Respiratory acidosis
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Definition
Respiratory acidosis occurs when CO₂ retention leads to an increase in arterial PaCO₂, causing a decrease in blood pH (acidaemia). It results from hypoventilation with inadequate alveolar ventilation to remove CO₂.Pathophysiology
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻↑ CO₂ drives the reaction to the right leading to an increase in H⁺ which lowers pH.
The kidneys respond by retaining bicarbonate (HCO₃⁻) to compensate but this takes hours to days.
Causes
- Acute respiratory acidosis (develops quickly, minimal renal compensation)
- Airway obstruction (foreign body, severe asthma, COPD exacerbation)
- Respiratory depression (drugs such as benzodiazepines and opiates, head injury)
- Neuromuscular weakness (Guillain–Barré, myasthenia gravis)
- Chest wall restriction (severe kyphoscoliosis)
- Chronic respiratory acidosis (gradual onset, renal compensation present)
- COPD
- Obesity hypoventilation syndrome
- Slowly progressive neuromuscular disorders
ABG Findings
| Parameter | Acute | Chronic |
|---|---|---|
| pH | ↓ | Near-normal (partially compensated) |
| PaCO₂ | ↑ | ↑↑ |
| HCO₃⁻ | Normal (initially) | ↑ (renal compensation) |
High CO₂, low pH = Respiratory acidosis.
Symptoms
- Headache, confusion, drowsiness (Increased CO₂ leads to cerebral vasodilation)
- Dyspnoea
- Tremor, flapping tremor (CO₂ narcosis in severe cases)
- Tachycardia, arrhythmia (due to acidaemia)
Management
- Treat the underlying cause (clear airway, reverse sedation, treat COPD exacerbation).
- Oxygen therapy (carefully in chronic CO₂ retainers)
- Non-invasive ventilation (BiPAP) if hypercapnia persists
- Intubation and mechanical ventilation if severe or worsening.
Summary
| Type | Mechanism | Compensation | Examples |
|---|---|---|---|
| Acute | Sudden CO₂ rise | Minimal renal | Asthma attack, opioid overdose |
| Chronic | Sustained CO₂ retention | Renal HCO₃⁻ retention | COPD, obesity hypoventilation |