Which of the following best represents appropriate compensation according to Winter's formula for metabolic acidosis with HCO3- 14 mEq/L?

• A. pH 7.31, PaCO2 40, HCO3- 14
• B. pH 7.29, PaCO2 25, HCO3- 14
• C. pH 7.41, PaCO2 20, HCO3- 14
• D. pH 7.31, PaCO2 29, HCO3- 14

Answer: (D)

Winter’s formula is used to judge respiratory compensation in metabolic acidosis. It says the expected PaCO2 = 1.5 × [HCO3−] + 8, with a ±2 mmHg range. For an HCO3− of 14, that’s 1.5 × 14 = 21, plus 8 equals about 29 mmHg, so the expected PaCO2 is roughly 27–31. The best match has a PaCO2 of 29, which fits that range and produces an acidemic pH consistent with metabolic acidosis with appropriate respiratory compensation. If PaCO2 were as high as 40, there’d be insufficient compensation (a concurrent respiratory acidosis). If it were 25, that’s lower than expected (overcompensation or a concurrent respiratory alkalosis). If the pH were normal or near normal with a low HCO3−, that would imply a mixed disorder. So the pairing with PaCO2 around 29 and HCO3− 14 reflects proper respiratory compensation for metabolic acidosis.