A guide for predicting arterial CO2 tension in metabolic acidosis

Abstract

This presentation examines the relation of arterial CO2 tension to the severity of acidemia in three large groups of cases of diabetic ketoacidosis, totalling 405 episodes. In particular, it evaluates the previously reported anecdotal observation that PaCO2 (in torr) in such patients is usually numerically close to the two-digit number to the right of the pH decimal point, down to a pH of 7.10-7.15. The relations between measured arterial CO2 tension levels and calculated plasma bicarbonate concentration in the three groups were very similar to those previously reported by others and us, the regression equations approximating PaCO2 = 1.5 x [HCO3-] + 8. Further, in the 262 episodes with blood pH 7.10-7.37, down to a pH of 7.10-7.15, the average PaCO2 in torr often did approximate the two-digit number to the right of the decimal point in the pH value. This relation provides a quick and easily remembered alternative guide for predicting the approximate expected PaCO2 in patients with metabolic acidosis. The basis for this apparently fortuitous relation between PaCO2 and blood pH may be the closely similar correlation between PaCO2 and 1/cH+ (i.e., 10PH), which in turn derives from the physiologically significant relation of alveolar ventilation to blood pH, and the inverse relation between PaCO2 and alveolar ventilation.