Winter's formula for expected PaCO2 in a metabolic acidosis with bicarbonate 12 mEq/L is approximately:

• A. 40 mmHg
• B. 34 mmHg
• C. 26 mmHg (1.5 × 12 + 8)
• D. 20 mmHg

Answer: (C)

In metabolic acidosis with low bicarbonate, the lungs compensate by blowing off CO2. Winter's formula gives the expected PaCO2 as 1.5 × [HCO3-] + 8, with a typical range of ±2. For an HCO3- of 12 mEq/L, that’s 1.5 × 12 = 18, plus 8 = 26 mmHg (roughly 24–28 mmHg). So the expected PaCO2 around 26 mmHg fits best, reflecting appropriate respiratory compensation. If the measured PaCO2 were significantly higher, there would be an additional respiratory acidosis; if it were significantly lower, a concurrent respiratory alkalosis could be present.