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1 Ontario Veterinary Studies, Clinical Studies, University of Guelph, Guelph, Ontario, Canada
2 Department of Veterinary Clinical Medicine, University of Illinois, Urbana, Illinois, USA
* To whom correspondence should be addressed. E-mail: hstaempf{at}uoguelph.ca.
The mechanism for an acid-base disturbance can be determined using the strong ion approach, which requires species-specific values for the total concentration of plasma non-volatile buffers (Atot) and the effective dissociation constant for plasma weak acids (Ka). The aim of this study was to experimentally determine Atot and Ka values for human plasma using in vitro CO2 tonometry. Plasma Pco2 was systematically varied from 25 to 145 mmHg at 37°C, thereby altering plasma pH over the physiologic range of 6.90 to 7.55, and plasma pH, Pco2, 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 Atot and Ka from the measured pH and Pco2 and estimated strong ion difference; the Atot and Ka values were then validated using a published data set (J Lab Clin Med 117: 453-467, 1991). The values (mean±SD) were: Atot=(17.2±3.5) mmol/l (equivalent to 0.224 mmol/g of protein or 0.378 mmol/g of albumin); Ka=(0.80±0.60) x 10-7; pKa=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 Atot, Ka, and net protein charge should facilitate the diagnosis and treatment of acid-base disturbances in critically ill humans.
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