Effect of changes in arterial-mixed venous oxygen content difference (C(a-v)O2) on indices of pulmonary oxygen transfer in a model ARDS lung.

Many indices are used to quantify pulmonary oxygen transfer. Indices that use only measurements from arterial blood and inspired gas assume a constant C(a-v)O2. Though variations in C(a-v)O2 are recognized, indices such as PaO2/FIO2 remain popular and are often considered the best measure of pulmonary oxygen transfer in critically ill patients. This study estimated the effect of within-subject variations in C(a-v)O2 and FIO2 on venous admixture (Qs/Qt), the calculated oxygen content difference between end-capillary and arterial blood (Cc'O2-CaO2), the alveolar-arterial oxygen tension gradient (P(A-a)O2) and PaO2/FIO2, using a validated lung model of acute respiratory distress syndrome (ARDS). All four indices showed changes with FIO2 and C(a-v)O2, although the magnitude of changes in Qs/Qt was clinically unimportant (<2%). The other three indices showed larger variations that may potentially be misleading. At an FlO2 of 0.7, PaO2 /FIO2 varied between 18 and 10 kPa and at an FIO2 of 0.9 the ratio varied between 22 and 8 kPa. These changes, which were unrelated to underlying lung pathology, are sufficiently large to result in misclassification on the gas exchange scale suggested by the American European Consensus Conference on ARDS. This study shows there is no reliable alternative to Qs/Qt to quantify pulmonary oxygen transfer in critically ill patients.