A marathon runner collapses after a race. ABG shows: pH 7.20, PaCO2 20 mmHg, HCO3 7.5 mEq/L. Serum Na 140, Cl 102 mEq/L. What is the primary disorder and is respiratory compensation adequate?

• A Primary metabolic acidosis; respiratory compensation is INADEQUATE (expected PaCO2 = 13.5 mmHg)
• B Primary respiratory alkalosis with appropriate metabolic compensation
• C Mixed metabolic acidosis and respiratory alkalosis
• D Primary metabolic acidosis; respiratory compensation is ADEQUATE (expected PaCO2 = 20 mmHg) ✓

Explanation

Primary metabolic acidosis (low pH, low HCO3). Winters' formula for expected PaCO2 = (1.5 × HCO3) + 8 ± 2 = (1.5 × 7.5) + 8 = 11.25 + 8 = 19.25 ± 2 mmHg (range 17.25–21.25). Measured PaCO2 is 20 mmHg, which falls within the expected range — compensation is ADEQUATE, indicating a pure metabolic acidosis. Anion gap = 140 – (102 + 7.5) = 30.5, markedly elevated (lactic acidosis from exercise). Options A and D erroneously suggest inadequate compensation or a mixed disorder when the PaCO2 is within expected range.

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

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