Winter's formula

"Winters' Formula", named for Dr. R.W. Winters, is a formula used to evaluate respiratory compensation when analyzing acid-base disorders and a metabolic acidosis is present. It can be given as

$P_{CO_2} = (1.5 \times HCO_3^-) + 8 \pm 2$,

where HCO3⁻ is given in units of mEq/L and pCO₂ will be in units of mmHg.

Winters' formula gives an expected value for the patient's P_CO2; the patient's actual (measured) P_CO2is then compared to this.
If the two values correspond, respiratory compensation is considered to be adequate.
If the measured P_CO2 is higher than the calculated value, there is also a primary respiratory acidosis.
If the measured P_CO2 is lower than the calculated value, there is also a primary respiratory alkalosis.

Alkalosis

Note that Winter's formula pertains to settings of metabolic acidosis.

To calculate the expected pCO2 in the setting of metabolic alkalosis, the following equations are used:

• pCO2 = 0.7 [HCO3] + 20 mmHg +/- 5
• pCO2 = 0.7 [HCO3] + 21 mmHg