Bench-to-bedside review: a brief history of clinical acid-base

Abstract

The history of assessing the acid-base equilibrium and associated disorders is intertwined with the evolution of the definition of an acid. In the 1950s clinical chemists combined the Henderson-Hasselbalch equation and the Bronsted-Lowry definition of an acid to produce the current bicarbonate ion-centred approach to metabolic acid-base disorders. Stewart repackaged pre-1950 ideas of acid-base in the late 1970s, including the Van Slyke definition of an acid. Stewart also used laws of physical chemistry to produce a new acid-base approach. This approach, using the strong ion difference (particularly the sodium chloride difference) and the concentration of weak acids (particularly albumin), pushes bicarbonate into a minor role as an acid-base indicator rather than as an important mechanism. The Stewart approach may offer new insights into acid-base disorders and therapies.

Figure 1

The Henderson–Hasselbalch equation. pH, plasma pH; pKa, negative log to base 10 of the apparent, overall dissociation constant of carbonic acid; [HCO₃^-], plasma bicarbonate concentration; α, solubility of carbon dioxide in blood at 37°C; pCO₂, partial pressure of carbon dioxide in blood.

Figure 2

The isohydric principal expressed in (a) the law of mass action form and (b) the Henderson–Hasselbalch form. Because all weak acids in a solution are in equilibrium with a single pool of hydrogen ions, the ratio of any of the conjugate anion and its undissociated acid will be able to describe the pH.

Figure 3

Important factors in the control of hydrogen and bicarbonate ions using the Stewart approach.