Venous crossover PO2. A reference point for assessing the physiological significance of alterations in oxyhaemoglobin equilibrium
A method is described for the quantitative analysis of the physiological significance of either an increase or decrease in P50. The venous crossover PO2 (Pv̄XO2) refers to that PO2 of the mixed venous blood on the standard oxyhaemoglobin equilibrium (OHE) curve at which the extraction of oxygen from the blood is the same for both the standard and the in vivo arterial and venous OHE conditions. A measured Pv̄O2 which is greater than the PV̄XO2 indicates that the in vivo OHE conditions conferred a benefit over the standard conditions inasmuch as the same volume of oxygen was delivered to the tissue at a higher venous PO2. The method of determining the venous crossover PO2 from the measured arteriovenous difference in haemoglobin saturation of oxygen is described using a mathematical model of the oxyhaemoglobin equilibrium curve. The presence of carboxyhaemoglobin is accounted for in this analysis. Clinical examples in which the method is applied are presented.