CLINICAL-APPLICATIONS OF QUANTITATIVE ACID-BASE CHEMISTRY

Stewart used physicochemical principles of aqueous solutions to develo p an understanding of variables that control hydrogen ion concentratio n (H+) in body fluids. He proposed that H+ concentration in body fluid s was determined by PCO2, strong ion difference (SID = sum of strong p ositive ion concentrations minus the sum of the strong anion concentra tions) and the total concentration of nonvolatile weak acid (A(tot)) u nder normal circumstances. Albumin is the major weak acid in plasma an d represents the majority of A(tot). These 3 variables were defined as independent variables, which determined the values of all other relev ant variables (dependent) in plasma, including H+. The major strong io ns in plasma are sodium and chloride. The difference between Na+ and C l- may be used as an estimation of SID. A decrease in SID below normal results in acidosis alkalosis (decrease in H+). Unidentified strong a nions such as lactate will decrease the SID, if present. Equations dev eloped by Fencl allow Stewart's work to be easily applied clinically f or evaluating the metabolic (nonrespiratory) contribution to acid-base balance. This approach separates the net metabolic abnormality into c omponents, and allows one to easily detect mixed metabolic acid-base a bnormalities. The Fencl approach provides insight into the nature and severity of the disturbances that exist in the patient. Sodium, chlori de, protein, and unidentified anion derangements may contribute to the observed metabolic acid-base imbalance. (C) 1995 by the American Coll ege of Veterinary internal Medicine.