A patient with severe vomiting develops ABG: pH 7.52, HCO3- 40 mEq/L, PaCO2 48 mmHg. The expected respiratory compensation for this degree of metabolic alkalosis is:

• A PaCO2 should rise by 0.7 mmHg per mEq/L rise in HCO3-; expected PaCO2 = 40 + 0.7×16 = 51.2 mmHg ✓
• B PaCO2 should fall by 1 mmHg per mEq/L rise in HCO3-
• C PaCO2 should not change as the lung cannot compensate for alkalosis
• D PaCO2 rises by 1.5 mEq/L per mEq/L rise in HCO3-

Explanation

For metabolic alkalosis, respiratory compensation involves hypoventilation to retain CO2. The Winter's formula equivalent for metabolic alkalosis: expected PaCO2 = 0.7 × ΔHCO3- + 40 (±5). Here ΔHCO3- = 40-24 = 16 mEq/L; expected PaCO2 = 40 + (0.7 × 16) = 40 + 11.2 = 51.2 mmHg (range 46–56). The observed PaCO2 of 48 mmHg falls within the expected range, confirming appropriate respiratory compensation without a concurrent respiratory disorder. Respiratory compensation for metabolic alkalosis is often incomplete because the resulting hypoventilation causes hypoxia (which stimulates ventilation, limiting compensatory CO2 retention). Lungs do compensate for metabolic alkalosis, making Option C incorrect.

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

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