A patient with pyloric stenosis from chronic vomiting presents with pH 7.58, HCO3− 44 mEq/L, PaCO2 50 mmHg. The expected respiratory compensation for metabolic alkalosis is:

• A PaCO2 rises 1.5 mmHg per 1 mEq/L rise in HCO3−
• B PaCO2 falls 1.2 mmHg per 1 mEq/L rise in HCO3−
• C PaCO2 rises 0.7 mmHg per 1 mEq/L rise in HCO3− ✓
• D No respiratory compensation occurs for metabolic alkalosis

Explanation

The expected respiratory compensation for metabolic alkalosis is hypoventilation (CO2 retention): PaCO2 rises approximately 0.7 mmHg per 1 mEq/L rise in HCO3− above 24. With HCO3− at 44 mEq/L (rise of 20), expected PaCO2 = 40 + (20 × 0.7) = 40 + 14 = 54 mmHg. The measured PaCO2 of 50 mmHg is close to expected, confirming appropriate compensation without a superimposed respiratory acidosis. Respiratory compensation for alkalosis is limited by the hypoxic drive preventing excessive hypoventilation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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