Circumventing the Henderson-Hasselbalch equation.
Prompt blood-gas analysis consisting of pH, PCO2, and PO2 is now almost universally accessible. PCO2 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 PCO2 that is most faithfully expressed by the cumbersome Henderson-Hasselbalch equation (HHE). Two methods of quickly determining metabolic status from pH and PCO2, 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 [HCO3-]/PCO2, which, when multiplied by PCO2, equals bicarbonate ion concentration.