Winters’ formula is used to evaluate respiratory compensation when analyzing acid-based disorders and a metabolic acidosis is present.
PCO2 = (1.5 x HCO3-) + 8 ± 2
- Winters’ formula gives an expected value for the patients’ PCO2.
- If the patient’s actual (measured) PCO2 is the same as the expected value, then respiratory compensation is considered adequate. This may be present in a patient with chronic obstructive pulmonary disease (COPD) who has a chronically elevated CO2 and whose kidneys have compensated so that the pH remains normal.
- If PCO2 is higher than the calculated value, there is also a secondary respiratory acidosis or mixed acid-base disorder. This may be present in a patient that has both poor perfusion and hypoventilation, for example, a patient with cardiac arrest.
- If the measured PCO2 is lower than the calculated value, there is a secondary respiratory alkalosis or mixed acid base disorder. This may be present in a patient with diabetic ketoacidosis (DKA) who is hyperventilating to compensate for the metabolic acidosis associated with DKA.
Albert, M., Dell, R., & Winters, R. (1967). Quantitative Displacement of Acid-Base Equilibrium in Metabolic Acidosis. Annals of Internal Medicine, 66 (312-322). doi: 10.7326/0003-4819-66-2-312