Respiratory Acidosis
Respiratory acidosis is a condition characterized by decreased alveolar ventilation leading to carbon dioxide retention (hypercapnia) and a resultant drop in blood pH.
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References
[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf [2] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (pp. 2–4, 7–9); Ryan Ho Urogenital.pdf (Section 2.4 Disorders of Acid-base Balance, pp. 34–35) [3] Senior notes: Ryan Ho Respiratory.pdf (AECOPD and respiratory failure sections); Maksim Medicine Notes.pdf (Respiratory medicine, pp. 284–286, 300–302) [4] Lecture slides: Handbook of Internal Medicine 2024.pdf (NIV section, p. 421)
Respiratory acidosis is a primary acid-base disorder characterised by an increase in arterial pCO₂ (hypercapnia) above the normal range (> 6 kPa / > 45 mmHg), resulting in a decrease in blood pH (acidaemia if uncompensated). [1][2]
Let's break this down from first principles:
- The Henderson-Hasselbalch equation governs body acid-base balance: pH ∝ [HCO₃⁻] / [pCO₂] [2]
- The respiratory component is represented by pCO₂ — it is the denominator. When pCO₂ rises, the ratio falls, and pH falls (becomes more acidic). [2]
- "Respiratory" = the primary disturbance is in CO₂ (a volatile acid, handled by the lungs)
- "Acidosis" = a process that pushes pH downward (increases [H⁺])
- Acidosis refers to the ongoing process; acidaemia refers to the actual measured pH < 7.35 — you can have respiratory acidosis without acidaemia if compensation is adequate [1][3]
Key distinction (GC lecture slide):
The fundamental problem in respiratory acidosis is failure of the lungs (or the ventilatory apparatus driving them) to eliminate CO₂ at a rate matching its metabolic production — i.e., alveolar hypoventilation.
Why CO₂ and not O₂?
CO₂ is ~20× more diffusible than O₂ across the alveolar-capillary membrane. Therefore, CO₂ retention (hypercapnia) almost always reflects a ventilation problem (inadequate movement of air), not a diffusion problem. By contrast, hypoxaemia can occur from V/Q mismatch, shunting, or diffusion impairment even when ventilation is preserved. This is why Type 1 respiratory failure (hypoxaemic) and Type 2 respiratory failure (hypoxaemic + hypercapnic) have different aetiologies.
Epidemiology and Risk Factors
- Respiratory acidosis is extremely common in clinical practice — encountered daily in emergency departments, ICUs, and general medical wards
- In Hong Kong, the most common cause of chronic respiratory acidosis is COPD (chronic obstructive pulmonary disease), which accounts for ~10% of public medical bed-days and is increasing due to prior high smoking prevalence and worsening air pollution [4][5]
- Acute respiratory acidosis is frequently seen in:
- COPD acute exacerbations (the single most common scenario in HK hospitals)
- Acute severe asthma (late/severe presentations)
- Opioid/sedative overdoses
- Neuromuscular emergencies (e.g., Guillain-Barré syndrome, myasthenic crisis)
- Post-operative hypoventilation
- Obesity hypoventilation syndrome (OHS) is increasingly recognised as obesity prevalence rises in HK
| Category | Risk Factors | Why they predispose |
|---|---|---|
| Pulmonary | COPD, severe asthma, bronchiectasis, ILD (end-stage) | Airflow obstruction / lung destruction → cannot expel CO₂ |
| Neuromuscular | MG, GBS, muscular dystrophies, ALS, spinal cord injury | Respiratory muscle weakness → cannot ventilate |
| CNS / Drive | Sedative/opioid use, brainstem stroke, central sleep apnoea, hypothyroidism | ↓Respiratory drive → hypoventilation |
| Chest wall | Kyphoscoliosis, flail chest, OHS, morbid obesity | ↓Chest wall compliance → cannot expand lungs adequately |
| Upper airway | Obstructive sleep apnoea, upper airway obstruction | Intermittent or sustained obstruction → ↓ventilation |
| Age | Elderly | ↓Respiratory reserve, ↑prevalence of COPD, ↓muscle mass |
| Iatrogenic | Excessive O₂ in COPD, sedation, post-anaesthesia | Removes hypoxic drive or depresses respiratory centre |
Anatomy and Physiology of CO₂ Homeostasis
To understand respiratory acidosis, you need to understand the entire chain of ventilation:
A failure at ANY level of this chain causes hypoventilation and CO₂ retention → respiratory acidosis.
- CO₂ production: ~200 mL/min at rest, generated by aerobic metabolism in all cells
- CO₂ transport in blood:
- Dissolved CO₂ (~7%)
- Bound to haemoglobin as carbamino compounds (~23%)
- Converted to HCO₃⁻ by carbonic anhydrase in RBCs (~70%): CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- CO₂ elimination: At the alveoli, dissolved CO₂ diffuses down its partial pressure gradient into alveolar gas and is exhaled
- Alveolar ventilation (VA) is the key determinant: pCO₂ = VCO₂ / VA × k
- If VA falls (hypoventilation), pCO₂ rises
- If VA rises (hyperventilation), pCO₂ falls
| Chemoreceptor | Location | Stimulus | Response |
|---|---|---|---|
| Central | Medullary surface | ↑pCO₂ (via ↑CSF H⁺) | ↑Ventilation (major driver, ~80%) |
| Peripheral | Carotid bodies (CN IX), aortic bodies (CN X) | ↓pO₂ < 60 mmHg, ↑pCO₂, ↓pH | ↑Ventilation |
The 'Hypoxic Drive' Concept in COPD
In chronic CO₂ retainers (e.g., severe COPD), the central chemoreceptors become desensitised to chronically elevated pCO₂. Their remaining ventilatory drive is predominantly from the peripheral chemoreceptors responding to hypoxaemia. If you give these patients excessive supplemental O₂ and abolish their hypoxaemia, you may remove their remaining ventilatory drive, worsening hypoventilation and CO₂ retention. This is why target SpO₂ in COPD exacerbation is 88–92% rather than > 94%. However, the Haldane effect (O₂ displacing CO₂ from Hb) and loss of hypoxic pulmonary vasoconstriction also contribute to O₂-induced hypercapnia.
Etiology (with Focus on Hong Kong)
The aetiologies map directly onto the ventilatory chain described above. Think systematically: where in the chain has something gone wrong?
Classification by Anatomical Level
| Cause | Mechanism | HK Relevance |
|---|---|---|
| Sedative overdose (opioids, benzodiazepines) [6] | Direct depression of medullary respiratory centre | Common — accidental or intentional overdose; post-operative |
| CNS depression (brainstem stroke, raised ICP, encephalitis) [6] | Damage to or compression of respiratory centres | Stroke is leading cause of morbidity in HK |
| Central sleep apnoea | Loss of automatic ventilatory drive during sleep | Less common than OSA |
| Metabolic alkalosis [6] | Compensatory hypoventilation (↓drive as pH already high) | Iatrogenic (over-correction of acidosis, diuretics) |
| Hypothermia [6] | ↓Metabolic rate and ↓neuronal function in respiratory centre | Seen in drowning, exposure, elderly in winter |
| Hypothyroidism [6] | ↓Metabolic drive, ↓respiratory muscle strength, ↓central drive | Common in HK, especially elderly women |
| Cause | Mechanism | HK Relevance |
|---|---|---|
| Guillain-Barré syndrome (GBS) [6] | Ascending demyelinating polyneuropathy → respiratory muscle paralysis | Acute presentations; post-infectious |
| Myasthenia gravis (MG) [6] | Antibodies against AChR at NMJ → fatigable weakness of respiratory muscles | Myasthenic crisis is a medical emergency |
| Spinal cord injury (C3-C5) [6] | Loss of phrenic nerve innervation → diaphragmatic paralysis | Trauma (RTAs, falls) |
| Phrenic nerve injury [6] | Unilateral or bilateral diaphragmatic paralysis | Post-cardiac surgery, tumour infiltration |
| Motor neuron disease (ALS) | Progressive motor neuron degeneration | Chronic progressive T2RF |
| Muscular dystrophies | Progressive respiratory muscle weakness | Duchenne (childhood), myotonic dystrophy |
| Critical illness myopathy/polyneuropathy | ICU-acquired weakness from prolonged ventilation, steroids, NMBAs | Very common in HK ICUs |
| Cause | Mechanism |
|---|---|
| Kyphoscoliosis [6] | Restrictive defect → ↓lung volumes → hypoventilation |
| Flail chest [6] | Paradoxical chest wall movement → ineffective ventilation |
| Obesity hypoventilation syndrome (OHS) [6] | ↑Abdominal mass → ↓diaphragmatic excursion + ↓chest wall compliance + ↓respiratory drive (leptin resistance) |
| Ankylosing spondylitis | Fusion of costovertebral joints → ↓chest wall expansion |
| Cause | Mechanism | HK Relevance |
|---|---|---|
| COPD (acute exacerbation or decompensated) [5][6] | Airflow obstruction → air trapping → ↑dead space → V/Q mismatch → eventually global hypoventilation + respiratory muscle fatigue | THE most common cause in HK |
| Severe acute asthma (status asthmaticus) [6] | Severe bronchospasm + mucus plugging → ↓VA | Common; rising "normal" pCO₂ in acute asthma is ominous |
| Upper airway obstruction [6] | Mechanical obstruction → cannot move air | Foreign body, tumour, anaphylaxis, OSA |
High Yield: A 'Normal' pCO₂ in Acute Severe Asthma is Dangerous
In acute asthma, patients initially hyperventilate (pCO₂ is LOW). A "normalising" pCO₂ in a breathless asthmatic means they are tiring and heading towards respiratory arrest. Do NOT be falsely reassured.
| Cause | Mechanism |
|---|---|
| ARDS (severe) | Diffuse alveolar damage → massive shunting + ↑dead space |
| Severe pneumonia | Consolidation → shunt; if extensive → eventual hypoventilation |
| End-stage ILD | Destruction of gas exchange surface + respiratory muscle fatigue |
Note: Most parenchymal diseases initially cause Type 1 respiratory failure (V/Q mismatch, shunt, diffusion impairment) with LOW or normal pCO₂ (because CO₂ is so diffusible). Only when disease is very severe or when respiratory muscles fatigue does Type 2 respiratory failure (and hence respiratory acidosis) supervene. [6]
| Acute Respiratory Acidosis | Chronic Respiratory Acidosis |
|---|---|
| COPD exacerbation | Stable severe COPD |
| Severe asthma | OHS |
| Sedative/opioid overdose | Neuromuscular disease (MG, MND, DMD) |
| GBS / myasthenic crisis | Kyphoscoliosis |
| Pneumothorax | Central/obstructive sleep apnoea |
| Pulmonary oedema (severe) | Chronic CNS disease |
| Upper airway obstruction | |
| Flail chest |
Acute vs Chronic — Why Does it Matter?
The distinction is critical because:
- Compensation differs (see below) — acute has minimal renal compensation; chronic has significant renal compensation
- Clinical urgency differs — acute respiratory acidosis with pH < 7.25 may be immediately life-threatening
- Treatment differs — acute may need emergent intubation; chronic may be managed with NIV at home
Pathophysiology
Why does pCO₂ rise? Because:
- paCO₂ = (VCO₂ × 0.863) / VA
- VA (alveolar ventilation) = (Tidal volume − Dead space) × Respiratory rate
- ANY reduction in VA relative to CO₂ production → ↑paCO₂
The body then mounts compensation:
The body compensates for respiratory acidosis by conserving bicarbonate via the kidneys [2][3]:
| Time Frame | Mechanism | Expected Compensation | Magnitude |
|---|---|---|---|
| Acute (minutes–hours) | Buffering by intracellular proteins and phosphate; slight renal adjustment | For every 10 mmHg ↑pCO₂ → HCO₃⁻ ↑ by ~1 mEq/L | Modest — pH drops significantly |
| Chronic (3–5 days) | Kidneys ↑H⁺ excretion (↑NH₄⁺, ↑titratable acidity) and ↑HCO₃⁻ reabsorption [2] | For every 10 mmHg ↑pCO₂ → HCO₃⁻ ↑ by ~3.5 mEq/L | More substantial — pH returns towards (but does not reach) normal |
Respiratory compensation (for metabolic disorders) occurs immediately and reaches maximum in 12 hours. Metabolic compensation (for respiratory disorders) occurs via acute buffering (modest) then chronic renal acid excretion changes (significant). [2]
Rule of thumb: ONLY in chronic respiratory alkalosis may there be complete compensation (normal pH). In all other acid-base disorders, compensation is always incomplete. [2]
So in chronic respiratory acidosis (e.g., stable severe COPD), you see: ↑pCO₂, ↑HCO₃⁻, near-normal pH. If pH is still very low despite a high HCO₃⁻, suspect an acute-on-chronic process or a superimposed metabolic acidosis.
| System | Effect | Mechanism |
|---|---|---|
| CNS | Headache, drowsiness, confusion → coma (CO₂ narcosis) | CO₂ crosses BBB freely → ↑CSF [H⁺] → cerebral vasodilation (headache) + direct neuronal depression |
| Cardiovascular | Initially ↑HR, ↑BP (sympathetic activation); later ↓contractility, vasodilation, arrhythmias | Acidaemia → ↑catecholamines (compensatory), but severe acidosis → ↓myocardial contractility, ↓response to catecholamines |
| Respiratory | Initially ↑respiratory drive (via chemoreceptors); later fatigue | Central chemoreceptors stimulated; but in chronic states, desensitisation occurs |
| Electrolytes | Hyperkalaemia | H⁺/K⁺ exchange: H⁺ moves intracellularly, K⁺ moves out (for every 0.1 ↓pH, K⁺ ↑ ~0.5 mEq/L — though less reliable for respiratory than metabolic acidosis) |
| Oxygen delivery | Right-shift of Hb-O₂ dissociation curve (Bohr effect) | Acidosis ↓Hb affinity for O₂ → facilitates O₂ unloading to tissues (partial compensatory benefit) |
| Renal | ↑Renal HCO₃⁻ reabsorption, ↑H⁺ excretion, ↑ammoniagenesis | Proximal tubule senses ↑pCO₂ → upregulates Na⁺/H⁺ exchanger and H⁺-ATPase → ↑HCO₃⁻ reclamation; collecting duct ↑NH₃ synthesis |
| Bone | Chronic: calcium and phosphate release from bone | Buffering of chronic acid load → contributes to osteoporosis in long-standing respiratory acidosis |
This distinction is important:
- Pure hypoventilation (e.g., opioid OD in a person with normal lungs): Both O₂ and CO₂ are affected proportionally. The A-a gradient is normal. Giving supplemental O₂ corrects hypoxaemia but does NOT fix hypercapnia (you still need to restore ventilation).
- V/Q mismatch (e.g., COPD): There are areas of low V/Q (poorly ventilated but perfused alveoli). The A-a gradient is widened. Initially, CO₂ elimination may be maintained by hyperventilation of unaffected areas (because CO₂ diffusion is very efficient). Only when the disease is severe or muscles fatigue does CO₂ retention occur.
- In COPD, respiratory acidosis develops because:
- Airway obstruction with non-uniform distribution of inspired air → V/Q mismatch [5]
- Destruction of alveoli with loss of capillary bed → ↓Q [5]
- Alveolar hypoventilation due to respiratory muscle fatigue (↑work of breathing from ↑airway resistance and hyperinflation) + inadequate supply of fuel to respiratory muscles (hypoxaemia, poor nutrition) [5]
PAO₂ = FiO₂ × (Patm − PH₂O) − (PaCO₂ / R)
Where R = respiratory quotient (~0.8). This shows that as PaCO₂ rises, PAO₂ must fall (assuming constant FiO₂). This is why respiratory acidosis is almost always accompanied by hypoxaemia unless the patient is on supplemental O₂.
Classification
| Acute | Chronic | Acute-on-Chronic | |
|---|---|---|---|
| Duration | Hours to days | Weeks to years | Acute deterioration superimposed on chronic |
| Compensation | Minimal (HCO₃⁻ ↑ ~1 per 10 mmHg ↑pCO₂) | Substantial (HCO₃⁻ ↑ ~3.5 per 10 mmHg ↑pCO₂) | Intermediate — HCO₃⁻ elevated from baseline but insufficient for the new pCO₂ |
| pH | Significantly ↓ | Near-normal or mildly ↓ | ↓ (more than expected for chronic, less than expected for purely acute) |
| Example | Opioid OD, acute severe asthma, GBS | Stable severe COPD, OHS, chronic NM disease | COPD exacerbation, pneumonia in patient with OHS |
| Level | Examples |
|---|---|
| ↓Respiratory drive | Sedative overdose, CNS depression, metabolic alkalosis, hypothermia, hypothyroidism |
| Neuromuscular | Spinal cord injury, GBS, phrenic nerve injury, myasthenia gravis |
| Thoracic cage disorder | Kyphoscoliosis, flail chest, OHS |
| Global lung hypoventilation | Upper airway obstruction, decompensated diffuse lower airway disorder (COPD, asthma) |
Clinical Features
| Symptom | Pathophysiological Basis |
|---|---|
| Dyspnoea (breathlessness) | ↑Work of breathing as the body attempts to increase ventilation to blow off CO₂; stimulation of central/peripheral chemoreceptors by ↑pCO₂ and ↓pO₂ |
| Headache | CO₂ is a potent cerebral vasodilator — ↑pCO₂ → cerebral vasodilation → ↑intracranial pressure → headache (often worse on waking, as CO₂ accumulates overnight) [6] |
| Morning headache (chronic) | Nocturnal hypoventilation (↓respiratory drive during sleep + ↓tidal volume supine) → CO₂ accumulates overnight → morning vasodilation headache |
| Drowsiness / somnolence | CO₂ narcosis — very high pCO₂ has a direct anaesthetic effect on the CNS; also ↑CSF H⁺ depresses neuronal function [6] |
| Confusion / altered mentation | Same as above — progressive CNS depression; also ↓cerebral O₂ delivery if concurrent hypoxaemia [6] |
| Anxiety, agitation (early) | Sympathetic nervous system activation triggered by hypercapnia and hypoxaemia |
| Sleep disturbance, daytime somnolence (chronic) | Nocturnal hypoventilation → sleep fragmentation; especially prominent in OHS and neuromuscular disease |
| Cough, sputum | Underlying airway disease (COPD, bronchiectasis) driving the respiratory acidosis — not a direct consequence of hypercapnia itself |
| Sign | Pathophysiological Basis |
|---|---|
| Tachypnoea | Compensatory ↑respiratory rate driven by chemoreceptors detecting ↑pCO₂ and ↓pO₂ (unless respiratory drive is the problem, e.g., opioid OD where you see bradypnoea) [6] |
| Use of accessory respiratory muscles (sternocleidomastoid, scalenes, abdominal muscles) | Recruited when the diaphragm alone cannot generate sufficient ventilation — signifies ↑work of breathing [6] |
| Paradoxical (abdominal) breathing | Diaphragmatic fatigue: chest wall and abdominal movements out of phase — the abdomen is drawn IN during inspiration instead of pushing OUT, because a fatigued/paralysed diaphragm is sucked upward by negative intrathoracic pressure rather than contracting downward [6] |
| Central cyanosis | Concurrent hypoxaemia (↓PaO₂) → ↑deoxyhaemoglobin > 5 g/dL → blue discolouration of tongue, lips, mucous membranes [6] |
| Flushing / plethora | CO₂-induced peripheral vasodilation |
| Bounding pulse | CO₂-induced vasodilation → ↓SVR → ↑stroke volume (high-output state) |
| Sweating | ↑Sympathetic nervous system discharge from hypercapnia and hypoxaemia [6] |
| Tachycardia, hypertension | ↑SN discharge: ↑HR, ↑BP — sympathetic activation as compensatory response to hypoxaemia and hypercapnia [6] |
| Flapping tremor (asterixis) | CO₂ retention sign — metabolic encephalopathy from hypercapnia causes transient loss of muscle tone → characteristic "liver flap"–like tremor of dorsiflexed hands (also seen in hepatic encephalopathy, uraemia — any metabolic encephalopathy) [6] |
| Papilloedema | ↑pCO₂ → cerebral vasodilation → ↑intracranial pressure → swelling of optic disc [6] |
| Altered consciousness → coma (CO₂ narcosis) | Severe hypercapnia (pCO₂ > 10–12 kPa) → direct CNS depression; CSF acidosis → progressive obtundation |
| Warm peripheries | CO₂-induced vasodilation (unlike shock where peripheries are cold) |
| Miosis (constricted pupils) — if opioid-induced | Direct opioid effect on Edinger-Westphal nucleus (pinpoint pupils = classic sign) |
Signs of the Underlying Cause
| Cause | Additional Signs |
|---|---|
| COPD | Barrel chest, hyperresonance, ↓air entry, prolonged expiratory phase, wheeze, pursed lip breathing |
| Neuromuscular disease | Wasting, fasciculations (MND), fatigable weakness (MG), areflexia (GBS) |
| OHS | Morbid obesity (BMI ≥ 30), small neck circumference |
| Kyphoscoliosis | Visible spinal deformity |
| Opioid overdose | Pinpoint pupils, ↓GCS, needle marks, ↓respiratory rate |
High Yield: Clinical Features Summary Table from GC Lecture / Senior Notes
| Feature Category | T1 + T2 Respiratory Failure | T2 Respiratory Failure ONLY (Hypercapnia) |
|---|---|---|
| Respiratory effort | Tachypnoea, accessory muscle use | |
| Diaphragmatic fatigue | Paradoxical breathing | |
| Sympathetic activation | ↑HR, ↑BP, sweating, agitation | |
| Hypoxaemia | Central cyanosis, confusion | |
| CO₂ retention | Headache (vasodilation), altered mentation, flushing, papilloedema, HTN, flapping tremor | |
| Acidosis | Air hunger, Kussmaul breathing, gasping |
Watch Out — Kussmaul Breathing vs Tachypnoea
Kussmaul breathing (deep, laboured, sighing respirations) is classically described as a compensation for metabolic acidosis (e.g., DKA), not respiratory acidosis. In respiratory acidosis, the problem IS the inadequate ventilation — you cannot compensate by breathing more if the lungs/muscles/drive are the problem. If you see Kussmaul breathing in someone with respiratory acidosis, think of a mixed disorder (concomitant metabolic acidosis). However, the "air hunger" and gasping described in severe acidaemia of any cause may superficially resemble Kussmaul breathing. [6][3]
To consolidate understanding, here is what the ABG looks like:
| Parameter | Acute Respiratory Acidosis | Chronic Respiratory Acidosis |
|---|---|---|
| pH | ↓↓ (< 7.35, may be < 7.25) | Mildly ↓ or low-normal (~7.32–7.38) |
| pCO₂ | ↑↑ (> 6 kPa / > 45 mmHg) | ↑↑ (> 6 kPa / > 45 mmHg) |
| HCO₃⁻ | Mildly ↑ (acute buffering) | Significantly ↑ (renal compensation) |
| pO₂ | ↓ (unless on supplemental O₂) | ↓ (unless on supplemental O₂) |
| Base excess | Near zero (acute) | Positive (chronic compensation) |
| A-a gradient | Normal (pure hypoventilation) or ↑ (V/Q mismatch, shunt) | Depends on underlying disease |
Quantitative Compensation Rules (Must Know for Exams)
| Disorder | Expected Compensation |
|---|---|
| Acute respiratory acidosis | ΔpCO₂ ↑10 mmHg → ΔHCO₃⁻ ↑1 mEq/L |
| Chronic respiratory acidosis | ΔpCO₂ ↑10 mmHg → ΔHCO₃⁻ ↑3.5 mEq/L |
If the measured HCO₃⁻ is higher than expected → concomitant metabolic alkalosis If the measured HCO₃⁻ is lower than expected → concomitant metabolic acidosis
High Yield Summary
Definition: Respiratory acidosis = primary ↑pCO₂ (> 45 mmHg / > 6 kPa) due to alveolar hypoventilation, resulting in ↓pH.
Most common cause in HK: COPD acute exacerbation.
Core pathophysiology: Any failure along the ventilatory chain (CNS drive → spinal cord → NMJ → respiratory muscles → chest wall → airways → alveoli) → ↓alveolar ventilation → CO₂ retention.
Compensation: Kidneys retain HCO₃⁻ — acute: +1 mEq/L per 10 mmHg rise in pCO₂; chronic: +3.5 mEq/L per 10 mmHg.
Key clinical features of hypercapnia: headache, flushing, flapping tremor, papilloedema, drowsiness → CO₂ narcosis/coma.
Danger sign in acute asthma: "normalising" pCO₂ = impending respiratory arrest.
O₂ therapy in COPD: Target SpO₂ 88–92% to avoid abolishing hypoxic drive.
Type 2 RF causes (mnemonic: D-N-T-G = Drive, Nerves/NMJ, Thoracic cage, Global lung hypoventilation): sedative OD / CNS depression / hypothyroidism; GBS / MG / spinal cord; kyphoscoliosis / flail chest / OHS; COPD / severe asthma / upper airway obstruction.
Active Recall - Respiratory Acidosis (Definition to Clinical Features)
[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf (Slide on ABG interpretation: pH, pCO2, HCO3) [2] Senior notes: Ryan Ho Urogenital.pdf (Section 2.4 Disorders of Acid-base Balance, pp. 34, 50) [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (pp. 2–3, 7) [4] Senior notes: Ryan Ho Respiratory.pdf (Section 3.2.2 COPD, p. 107–108) [5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (COPD Pathophysiology, p. 217–219) [6] Senior notes: Ryan Ho Fundamentals.pdf / Ryan Ho Respiratory.pdf / Ryan Ho Critical Care.pdf (Section 2.5 / 3.2.5 Respiratory Failure, pp. 29–30 / 230)
Differential Diagnosis of Respiratory Acidosis
When you encounter respiratory acidosis (↑pCO₂ with ↓pH) on an ABG, the differential diagnosis is really asking: "Why is this patient hypoventilating (or failing to eliminate CO₂)?" The ABG result is the consequence — you need to find the cause.
But there is a crucial prior step: Are you sure this is a primary respiratory acidosis, or could the elevated pCO₂ be compensatory for a primary metabolic alkalosis? [3]
Primary metabolic alkalosis → Compensatory respiratory acidosis = hypoventilation to conserve CO₂, increased pCO₂ [3]
This is the single most important differential to exclude before you start hunting for lung/neuromuscular/CNS causes. If the pH is > 7.45 (alkalotic) and pCO₂ is high, the primary process is metabolic alkalosis, and the "respiratory acidosis" is just compensation. If pH is < 7.35 (acidotic) and pCO₂ is high, the primary process is respiratory acidosis.
The A-a Gradient Is Your Key Differentiator
A normal A-a gradient (< 15 mmHg in young, < 25 in elderly) with ↑pCO₂ tells you the lungs themselves are fine — the problem is before the alveolus (drive, nerves, muscles, chest wall, upper airway). A widened A-a gradient with ↑pCO₂ tells you there is intrinsic lung disease. This single calculation dramatically narrows your differential.
Systematic Differential Diagnosis
Below is the full differential, organised by the anatomical level of failure along the ventilatory chain. This mirrors the Type 2 respiratory failure aetiology framework directly from the GC lecture and senior notes [1][6]:
| Cause | Key Features Distinguishing It | A-a Gradient |
|---|---|---|
| Sedative / opioid overdose [6] | History of ingestion or exposure; pinpoint pupils (opioids); ↓GCS; ↓respiratory rate (bradypnoea); responds to naloxone (opioid) or flumazenil (benzodiazepine) | Normal |
| CNS depression — brainstem stroke, raised ICP, encephalitis [6] | Focal neurological signs (stroke); papilloedema, Cushing reflex (raised ICP); fever + meningism (encephalitis); CT brain abnormal | Normal |
| Metabolic alkalosis (compensatory hypoventilation) [3][6] | pH actually alkalotic (> 7.45); history of vomiting, NG suctioning, diuretic use, alkali ingestion; HCO₃⁻ very high; Cl⁻ low | Normal |
| Hypothermia [6] | Core temperature < 35°C; exposure history; J (Osborn) waves on ECG | Normal |
| Hypothyroidism (severe / myxoedema coma) [6] | Hypothermic, bradycardic, hyporeflexic, altered mental status; ↑TSH, ↓fT4 | Normal |
| Central sleep apnoea | Nocturnal hypoventilation, daytime somnolence; diagnosed on polysomnography | Normal |
Why do CNS causes produce respiratory acidosis? Because the brainstem respiratory centres (dorsal and ventral respiratory groups in the medulla) generate the rhythmic neural signals that drive breathing. If these centres are depressed (drugs, structural damage, metabolic derangement), the respiratory rate and/or tidal volume falls → ↓alveolar ventilation → CO₂ accumulates. The lungs are structurally normal, so gas exchange per unit of ventilation is fine → normal A-a gradient.
| Cause | Key Features Distinguishing It | A-a Gradient |
|---|---|---|
| Guillain-Barré syndrome (GBS) [6] | Ascending weakness, areflexia; preceding infection (Campylobacter, CMV); progressive over days-weeks; forced vital capacity (FVC) monitoring critical | Normal |
| Myasthenia gravis — myasthenic crisis [6] | Fatigable weakness, diplopia, ptosis, bulbar symptoms; +ve AChR or MuSK antibodies; responds to edrophonium/neostigmine | Normal |
| Spinal cord injury (C3-C5) [6] | Trauma history; quadriplegia; sensory level; acute urinary retention | Normal |
| Phrenic nerve injury (bilateral) [6] | Post-cardiac surgery, tumour infiltration, neuralgic amyotrophy; orthopnoea out of proportion; paradoxical abdominal breathing; elevated hemidiaphragm on CXR; confirmed by ultrasound or nerve conduction | Normal |
| Motor neuron disease (ALS) | Progressive weakness, fasciculations, UMN + LMN signs; bulbar symptoms; chronic progressive course | Normal |
| Muscular dystrophies (Duchenne, myotonic) | Known diagnosis; progressive proximal weakness; CK elevated | Normal |
| Critical illness myopathy / polyneuropathy | ICU setting; prolonged ventilation; steroids/NMBAs use; difficulty weaning | Normal |
| Botulism | Descending paralysis (opposite to GBS); autonomic features; contaminated food/wound history | Normal |
Why do neuromuscular causes give normal A-a gradient? Same logic — the lungs are pristine. The problem is the "bellows" mechanism can't move air in and out.
| Cause | Key Features Distinguishing It | A-a Gradient |
|---|---|---|
| Kyphoscoliosis [6] | Visible spinal deformity; restrictive pattern on spirometry; chronic and insidious; often presents in middle age when respiratory reserve declines | Normal (early) → may widen as secondary atelectasis develops |
| Flail chest [6] | Trauma history; paradoxical chest wall segment movement; multiple rib fractures on CXR/CT | Normal to mildly widened (concurrent contusion) |
| Obesity hypoventilation syndrome (OHS) [6] | BMI ≥ 30 kg/m² (usually ≥ 40); daytime hypercapnia; often coexists with OSA; polysomnography diagnostic | Normal to mildly widened |
| Ankylosing spondylitis (severe) | Young male; bamboo spine on X-ray; ↓chest expansion; HLA-B27+ | Normal |
| Cause | Key Features Distinguishing It | A-a Gradient |
|---|---|---|
| COPD — acute exacerbation or decompensated chronic [6][7] | THE #1 cause in HK. Smoking history; chronic cough/sputum; obstructive spirometry (FEV₁/FVC < 0.7); hyperinflation on CXR; often triggered by infection | Widened |
| Severe acute asthma (status asthmaticus) [6] | Known asthma; acute wheeze + dyspnoea; "normalising" pCO₂ = danger sign of fatigue; responds to bronchodilators + steroids | Widened |
| Upper airway obstruction [6] | Stridor (inspiratory); drooling; history of FB, tumour, anaphylaxis, post-extubation oedema | Normal to widened |
| Severe pneumonia / ARDS | Fever, consolidation on CXR (pneumonia); bilateral infiltrates, ↓P/F ratio (ARDS); usually T1RF initially → T2RF when severe/fatigued | Widened |
| End-stage interstitial lung disease | Known ILD; restrictive spirometry; bilateral reticular/honeycomb pattern on HRCT; clubbing | Widened |
| Massive pleural effusion | Dullness to percussion; ↓breath sounds; CXR shows large effusion compressing lung | Widened |
High Yield: COPD Is the Dominant Cause
In any HKUMed exam scenario, when you see an elderly male smoker with chronic cough, ↑pCO₂, and ↓pH on ABG — think COPD exacerbation first. It accounts for the vast majority of respiratory acidosis presentations in Hong Kong hospitals. The interactive tutorial case (70-year-old, 40 pack-years, increased breathlessness, purulent sputum) is the classic setup [7][8].
Sometimes the ABG shows respiratory acidosis plus another acid-base disorder. Common mixed scenarios:
| Mixed Disorder | Clinical Scenario | How to Recognise |
|---|---|---|
| Respiratory acidosis + metabolic acidosis | COPD exacerbation + lactic acidosis from sepsis/shock; cardiac arrest | pH much lower than expected for the pCO₂; HCO₃⁻ lower than expected for compensation |
| Respiratory acidosis + metabolic alkalosis | COPD patient on diuretics (→ contraction alkalosis); COPD + vomiting | pH near-normal despite very high pCO₂; HCO₃⁻ higher than expected for compensation alone |
| Acute-on-chronic respiratory acidosis | Stable COPD (chronically ↑pCO₂) develops pneumonia/exacerbation → further ↑pCO₂ | Known baseline ABG with chronic ↑pCO₂; acute deterioration with ↓pH beyond usual baseline |
Low bicarbonate does not always mean metabolic acidosis — it could be the compensatory mechanism to a primary respiratory alkalosis (e.g., something causing hyperventilation) [3][9]. Similarly, high bicarbonate does not always mean metabolic alkalosis — it could be renal compensation for chronic respiratory acidosis. Context is everything.
| Mimic | Why It Can Confuse | How to Differentiate |
|---|---|---|
| Compensatory hypoventilation for metabolic alkalosis | ↑pCO₂ present; could be misread as primary respiratory acidosis | pH is > 7.45 (alkalotic); primary process is ↑HCO₃⁻; pCO₂ rise is proportional and expected (ΔpCO₂ ≈ 0.7 × ΔHCO₃⁻) [3] |
| Lab error / venous sample mislabelled as arterial | Venous pCO₂ is ~6–8 mmHg higher than arterial; could falsely elevate pCO₂ | Check sample type; venous pH is ~0.03–0.05 lower than arterial |
| Permissive hypercapnia in ventilated patients | Deliberately allowing ↑pCO₂ (lung-protective ventilation in ARDS) | Clinical context: ICU, known ventilation strategy; not a pathological process |
| Acute Respiratory Acidosis | Chronic Respiratory Acidosis |
|---|---|
| AECOPD | Stable severe COPD |
| Status asthmaticus | OHS |
| Opioid / sedative overdose | Neuromuscular disease (MG, MND, DMD) |
| GBS / myasthenic crisis | Kyphoscoliosis |
| Acute upper airway obstruction | Central / obstructive sleep apnoea |
| Flail chest / tension pneumothorax | Chronic CNS disease |
| Severe pneumonia → T2RF | End-stage ILD |
| Cardiac arrest |
For completeness (and because paediatrics questions may overlap), non-respiratory tract causes of respiratory distress in neonates include congenital heart disease, CNS disorders causing apnoea (cerebral haemorrhage, oedema, drugs), GI disorders (OA/TOF, gastro-oesophageal reflux), and systemic causes (metabolic acidosis, sepsis) [10]. In children, important additional causes include:
- Foreign body aspiration (upper airway obstruction)
- Croup / epiglottitis (mural causes)
- Inborn errors of metabolism causing secondary respiratory depression [11]
- Respiratory distress syndrome in premature neonates (surfactant deficiency → atelectasis → T2RF)
| Feature | Central/Drive | Neuromuscular | Chest Wall | Airway/Lung |
|---|---|---|---|---|
| A-a gradient | Normal | Normal | Normal (early) | Widened |
| Respiratory rate | ↓↓ (bradypnoea) | Variable (may be ↑ early) | ↑ (shallow) | ↑↑ |
| Muscle strength | Normal | ↓↓ | Normal | Normal |
| Lung exam | Clear | Clear | Restrictive | Wheeze / creps / ↓AE |
| CXR | Normal | Normal (± elevated hemi-diaphragm) | Skeletal deformity; flail | Hyperinflation / consolidation |
| Key Ix | Tox screen, CT brain, TFT | NCS/EMG, AChR Ab, FVC | Sleep study, spirometry | Spirometry, CXR, CT |
| Responds to O₂ alone? | Partially (fixes hypoxia, not pCO₂) | Partially | Partially | Partially |
| Definitive Rx | Antidote / treat cause | Plasmapheresis/IVIg (GBS); anti-ChE (MG) | NIV / weight loss | Bronchodilators, NIV, intubation |
High Yield: The 'D-N-T-G' Framework for T2RF Causes
Causes of Type 2 respiratory failure (and therefore respiratory acidosis): [6]
- D = ↓Respiratory drive: sedative overdose, CNS depression, metabolic alkalosis, hypothermia, hypothyroidism
- N = Neuromuscular: spinal cord injury, GBS, phrenic nerve injury, myasthenia gravis
- T = Thoracic cage disorder: kyphoscoliosis, flail chest, OHS
- G = Global lung hypoventilation: upper airway obstruction, decompensated diffuse lower airway disorder (COPD, asthma)
This is directly from the lecture slides and senior notes — learn this framework cold.
Active Recall - Differential Diagnosis of Respiratory Acidosis
References
[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf (Slide on ABG interpretation) [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (pp. 3–4) [6] Senior notes: Ryan Ho Fundamentals.pdf / Ryan Ho Respiratory.pdf (Section 2.5 / 3.2.5 Respiratory Failure, pp. 29–30 / 230) [7] Senior notes: Maksim Medicine Notes.pdf (Respiratory medicine, pp. 280, 302) [8] Lecture slides: GC_Interactive tutorial (Resp-COPD case) student copy.pdf (p. 1) [9] Senior notes: Ryan Ho Chemical Path.pdf (Section E. Salicylate, p. 42) [10] Lecture slides: GC 204. The newborn baby cannot breathe Oesophageal atresia, diaphragmatic hernia, and other surgery of lung.pdf (p. 10) [11] Senior notes: Adrian Lui Pediatrics Notes.pdf (pp. 299, 463)
Diagnostic Criteria for Respiratory Acidosis
Unlike diseases such as rheumatoid arthritis or SLE that have formal classification criteria, respiratory acidosis is defined purely by arterial blood gas (ABG) findings. The "diagnosis" is the ABG pattern itself, and the clinical task is then to identify the underlying cause.
The GC lecture slide gives a concise 3-step framework [1]:
| Step | Parameter | Interpretation |
|---|---|---|
| 1. pH | < 7.35 = acidaemic; 7.35–7.45 = normal; > 7.45 = alkalaemic | Determines whether there is acidaemia |
| 2. pCO₂ (N: 4–6 kPa) | If elevated with acidaemia → respiratory acidosis | Identifies the respiratory component |
| 3. HCO₃⁻ (N: 21–27 mEq/L) | Metabolic status; renal compensation; chronicity | Distinguishes acute from chronic and identifies mixed disorders |
This is not just academic — it determines urgency and management:
| Feature | Acute | Chronic | Acute-on-Chronic |
|---|---|---|---|
| pH | Significantly ↓ (< 7.35, often < 7.25) | Mildly ↓ or low-normal (7.32–7.38) | ↓ more than expected for chronic baseline |
| pCO₂ | ↑ | ↑ | ↑↑ (above patient's known baseline) |
| HCO₃⁻ | Mildly ↑ | Significantly ↑ | ↑ but insufficient for the acute pCO₂ rise |
| Compensation rule | Acute: every 10 mmHg (1.3 kPa) rise in pCO₂ → 1 mmol/L rise in HCO₃⁻ [2][12][13] | Chronic: every 10 mmHg (1.3 kPa) rise in pCO₂ → 3 mmol/L rise in HCO₃⁻ [2][12][13] | HCO₃⁻ between acute and chronic expected values |
| HCO₃⁻ ceiling | Cannot go higher than ~30 mmol/L [2] | Cannot go higher than ~45 mmol/L [2] | — |
High Yield: The '1-2-3-4' Rule for Respiratory Compensation
The "1-2-3-4" rule for compensation of acute and chronic respiratory acidosis and alkalosis: [13]
| Disorder | ΔpCO₂ | ΔHCO₃⁻ |
|---|---|---|
| Acute respiratory acidosis | ↑10 mmHg | ↑1 mmol/L |
| Acute respiratory alkalosis | ↓10 mmHg | ↓2 mmol/L |
| Chronic respiratory acidosis | ↑10 mmHg | ↑3 mmol/L |
| Chronic respiratory alkalosis | ↓10 mmHg | ↓4 mmol/L |
This is the single most important set of numbers to memorise for ABG interpretation in exams.
Once you've confirmed respiratory acidosis, you must check whether the compensation is appropriate. If it isn't, there's a mixed disorder lurking:
- If actual HCO₃⁻ > expected HCO₃⁻ → concomitant primary metabolic alkalosis (e.g., COPD patient on thiazide diuretics) [2]
- If actual HCO₃⁻ < expected HCO₃⁻ → concomitant primary metabolic acidosis (e.g., COPD patient with septic shock causing lactic acidosis) [2]
Three-step approach when interpreting ABG: [2]
- Look at pH: direction of change often indicates primary/predominant acidosis/alkalosis
- Pattern recognition: "3 ups/3 downs rule" — if ALL 3 components of H-H equation move in same direction, then there must be a simple metabolic acid-base disturbance; "Not up, not down rule" — if HCO₃⁻ and CO₂ move in different directions, then there must be mixed acid-base disturbance
- Evaluate compensation to uncover subtle mixed disorders
Diagnostic Algorithm
Step 1: Look at the pH — Is the patient acidaemic (< 7.35), alkalaemic (> 7.45), or normal?
Step 2: Identify the primary disorder — Look at pCO₂ and HCO₃⁻:
- If pH < 7.35 AND pCO₂ > 45 mmHg → primary respiratory acidosis
- If pH < 7.35 AND HCO₃⁻ < 22 → primary metabolic acidosis
- If both are abnormal, the one that "explains" the pH direction is the primary disorder
Step 3: Assess compensation using the 1-2-3-4 rule — Is the HCO₃⁻ response appropriate?
- If yes → simple respiratory acidosis (acute or chronic)
- If HCO₃⁻ is too high or too low → mixed disorder
Step 4: Calculate the A-a gradient — Localise the problem:
- A-a gradient = PAO₂ − PaO₂
- PAO₂ = FiO₂ × (Patm − PH₂O) − (PaCO₂ / 0.8)
- On room air at sea level: PAO₂ ≈ 0.21 × (760 − 47) − (PaCO₂ / 0.8)
- Normal: < 15 mmHg (young), < 25 mmHg (elderly); add 3 mmHg per decade over 30
Step 5: Integrate with clinical context — History, examination, and targeted investigations to identify the specific cause
Investigation Modalities with Key Findings and Interpretations
Learning point: Interpret ABGs using a stepwise approach — assess oxygenation via PaO₂ and A-a gradient, determine acid-base status through pH, identify primary disorder from PaCO₂ and bicarbonate, then calculate compensation. Type 2 respiratory failure with hypercapnia indicates ventilatory failure requiring urgent intervention. [14]
| Parameter | Normal Range | Finding in Respiratory Acidosis | Interpretation |
|---|---|---|---|
| pH | 7.35–7.45 | ↓ (< 7.35) or near-normal if chronic | Confirms acidaemia |
| pCO₂ | 4–6 kPa (35–45 mmHg) [1] | ↑ (> 6 kPa) | Elevated with acidaemia → respiratory acidosis [1] |
| HCO₃⁻ | 21–27 mEq/L [1] | ↑ (compensation) | Reflects metabolic status and renal compensation / chronicity [1] |
| PaO₂ | 10–13 kPa (75–100 mmHg) | ↓ (unless on O₂) | Concurrent hypoxaemia expected |
| Base excess | −2 to +2 | +ve (chronic) | Reflects degree of metabolic compensation |
Interpretation of arterial blood gases, esp. different settings of type 2 respiratory failure — explicitly listed as a key learning objective in respiratory medicine [14][15]
Why ABG and not VBG? An ABG is essential because:
- Only arterial sampling gives accurate PaO₂ (to calculate A-a gradient and confirm hypoxaemia)
- Venous pCO₂ is 3–8 mmHg higher than arterial; venous pH is 0.02–0.04 lower [2]
- However, peripheral venous BG can be used as a screening tool: usually add 0.02–0.04 to pH, subtract 1–2 mmol/L from HCO₃⁻ and 3–8 mmHg from pCO₂ [2]
When to Order an ABG
Indications for ABG in the context of suspected respiratory acidosis:
- Any patient with acute respiratory distress, ↓GCS, or suspected T2RF
- COPD exacerbation (to guide O₂ therapy and NIV decision)
- Respiratory acidosis (i.e., T2RF with pH ≤ 7.35 / PaCO₂ > 6.0 kPa) — indication for mechanical ventilation consideration [7]
- Monitoring response to NIV — check ABG within 1–2 hours after initiation
- Suspicion of mixed acid-base disorder
| Normal A-a Gradient | Widened A-a Gradient | |
|---|---|---|
| Means | Lungs are normal; hypoventilation is extrapulmonary | Intrinsic lung pathology |
| Causes of resp acidosis | CNS depression, NM disease, chest wall disorder, UAO | COPD, severe asthma, pneumonia, ARDS, ILD |
| Calculation | PAO₂ − PaO₂ | Same formula |
| Normal value | < 15 mmHg (young), increases with age | > 25 mmHg is definitely abnormal |
| Parameter | Expected Finding | Why |
|---|---|---|
| K⁺ | ↑ (mild) | H⁺/K⁺ exchange — acidosis drives K⁺ out of cells; less predictable in respiratory than metabolic acidosis |
| Na⁺, Cl⁻ | Usually normal | Not directly affected by respiratory acidosis |
| HCO₃⁻ (venous) | ↑ (matches ABG) | Compensation marker; can screen for chronic respiratory acidosis |
| Urea, Creatinine | May be ↑ | If CKD is contributing (metabolic acidosis component) or if shock/dehydration |
| Finding | Significance |
|---|---|
| Polycythaemia (↑Hb, ↑Hct) | Secondary to chronic hypoxaemia — the kidneys sense low O₂ → ↑EPO → ↑RBC production [7] |
| Leukocytosis | Suggests infection as precipitant (e.g., pneumonia triggering AECOPD) |
| Anaemia | May worsen tissue hypoxia; rule out as contributing factor to symptoms |
| Finding | Suggests |
|---|---|
| Hyperinflated lungs, flattened diaphragm, long narrow heart shadow, hyperlucent lung fields [7] | COPD |
| Consolidation | Pneumonia (precipitant of T2RF) |
| Bilateral infiltrates | ARDS, pulmonary oedema |
| Pneumothorax (absent lung markings) | Tension pneumothorax causing acute T2RF |
| Elevated hemidiaphragm | Phrenic nerve palsy |
| Skeletal deformity | Kyphoscoliosis |
| Widened mediastinum | Aortic pathology, mediastinal mass |
| Finding | Suggests |
|---|---|
| Post-bronchodilator FEV₁/FVC < 0.7 [7] | COPD — mandatory for diagnosis |
| ↓FVC with normal FEV₁/FVC | Restrictive pattern — NM disease, chest wall disorder, ILD |
| FVC trending (serial measurements) | Critical in GBS/MG — FVC < 20 mL/kg or declining → imminent respiratory failure, consider intubation |
| ↓MIP/MEP (maximal inspiratory/expiratory pressure) | Respiratory muscle weakness — NM cause |
| Finding | Suggests |
|---|---|
| P pulmonale (tall peaked P waves in II) | Right atrial enlargement from chronic pulmonary hypertension (cor pulmonale) [7] |
| Right ventricular hypertrophy (RVH), RV strain, RBBB | Cor pulmonale secondary to chronic COPD [7] |
| Peaked T waves, widened QRS | Hyperkalaemia (consequence of acidosis) |
| Low voltage | Hyperinflation (COPD) or pericardial effusion |
- Indicated when ≥2 cardinal symptoms (including ↑sputum purulence) or when need for mechanical ventilation/intubation [7] — these are the indications for antibiotics in AECOPD
- Helps identify bacterial precipitant: H. influenzae, S. pneumoniae, M. catarrhalis are the classic trio in AECOPD
- ZN stain and TB culture if TB suspected (especially in HK)
| Investigation | When to Order | Key Findings |
|---|---|---|
| Toxicology screen / serum drug levels | Suspected overdose (opioids, benzodiazepines, barbiturates) | Confirms specific agent |
| CT brain | Suspected brainstem stroke, raised ICP, encephalitis | Infarct, haemorrhage, mass, oedema |
| TFT (TSH, fT4) | Suspected hypothyroidism / myxoedema coma | ↑↑TSH, ↓↓fT4 |
| Nerve conduction studies / EMG | Suspected NM cause (GBS, MG, MND) | Demyelination pattern (GBS), decremental response (MG), denervation (MND) |
| Anti-AChR / Anti-MuSK antibodies | Suspected MG | Positive = confirms MG |
| Lumbar puncture | Suspected GBS (albuminocytological dissociation) | ↑Protein, normal WCC |
| Diaphragm ultrasound / fluoroscopy | Suspected phrenic nerve palsy | Paradoxical diaphragm movement |
| Polysomnography (sleep study) | Suspected OSA/OHS/central sleep apnoea | AHI ≥ 5 (OSA); nocturnal hypoventilation (OHS) |
| CT thorax (HRCT) | ILD, bronchiectasis, emphysema characterisation | Honeycombing (UIP), ground-glass (NSIP), emphysema distribution |
| CTPA | Suspected PE as cause of acute decompensation | Filling defect in pulmonary arteries |
| Echocardiography | Suspected cor pulmonale, cardiac cause of dyspnoea | RV dilatation, ↑PASP, TR |
| BNP / NT-proBNP | Differentiate cardiac vs respiratory cause of dyspnoea | Can be used to rule out HF, but not rule in (very sensitive, not very specific) [16] |
| Serum lactate | Assess tissue perfusion if sepsis/shock suspected | ↑ lactate → poor tissue perfusion [17] |
| Investigation | Timing | Purpose |
|---|---|---|
| Repeat ABG | 30–60 min after initiating O₂ therapy; 1–2 hours after starting NIV [7] | Assess response; decide whether to escalate (intubation) |
| Continuous pulse oximetry | Ongoing | Target SpO₂ 88–92% in COPD to avoid O₂-induced hypercapnia |
| Serial FVC | Every 4–6h in GBS/MG | FVC < 15–20 mL/kg or falling > 30% → intubation |
| Serum K⁺ | Every 6h with bronchodilator use | SABA can cause hypokalaemia [7] |
High Yield Summary — Diagnosis of Respiratory Acidosis
- ABG is the only way to diagnose respiratory acidosis — pH < 7.35 with pCO₂ > 6 kPa (45 mmHg).
- 3-step GC lecture approach: pH → pCO₂ → HCO₃⁻ (metabolic status / compensation / chronicity).
- 1-2-3-4 rule: Acute resp acidosis +1; acute resp alkalosis −2; chronic resp acidosis +3; chronic resp alkalosis −4 (mmol/L HCO₃⁻ change per 10 mmHg pCO₂ change).
- A-a gradient distinguishes extrapulmonary (normal) from intrapulmonary (widened) causes.
- Mixed disorders are identified when measured HCO₃⁻ deviates from expected compensation.
- CXR + spirometry are essential ancillary tests for intrapulmonary causes (COPD, asthma).
- Repeat ABG at 30–60 min after starting O₂ or NIV to assess response.
Active Recall - Diagnosis of Respiratory Acidosis
[1] Lecture slides: GC 023. A cyanotic, dyspneic elderly man_respiratory failure.pdf (Slide: pH, pCO₂, HCO₃⁻ interpretation steps) [2] Senior notes: Ryan Ho Urogenital.pdf (Section 2.4 Disorders of Acid-base Balance, pp. 34–35) [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (pp. 3–4) [7] Senior notes: Maksim Medicine Notes.pdf (Respiratory medicine, pp. 300–302) [12] Senior notes: Maksim Medicine Notes.pdf (Nephrology acid-base, p. 213) [13] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Compensation table, pp. 85–87) [14] Senior notes: Learning_Points_All_Lectures.txt (Respiratory Medicine learning points) [15] Lecture slides: Data Interpretation - Respiratory - by Dr DCL Lam (Rotation 2).ppt.pdf (p. 35); Respiratory- Introduction to Resp investigations.pdf (p. 35) [16] Senior notes: Block A - Introduction to CVS investigations (including ECG).pdf (NT-proBNP discussion, p. 3) [17] Senior notes: Ryan Ho Critical Care.pdf (Approach to shock investigations, p. 17)
The management of respiratory acidosis is fundamentally about restoring the balance of the Henderson-Hasselbalch equation: pH ∝ HCO₃⁻ / pCO₂. Since the primary problem is ↑pCO₂, the definitive treatment is to increase alveolar ventilation — either by treating the underlying cause that impairs ventilation, or by mechanically assisting ventilation, or both.
You are NOT treating an ABG number. You are treating the patient by:
- Securing the airway and supporting breathing (ABC)
- Treating the underlying cause (e.g., bronchodilators for COPD, naloxone for opioid OD)
- Supporting ventilation (O₂, NIV, or invasive ventilation) to buy time while the cause is addressed
- Monitoring response and escalating if needed
The Cardinal Rule of Respiratory Acidosis Management
You cannot correct respiratory acidosis by giving bicarbonate. Unlike metabolic acidosis where NaHCO₃ may have a role, in respiratory acidosis the problem is CO₂ accumulation. Giving bicarbonate would transiently raise HCO₃⁻ but would also generate more CO₂ (HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O), which the patient cannot excrete because their ventilation is already impaired. This would paradoxically worsen intracellular and CSF acidosis. The treatment is ventilation, not bicarbonate. [3]
Treatment Modalities — Detailed
Why is O₂ needed? Respiratory acidosis is almost always accompanied by hypoxaemia (from the alveolar gas equation: ↑PaCO₂ → ↓PAO₂ → ↓PaO₂). Hypoxaemia kills faster than hypercapnia, so you must correct it — but carefully.
Why "controlled"? Over-aggressive O₂ therapy will lead to suppression of hypoxic respiratory drive. Patients suffering from chronic hypercapnia have ↑HCO₃⁻ levels in blood and CSF to accommodate high pCO₂ without significant ↑ in pH. As a result, their chemoreceptor response to ↑pCO₂ will be blunted and will depend on hypoxic drive to maintain their respiratory rate. A high pO₂ is not good for these patients because this will lead to ↓RR and therefore CO₂ retention and narcosis. [18]
| Aspect | Detail |
|---|---|
| Target SpO₂ | 88–92% in known/suspected CO₂ retainers (COPD) [7][18][19] |
| Starting device | 24% Venturi mask or 1–2 L/min by nasal prongs [19] |
| Monitoring | ABG 30–60 min after initiation of O₂ therapy; modify flow rate according to PaO₂ and pH [7][19] |
| If no CO₂ retention risk | Can target SpO₂ ≥ 94% (e.g., GBS, acute NM cause with previously healthy lungs) |
Devices and FiO₂:
| Device | Flow Rate | FiO₂ | When to Use |
|---|---|---|---|
| Nasal cannula | 1–2 L/min | ~24–28% | Initial low-flow; COPD |
| Venturi mask | Variable | Accurate FiO₂ 24–50% by changing ports [18] | COPD — most precise control |
| Humidified high-flow nasal cannula | Up to 60 L/min | Set FiO₂ | Hypoxaemia refractory to low-flow; can provide some dead-space washout |
| Non-rebreather mask | 10–15 L/min | ~60–90% | Severe hypoxaemia without CO₂ retention risk |
High Yield Exam Point
NEVER give uncontrolled high-flow O₂ to a COPD patient without monitoring. Always start low and titrate. If you write "give 100% O₂" in an exam for a COPD exacerbation, you will lose marks.
2. Non-Invasive Ventilation (NIV) / BiPAP
NIV is the single most important intervention for respiratory acidosis in COPD. "BiPAP" = bi-level positive airway pressure: "bi" = two, meaning two different pressures for inspiration and expiration.
- IPAP (Inspiratory Positive Airway Pressure): augments tidal volume during inspiration → ↓work of breathing → ↑alveolar ventilation → reduces CO₂ [7]
- EPAP (Expiratory Positive Airway Pressure): maintains airway patency during expiration → prevents upper airway obstruction, counteracts intrinsic PEEP in COPD [7]
Strongest evidence (must know): [7][18][19][20]
| Indication | Evidence Level |
|---|---|
| Acute COPD exacerbation with hypercapnic acidosis (PaCO₂ > 6.0 kPa or pH < 7.35) | Strongest [20] |
| Hypercapnic respiratory failure secondary to obesity hypoventilation syndrome, chest wall deformity, or neuromuscular disease | Strongest [20] |
| Cardiogenic pulmonary oedema (CPAP more commonly used here) | Strongest [20] |
| To facilitate weaning from invasive ventilation (esp. in COPD) | Strongest [20] |
| Acute respiratory failure in immunocompromised patients | Strongest [20] |
| Post-operative hypoxaemia (except upper GI surgery) | Reasonable evidence [20] |
| Patients decided not for intubation | Reasonable evidence [20] |
Less efficacious or even harmful: [20]
| Setting | Why Less Effective |
|---|---|
| Acute severe asthma | Dynamic hyperinflation; risk of barotrauma |
| ARDS | Risk of barotrauma; may delay intubation |
| Pneumonia, esp. if purulent sputum | Cannot protect airway with copious secretions |
From the AECOPD management framework: NIPPV indications: respiratory acidosis (i.e., T2RF with pH ≤ 7.35 / PaCO₂ > 6.0 kPa); SOB with signs of respiratory fatigue (e.g., use of accessory muscles, paradoxical abdominal motion, RR > 25) [7][19]
Absolute contraindications (must intubate instead) [20]:
| Contraindication | Rationale |
|---|---|
| Lack of spontaneous breathing / cardiopulmonary arrest / gasping | NIV requires spontaneous respiratory effort to trigger |
| Anatomical or functional airway obstruction | Cannot deliver pressure past obstruction |
| Severe facial deformity / trauma / burns | Cannot fit mask |
| GI bleeding or ileus | High aspiration risk |
Relative contraindications (use with enhanced monitoring) [20]:
| Contraindication | Rationale |
|---|---|
| Haemodynamic instability | Positive pressure ↓venous return → ↓CO → worsen hypotension |
| Severe acidosis (pH < 7.15) | May need immediate intubation; NIV unlikely sufficient |
| Inability to protect airway, severely impaired consciousness (GCS < 8) | Aspiration risk |
| Inability to cooperate / agitated status | Cannot maintain mask seal |
| Excessive secretions | Cannot clear secretions with mask on |
| Recent upper airway or GI surgery | Risk of disrupting surgical site |
| Parameter | Typical Initial Setting | Target |
|---|---|---|
| EPAP | 4–5 cmH₂O [7][18] | Maintain airway patency; counteract intrinsic PEEP |
| IPAP | 8–15 cmH₂O, titrate up to 20 cmH₂O [18] | Aim TV ~6–8 mL/kg, RR ≤ 25 [7] |
| Mode | S/T (spontaneous/timed) — provides backup breaths if patient fails to trigger | Ensure minimum ventilation even if drive is ↓ |
| FiO₂ | Titrate to SpO₂ 88–92% | Avoid over-oxygenation |
| Monitoring | Check ABG at 30–60 min [7][19][20] | If improving → continue; if not → consider intubation |
Complications of NIV: [7]
- Mask-related: air leak, eye irritation, facial skin necrosis (most common complication)
- Hypotension (reduced venous return from positive pressure)
- Vomiting, aspiration (gastric distension)
- Breathing discomfort: too high IPAP or EPAP
NIV: The Critical Decision Point
Do not delay intubation and mechanical ventilation if no improvement on NIV [19]. The ABG at 30–60 min is your decision point:
- If pH improving and pCO₂ falling → continue NIV
- If no improvement or worsening → repeat at 4–6h if little improvement → consider intubation [7]
- If clinical deterioration (↓GCS, worsening fatigue, haemodynamic instability) → immediate intubation
NIV is a bridge, not a destination. Used well, it avoids intubation in ~75% of COPD exacerbations with respiratory acidosis.
When NIV fails or is contraindicated, the patient needs endotracheal intubation and invasive positive pressure ventilation (IPPV) in an ICU setting.
Indications for invasive ventilation: [7][19][20]
| Indication | Explanation |
|---|---|
| Unable to tolerate or failed NIV | NIV not improving pH/pCO₂ after 30–60 min |
| Unable to protect airway: impaired consciousness (GCS < 10), aspiration risk (UGIB, severe gastric distension) [7] | Risk of aspiration with mask ventilation |
| Severe haemodynamic instability / ventricular arrhythmia / cardiac or respiratory arrest [7] | Need definitive airway and circulatory control |
| PaCO₂ > 6.7 kPa with pH < 7.32 (laboratory criteria) [19] | Severe respiratory acidosis not responding to medical Rx |
| PaO₂ < 7.3 kPa despite O₂ supplementation [19] | Refractory hypoxaemia |
| Signs of respiratory distress: use of accessory muscles, insucking, abdominal paradox with failure of NIV [19] | Impending respiratory arrest |
Advantages of mechanical ventilation: [19]
- Improves gaseous exchange: ↑oxygenation by improving V/Q matching, ↑alveolar ventilation
- Reverse acute respiratory acidosis
- Relieve respiratory distress: ↓work of breathing, ↓respiratory muscle fatigue
Ventilation strategy in respiratory acidosis:
- Mode: typically pressure support (PS) or assist-control (AC)
- Target: gradual reduction of pCO₂ — do NOT rapidly normalise pCO₂ in chronic retainers
- Why? Because in chronic respiratory acidosis, the kidneys have retained HCO₃⁻. If you rapidly blow off CO₂, the persistent ↑HCO₃⁻ will cause a severe metabolic alkalosis ("post-hypercapnic alkalosis"), which can cause seizures, arrhythmias, and ↓cerebral perfusion
- Rule: aim for the patient's baseline pCO₂, not 40 mmHg
4. Treatment of the Underlying Cause (Cause-Specific)
This is the most important long-term management step. All ventilatory support is buying time for this.
| Target | Treatment | Key Details |
|---|---|---|
| Hypoxia | O₂ | Start with 1–2 L/min by nasal cannula or Venturi mask; target SpO₂ 88–92% [7] |
| Respiratory acidosis | NIV (NIPPV) | BiPAP: EPAP 4, IPAP 14, aim TV 6–8 mL/kg, RR ≤ 25; check ABG at 30–60 min [7] |
| Expiratory obstruction | Bronchodilators: SABA (salbutamol) ± SAMA (ipratropium) with spacer | S/E: tachycardia, hypokalaemia (check blood K Q6h) [7] |
| Airway inflammation | Corticosteroid: PO Prednisolone 40 mg for 5–10 days; discontinue after acute episode | S/E: hyperglycaemia, fluid retention, hypertension [7] |
| Underlying infection | Antibiotics | Indications (need to know!): ≥2 cardinal symptoms (including ↑sputum purulence); OR need of mechanical ventilation/intubation [7] |
| Choice: Augmentin / cefotaxime × 5–7 days — need to cover S. pneumoniae, H. influenzae, M. catarrhalis [7][18] |
Augmentin (amoxicillin-clavulanate) is sufficient and clavulanate is required to cover for β-lactamase resistance of Haemophilus influenzae and Moraxella catarrhalis [19]
| Treatment | Mechanism | Details |
|---|---|---|
| Naloxone | Competitive μ-opioid receptor antagonist | 0.4–2 mg IV, repeat Q2–3 min; short half-life (~30–90 min) — may need infusion or repeated doses as opioids may outlast naloxone |
| Flumazenil | Competitive GABA-A/benzodiazepine receptor antagonist | 0.2 mg IV, titrate; CAUTION: may precipitate seizures in chronic benzodiazepine users or mixed OD with pro-convulsants |
| Intubation | Definitive airway protection | If GCS < 8 or antidote insufficient |
| Cause | Treatment |
|---|---|
| GBS | IVIg or plasmapheresis; ICU monitoring of FVC (intubate if FVC < 15–20 mL/kg or falling > 30%); supportive NIV/IPPV |
| Myasthenic crisis | IV anticholinesterase (neostigmine), IVIg or plasmapheresis; avoid aminoglycosides and other NMJ-blocking drugs; intubation if FVC declining |
| Spinal cord injury | Stabilise spine; ICU admission; often requires long-term ventilatory support |
| Cause | Treatment |
|---|---|
| OHS | NIV (BiPAP) is first-line; weight loss (bariatric surgery if indicated); treat concurrent OSA with CPAP |
| Kyphoscoliosis | Long-term domiciliary NIV; respiratory physiotherapy |
| Hypothyroidism | IV levothyroxine (in myxoedema coma); supportive ventilation |
| Upper airway obstruction | Remove FB; treat anaphylaxis (adrenaline); surgical airway if needed |
| Problem | Management | Rationale |
|---|---|---|
| Hyperkalaemia (from acidosis) | IV calcium gluconate (cardioprotection); insulin-dextrose drip; inhaled β₂-agonist; K⁺ binders; dialysis if severe [21] | H⁺/K⁺ exchange pushes K⁺ out of cells; correcting the respiratory acidosis (by improving ventilation) will itself help resolve the hyperkalaemia |
| Arrhythmias | Correct hyperkalaemia; continuous cardiac monitoring | Acidaemia + hyperkalaemia predispose to arrhythmias |
| Post-hypercapnic metabolic alkalosis | Avoid rapid pCO₂ correction; give acetazolamide (carbonic anhydrase inhibitor → promotes HCO₃⁻ excretion); replace KCl and Cl⁻ | When pCO₂ is rapidly lowered by ventilation, the retained HCO₃⁻ (from chronic renal compensation) persists → alkalosis |
| Modality | Indication | Details |
|---|---|---|
| Long-term O₂ therapy (LTOT) | Stopped smoking + resting SaO₂ ≤ 88% / PaO₂ ≤ 7.3 kPa; OR SaO₂ ≥ 89% with complications (3Ps: peripheral oedema/cor pulmonale, P pulmonale on ECG, polycythaemia Hct > 56%) [7] | ≥ 15 h/day; target SaO₂ ≥ 90% [7] |
| Domiciliary NIV | Chronic hypercapnic COPD with recurrent exacerbations requiring NIV; OHS; NM disease; kyphoscoliosis | ↓Hospitalisations, ↑quality of life, possibly ↑survival |
| Pulmonary rehabilitation | All stable COPD patients | ↑Exercise capacity, ↓dyspnoea, ↓exacerbations |
| Smoking cessation | All smokers | Single most important modifiable risk factor |
| Surgical options for COPD | Bullectomy; lung volume reduction surgery; lung transplant (definitive treatment) [7] | Selected patients only |
| Severity | pH | Immediate Action | Ventilatory Support | Key Monitoring |
|---|---|---|---|---|
| Mild | 7.30–7.35 | Controlled O₂ + treat cause | Usually medical Rx sufficient; consider NIV if fatiguing | ABG at 1–2 h |
| Moderate | 7.25–7.30 | Controlled O₂ + treat cause + NIV | BiPAP: IPAP 8–20, EPAP 4–5 | ABG at 30–60 min |
| Severe | < 7.25 | Controlled O₂ + treat cause + NIV/intubation | NIV if cooperative and can protect airway; intubation if not | ABG at 30 min; ICU referral |
| Life-threatening | < 7.15 or arrest | Intubation + IPPV | Absolute indication for invasive ventilation | Continuous ICU monitoring |
High Yield Summary — Management of Respiratory Acidosis
- ABC first — airway, breathing, circulation. Intubate if GCS < 8 or arrest.
- Controlled O₂: target SpO₂ 88–92% in COPD; start with 1–2 L/min nasal cannula or 24% Venturi mask.
- NIV (BiPAP) is the key intervention for moderate respiratory acidosis (pH 7.25–7.35): IPAP 8–20 cmH₂O, EPAP 4–5 cmH₂O. Check ABG at 30–60 min.
- Intubation + IPPV if NIV fails, GCS < 8, haemodynamically unstable, or arrest.
- Treat the underlying cause: COPD → bronchodilators + steroids + Abx (if indicated); opioid OD → naloxone; GBS → IVIg/plasmapheresis.
- Do NOT give bicarbonate for respiratory acidosis — the treatment is ventilation.
- Do NOT rapidly normalise pCO₂ in chronic retainers — risk of post-hypercapnic metabolic alkalosis.
- Antibiotics for AECOPD: only if ≥2 cardinal symptoms including ↑purulence, or requiring mechanical ventilation. Choice: Augmentin.
- Long-term: LTOT ≥ 15 h/day if criteria met; domiciliary NIV; smoking cessation; pulmonary rehab.
Active Recall - Management of Respiratory Acidosis
[3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (pp. 7–8) [6] Senior notes: Ryan Ho Fundamentals.pdf (Section 3.2.5 Respiratory Failure, p. 230) [7] Senior notes: Maksim Medicine Notes.pdf (Respiratory medicine, pp. 284–286, 300–302) [18] Senior notes: Ryan Ho Respiratory.pdf (Section on AECOPD management, pp. 115–116; NIV section, pp. 32–33; footnote 84 on hypoxic drive) [19] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (AECOPD management, pp. 226–228; Mechanical ventilation, pp. 107–109) [20] Lecture slides: Handbook of Internal Medicine 2024.pdf (NIV section, p. 421) [21] Senior notes: Block A - Chronic Kidney Disease and its Complications.pdf (Hyperkalaemia/acidosis management, p. 28)
Complications of Respiratory Acidosis
The complications of respiratory acidosis arise from two interrelated pathological processes: (1) the direct effects of hypercapnia (↑CO₂) and acidaemia (↓pH) on organ systems, and (2) the consequences of the underlying disease process itself. Additionally, (3) complications of treatment (iatrogenic) must be considered, as they are commonly examined.
Think of it this way: CO₂ is both a vasodilator and a CNS depressant. Acidaemia impairs enzyme function, disrupts electrolyte balance, and depresses myocardial contractility. Every organ system is affected once pH falls below ~7.2.
| Complication | Pathophysiological Mechanism | Clinical Significance |
|---|---|---|
| CO₂ narcosis / hypercapnic encephalopathy | CO₂ freely crosses the blood-brain barrier (BBB) → ↑CSF [H⁺] → direct neuronal depression and cerebral vasodilation → progressive obtundation from drowsiness → confusion → altered mentation → stupor → coma [6] | The most feared acute complication. Once GCS drops below 8, the patient can no longer protect their airway → aspiration risk → vicious cycle of worsening respiratory failure. This is why GCS < 8 mandates intubation. |
| Cerebral oedema | Severe acute hypercapnia → profound cerebral vasodilation → ↑cerebral blood flow → ↑intracranial pressure → papilloedema, headache, and if extreme, herniation | Primarily seen in acute, severe respiratory acidosis. Chronic patients are partially protected because CSF bicarbonate gradually rises to buffer the CSF pH. |
| Headache | CO₂-induced cerebral vasodilation [6] | Particularly prominent on waking (nocturnal CO₂ accumulation during sleep when ventilation is even more depressed). Classic early warning sign in chronic hypoventilators (OHS, NMD). |
| Seizures | Severe acidaemia → neuronal membrane instability; also may occur with rapid correction (post-hypercapnic alkalosis — see iatrogenic complications below) | Uncommon unless pH < 7.1 |
Why is the CNS so vulnerable? The brain has no buffer reserves of its own — it relies on the choroid plexus to slowly adjust CSF HCO₃⁻. In acute respiratory acidosis, CO₂ floods the CSF (it crosses the BBB instantly), but HCO₃⁻ cannot follow quickly → the CSF pH plummets faster than blood pH. This is why neurological symptoms can seem disproportionate to the arterial pH in acute hypercapnia.
| Complication | Pathophysiological Mechanism | Clinical Significance |
|---|---|---|
| Cardiac arrhythmias | Acidaemia → ↓myocardial contractility + ↓threshold for arrhythmia; concurrent hyperkalaemia (H⁺/K⁺ shift) → peaked T waves, widened QRS, VF/VT | Both hypokalaemia and hyperkalaemia have lethal consequences [22] — in respiratory acidosis, the K⁺ shifts extracellularly, but during treatment (correction of acidosis), K⁺ shifts back intracellularly and may cause dangerous hypokalaemia if not monitored |
| ↓Myocardial contractility | Acidaemia → ↓Ca²⁺ sensitivity of troponin C, ↓Na⁺/K⁺/ATPase activity → ↓contractility [23] | Can precipitate or worsen heart failure, especially in patients with pre-existing cardiac disease |
| ↓Response to catecholamines | Severe acidaemia → ↓receptor affinity for endogenous and exogenous catecholamines [2] | This is why cardiovascular collapse in severe acidosis is so difficult to treat — vasopressors become less effective |
| Hypotension | ↓Contractility + ↓catecholamine response + CO₂-induced peripheral vasodilation | May present as shock in severe cases |
| Pulmonary hypertension and cor pulmonale (chronic) | Chronic hypoxaemia → hypoxic pulmonary vasoconstriction (Euler-Liljestrand reflex) → pulmonary arterial remodelling → ↑pulmonary vascular resistance → RV afterload ↑ → RV hypertrophy → RV failure → cor pulmonale | A major complication of chronic respiratory acidosis in COPD — manifests as the "3Ps": peripheral oedema (cor pulmonale), P pulmonale on ECG, polycythaemia (Hct > 56%) [7] |
From the GC COPD lecture: Management of complications of COPD — Cor pulmonale: diuretics, salt and fluid restriction [24]
| Complication | Mechanism | Clinical Consequence |
|---|---|---|
| Hyperkalaemia | Acidaemia → H⁺ moves intracellularly in exchange for K⁺ moving extracellularly (H⁺/K⁺ transcellular shift). For every 0.1 ↓pH, K⁺ rises ~0.1–0.5 mEq/L (less predictable in respiratory than metabolic acidosis, but still clinically significant) | ECG changes (peaked T, widened QRS); risk of fatal arrhythmia if K⁺ > 6.5 mEq/L |
| Secondary metabolic alkalosis (compensatory) | Compensatory metabolic alkalosis = kidney conserves more bicarbonate, increased HCO₃⁻ [3] | In chronic respiratory acidosis, serum HCO₃⁻ may be 30–45 mEq/L. This is physiologically appropriate but creates a vulnerability: if the respiratory acidosis is rapidly corrected (see iatrogenic complications), the retained HCO₃⁻ causes severe alkalosis. |
| Chronic bone loss / osteoporosis | Chronic acidaemia → bone is used as a buffer reservoir (calcium carbonate/phosphate is mobilised from bone to buffer H⁺) → gradual demineralisation | Contributes to osteoporosis in chronic COPD and other long-standing hypoventilation syndromes. CKD-related metabolic acidosis has a similar effect: ↑bone resorption, ↑protein catabolism [23] |
| Protein catabolism and muscle wasting | Chronic acidaemia → ↑protein degradation via ubiquitin-proteasome pathway; ↓protein synthesis | Contributes to respiratory muscle weakness → worsening ventilatory failure (a vicious cycle) |
These are complications of the diseases that cause respiratory acidosis, but they are intimately linked:
| Complication | Context | Mechanism |
|---|---|---|
| Respiratory arrest | Progression of any severe acute respiratory acidosis | Progressive respiratory muscle fatigue → CO₂ narcosis → loss of respiratory drive → apnoea → cardiac arrest if not intervened |
| Secondary polycythaemia | Chronic hypoxaemia → ↑EPO → ↑RBC production → Hct > 56% [7] | ↑Blood viscosity → ↑risk of thromboembolism (DVT, PE, stroke). This is one of the "3Ps" indicating need for LTOT. |
| Pulmonary infections | Particularly in COPD (mucociliary dysfunction, mucus stasis), NMD (weak cough, aspiration) | Predisposed by mucus accumulation and stasis airflow [25]; may precipitate acute-on-chronic respiratory acidosis |
| Pneumothorax | COPD with emphysematous bullae | Excessive air trapping → leakage from weak spots of the pleura [25]. Can cause sudden deterioration of a previously stable chronic T2RF patient. |
| Aspiration pneumonia | CO₂ narcosis → ↓GCS → loss of airway protective reflexes; NIV with gastric distension | Common in patients with ↓consciousness from severe hypercapnia or during NIV with excessive gastric inflation |
| Complication | Mechanism |
|---|---|
| Renal bicarbonate retention (chronic compensation, not truly a "complication" but clinically relevant) | Kidneys ↑H⁺ excretion and ↑HCO₃⁻ reabsorption over 3–5 days to compensate for chronic respiratory acidosis [2][3]. This is physiologically appropriate but creates the substrate for post-hypercapnic alkalosis if pCO₂ is corrected too rapidly. |
| AKI (in acute severe cases) | Severe acidaemia → ↓cardiac output → ↓renal perfusion → pre-renal AKI; also ↓renal vascular response to catecholamines |
6. Iatrogenic Complications (Complications of Treatment)
These are extremely high-yield for exams because they test understanding of pathophysiology:
| Aspect | Detail |
|---|---|
| When | When chronic respiratory acidosis is corrected too rapidly (e.g., aggressive mechanical ventilation targeting pCO₂ of 40 mmHg in a patient whose baseline was 65 mmHg) |
| Why | In chronic respiratory acidosis, the kidneys have retained large amounts of HCO₃⁻ (compensation). If you rapidly blow off CO₂, the pCO₂ drops but the HCO₃⁻ remains high (renal excretion of HCO₃⁻ takes days to adjust). pH = HCO₃⁻ / pCO₂ → the ratio swings dramatically upward → severe metabolic alkalosis |
| Consequences | Severe alkalosis (pH > 7.55) → seizures, arrhythmias, ↓cerebral perfusion (alkalosis causes cerebral vasoconstriction — the opposite of hypercapnia), ↓ionised Ca²⁺ (alkalosis ↑albumin binding of Ca²⁺ → tetany), hypokalaemia (H⁺/K⁺ shift reversal) |
| Prevention | Target the patient's chronic baseline pCO₂, NOT 40 mmHg; slow gradual reduction |
Overcorrection may increase CO₂ production, which can aggravate respiratory acidosis in a poorly ventilated patient. Watch out for hypercapnia which may cause paradoxical increase in acidaemia after NaHCO₃ therapy. [26]
| Aspect | Detail |
|---|---|
| When | Excessive O₂ therapy in chronic CO₂ retainers (COPD) |
| Why | Three mechanisms: (1) Suppression of hypoxic respiratory drive [18]; (2) Haldane effect (O₂ displaces CO₂ from Hb → ↑dissolved CO₂); (3) Release of hypoxic pulmonary vasoconstriction → ↑perfusion to poorly ventilated units → ↑dead space |
| Consequence | Worsening CO₂ retention → deepening CO₂ narcosis → respiratory arrest |
| Prevention | Target SpO₂ 88–92% in COPD; start with 1–2 L/min by nasal cannula; check ABG at 30–60 min [7][24] |
| Complication | Mechanism |
|---|---|
| Facial skin necrosis (most common) [7] | Prolonged pressure from tight-fitting mask |
| Gastric distension → vomiting → aspiration [7] | Positive pressure drives air into oesophagus/stomach |
| Hypotension [7] | Positive intrathoracic pressure ↓venous return → ↓preload → ↓cardiac output |
| Eye irritation, conjunctivitis [7] | Air leak from poorly fitting nasal bridge of mask |
| Pneumothorax (barotrauma) | Excessive positive pressure → alveolar rupture (especially in emphysema) |
| Complication | Mechanism |
|---|---|
| Ventilator-associated pneumonia (VAP) | Biofilm on ET tube + ↓mucociliary clearance + micro-aspiration around cuff |
| Barotrauma / volutrauma | Excessive airway pressures or tidal volumes → pneumothorax, pneumomediastinum |
| Ventilator-induced diaphragmatic dysfunction (VIDD) | Prolonged controlled ventilation → diaphragmatic muscle atrophy → difficulty weaning |
| Critical illness myopathy / polyneuropathy | Prolonged ICU stay, steroids, NMBAs → generalised weakness → prolonged ventilator dependence |
Lethal pH levels: < 7.1 or > 7.7 → all cellular function becomes disrupted [3]
The common final pathways to death in severe respiratory acidosis are:
- Cardiac arrest — from severe acidaemia + hyperkalaemia → VF/asystole
- Respiratory arrest — from CO₂ narcosis → complete loss of respiratory drive
- Multiorgan failure — from prolonged hypoxaemia + acidaemia → ↓tissue perfusion and cellular dysfunction
- Complications of the underlying disease — e.g., sepsis from pneumonia that precipitated the exacerbation
High Yield Summary — Complications of Respiratory Acidosis
Acute complications (directly from hypercapnia + acidaemia):
- CNS: CO₂ narcosis, cerebral oedema, headache, confusion → coma
- CVS: arrhythmias, ↓contractility, ↓catecholamine response, hypotension
- Electrolyte: hyperkalaemia (H⁺/K⁺ shift)
- Respiratory arrest if not treated
Chronic complications (from long-standing respiratory acidosis, typically COPD):
- Cor pulmonale (the "3Ps": peripheral oedema, P pulmonale, polycythaemia)
- Secondary polycythaemia → ↑thromboembolism risk
- Bone loss / osteoporosis
- Muscle wasting
Iatrogenic complications (from treatment — very exam-relevant):
- Post-hypercapnic metabolic alkalosis (rapid correction of chronic CO₂)
- O₂-induced hypercapnia (excessive O₂ in COPD)
- NIV complications: skin necrosis, gastric distension/aspiration, hypotension
- Ventilator complications: VAP, barotrauma, VIDD
Active Recall - Complications of Respiratory Acidosis
References
[2] Senior notes: Ryan Ho Urogenital.pdf (Section 2.4 Disorders of Acid-base Balance, pp. 34, 39) [3] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (pp. 2–3, 7, 9) [6] Senior notes: Ryan Ho Fundamentals.pdf / Ryan Ho Respiratory.pdf (Section 2.5 / 3.2.5 Respiratory Failure, pp. 29–30 / 230) [7] Senior notes: Maksim Medicine Notes.pdf (Respiratory medicine, pp. 284–286, 300–302) [18] Senior notes: Ryan Ho Respiratory.pdf (Footnote 84 on hypoxic drive, p. 116) [22] Senior notes: Block A - Introduction to Renal Investigations (RFT, urine tests and US kidneys).pdf (p. 6 — K+ lethal consequences) [23] Senior notes: Ryan Ho Urogenital.pdf (Section on CKD metabolic acidosis consequences, p. 106) [24] Lecture slides: GC 041. Cough in a chronic smoker_COPD; smoking cessation.pdf (p. 25 — COPD complications management) [25] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Asthma complications — pneumothorax, infections, T2RF, p. 213) [26] Lecture slides: Handbook of Internal Medicine 2024.pdf (p. 302 — NaHCO₃ risks and paradoxical acidosis warning)
High Yield Summary
Definition: Respiratory acidosis = primary ↑pCO₂ (> 45 mmHg / > 6 kPa) due to alveolar hypoventilation, resulting in ↓pH.
Most common cause in HK: COPD acute exacerbation.
Core pathophysiology: Any failure along the ventilatory chain (CNS drive → spinal cord → NMJ → respiratory muscles → chest wall → airways → alveoli) → ↓alveolar ventilation → CO₂ retention.
Compensation: Kidneys retain HCO₃⁻ — acute: +1 mEq/L per 10 mmHg rise in pCO₂; chronic: +3.5 mEq/L per 10 mmHg.
Key clinical features of hypercapnia: headache, flushing, flapping tremor, papilloedema, drowsiness → CO₂ narcosis/coma.
Danger sign in acute asthma: "normalising" pCO₂ = impending respiratory arrest.
O₂ therapy in COPD: Target SpO₂ 88–92% to avoid abolishing hypoxic drive.
Type 2 RF causes (mnemonic: D-N-T-G = Drive, Nerves/NMJ, Thoracic cage, Global lung hypoventilation): sedative OD / CNS depression / hypothyroidism; GBS / MG / spinal cord; kyphoscoliosis / flail chest / OHS; COPD / severe asthma / upper airway obstruction.
High Yield Summary — Diagnosis of Respiratory Acidosis
- ABG is the only way to diagnose respiratory acidosis — pH < 7.35 with pCO₂ > 6 kPa (45 mmHg).
- 3-step GC lecture approach: pH → pCO₂ → HCO₃⁻ (metabolic status / compensation / chronicity).
- 1-2-3-4 rule: Acute resp acidosis +1; acute resp alkalosis −2; chronic resp acidosis +3; chronic resp alkalosis −4 (mmol/L HCO₃⁻ change per 10 mmHg pCO₂ change).
- A-a gradient distinguishes extrapulmonary (normal) from intrapulmonary (widened) causes.
- Mixed disorders are identified when measured HCO₃⁻ deviates from expected compensation.
- CXR + spirometry are essential ancillary tests for intrapulmonary causes (COPD, asthma).
- Repeat ABG at 30–60 min after starting O₂ or NIV to assess response.
High Yield Summary — Management of Respiratory Acidosis
- ABC first — airway, breathing, circulation. Intubate if GCS < 8 or arrest.
- Controlled O₂: target SpO₂ 88–92% in COPD; start with 1–2 L/min nasal cannula or 24% Venturi mask.
- NIV (BiPAP) is the key intervention for moderate respiratory acidosis (pH 7.25–7.35): IPAP 8–20 cmH₂O, EPAP 4–5 cmH₂O. Check ABG at 30–60 min.
- Intubation + IPPV if NIV fails, GCS < 8, haemodynamically unstable, or arrest.
- Treat the underlying cause: COPD → bronchodilators + steroids + Abx (if indicated); opioid OD → naloxone; GBS → IVIg/plasmapheresis.
- Do NOT give bicarbonate for respiratory acidosis — the treatment is ventilation.
- Do NOT rapidly normalise pCO₂ in chronic retainers — risk of post-hypercapnic metabolic alkalosis.
- Antibiotics for AECOPD: only if ≥2 cardinal symptoms including ↑purulence, or requiring mechanical ventilation. Choice: Augmentin.
- Long-term: LTOT ≥ 15 h/day if criteria met; domiciliary NIV; smoking cessation; pulmonary rehab.
High Yield Summary — Complications of Respiratory Acidosis
Acute complications (directly from hypercapnia + acidaemia):
- CNS: CO₂ narcosis, cerebral oedema, headache, confusion → coma
- CVS: arrhythmias, ↓contractility, ↓catecholamine response, hypotension
- Electrolyte: hyperkalaemia (H⁺/K⁺ shift)
- Respiratory arrest if not treated
Chronic complications (from long-standing respiratory acidosis, typically COPD):
- Cor pulmonale (the "3Ps": peripheral oedema, P pulmonale, polycythaemia)
- Secondary polycythaemia → ↑thromboembolism risk
- Bone loss / osteoporosis
- Muscle wasting
Iatrogenic complications (from treatment — very exam-relevant):
- Post-hypercapnic metabolic alkalosis (rapid correction of chronic CO₂)
- O₂-induced hypercapnia (excessive O₂ in COPD)
- NIV complications: skin necrosis, gastric distension/aspiration, hypotension
- Ventilator complications: VAP, barotrauma, VIDD
Metabolic Alkalosis
Metabolic alkalosis is a primary acid-base disorder characterized by an elevated serum bicarbonate concentration and increased arterial pH, often resulting from excessive loss of hydrogen ions or gain of bicarbonate.
Respiratory Alkalosis
Respiratory alkalosis is a condition of elevated blood pH resulting from excessive alveolar ventilation that leads to a decrease in arterial carbon dioxide (PaCO₂) below normal levels.