A patient with severe vomiting has pH 7.55, PaCO2 48 mmHg, HCO3⁻ 40 mEq/L, Na+ 138, K+ 2.8 mEq/L, Cl⁻ 88 mEq/L. The anion gap is ___. What acid-base diagnosis is MOST accurate?

• A Metabolic alkalosis with appropriate respiratory compensation; anion gap = 12
• B Metabolic alkalosis with appropriate respiratory compensation; anion gap = 10 ✓
• C Metabolic alkalosis with superimposed respiratory acidosis; anion gap = 10
• D Mixed metabolic alkalosis and metabolic acidosis; anion gap = 12

Explanation

Anion gap = Na⁺ − (Cl⁻ + HCO3⁻) = 138 − (88 + 40) = 10 mEq/L (normal). In metabolic alkalosis, expected respiratory compensation: PaCO2 = 0.7 × HCO3⁻ + 21 ± 2 = (0.7 × 40) + 21 = 49 ± 2 = 47–51 mmHg. Observed PaCO2 of 48 mmHg falls within this range, confirming appropriate hypoventilatory compensation. The picture is pure metabolic alkalosis (contraction alkalosis from vomiting) with appropriate compensation. No superimposed respiratory acidosis is present.

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

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