Total weak acid concentration and effective dissociation constant of nonvolatile buffers in human plasma

The strong ion approach provides a quantitative physicochemical method for describing the mechanism for an acid-base disturbance. The approach require s species-specific values for the total concentration of plasma nonvolatile buffers (A(tot)) and the effective dissociation constant for plasma nonvol atile buffers (K-a), but these values have not been determined for human pl asma. Accordingly, the purpose of this study was to calculate accurate A(to t) and K-a values using data obtained from in vitro strong ion titration an d CO2 tonometry. The calculated values for A(tot) (24.1 mmol/l) and K-a (1. 05 x 10(-7)) were significantly (P<0.05) different from the experimentally determined values for horse plasma and differed from the empirically assume d values for human plasma (A(tot) = 19.0 meq/l and K-a = 3.0 x 10(-7)). The derivatives of pH with respect to the three independent variables [strong ion difference (SID), PCO2, and A(tot)] of the strong ion approach were cal culated as follows: dpH/dSID(+) = [1+10((pK a-pH))]2/(2.303x{SPCO(2)10((pH- pK' 1))[1+ 10((pKa-pH))](2) + A(tot)10((pKa-pH))}); dpH/dPCO(2) = S10(-pK' 1)/{2.303[A(tot)10(pH) (10(pH) + 10(pKa))(-2) -SID(+)10(-pH)]}, dpH/dA(tot) = -1/{2.303 [SPCO(2)10((pH-pK'1)) + SID(+)10((pKa-pH))]}, where S is solub ility of CO2 in plasma. The derivatives provide a useful method for calcula ting the effect of independent changes in SID+,PCO2, and A(tot) on plasma p H. The calculated values for A(tot) and K-a should facilitate application o f the strong ion approach to acid-base disturbances in humans.