A previously healthy man acutely retains CO2 after an opioid overdose; PaCO2 rises from 40 to 80 mmHg within minutes. His serum bicarbonate will be closest to:

• A 24 mEq/L, unchanged, since renal compensation takes days
• B 18 mEq/L, reflecting intracellular shift of bicarbonate
• C 36 mEq/L, reflecting full renal retention of bicarbonate
• D 28 mEq/L, reflecting acute buffering of added CO2 ✓

Explanation

Acute respiratory acidosis allows only chemical buffering by non-bicarbonate buffers, which adds roughly 1 mEq/L of bicarbonate per 10 mmHg rise in PaCO2, giving about 28 mEq/L here. Full renal compensation, generating approximately 4 mEq/L per 10 mmHg rise, requires 3 to 5 days of sustained hypercapnia and cannot occur within minutes. A bicarbonate of 24 would indicate absent buffering, which does not occur physiologically.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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