A ventilated ICU patient has pH 7.21, PaCO2 48 mmHg, HCO3 18 mEq/L. Using Winter's formula, what is the correct interpretation?

• A Pure metabolic acidosis with appropriate respiratory compensation
• B Mixed metabolic and respiratory alkalosis
• C Metabolic acidosis with a superimposed respiratory alkalosis
• D Metabolic acidosis with a superimposed respiratory acidosis ✓

Explanation

Winter's formula gives the expected PaCO2 as 1.5 times the bicarbonate plus 8, allowing plus or minus 2. Here that is 1.5 times 18 equals 27, plus 8 equals 35, so the expected range is 33 to 37 mmHg. The measured PaCO2 of 48 mmHg is well above this range, meaning ventilation is inadequate for the degree of metabolic acidosis and a separate respiratory acidosis coexists. Appropriate compensation would have required the PaCO2 to fall toward 35 mmHg.

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

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