34]	Moderate target; avoid excessive O₂

Start with 1–2 L/min by nasal cannula or 24–28% Venturi mask in COPD. Monitor ABG 30–60 minutes after initiation of O₂ therapy and modify flow rate according to PaO₂ and pH. ^[33]

As explained in the pathophysiology section, three mechanisms contribute to O₂-induced hypercapnia in COPD ^[1]:

Haldane effect: Oxygenated Hb releases CO₂ → PaCO₂ rises
Release of hypoxic pulmonary vasoconstriction (HPV): O₂ dilates vessels to poorly ventilated alveoli → worsened V/Q mismatch
Reduced hypoxic ventilatory drive: Minor contribution

Avoid high-dose O₂ therapy to prevent removal of hypoxic drive, especially in patients without hypercapnic drive. ^[33]

Common Exam Error

Students often state that "COPD patients rely on hypoxic drive alone, so giving O₂ removes their drive to breathe." This is an oversimplification. The Haldane effect and release of HPV are actually the dominant mechanisms. State all three in an exam answer.

10.4 Non-Invasive Ventilation (NIV)

NIV delivers positive-pressure ventilation through a mask (nasal, oronasal, or full face) without an endotracheal tube. The name tells you: "non-invasive" = no tube into the trachea; "ventilation" = assisting the breathing ^[1]^[33]^[35].

Type	Mechanism	Clinical Use
CPAP (Continuous Positive Airway Pressure)	Delivers a constant fixed positive pressure throughout the breathing cycle. Splints the airways open, recruits collapsed alveoli, improves oxygenation. Does NOT augment ventilation (does not increase tidal volume). ^[35]	Cardiogenic pulmonary oedema (APO) ^[35]^[36]; obstructive sleep apnoea; chest wall trauma
BiPAP (Bilevel Positive Airway Pressure)	Delivers higher pressure during inspiration (IPAP) and lower pressure during expiration (EPAP). IPAP augments tidal volume (assists ventilation → blows off CO₂). EPAP = functions like CPAP (splints airways open, recruits alveoli). ^[35]	Type 2 RF, especially AECOPD with respiratory acidosis; neuromuscular disease; OHS; weaning from invasive ventilation ^[35]^[36]

Why does CPAP work in APO?

Positive pressure throughout the cycle → pushes fluid out of alveoli back into interstitium → improves V/Q matching
Reduces preload (↓venous return to the thorax) → reduces pulmonary congestion
Reduces afterload (positive intrathoracic pressure makes it easier for the LV to eject) → improves cardiac output
Net effect: dramatic improvement in oxygenation and dyspnoea within minutes

Why does BiPAP work in COPD with hypercapnic acidosis?

IPAP (start at 10–15 cmH₂O, titrate up to 20 cmH₂O) ^[35]: augments tidal volume → increases alveolar ventilation → blows off CO₂ → corrects respiratory acidosis
EPAP (start at 4–5 cmH₂O) ^[35]: counterbalances intrinsic PEEP (auto-PEEP) from air trapping, prevents airway collapse during expiration
Reduces work of breathing → rests fatigued respiratory muscles

Levels of evidence for use of NIV in acute respiratory failure ^[36]:

Strength of Evidence	Indication
Strongest evidence	1. Acute COPD exacerbation with hypercapnic acidosis (PaCO₂ > 6.0 kPa or pH < 7.35)
	2. Hypercapnic respiratory failure secondary to obesity hypoventilation syndrome (OHS), chest wall deformity, or neuromuscular disease
	3. Cardiogenic pulmonary oedema
	4. To facilitate weaning from invasive ventilation (esp. in COPD)
	5. Acute respiratory failure in immunocompromised patients
Reasonable evidence	1. Post-operative hypoxaemia (except upper GI surgery)
	2. Patients decided not for intubation
Less efficacious or even harmful	1. Acute severe asthma
	2. Acute respiratory distress syndrome (ARDS)
	3. Pneumonia, especially if purulent sputum

The strongest indication for NIV is AECOPD with respiratory acidosis (pH < 7.35). Check ABG 30–60 minutes after initiation of NIV. Do not delay intubation and mechanical ventilation if no improvement. ^[33]^[36]

Category	Contraindications ^[35]^[36]
Absolute	1. Lack of spontaneous breathing in cardiopulmonary arrest / gasping
	2. Anatomical or functional airway obstruction
	3. Severe facial deformity / trauma / burns
	4. Gastrointestinal bleeding or ileus
Relative (warrants enhanced observation)	1. Haemodynamic instability
	2. Severe acidosis (pH < 7.15)
	3. Inability to protect airway in severely impaired consciousness (GCS < 8)
	4. Inability to cooperate in agitated status
	5. Excessive secretions
	6. Recent upper airway or gastrointestinal surgery

High Yield: When to Escalate from NIV to Intubation

Monitor ABG within 1–2 hours of starting NIV. If little improvement, repeat at 4–6 hours. If still no improvement → consider intubation. ^[35] Signs of NIV failure:

Persistent or worsening acidosis (pH still < 7.25)
Rising PaCO₂ despite adequate IPAP
Worsening consciousness (GCS dropping)
Inability to tolerate the mask
Haemodynamic instability

Complication	Mechanism
Mask-related: air leak, eye irritation, facial skin necrosis (most common) ^[35]	Pressure from tight-fitting mask over prolonged periods
Hypotension	Positive pressure reduces venous return → ↓preload → ↓cardiac output
Vomiting, aspiration	Gastric distension from swallowed air
Breathing discomfort	IPAP or EPAP set too high
Pneumothorax (rare)	Barotrauma from excessive airway pressure

10.5 Invasive Mechanical Ventilation

When NIV fails or is contraindicated, the patient requires endotracheal intubation and invasive positive-pressure ventilation (IPPV), managed in an ICU setting ^[3]^[37].

General indications ^[37]:

Respiratory failure not adequately controlled by other means (including NIV failure)
Cardiac or respiratory arrest
Failure to protect airway with GCS < 8
Haemodynamic instability (severe hypotension)

Clinical criteria ^[37]:

Stridor (upper airway obstruction)
Apnoea / hypopnoea
Signs of respiratory distress: accessory muscle use, intercostal recession, abdominal paradox
Decreased GCS < 8

Laboratory criteria ^[37]:

Vital capacity < 10 mL/kg (or FVC < 15–20 mL/kg in GBS/neuromuscular disease)
FEV₁ < 10 mL/kg
PaO₂ < 7.3 kPa (55 mmHg) despite O₂ supplementation
PaCO₂ > 6.7 kPa (50 mmHg) with pH < 7.32

Specific indications by cause ^[33]:

AECOPD: Patients unable to tolerate or failed NIV; impaired consciousness, massive aspiration, unable to clear secretions; severe haemodynamic instability; severe ventricular arrhythmia; cardiac or respiratory arrest
GBS: FVC < 15–20 mL/kg, inefficient cough, dysphagia, atelectasis ^[38]
Severe acute asthma: Silent chest, exhaustion, rising PaCO₂, drowsiness

Contraindication	Alternative
Trauma to the upper airway ^[37]	Surgical airway (cricothyrotomy / tracheostomy)
Total upper airway obstruction (severe laryngeal oedema from infection, burns, anaphylaxis) ^[37]	Needle cricothyrotomy (emergency) → surgical tracheostomy
Cervical spine injury ^[37]	Fibreoptic-guided intubation with manual in-line stabilisation; or surgical airway

Mechanical ventilation (positive pressure ventilation) ^[37]:

Improves gaseous exchange: ↑oxygenation by improving V/Q matching; ↑alveolar ventilation; reverses acute respiratory acidosis
Relieves respiratory distress: ↓work of breathing; ↓respiratory muscle fatigue

Feature	Endotracheal Tube	Tracheostomy
Advantages	Easier and quicker insertion; no surgical procedure; no stoma complications	Ease of suctioning; ease of replacement; patient comfort; reduced sedation; enhanced mobility/speech/swallowing; reduces dead space ^[37]
Disadvantages	Injury to nose/mouth/vocal cords; difficult replacement; patient discomfort; needs sedation	More invasive; stomal complications ^[37]
When	Immediate / emergency	Prolonged ventilation expected (> 7–14 days); failed intubation; upper airway obstruction

Mode	How It Works	When Used
Volume-controlled ventilation (VCV)	Delivers a set tidal volume at a set rate; airway pressure varies	General use; guarantees minute ventilation
Pressure-controlled ventilation (PCV)	Delivers a set pressure; tidal volume varies depending on compliance	ARDS (with lung-protective strategy)
Pressure support ventilation (PSV)	Patient-triggered; ventilator augments each breath to a set pressure	Weaning; partially spontaneous breathing
SIMV	Set number of mandatory breaths; patient can breathe spontaneously between	Weaning

Lung-protective ventilation in ARDS ^[5]:

Low tidal volume: 6 mL/kg ideal body weight (to prevent volutrauma — overdistension of alveoli)
PEEP (Positive End-Expiratory Pressure): Keeps alveoli recruited at end-expiration → prevents cyclical opening and closing (atelectrauma)
Plateau pressure < 30 cmH₂O (to prevent barotrauma)
Permissive hypercapnia: Accept higher PaCO₂ (and mild acidosis) to avoid injurious ventilator settings
Prone positioning: In moderate-severe ARDS (P/F < 150), prone positioning for ≥ 12–16 hours/day improves V/Q matching and reduces mortality

10.6 Treatment of Specific Underlying Causes

This is the definitive aspect of management — respiratory failure is a syndrome, so you must treat the cause ^[1]:

Treatment	Details
Controlled O₂	1–2 L/min nasal cannula or 24–28% Venturi mask. Target SpO₂ 88–92%. ABG at 30–60 min ^[33]
Bronchodilators	SABA (salbutamol/Ventolin) ± SAMA (ipratropium/Atrovent) via nebuliser or MDI with spacer ^[33]
Systemic corticosteroids	IV hydrocortisone or oral prednisolone — reduce airway inflammation and shorten exacerbation ^[33]
Antibiotics	If ≥ 2 cardinal symptoms with one being increased sputum purulence, or if requiring ventilation. Augmentin (amoxicillin-clavulanate) is sufficient (covers H. influenzae and M. catarrhalis β-lactamase) ^[33]
NIV (BiPAP)	If respiratory acidosis pH < 7.35 despite controlled O₂. IPAP 10–15 → titrate to 20 cmH₂O; EPAP 4–5 cmH₂O ^[33]^[35]
IV methylxanthines	Aminophylline: second-line if no response to above ^[33] — narrow therapeutic window, monitor levels
Invasive ventilation	If NIV fails or contraindicated

Severity	Management ^[34]
Moderate	O₂ by nasal cannula/mask (aim SpO₂ 93–95%); salbutamol 4 puffs Q4h + PRN with spacer; prednisolone PO 1 mg/kg
Severe / Life-threatening	As above PLUS: ipratropium bromide 4 puffs with spacer; IV steroids; IV MgSO₄ 2g over 20 min; mechanical ventilation if respiratory muscles fatigued ^[34]
ICU indications	Life-threatening features (drowsiness, confusion, silent chest); deteriorating PEF/FEV₁; worsening/persistent hypoxia or hypercapnia; cardiorespiratory arrest ^[34]

Component	Detail
O₂ therapy	Aim PaO₂ ≥ 60 mmHg or SpO₂ ≥ 92%. High-concentration (> 35%) O₂. CPAP or mechanical ventilation if remain hypoxic ^[30]
Empirical antibiotics	Based on CAP severity scoring (CURB-65 or PSI) and local guidelines. IV amoxicillin-clavulanate ± macrolide for moderate-severe CAP
Fluid resuscitation	IV if oral intake not possible
Analgesia	For pleuritic pain: paracetamol, NSAIDs; opiates with caution in poor respiratory function ^[30]

Stability	Management ^[11]
Haemodynamically stable	Anticoagulation: LMWH (enoxaparin) or fondaparinux → transition to DOAC or warfarin
Haemodynamically unstable (sBP < 90)	Heparin immediately + thrombolysis (systemic tPA). If thrombolysis fails or contraindicated → surgical/catheter embolectomy ^[11]
Respiratory support	RV failure: give HFNC O₂ rather than positive pressure → positive pressure increases RV workload ^[11]

Component	Detail
Position	Sit patient upright (reduces venous return → reduces preload)
Oxygen	High-flow O₂ to maintain SpO₂ > 94% ^[3]
CPAP	Strongest evidence for NIV in APO ^[36] — recruits alveoli, reduces preload and afterload
IV furosemide	Loop diuretic → reduces preload by promoting diuresis
IV GTN (glyceryl trinitrate)	Vasodilator → reduces preload (venodilation) and afterload
Morphine	Reduces anxiety and sympathetic drive; mild vasodilation. Use cautiously — can cause respiratory depression
Treat underlying cause	ACS → PCI/CABG; arrhythmia → cardioversion/rate control; valvular disease → surgery

Step	Detail
1. Clinical diagnosis	Do NOT wait for CXR. Tracheal deviation + hyperresonance + absent breath sounds + hypotension ^[12]
2. Needle decompression	14G cannula into 2nd intercostal space, mid-clavicular line, affected side. Converts tension to simple pneumothorax
3. Chest drain (tube thoracostomy)	Definitive treatment: 4th–5th intercostal space, mid-axillary line, safe triangle

Component	Detail
Naloxone	Opioid antagonist. Reverses respiratory depression rapidly. Short half-life → may need repeated doses or infusion. Start 0.4–2 mg IV, repeat every 2–3 minutes
Supportive	Bag-valve-mask ventilation until naloxone takes effect; intubation if unresponsive

Component	Detail
Monitor FVC serially	FVC < 15–20 mL/kg → intubate ^[38]. Also intubate if inefficient cough, bulbar dysfunction, atelectasis
GBS: Immunotherapy	Plasma exchange (5 sessions over 2 weeks) OR high-dose IVIg (0.4 g/kg/day for 5 days). Do NOT combine. ^[38] Steroids are NOT effective in GBS
MG crisis	Plasmapheresis or IVIg; stop/reduce offending drugs; avoid aminoglycosides (worsen NMJ block)

Measure	Rationale
Haemodynamic support	IV fluids for hypovolaemia; vasopressors (noradrenaline) for septic shock; inotropes (dobutamine) for cardiogenic shock ^[3]
Correct metabolic acidosis	Identify and treat the underlying disorder. NaHCO₃ only for severe metabolic acidosis (pH < 7.1) as a temporising measure — it does NOT treat the root cause ^[28]^[39]
Fluid and electrolyte management	Correct hypokalaemia (risk of arrhythmia), hypomagnesaemia, hypophosphataemia (respiratory muscle weakness)
Nutritional support	Respiratory muscle weakness from malnutrition → respiratory failure and chest infections ^[7]. In COPD, consider Pulmocare (low carbohydrate, high calorie — reduces CO₂ production) ^[40]
DVT prophylaxis	Immobilised patients in ICU → LMWH unless contraindicated; TED stockings
Stress ulcer prophylaxis	PPI or H₂ blocker in ventilated ICU patients
Physiotherapy / Pulmonary rehabilitation	Chest physiotherapy (deep breathing, postural drainage, chest percussion); mobilisation as tolerated; pursed-lip breathing, diaphragmatic breathing, inspiratory muscle training ^[41]
Smoking cessation	Most important long-term intervention in COPD ^[14]

LTOT improves 3-year survival by 50% in COPD patients with chronic hypoxaemia — this is one of the most evidence-based interventions in respiratory medicine ^[42]:

Feature	Detail
Prerequisite	Start only when patient is stable with ABG × 2 at least 3–4 weeks apart, and after optimisation of medical therapy. Must have stopped smoking ^[42]
Indications	1. Resting hypoxaemia: SpO₂ ≤ 88% or PaO₂ ≤ 7.3 kPa
	2. SpO₂ 89% / PaO₂ 7.4–7.9 kPa WITH complications (3Ps): Peripheral oedema (cor pulmonale); Pulmonary hypertension (P pulmonale on ECG); Polycythaemia (Hct > 56%) ^[42]
	Note: NO survival benefit if moderate hypoxaemia only (SpO₂ 89–92%) without complications ^[42]
Duration	≥ 15 hours/day ^[42]
Target	SpO₂ ≥ 90% ^[42]
Delivery	Oxygen concentrator at home; portable O₂ for ambulation

10.9 Special Situations

ARDS has no specific pharmacological cure — management is supportive with lung-protective ventilation ^[5]:

Strategy	Detail	Rationale
Low tidal volume ventilation	6 mL/kg ideal body weight	Prevents volutrauma (alveolar overdistension)
PEEP	Titrate to maintain recruitment; ≥ 5 cmH₂O minimum by Berlin definition	Keeps alveoli open at end-expiration; prevents atelectrauma
Plateau pressure	< 30 cmH₂O	Prevents barotrauma
Permissive hypercapnia	Accept PaCO₂ up to ~60–80 mmHg	Allows smaller tidal volumes; accept mild respiratory acidosis
Prone positioning	≥ 12–16 h/day in moderate-severe ARDS (P/F < 150)	Improves V/Q matching in dorsal lung zones; proven mortality benefit
Conservative fluid strategy	Keep patient dry (negative fluid balance) if haemodynamically stable	Reduces pulmonary oedema and improves oxygenation
Neuromuscular blockade	Cisatracurium in early severe ARDS (P/F < 150) for 48h	Improves chest wall compliance and oxygenation; prevents patient-ventilator asynchrony
ECMO	Veno-venous ECMO for refractory hypoxaemia despite optimal ventilatory support	Provides extracorporeal gas exchange; bridge to recovery

Cor pulmonale = RV failure due to respiratory disease ^[43]:

Treatment	Rationale
Treat underlying respiratory disease	Remove the cause of pulmonary hypertension
LTOT	Reverses hypoxic pulmonary vasoconstriction → reduces pulmonary artery pressure
Diuretics	For peripheral oedema from RV failure
Anticoagulation	Reduces risk of thromboembolism
Digoxin	Only if atrial tachyarrhythmia (e.g., AF)

Cause	O₂ Target	Key Specific Treatment	Ventilatory Support
AECOPD	88–92%	Bronchodilators, steroids, antibiotics	BiPAP if pH < 7.35
Acute severe asthma	93–95%	Salbutamol, ipratropium, IV steroids, IV MgSO₄	Intubation if exhaustion/silent chest
Pneumonia	≥ 92%	Empirical antibiotics	CPAP/IPPV if refractory
APO	> 94%	Furosemide, GTN, treat ACS/arrhythmia	CPAP (first choice NIV)
PE (massive)	As needed	Anticoagulation ± thrombolysis	HFNC preferred; avoid positive pressure
ARDS	Per P/F ratio	Lung-protective ventilation, prone positioning	IPPV always; ECMO if refractory
Tension PTX	100% initially	Needle decompression → chest drain	Usually not needed after decompression
Opioid OD	Bag-valve-mask first	Naloxone IV	Intubation if no response
GBS / MG crisis	Per FVC monitoring	IVIg or plasmapheresis	Intubation when FVC < 15–20 mL/kg
OHS	88–92%	Weight loss (long-term); CPAP/BiPAP (chronic)	BiPAP for acute decompensation

High Yield Summary — Management of Respiratory Failure

O₂ therapy is first-line: Target SpO₂ 94–98% (general) or 88–92% in COPD/CO₂ retainers. Always re-check ABG 30–60 min after starting O₂.
Venturi mask delivers precise FiO₂ — use in COPD. Start 24–28%.
NIV (BiPAP): Strongest indication = AECOPD with pH < 7.35. Also: APO (CPAP), OHS, neuromuscular disease, immunocompromised, weaning.
NIV contraindications: Respiratory arrest, GCS < 8, haemodynamic instability, facial trauma, GI bleeding, excessive secretions.
Check ABG 1–2 hours after starting NIV. No improvement → do not delay intubation.
Intubation criteria: NIV failure, GCS < 8, apnoea, FVC < 10–20 mL/kg, persistent acidosis, respiratory arrest.
ARDS: Lung-protective ventilation (Vt 6 mL/kg, plateau pressure < 30, adequate PEEP, permissive hypercapnia, prone positioning).
Treat the cause: Antibiotics (pneumonia), bronchodilators (COPD/asthma), anticoagulation/thrombolysis (PE), diuretics/CPAP (APO), naloxone (opioid OD), IVIg/plasmapheresis (GBS/MG).
LTOT: Improves survival in COPD by 50%. Indications: SpO₂ ≤ 88% or with 3Ps (Peripheral oedema, Pulmonary HTN, Polycythaemia). ≥ 15 h/day. Patient MUST have stopped smoking.
In massive PE with RV failure: HFNC preferred over positive pressure ventilation (positive pressure increases RV workload).

Active Recall - Management of Respiratory Failure

References

^[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf ^[3] Lecture slides: GC 005 - The patient is critically ill - Intensive care medicine; Unstable vital organ function [Updated 2025].pdf ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, Section on ARDS (Berlin definition) ^[7] Senior notes: Ryan Ho Fluids and Nutrition.pdf, p.7 ^[11] Senior notes: Block A - Chest Pain - Department of Medicine.pdf, p.3 (PE management) ^[12] Senior notes: Ryan Ho Critical Care.pdf, p.6 (tension pneumothorax) ^[14] Lecture slides: GC 041. Cough in a chronic smoker_COPD; smoking cessation.pdf ^[28] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf, p.7-8 (metabolic acidosis management) ^[30] Senior notes: Adrian Lui Pediatrics Notes.pdf, p.165 (CAP management) ^[32] Senior notes: Block A - Shortness of breath on exertion_ heart failure.pdf, p.23 (acute HF management) ^[33] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, p.226 (AECOPD management) ^[34] Senior notes: Maksim Medicine Notes.pdf, p.299 (asthma management) ^[35] Senior notes: Maksim Medicine Notes.pdf, p.284 (NIV); Ryan Ho Respiratory.pdf, p.33 (NIV) ^[36] Lecture slides: Handbook of Internal Medicine 2024.pdf, p.421 (NIV evidence and contraindications) ^[37] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, p.107 (mechanical ventilation indications); MBBS Final MB (Surgery) (Felix PY Lai).pdf, p.55 (ET intubation) ^[38] Senior notes: Ryan Ho Neurology.pdf, p.183 (GBS management) ^[39] Senior notes: Ryan Ho Urogenital.pdf, p.39 (metabolic acidosis management) ^[40] Senior notes: Ryan Ho Fluids and Nutrition.pdf, p.9 (Pulmocare for COPD) ^[41] Senior notes: Maksim Medicine Notes.pdf, p.304 (pulmonary rehabilitation) ^[42] Senior notes: Maksim Medicine Notes.pdf, p.300 (LTOT) ^[43] Senior notes: Ryan Ho Respiratory.pdf, p.39 (cor pulmonale)

Complications of Respiratory Failure

Respiratory failure is not just a problem of gas exchange — it triggers a cascade of systemic consequences. The hypoxaemia, hypercapnia, acidosis, and the treatments themselves (mechanical ventilation, prolonged ICU stay) all generate complications across every organ system. Understanding why each complication occurs from first principles is essential.

11.1 Complications of Hypoxaemia

When PaO₂ drops below 60 mmHg, tissue oxygen delivery falls precipitously (remember the steep part of the oxyhaemoglobin dissociation curve). Every organ that depends on aerobic metabolism is at risk.

Complication	Pathophysiological Basis	Clinical Significance
Tachycardia and arrhythmias	Hypoxaemia stimulates peripheral chemoreceptors (carotid bodies) → sympathetic activation → ↑HR, ↑catecholamines. Myocardial hypoxia destabilises cardiac conduction. Cardiac arrhythmia (especially AF) is a recognised complication of pneumonia ^[44]	AF is the most common arrhythmia in acute respiratory failure. Can worsen haemodynamics and increase stroke risk
Myocardial ischaemia / infarction	Myocardial O₂ demand increases (tachycardia, sympathetic drive) while supply decreases (low PaO₂). Pre-existing coronary artery disease amplifies this mismatch	Acute MI is a recognised perioperative and post-pneumonia complication ^[44]^[45]. Check serial troponin and ECG
Pulmonary hypertension	Chronic hypoxaemia → hypoxic pulmonary vasoconstriction (HPV) → sustained elevation of pulmonary vascular resistance → pulmonary hypertension ^[14]^[46]	This is the mechanism linking chronic lung disease (COPD, ILD) to pulmonary hypertension. HPV is normally a protective reflex (diverts blood away from poorly ventilated alveoli), but when global and chronic, it becomes pathological
Cor pulmonale	Chronic hypoxaemia → pulmonary HTN → RVH → RV failure ^[14]^[46]. The right ventricle, which is normally a thin-walled, compliant chamber designed for a low-pressure circulation, hypertrophies and eventually fails when chronically exposed to elevated pulmonary pressures	A major complication of COPD. Signs: peripheral oedema, raised JVP, hepatomegaly, parasternal heave, loud P2 ^[43]^[46]. Manage with LTOT, diuretics, treating the underlying lung disease
Hypotension / shock	Severe hypoxaemia → myocardial depression → decreased cardiac output. Also, the underlying cause (e.g., massive PE, tension pneumothorax) may directly cause obstructive shock	Requires immediate haemodynamic resuscitation

Complications of COPD: Acute exacerbation (pneumothorax, increased airflow obstruction, infection, respiratory failure), chronic respiratory failure, cor pulmonale (chronic hypoxaemia → pulmonary HTN → RVH and failure), pulmonary thromboembolism. ^[14]

Complication	Pathophysiological Basis
Confusion and agitation	The brain is exquisitely sensitive to hypoxia — neurons begin to suffer within 4–6 minutes of complete anoxia. Moderate hypoxaemia causes cortical dysfunction → confusion, restlessness, poor judgement
Seizures	Severe hypoxaemia disrupts cortical electrical stability → generalised seizures
Hypoxic encephalopathy	Prolonged or severe hypoxaemia → neuronal death → permanent brain injury. The hippocampus, cortex, and basal ganglia are most vulnerable ("watershed zones")
Coma and death	End-stage consequence of uncorrected severe hypoxaemia

Complication	Pathophysiological Basis
Acute kidney injury (AKI)	Hypoxaemia + haemodynamic instability → renal hypoperfusion → pre-renal AKI. If prolonged → acute tubular necrosis (intrinsic AKI). Also, acidosis and sepsis (common co-existing conditions) directly injure renal tubules

Complication	Pathophysiological Basis
Secondary polycythaemia	Chronic hypoxaemia → ↑EPO production by the kidney → ↑erythropoiesis → elevated Hb and Hct ^[8]. This is a compensatory mechanism to increase O₂ carrying capacity, but it increases blood viscosity → risk of thrombosis. Polycythaemia (secondary to hypoxaemia) is a complication of COPD ^[8]
Venous thromboembolism (VTE)	Polycythaemia + immobility + systemic inflammation → hypercoagulable state → DVT/PE. Pulmonary thromboembolism is a complication of COPD ^[14]

Complication	Pathophysiological Basis
Lactic acidosis	Tissue hypoxia → anaerobic glycolysis → L-lactate accumulates (Type A lactic acidosis) ^[28]. This creates a vicious cycle: acidosis depresses myocardial function → worsens cardiac output → worsens tissue hypoxia
Electrolyte abnormalities (hyponatraemia from SIADH)	Pneumonia and other respiratory infections can cause SIADH → hyponatraemia ^[44]^[47]. Positive pressure ventilation also stimulates ADH release

CO₂ is not just a "waste gas" — it has profound physiological effects when it accumulates. PaCO₂ > 50 mmHg causes respiratory acidosis (CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻), and CO₂ itself is a potent cerebral vasodilator.

Complication	Pathophysiological Basis	Clinical Feature
CO₂ narcosis	High PaCO₂ → central nervous system depression → progressive drowsiness → stupor → coma. CO₂ at very high levels acts as a CNS depressant	Progressive drowsiness and confusion in a COPD patient = suspect CO₂ retention → urgent ABG ^[1]. Flapping tremor (asterixis)
Cerebral oedema and raised ICP	CO₂ is a potent cerebral vasodilator → ↑cerebral blood flow → ↑intracranial pressure → headache, papilloedema	Morning headache is a clue to chronic hypercapnia (hypoventilation worse during sleep → PaCO₂ peaks overnight) ^[1]
Respiratory acidosis	↑PaCO₂ → ↑H⁺ → pH drops	Acute acidosis (pH < 7.2) → cardiovascular depression: ↓myocardial contractility, ↓response to catecholamines, arrhythmias ^[39]
Sympathetic activation	Hypercapnia stimulates the sympathetic nervous system → tachycardia, hypertension, diaphoresis, flushing, bounding pulse	May mask the severity of the underlying respiratory failure
Cardiovascular depression (severe)	At very high PaCO₂ (> 80–100 mmHg), the direct depressant effect on the myocardium overwhelms the sympathetic stimulation → hypotension, bradycardia → cardiac arrest	Pre-terminal event

Respiratory failure is a syndrome — many complications are specific to the underlying cause:

Underlying Cause	Specific Complications
Pneumonia	Respiratory failure, lung abscess, septicaemia with multi-organ failure, parapneumonic effusion ± empyema, electrolyte abnormalities (hypoNa from SIADH), cardiac complications (acute MI, cardiac arrhythmia especially AF) ^[44]
COPD	Acute exacerbation (pneumothorax, ↑ airflow obstruction, infection, respiratory failure), chronic respiratory failure, cor pulmonale, pulmonary thromboembolism ^[14]^[46]
Acute severe asthma	Pneumothorax or pneumomediastinum (excessive air trapping, leakage from weak spots of the pleura), pulmonary infections (predisposed by mucus accumulation and stasis), respiratory failure (Type 2 — ventilatory failure) ^[48]
ARDS	Multi-organ failure (ARDS rarely occurs in isolation — usually part of systemic inflammatory response), pulmonary fibrosis (fibroproliferative phase > 7 days), barotrauma/volutrauma from ventilation, nosocomial infection
PE	RV failure, obstructive shock, post-PE syndrome (chronic thromboembolic pulmonary hypertension — CTEPH), recurrent VTE

11.4 Complications of Treatment

Complication	Pathophysiological Basis
Oxygen-induced hypercapnia	In COPD/chronic CO₂ retainers: Haldane effect, release of HPV, reduced hypoxic drive → PaCO₂ rises when high-flow O₂ is given. Target SpO₂ 88–92% ^[1]
Absorption atelectasis	High FiO₂ (especially 100%) washes out nitrogen from alveoli. Nitrogen normally acts as a "splint" keeping alveoli open (poorly soluble, stays in the alveolus). When replaced by O₂ (highly soluble, rapidly absorbed into blood), alveoli collapse → atelectasis → paradoxical worsening of hypoxaemia
Oxygen toxicity (pulmonary)	Prolonged exposure to FiO₂ > 0.6 generates reactive oxygen species (ROS) → oxidative damage to alveolar epithelium and endothelium → exudative phase similar to ARDS. Clinically relevant after > 24–48 hours of high FiO₂
Retinopathy of prematurity	In premature neonates: high PaO₂ → retinal vasoconstriction → ischaemia → neovascularisation → retinal detachment. Target lower SpO₂ in preterm neonates (89–94%)

Complication	Mechanism
Facial skin necrosis (most common complication overall) ^[35]	Pressure from the mask interface over prolonged periods
Air leak and eye irritation	Imperfect mask seal → air escapes upward toward the eyes
Gastric distension, vomiting, aspiration	Air is swallowed (aerophagia) → stomach inflates → risk of vomiting and aspiration ^[35]^[49]
Hypotension	Positive intrathoracic pressure reduces venous return → ↓preload → ↓cardiac output ^[35]
Pneumothorax (barotrauma)	Excessive airway pressure → alveolar rupture → air leak into pleural space. Rare with NIV
Claustrophobia / discomfort	Tight mask → patient anxiety → poor compliance

These are among the most important and examinable complications in critical care ^[44]^[45]:

Complication	Pathophysiological Basis	Prevention
Ventilator-associated pneumonia (VAP)	Endotracheal tube bypasses upper airway defences (cough, mucociliary clearance). Microaspiration of contaminated oropharyngeal secretions around the ETT cuff → lower respiratory tract infection. Most common nosocomial infection in ICU	Head-of-bed elevation to 30–45°; oral hygiene with chlorhexidine; subglottic secretion drainage; sedation holidays; minimise duration of intubation
Barotrauma	Excessive airway pressures → alveolar overdistension → rupture → pneumothorax, pneumomediastinum, subcutaneous emphysema	Lung-protective ventilation: plateau pressure < 30 cmH₂O ^[5]
Volutrauma	Excessive tidal volumes → alveolar overdistension even if pressure is acceptable → inflammatory cascade → worsening lung injury	Low tidal volume: 6 mL/kg ideal body weight ^[5]
Atelectrauma	Cyclic opening and closing of alveoli at low lung volumes → shear stress → inflammation	Adequate PEEP to keep alveoli recruited
Biotrauma	Mechanical ventilation-induced release of pro-inflammatory cytokines from injured alveoli → systemic inflammatory response → multi-organ failure	Lung-protective strategy reduces biotrauma
Oxygen toxicity	Prolonged high FiO₂ → ROS → alveolar damage	Wean FiO₂ to < 0.6 as soon as possible
Haemodynamic compromise	Positive pressure reduces venous return → ↓cardiac output → hypotension. Also increases RV afterload	Adequate fluid resuscitation; avoid excessive PEEP
Laryngeal/tracheal injury	ETT cuff pressure → mucosal ischaemia → ulceration → subglottic stenosis (late complication)	Maintain cuff pressure 20–30 cmH₂O; consider tracheostomy if prolonged ventilation expected
Diaphragm atrophy (ventilator-induced diaphragmatic dysfunction)	Prolonged controlled ventilation → disuse atrophy of diaphragm → difficulty weaning	Early spontaneous breathing trials; avoid full control mode when possible
Critical illness polyneuropathy / myopathy	Prolonged ICU stay + sepsis + corticosteroids + neuromuscular blockers → axonal neuropathy and/or myopathy → limb weakness and difficulty weaning from ventilator	Minimise sedation; avoid prolonged neuromuscular blockade; early mobilisation

Pre-existing pulmonary diseases increase respiratory complications. Post-operative complications include mucous retention → pneumonia → respiratory failure. ^[45]

Complication	Pathophysiological Basis
ICU-acquired weakness	Critical illness polyneuropathy + myopathy + disuse atrophy. Steroids and neuromuscular blockers are major risk factors
Delirium	Sedatives, sleep deprivation, sensory overload/deprivation, sepsis, metabolic derangements
Pressure ulcers	Immobility → sustained pressure on skin → tissue ischaemia → necrosis
Deep vein thrombosis / PE	Immobility + systemic inflammation + central venous catheters → Virchow's triad fulfilled
Nosocomial infections	VAP, catheter-related bloodstream infection (CRBSI), catheter-associated UTI (CAUTI)
GI complications	Stress ulcers (mucosal ischaemia from splanchnic vasoconstriction during shock/hypoxia), ileus, Clostridium difficile infection (from broad-spectrum antibiotics)
Psychological sequelae (Post-ICU syndrome)	PTSD, anxiety, depression, cognitive impairment — affects up to 50% of ICU survivors
Malnutrition and muscle wasting	Catabolic state + inadequate nutrition → respiratory muscle weakness → difficulty weaning → further respiratory failure ^[7]

Severe respiratory failure (especially ARDS) rarely occurs in isolation — it is often part of a systemic inflammatory response that damages multiple organs ^[5]^[50]:

System	Pathology	Mechanism
Respiratory	ARDS	Diffuse alveolar damage → non-cardiogenic pulmonary oedema
Cardiovascular	Septic shock	Vasodilation + myocardial depression from inflammatory mediators
Hepatic	Jaundice, GI bleeding, paralytic ileus	Hepatic ischaemia + inflammatory injury
Renal	Acute tubular necrosis / AKI	Renal hypoperfusion + direct nephrotoxicity from inflammatory mediators
Neurological	Septic encephalopathy, critical illness polyneuropathy	Cerebral microvascular dysfunction; axonal injury
Haematological	Disseminated intravascular coagulopathy (DIC)	Widespread endothelial activation → consumption of clotting factors → simultaneous bleeding and thrombosis

MODS includes: Respiratory (ARDS), Cardiovascular (septic shock), Hepatic (jaundice/GI bleeding/paralytic ileus), Renal (ATN/AKI), Neurological (septic encephalopathy/critical illness polyneuropathy), Haematological (DIC). ^[50]

Complication	Pathophysiological Basis
Cor pulmonale	Chronic hypoxaemia → pulmonary HTN → RVH and failure ^[14]^[43]^[46]. The most important long-term cardiovascular complication of chronic respiratory failure
Secondary polycythaemia	Chronic hypoxaemia → ↑EPO → ↑RBC mass → hyperviscosity → risk of thrombosis ^[8]
Pneumothorax	Secondary to rupture of emphysematous bullae in COPD ^[8]^[14]. Even a small pneumothorax can be life-threatening in a patient with limited respiratory reserve
Pulmonary cachexia / sarcopenia	Chronic inflammation + increased work of breathing (↑caloric expenditure) + poor appetite → muscle wasting → further respiratory muscle weakness → worsening ventilatory failure. A vicious cycle
Depression and anxiety	Chronic dyspnoea, reduced exercise tolerance, loss of independence, social isolation → significant psychological morbidity
Sleep-disordered breathing	Hypoventilation worsens during sleep (loss of wakefulness drive) → nocturnal desaturation → morning headaches, poor sleep quality, daytime somnolence
Recurrent infections	Impaired mucociliary clearance (COPD), structural damage (bronchiectasis), immunosuppression from steroids → frequent lower respiratory tract infections → further exacerbations → progressive decline in lung function

The Vicious Cycle of Chronic Respiratory Failure

Chronic respiratory failure creates multiple self-reinforcing loops:

Hypoxaemia → polycythaemia → hyperviscosity → PE → worsening hypoxaemia
Hypoxaemia → pulmonary HTN → cor pulmonale → RV failure → peripheral oedema → fluid overload → worsening gas exchange
Increased work of breathing → cachexia → respiratory muscle weakness → worsening ventilatory failure
Mucus retention → infection → inflammation → further airway damage → more mucus retention

This is why comprehensive management (LTOT, pulmonary rehabilitation, nutritional support, smoking cessation, vaccination, regular follow-up) is essential to break these cycles.

Preventive Strategy	Target Complication	Detail
Controlled O₂ therapy (SpO₂ 88–92% in COPD)	O₂-induced hypercapnia	Monitor ABG 30–60 min after starting O₂ ^[1]
Lung-protective ventilation	Barotrauma, volutrauma, biotrauma	Vt 6 mL/kg, plateau pressure < 30, adequate PEEP ^[5]
Head-of-bed elevation 30–45°	VAP, aspiration	Reduces microaspiration of oropharyngeal secretions
DVT prophylaxis	PE, DVT	LMWH + TED stockings unless contraindicated
Stress ulcer prophylaxis	GI bleeding	PPI or H₂ blocker in ventilated patients
Sedation holidays and spontaneous breathing trials	Prolonged ventilation, ICU-acquired weakness, VAP	Shorten duration of mechanical ventilation
Early mobilisation	ICU-acquired weakness, DVT, deconditioning	As soon as haemodynamically stable
Smoking cessation	Progressive COPD, recurrent exacerbations	Most important modifiable risk factor ^[14]
Influenza and pneumococcal vaccination	Infective exacerbations	Especially in COPD, asthma, elderly
Pulmonary rehabilitation	Deconditioning, depression, respiratory muscle weakness	Breathing training, inspiratory muscle training, limb aerobic and resistive training, nutrition, psychological counselling, education ^[41]
LTOT	Progression of cor pulmonale, polycythaemia	≥ 15 h/day in those meeting criteria ^[42]

Prevention of complications — which account for most of the hospitalisations and mortality in COPD — is a key principle. Management includes: controlled O₂ therapy, exclude and treat pneumothorax, systemic corticosteroid, increased inhaled bronchodilators, antibiotics for infections, non-invasive ventilation for decompensated Type 2 respiratory failure. Cor pulmonale is managed with diuretics, salt and fluid restriction. ^[14]

High Yield Summary — Complications of Respiratory Failure

Hypoxaemia complications: Cardiac (arrhythmia, MI, cor pulmonale), neurological (confusion, seizures, encephalopathy), renal (AKI), haematological (secondary polycythaemia, VTE), metabolic (lactic acidosis, SIADH).
Hypercapnia complications: CO₂ narcosis (drowsiness → coma), cerebral oedema/raised ICP (headache), respiratory acidosis (cardiovascular depression), sympathetic activation.
Cor pulmonale is the cardinal cardiovascular complication of chronic respiratory failure: chronic hypoxaemia → HPV → pulmonary HTN → RV failure.
Secondary polycythaemia (Hct > 56%) is a compensatory response to chronic hypoxaemia but increases thrombotic risk.
Treatment complications: O₂-induced hypercapnia (COPD); NIV: facial skin necrosis, aspiration, hypotension; Mechanical ventilation: VAP, barotrauma, volutrauma, atelectrauma, oxygen toxicity, diaphragm atrophy, ICU-acquired weakness.
MODS in severe RF/ARDS/sepsis: ARDS, septic shock, AKI, DIC, hepatic failure, septic encephalopathy.
Chronic RF vicious cycles: Hypoxia → polycythaemia → VTE; hypoxia → cor pulmonale; ↑WOB → cachexia → muscle weakness → worse ventilation; mucus retention → infection → more damage.
Prevention: Lung-protective ventilation, controlled O₂, DVT prophylaxis, VAP prevention (HOB elevation, oral hygiene, sedation holidays), early mobilisation, smoking cessation, vaccination, LTOT, pulmonary rehabilitation.

Active Recall - Complications of Respiratory Failure

References

^[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, Section on ARDS (Berlin definition) ^[7] Senior notes: Ryan Ho Fluids and Nutrition.pdf, p.7 ^[8] Senior notes: Maksim Medicine Notes.pdf, p.300 (COPD complications) ^[14] Lecture slides: GC 041. Cough in a chronic smoker_COPD; smoking cessation.pdf ^[28] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf, p.7 (lactic acidosis) ^[35] Senior notes: Maksim Medicine Notes.pdf, p.284 (NIV complications) ^[39] Senior notes: Ryan Ho Urogenital.pdf, p.39 (consequences of severe metabolic acidosis) ^[41] Senior notes: Maksim Medicine Notes.pdf, p.304 (pulmonary rehabilitation) ^[42] Senior notes: Maksim Medicine Notes.pdf, p.300 (LTOT) ^[43] Senior notes: Ryan Ho Respiratory.pdf, p.39 (cor pulmonale) ^[44] Senior notes: Ryan Ho Respiratory.pdf, p.65 (pneumonia complications) ^[45] Lecture slides: GC 002 - Is he fit for surgery - Pre-operative assessment.pdf ^[46] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, p.228 (COPD complications) ^[47] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf, p.21 (SIADH from respiratory causes) ^[48] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf, p.213 (asthma complications) ^[49] Lecture slides: Handbook of Internal Medicine 2024.pdf, p.423 (NIV monitoring and complications) ^[50] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf, p.36 (MODS)

High Yield Summary

Definition: Respiratory failure = PaO₂ < 60 mmHg ± PaCO₂ > 50 mmHg on room air (ABG diagnosis).
Type 1 (Hypoxaemic): Low PaO₂, normal/low PaCO₂. Mechanism: V/Q mismatch, shunt, diffusion impairment. Elevated A-a gradient. Common causes: pneumonia, PE, ARDS, acute cardiogenic pulmonary oedema.
Type 2 (Hypercapnic/Ventilatory): Low PaO₂ AND high PaCO₂. Mechanism: alveolar hypoventilation ± V/Q mismatch. Normal A-a gradient if pure pump failure. Most common cause: COPD. Other causes: drug overdose, GBS, MND, obesity hypoventilation.
Type 1 → Type 2 progression occurs with respiratory muscle fatigue — ominous sign.
Acute vs. Chronic: Acute = uncompensated pH. Chronic = renal compensation (elevated HCO₃⁻, near-normal pH). Acute-on-chronic = elevated baseline HCO₃⁻ with falling pH.
A-a gradient: Normal = pure hypoventilation (pump failure). Elevated = lung parenchyma/vascular problem.
Signs of respiratory distress: tachypnoea, accessory muscle use, tripod position, retractions, inability to speak, cyanosis (late).
CO₂ retention signs: morning headache, drowsiness, confusion, CO₂ flap (asterixis), bounding pulse.
COPD & O₂: Target SpO₂ 88–92% in CO₂ retainers. Mechanisms of O₂-induced hypercapnia: Haldane effect, release of HPV, reduced hypoxic drive.
ARDS: Berlin criteria. P/F ratio classifies severity. Refractory hypoxaemia (shunt-mediated).
Tension pneumothorax: Clinical diagnosis → immediate needle decompression. Do NOT wait for CXR.
In children: respiratory failure is the commonest pathway to cardiac arrest.

High Yield Summary — Differential Diagnosis of Respiratory Failure

Type 1 RF (↓PaO₂, normal PaCO₂, ↑A-a gradient): Think lung parenchyma or vasculature — pneumonia, PE, ARDS, APO, ILD, pneumothorax.
Type 2 RF (↓PaO₂, ↑PaCO₂):
- Normal A-a gradient → pure pump failure: CNS (drug OD, stroke), nerves (GBS), NMJ (MG), muscles (MND, cachexia), chest wall (kyphoscoliosis, obesity).
- Elevated A-a gradient → pump failure PLUS lung disease: AECOPD (most common), severe asthma, upper airway obstruction.
Cannot-miss emergencies: tension pneumothorax, massive PE, acute severe asthma (silent chest), anaphylaxis, cardiac tamponade.
Cardiac vs respiratory: PND/orthopnoea favour cardiac; BNP helps distinguish but is not perfectly specific. Always get an ECG and CXR.
Mimics: Cyanotic CHD, severe anaemia, MetHb, CO poisoning, metabolic acidosis, anxiety — check ABG with co-oximetry, Hb, and clinical context.
Immunocompromised: Think PCP, CMV, TB, invasive aspergillosis.
Children: Respiratory failure is the commonest pathway to cardiac arrest; think bronchiolitis, croup, foreign body.

High Yield Summary — Diagnosis of Respiratory Failure

Respiratory failure is an ABG diagnosis: PaO₂ < 60 mmHg ± PaCO₂ > 50 mmHg.
Type 1 vs Type 2 separates oxygenation from ventilation failure.
A-a gradient separates lung disease (elevated) from pump failure (normal) — the most important diagnostic fork.
pH + HCO₃⁻ separates acute from chronic from acute-on-chronic.
CXR is the first-line imaging — it immediately narrows the differential.
BNP distinguishes cardiac from respiratory causes (sensitive but not specific).
CTPA is the gold standard for PE; D-dimer is for ruling OUT PE in stable, low-to-moderate risk patients.
Serial FVC is critical in neuromuscular disease — FVC < 20 mL/kg → intubate.
ARDS requires Berlin criteria (timing, bilateral opacities, non-cardiogenic, PaO₂/FiO₂ ratio on PEEP).
SpO₂ does not measure PaCO₂ — always get ABG in suspected Type 2 RF.
Co-oximetry is required to diagnose CO poisoning and methaemoglobinaemia.
In COPD: target SpO₂ 88–92%; always re-check ABG 30–60 minutes after changing O₂ therapy.

High Yield Summary — Management of Respiratory Failure

O₂ therapy is first-line: Target SpO₂ 94–98% (general) or 88–92% in COPD/CO₂ retainers. Always re-check ABG 30–60 min after starting O₂.
Venturi mask delivers precise FiO₂ — use in COPD. Start 24–28%.
NIV (BiPAP): Strongest indication = AECOPD with pH < 7.35. Also: APO (CPAP), OHS, neuromuscular disease, immunocompromised, weaning.
NIV contraindications: Respiratory arrest, GCS < 8, haemodynamic instability, facial trauma, GI bleeding, excessive secretions.
Check ABG 1–2 hours after starting NIV. No improvement → do not delay intubation.
Intubation criteria: NIV failure, GCS < 8, apnoea, FVC < 10–20 mL/kg, persistent acidosis, respiratory arrest.
ARDS: Lung-protective ventilation (Vt 6 mL/kg, plateau pressure < 30, adequate PEEP, permissive hypercapnia, prone positioning).
Treat the cause: Antibiotics (pneumonia), bronchodilators (COPD/asthma), anticoagulation/thrombolysis (PE), diuretics/CPAP (APO), naloxone (opioid OD), IVIg/plasmapheresis (GBS/MG).
LTOT: Improves survival in COPD by 50%. Indications: SpO₂ ≤ 88% or with 3Ps (Peripheral oedema, Pulmonary HTN, Polycythaemia). ≥ 15 h/day. Patient MUST have stopped smoking.
In massive PE with RV failure: HFNC preferred over positive pressure ventilation (positive pressure increases RV workload).

High Yield Summary — Complications of Respiratory Failure

Hypoxaemia complications: Cardiac (arrhythmia, MI, cor pulmonale), neurological (confusion, seizures, encephalopathy), renal (AKI), haematological (secondary polycythaemia, VTE), metabolic (lactic acidosis, SIADH).
Hypercapnia complications: CO₂ narcosis (drowsiness → coma), cerebral oedema/raised ICP (headache), respiratory acidosis (cardiovascular depression), sympathetic activation.
Cor pulmonale is the cardinal cardiovascular complication of chronic respiratory failure: chronic hypoxaemia → HPV → pulmonary HTN → RV failure.
Secondary polycythaemia (Hct > 56%) is a compensatory response to chronic hypoxaemia but increases thrombotic risk.
Treatment complications: O₂-induced hypercapnia (COPD); NIV: facial skin necrosis, aspiration, hypotension; Mechanical ventilation: VAP, barotrauma, volutrauma, atelectrauma, oxygen toxicity, diaphragm atrophy, ICU-acquired weakness.
MODS in severe RF/ARDS/sepsis: ARDS, septic shock, AKI, DIC, hepatic failure, septic encephalopathy.
Chronic RF vicious cycles: Hypoxia → polycythaemia → VTE; hypoxia → cor pulmonale; ↑WOB → cachexia → muscle weakness → worse ventilation; mucus retention → infection → more damage.
Prevention: Lung-protective ventilation, controlled O₂, DVT prophylaxis, VAP prevention (HOB elevation, oral hygiene, sedation holidays), early mobilisation, smoking cessation, vaccination, LTOT, pulmonary rehabilitation.

Respiratory Failure

1. Definition

2. Epidemiology and Risk Factors

2.1 Epidemiology

2.2 Risk Factors

3. Anatomy and Physiology Relevant to Respiratory Failure

3.1 The Respiratory Chain

3.2 Key Physiological Concepts

3.2.1 Ventilation (V) and Perfusion (Q) Matching

3.2.2 Shunt

3.2.3 Diffusion Impairment

3.2.4 Hypoventilation

3.2.5 The Alveolar-Arterial (A-a) Gradient

4. Classification of Respiratory Failure

4.1 By Blood Gas Pattern: Type 1 vs Type 2

Why is PaCO₂ normal or low in Type 1?

Why does Type 2 have high PaCO₂?

4.2 By Time Course: Acute vs Chronic

4.3 Other Classifications

5. Etiology (with Focus on Hong Kong)

5.1 Causes of Type 1 Respiratory Failure

5.2 Causes of Type 2 Respiratory Failure

5.3 Pathophysiology by Cause — Detailed

5.3.1 COPD → Respiratory Failure

5.3.2 Pneumonia → Type 1 Respiratory Failure

5.3.3 ARDS → Type 1 (Severe) Respiratory Failure

5.3.4 Pulmonary Embolism → Type 1 Respiratory Failure

5.3.5 Neuromuscular Disease → Type 2 Respiratory Failure

6. Clinical Features

6.1 Symptoms

6.2 Signs

6.2.1 General Inspection

6.2.2 Signs of Respiratory Distress (Indicates Need for Immediate Action) [12]

6.2.3 Respiratory Examination Signs

6.2.4 Cardiovascular Signs in Respiratory Failure

6.2.5 Signs Related to Specific Aetiologies

6.3 Pattern Recognition — Putting Symptoms and Signs Together

7. Special Considerations

7.1 Oxygen-Induced Hypercapnia in COPD

7.2 The P/F Ratio (PaO₂/FiO₂)

7.3 Respiratory Failure in Children

Active Recall - Respiratory Failure

References

8.1 The Clinical Problem

8.2 Framework for Differential Diagnosis

8.3 Differential Diagnosis Organized by Type of Respiratory Failure

8.3.1 Type 1 Respiratory Failure (Hypoxaemic: ↓PaO₂, Normal/↓PaCO₂)

8.3.2 Type 2 Respiratory Failure (Hypercapnic: ↓PaO₂ + ↑PaCO₂)

8.4 Differentiating Cardiac vs Respiratory Causes of Dyspnoea

8.5 Key Differentials That Must Not Be Missed (Life-Threatening)

8.6 Differential Diagnosis in Special Populations

8.6.1 The Immunocompromised Patient

8.6.2 The Post-operative Patient

8.6.3 Children [25]

8.7 Algorithmic Approach to the Differential Diagnosis

8.8 Distinguishing Mimics of Respiratory Failure

8.9 Putting It Together: A Practical Bedside Approach

Active Recall - Differential Diagnosis of Respiratory Failure

9.1 Diagnostic Criteria

9.1.1 ABG-Based Classification Criteria

9.1.2 Time Course Criteria (Acute vs Chronic vs Acute-on-Chronic)

9.1.3 Diagnostic Criteria for ARDS (As a Specific Cause of Respiratory Failure)

9.2 The Stepwise Diagnostic Algorithm

9.3 Investigations — Modalities, Key Findings, and Interpretation

9.3.1 Arterial Blood Gas (ABG)

9.3.2 Pulse Oximetry (SpO₂)

9.3.3 Chest X-Ray (CXR)

9.3.4 ECG

9.3.5 Blood Tests

9.3.6 CT Thorax / CT Pulmonary Angiography (CTPA)

9.3.7 V/Q (Ventilation/Perfusion) Scan

9.3.8 Echocardiography

9.3.9 Spirometry / Pulmonary Function Tests (PFTs)

9.3.10 Sputum Investigations

9.3.11 Special Investigations

9.4 Integrated Diagnostic Algorithm — From Bedside to Cause

9.5 Summary Table: Investigation Findings by Cause

Active Recall - Diagnostic Criteria, Algorithm, and Investigations in Respiratory Failure

10.1 Principles of Management

10.2 Management Algorithm