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Comparison of three strong ion models used for quantifying the acid-base status of human plasma with special emphasis on the plasma weak acids

Department of Intensive Care, Nambour Hospital, Nambour, Queensland, Australia

Submitted 22 November 2004 ; accepted in final form 6 January 2005

Currently, three strong ion models exist for the determination of plasma pH. Mathematically, they vary in their treatment of weak acids, and this study was designed to determine whether any significant differences exist in the simulated performance of these models. The models were subjected to a "metabolic" stress either in the form of variable strong ion difference and fixed weak acid effect, or vice versa, and compared over the range 25 PCO₂ 135 Torr. The predictive equations for each model were iteratively solved for pH at each PCO₂ step, and the results were plotted as a series of log(PCO₂)-pH titration curves. The results were analyzed for linearity by using ordinary least squares regression and for collinearity by using correlation. In every case, the results revealed a linear relationship between log(PCO₂) and pH over the range 6.8 pH 7.8, and no significant difference between the curve predictions under metabolic stress. The curves were statistically collinear. Ultimately, their clinical utility will be determined both by acceptance of the strong ion framework for describing acid-base physiology and by the ease of measurement of the independent model parameters.

acid-base balance; strong ion difference; mathematical model; pH

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