Effect of Opposite Changes in Cardiac Output and Arterial PO2 on the Relationship between Mixed Venous PO2 and Oxygen Transport

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  • BGBarry A. Gray
    Barry A. Gray
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Abstract
We examined the relationship between changes in systemic oxygen transport (SO2T) and mixed venous PO2 (P O2) in nine critically ill patients with acute respiratory failure and analyzed the effect of like and opposite changes in cardiac output (CO) and arterial PO2 (PaO2) on this relationship. Paired measurements of oxygen consumption ( O2), SO2T, and P O2 were obtained before and after changes in the level of positive end-expiratory pressure (PEEP) equal to or more than 5 cm H2O. O2 was measured with a rebreathing circuit adapted to a volume ventilator, and SO2T was calculated from thermodilution CO, PaO2, SaO2, and hemoglobin. In eight studies, CO and PaO2 changed in the same direction, and the absolute change in SO2T averaged 48 ± 38 ml/min/m2. In 12 studies, CO and PaO2 changed in opposite directions, and the absolute change in SO2T averaged 78 ± 69 ml/min/m2. When PaO2 and CO changed in the same direction, P O2 increased on the higher level of SO2T (average difference 3.0 ± 3.7 mm Hg, p < 0.05) and there was a strong positive correlation between the difference in SO2T on lower and higher levels of PEEP and the difference in P O2 (r = 0.83). When PaO2 and CO changed in opposite directions, P O2 was unchanged on the higher level of SO2T, and there was no correlation between the difference in SO2T on lower and higher levels of PEEP and the difference in P O2 (r = −0.45). O2 was not different at the lower and higher levels of SO2T in both groups, indicating that O2 was not transport-limited in these patients. We conclude that unidirectional changes in CO and PaO2 produce like changes in P O2, whereas the relationship between SO2T and P O2 is inconsistent when opposite changes in CO and PaO2 occur.

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