A patient with protracted vomiting has arterial pH of 7.48, PaCO2 of 48 mmHg, and HCO3 of 36 mEq/L. Which statement about the respiratory compensation is correct?
- A Inappropriate, indicating a superimposed respiratory acidosis
- B Excessive, since PaCO2 should fall in metabolic alkalosis
- C Appropriate, since PaCO2 should rise about 0.7 mmHg for every 1 mEq/L rise in bicarbonate ✓
- D Inappropriate, indicating a superimposed respiratory alkalosis
Correct answer: C. Appropriate, since PaCO2 should rise about 0.7 mmHg for every 1 mEq/L rise in bicarbonate
Explanation
In simple metabolic alkalosis, PaCO2 rises approximately 0.7 mmHg for each 1 mEq/L increase in bicarbonate above 24. With bicarbonate of 36, the expected PaCO2 is roughly 40 plus 0.7 times 12, about 48 mmHg, matching the measured value. Hypoventilation is the physiologic compensation, limited by hypoxemia, so a rising PaCO2 here is expected rather than pathological.
Reference: Costanzo Physiology, 6th ed.
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