The validity of oxygen content calculations.

Blood oxygen content calculated from haemoglobin concentration, measured haemoglobin oxygen saturation and measured oxygen tension was compared with three other methods of estimating oxygen content. These other methods were those of Van Slyke and Zander, which are direct methods, and a method using Kelman's equation to estimate the saturation from measured oxygen tension and hence content. The coefficients of correlation (corr coeff) (r) were 0.9050 (n = 22), 0.9919 (n = 24) and 0.9862 (n = 25) for the respective methods when compared with oxygen content calculated using measured saturation. The Van Slyke method proved to be imprecise in our hands. The direct measurement using the oxygen cuvette of Zander gave oxygen content values similar to those estimated from measured saturation. The oxygen content calculated from pO2 alone when compared to that derived from measured saturation had a corr coeff (r) of 0.9862 (n = 25), but the high residual standard deviation (So) of 6.939 ml/l indicates that the practice of calculating oxygen content from oxygen tension alone is imprecise. We conclude that oxygen content may be satisfactorily estimated by the Zander method when it becomes generally available, but until then the measurement of oxygen saturation is a necessary prerequisite to the estimation of blood oxygen content.