Acid-base disturbances in critically ill patients

Abstract

The current understanding of acid-base physiology states that all changes in the blood, pH, as well as variations in a healthy state and during disease occur through changes in 3 variables: (a) Carbon dioxide (CO2), (b) electrolyte relative concentration, and (c) weak acid concentration. Internal milieu and acid-base disturbances generate alterations in
three areas: a) direct damage in multiple organs and systems, (b) a compensatory response to the disturbance, which may be suitable or incomplete, and (c) functional alterations in immune cells. The classically used Henderson-Hasselbalch equation for the classification of acid-base disorders in respiratory (abnormal CO2) and metabolic (abnormal
bicarbonate) and for the “anion gap” calculation, but this equilibrium  equation assumes that albumin and phosphate concentrations are very close to normal, a condition that is not necessarily certain in most critically ill patients, so the anion gap must be corrected. The Stewart’s model proposes that pH varies according to 3 independent variables: the
strong ion difference (DIF), non-volatile weak acids (Atot), and pCO2. Consequently, the merit of this approach is to merge acid-base status and electrolyte changes in a single interpretation. Therapy of acid-base disturbances in critically ill patients is focused on early detection of such conditions, using a comprehensive approach involving both the
classical theory and Stewart’s theory so that adequate management of the underlying condition may be achieved.