Acidosis in the Critically Ill: Interpretation and Significance

Abstract

To the clinician acidosis is the increased production and/or decreased elimination of acid by the body. To the biochemist or the physiologist acidosis is a process that increases the concentration of hydrogen ion (H⁺) in a solution (such as blood). At first glance, this distinction seems minor, but its implications are enormous. The most important implication is that acidosis is not merely the addition (or the decreased elimination) of H⁺. One simple example of this is respiratory acidosis. The body “eliminates” most of its H⁺ by eliminating CO₂. Yet CO₂ does not contain H⁺. Where does the H⁺ go? The answer is that it becomes water (H⁺ + HCO ^-₃ → H₂CO₃ → CO₂ + H₂O). This answer seems satisfactory until one asks how an increase in CO₂ produces an increase in H⁺ concentration. The answer is that CO₂ is in equilibrium with HCO ^-₃ and thus an increase in CO₂ produces an increase in H⁺ and HCO₃” (i.e., the reaction is reversible). Here we see that the change in CO₂ produces a change in H⁺ concentration. The H⁺ itself comes from the dissociation of water. This chapter will focus on the other determinates of H⁺ concentration in biologic solutions and the clinical effects of changes in these determinants.