A 62-year-old man with longstanding COPD has a PaCO2 of 70 mmHg and a serum bicarbonate of 34 mEq/L on routine ABG. Comparing this with a previously well patient who acutely retains CO2 to the same PaCO2, the key difference lies in the renal contribution. The expected bicarbonate in ACUTE respiratory acidosis at a PaCO2 of 70 mmHg would be approximately:

• A 20 mEq/L
• B 27 mEq/L ✓
• C 34 mEq/L
• D 42 mEq/L

Explanation

Acute respiratory acidosis allows only chemical buffering, raising bicarbonate by roughly 1 mEq/L for every 10 mmHg rise in PaCO2. From a baseline of 24, a 30 mmHg rise gives about 27 mEq/L. Chronic retention permits renal adaptation, adding roughly 4 mEq/L per 10 mmHg, giving values near 36, close to the measured 34, confirming chronic compensation in this patient. Options A and D correspond to inappropriate compensation directions.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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