A 25-year-old woman with anorexia nervosa and self-induced vomiting has ABG: pH 7.52, PaCO2 48 mmHg, HCO3− 38 mEq/L. Which acid-base disturbance is present and what is the mechanism?

• A Respiratory alkalosis due to hyperventilation from anxiety
• B Metabolic alkalosis with respiratory compensation, due to loss of HCl in gastric juice ✓
• C Mixed respiratory alkalosis and metabolic alkalosis
• D Metabolic acidosis compensated by respiratory alkalosis

Explanation

Self-induced vomiting removes gastric juice rich in HCl. Loss of H+ (not balanced by loss of HCO3−) causes metabolic alkalosis: primary HCO3− rise (38 mEq/L) and compensatory CO2 retention (PaCO2 48 mmHg, expected compensation: PaCO2 = 40 + 0.7 × [HCO3− − 24] = 40 + 0.7 × 14 = 49.8 — consistent with appropriate respiratory compensation). The pH 7.52 confirms alkalemia. The kidneys also conserve HCO3− and excrete HCl in 'paradoxical aciduria' due to concurrent hypochloremia and hypokalemia. This is the biochemical basis of pure metabolic alkalosis with appropriate respiratory compensation.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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