A 22-year-old woman presents with vomiting for 3 days. ABG: pH 7.52, PaCO2 52 mmHg, HCO3⁻ 42 mEq/L. Serum electrolytes: Na⁺ 138, K⁺ 2.8, Cl⁻ 88 mEq/L. What is the acid-base disorder and is the respiratory compensation appropriate?

• A Metabolic alkalosis with appropriate respiratory compensation; expected PaCO2 = 40 + 0.7×(42−24) = 52.6 mmHg ✓
• B Mixed metabolic alkalosis and respiratory alkalosis; PaCO2 should be lower
• C Metabolic alkalosis with inadequate respiratory compensation; PaCO2 should be 48 mmHg
• D Respiratory acidosis with metabolic alkalosis; primary driver is the elevated PaCO2

Explanation

This is metabolic alkalosis (elevated pH, elevated HCO3⁻). The expected respiratory compensation is: PaCO2 = 40 + 0.7 × (ΔHCO3⁻) = 40 + 0.7 × (42 − 24) = 40 + 12.6 = 52.6 mmHg. The measured PaCO2 of 52 mmHg matches this prediction, confirming pure metabolic alkalosis with appropriate hypoventilation. The hypokalemia (2.8) and hypochloremia (88) are consistent with vomiting-induced alkalosis (loss of HCl). Option B would require PaCO2 lower than predicted; option C gives an incorrect calculation; option D is wrong because high pH rules out primary respiratory acidosis.

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

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