A 65-year-old man with stable chronic COPD has a baseline PaCO2 of 70 mmHg on room air during a quiescent phase. Assuming full renal compensation appropriate for chronic hypercapnia, his expected serum bicarbonate concentration is closest to:

• A 27 mEq/L
• B 31 mEq/L
• C 36 mEq/L ✓
• D 44 mEq/L

Explanation

In chronic respiratory acidosis, renal bicarbonate reabsorption and generation rise such that serum bicarbonate increases by approximately 3.5 to 4 mEq/L for every 10 mmHg chronic elevation in PaCO2, compared with only about 1 mEq/L acutely. For a 30 mmHg rise above 40 mmHg, the expected bicarbonate is roughly 24 plus 12, about 36 mEq/L. Values of 27 or 31 fit acute or partial compensation, and 44 suggests a superimposed metabolic alkalosis.

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

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