Circumventing the Henderson-Hasselbalch equation

Prompt blood-gas analysis consisting of pH, PCO 2 , and PO 2 is now almost universally accessible. PCO 2 directly conveys the role of the respiratory mechanism in any acid-base disturbance. The metabolic condition, however, is hidden in a relationship between pH and PCO 2 that is most faithfully expressed by the cumbersome Henderson-Hasselbalch equation (HHE). Two methods of quickly determining metabolic status from pH and PCO 2 , without resorting to the HHE, are discussed. The first method is well known and adjusts pH to represent only metabolic state by quantitatively cancelling the influence of hyper- or hypoventilation. The second method, not previously reported, uses measured pH to estimate the ratio HCO 3 − /PCO 2 , which, when multiplied by PCO 2 , equals bicarbonate ion concentration.