In CHRONIC respiratory acidosis, for every 10 mmHg rise in PaCO2, plasma bicarbonate increases by approximately:

• A 1 mEq/L
• B 2 mEq/L
• C 4 mEq/L ✓
• D 7 mEq/L

Explanation

Chronic respiratory acidosis allows full renal compensation: increased H+ secretion and ammoniagenesis raise bicarbonate by about 3.5 to 4 mEq/L per 10 mmHg rise in PaCO2. A rise of only 1 mEq/L per 10 mmHg characterises acute respiratory acidosis where only chemical buffering operates. A value of 7 mEq/L exceeds any standard rule and would suggest a coexisting metabolic alkalosis rather than simple compensation.

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

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