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

Objective - To calculate values for the total concentration of nonvolatile weak acids (A_tot) and the effective dissociation constant for nonvolatile weak acids (K_a) 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 A_tot and K_a. Four methods for quantifying the unmeasured strong anion concentration in plasma (anion gap, the Fencl base excess method [BE_ua], 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, A_tot was 25 mM/L, equivalent to 7.6 times the albumin concentration or 3.6 times the total protein concentration; K_a was 0.87 × 10^-7, equivalent to pK_a of 7.06. The A_tot and K_a 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 BE_ua (R², 0.56), [XA] (R², 0.50), and the anion gap (R², 0.41). Conclusions and Clinical Relevance - Calculated values for A_tot, K_a, and the SIG equation should facilitate application of the strong ion approach to acid-base disturbances in cattle.