Evaluation of Expected Ventilatory Response to Metabolic Acidosis in Severely Ill Patients

Winters' formula (pCO2 =1.5*HCO3 +8) is used worldwide to predict the ventilatory response to metabolic acidosis, namely to predict the pCO2 value complying with reduction of serum bicarbonate concentration (HCO3 ). This equation was obtained half a century ago in mostly pediatric subjects. Subsequently different and inconsistent rules have been suggested. The study was done to verify the reliability of Winters' formula in severely ill patients with respect of other modern and commonly used formulas.

We applied Winters' formula and some other formulas to a dataset of arterial gas analysis from 29 severely ill malaria patients (about half of them requiring ICU or hemodialysis). The expected pCO2 value was computed by each formula and the root mean square error (RMSE) was measured. Beyond predicting the expected pCO2 value, expected range of values was also computed (as expected value ± each own error) and agreement with the best fit equation (± its error) was assessed.

In this dataset featured by metabolic acidosis of moderate degree (mean pH 7.2, mean HCO3 : 15.3 mmol/l) a strong positive linear relationship between pCO2 and HCO3 was found (R squared =0.97). The best fit linear equation was in form of pCO2 = 1.28*HCO3 +11.55. Winters' formula exhibits the lowest RMSE (1 mmHg) and shows the better agreement (Cohen's kappa=0.7) with the best fit equation Conclusions: Winters' formula can still profitably used to compute the expected pCO2 value and in turn to infer mixed (metabolic plus respiratory) acid-base disorders in severely ill patients.