The TANH-equation modified for the hemoglobin, oxygen, and carbon monoxide equilibrium

The model of the hemoglobin-oxygen equilibrium represented by the TANH-equation is incorporated in the Oxygen Status Algorithm, a computer program for calculating and displaying the oxygen status and the acid-base status of the blood. In the presence of carbon monoxide it is necessary to take the Haldane equation into account. We here describe the necessary equations and methods for iterative solutions. The validity of the Haldane equation has previously been demonstrated by Zwart et al. (J Appl Physiol 1984; 57: 14-20). We have performed a few experiments to confirm this. Like Zwart et al. we find a small deviation from the theory, but in the opposite direction, i.e. the measured p50 values are slightly higher than predicted. We conclude that the Haldane equation adequately accounts for the carbon monoxide effect up to 30% carboxy-hemoglobin, but further studies are needed to confirm or exclude any minor deviation from the Haldane relationship which may be significant at higher carboxy-hemoglobin fractions.