In ACUTE respiratory alkalosis, for every 10 mmHg fall in PaCO2, plasma bicarbonate decreases by approximately:

• A 1 mEq/L
• B 2 mEq/L ✓
• C 4 mEq/L
• D 5 mEq/L

Explanation

Acute respiratory alkalosis compensation is purely chemical buffering by intracellular proteins and haemoglobin, lowering bicarbonate by about 2 mEq/L for each 10 mmHg reduction in PaCO2. The figure of 4 to 5 mEq/L per 10 mmHg applies to CHRONIC respiratory alkalosis where renal bicarbonate excretion adds to chemical buffering. Confusing the acute and chronic rules is the commonest error in compensation calculations.

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

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