Calculation of variables describing plasma nonvolatile weak acids for use in the strong ion approach to acid-base balance in cattle

Abstract

Objective - To calculate values for the total concentration of nonvolatile weak acids (Atot) and the effective dissociation constant for nonvolatile weak acids (Ka) of bovine plasma and to determine the best method for quantifying the unmeasured strong anion concentration in bovine plasma. Sample Population - Data sets from published and experimental studies. Procedure - The simplified strong ion model was applied to published and experimentally determined values for pH, Pco2, and strong ion difference (SID+). Nonlinear regression was used to solve simultaneously for Atot and Ka. Four methods for quantifying the unmeasured strong anion concentration in plasma (anion gap, the Fencl base excess method [BEua], the Figge unmeasured anion method [XA], and the strong ion gap [SIG]) were compared in 35 cattle with abomasal volvulus. Results - For bovine plasma at 37 C, Atot was 25 mM/L, equivalent to 7.6 times the albumin concentration or 3.6 times the total protein concentration; Ka was 0.87 × 10-7, equivalent to pKa of 7.06. The Atot and Ka values were validated, using data sets from in vivo and in vitro studies. Plasma unmeasured strong anion concentration was most accurately predicted in critically ill cattle by calculating SIG from serum albumin (R2, 0.66) or total protein concentration (R2, 0.60), compared with BEua (R2, 0.56), [XA] (R2, 0.50), and the anion gap (R2, 0.41). Conclusions and Clinical Relevance - Calculated values for Atot, Ka, and the SIG equation should facilitate application of the strong ion approach to acid-base disturbances in cattle.