New aspects of acid-base balance in intensive care

For 20 years, an alternative view of the universe has been available for acid-base physiology. The Stewart approach emphasizes mathematically independent and dependent variables. With the Stewart approach bicarbonate and hydrogen ions are dependent variables that represent the effects rather than the causes of acid-base derangements. Neither bicarbonate nor pH can be regulated directly; rather they are controlled by the independent variables. In plasma there are three independent variables: the partial pressure of carbon dioxide, strong ion difference, and weak acids. In plasma, sodium and chloride are the principal strong ions, and albumin is the principal weak acid. Critically ill patients often have changes in these variables.

Recent studies have examined various aspects of the Stewart approach, including the effects of buffers and haemofiltration as well as bedside assessment of a patient's acid-base status. While sodium bicarbonate increases the strong ion difference by increasing plasma sodium, tris-hydroxymethyl aminomethane acts by increasing plasma weak base concentration and weak cations. Several studies support correcting the anion gap for changes in albumin (and even phosphate). One study raises a cautionary note on the poor agreement between central laboratory and point-of-care measurements of important biochemical variables, including plasma sodium and chloride.

The Stewart approach to acid-base physiology continues to develop as a comprehensive method to diagnose and manage acid-base disorders.