Crossover PO2, a measure of the variable effect of increased P50 on mixed venous PO2.

The effect on mixed venous Po2 of a shift to the right in the oxygen hemoglobin equilibrium curve, without a change in either arterial Po2 or oxygen uptake, depends on the difference between the arterial oxygen saturation differences and the venous oxygen saturation differences of the shifted and standard curves. For every rightward-shifted oxygen hemoglobin equilibrium curve there is a Po2, which I call the crossover Po2, at which mixed venous Po2 is the same for the standard and shifted curve. Crossover Po2 depends upon the hemoglobin concentration and the arteriovenous oxygen content difference. I demonstrate that in hypoxemic patients, especially if anemia and shock are present, the arterial Po2 may be less than the crossover Po2; thus, an increased blood P50, the Po2 at oxygen saturation of 50 per cent (pH 7.40, temperature 37 degrees C), cannot increase the mixed venous Po2.