The Henderson-Hasselbalch equation describes the relationship between pH and bicarbonate. In a patient with metabolic alkalosis, compensatory hypoventilation is limited. What is the ceiling PaCO2 in respiratory compensation for metabolic alkalosis?
- A PaCO2 can rise to 75 mmHg without limit as long as the patient remains normoxic on supplemental oxygen
- B PaCO2 rises to a maximum of approximately 55 mmHg before hypoxia-driven respiratory drive overrides the compensation ✓
- C PaCO2 rises to normalise pH completely within 12–24 hours via pure respiratory compensation
- D No respiratory compensation occurs for metabolic alkalosis as the respiratory centre does not respond to alkaline pH
Correct answer: B. PaCO2 rises to a maximum of approximately 55 mmHg before hypoxia-driven respiratory drive overrides the compensation
Explanation
Respiratory compensation for metabolic alkalosis involves hypoventilation and CO2 retention to raise PaCO2. However, the compensation is limited because as PaCO2 rises and PaO2 falls, peripheral chemoreceptors (sensing hypoxia) override the central suppression of ventilation. The expected compensation is PaCO2 = 0.7 × [HCO3⁻] + 21 (±2 mmHg), with a practical ceiling of approximately 50–55 mmHg. Compensation never fully normalises pH. Full correction requires renal HCO3⁻ excretion.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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