A 58-year-old man presents with acute respiratory distress. ABG: pH 7.18, PaCO2 80 mmHg, HCO3- 27 mEq/L. If the same degree of hypoventilation had developed over several weeks, the expected HCO3- would be approximately:

• A 22 mEq/L
• B 28 mEq/L
• C 36 mEq/L ✓
• D 44 mEq/L

Explanation

In chronic respiratory acidosis, renal retention of HCO3- adds approximately 3.5 to 4 mEq/L per 10 mmHg rise in PaCO2. For a 40 mmHg increment, HCO3- rises about 16 mEq/L above 24, giving roughly 36 to 38 mEq/L. Acute compensation adds only about 1 mEq/L per 10 mmHg, matching the measured 27 in this acute presentation and confirming the answer choice of 36 applies only to chronic adaptation.

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

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