A 55-year-old diabetic patient has the following arterial blood gas: pH 7.22, PaCO2 28 mmHg, HCO3− 11 mEq/L. Na+ 138, Cl− 100, K+ 4.2. What is the anion gap, and is the respiratory compensation adequate?

• A Anion gap 27, respiratory compensation inadequate (expected PaCO2 = 27 mmHg) ✓
• B Anion gap 27, respiratory compensation adequate (expected PaCO2 = 28 mmHg)
• C Anion gap 16, respiratory compensation adequate
• D Anion gap 38, with concurrent respiratory alkalosis

Explanation

Anion gap = Na+ − (Cl− + HCO3−) = 138 − (100 + 11) = 27 mEq/L (high, normal 8–12). This is a high-anion-gap metabolic acidosis (likely DKA). Winter's formula for expected PaCO2 = (1.5 × HCO3−) + 8 ± 2 = (1.5 × 11) + 8 = 16.5 + 8 = 24.5 ± 2, so expected range is 22.5–26.5 mmHg. The measured PaCO2 of 28 mmHg is above this range, indicating inadequate respiratory compensation — the patient has a concurrent respiratory acidosis (or reduced ventilatory response) superimposed on the metabolic acidosis.

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

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