Experimental determination of net protein charge and A_tot and K_a of nonvolatile buffers in human plasma

The mechanism for an acid-base disturbance can be determined by using the strong ion approach, which requires species-specific values for the total concentration of plasma nonvolatile buffers (A_tot) and the effective dissociation constant for plasma weak acids (K_a). The aim of this study was to experimentally determine A_tot and K_a values for human plasma by using in vitro CO₂ tonometry. Plasma P_CO2 was systematically varied from 25 to 145 Torr at 37°C, thereby altering plasma pH over the physiological range of 6.90-7.55, and plasma pH, P_CO2, and concentrations of quantitatively important strong ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, lactate) and buffer ions (total protein, albumin, phosphate) were measured. Strong ion difference was estimated, and nonlinear regression was used to calculate A_tot and K_a from the measured pH and P_CO2 and estimated strong ion difference; the A_tot and K_a values were then validated by using a published data set (Figge J, Rossing TH, and Fencl V, J Lab Clin Med 117: 453-467, 1991). The values (mean ± SD) were as follows: A_tot = 17.2 ± 3.5 mmol/l (equivalent to 0.224 mmol/g of protein or 0.378 mmol/g of albumin); K_a = 0.80 ± 0.60 × 10^-7; negative log of K_a = 7.10. Mean estimates were obtained for strong ion difference (37 meq/l) and net protein charge (13.0 meq/l). The experimentally determined values for A_tot, K_a, and net protein charge should facilitate the diagnosis and treatment of acid-base disturbances in critically ill humans.