Assessment of the efficiency of pulmonary oxygenation. The choice of oxygenation index.

Oxygen-derived indices are often used as alternatives to determination of venous admixture (Qva/Qt) when a pulmonary arterial catheter is not in use, but the question of which more accurately indicates the efficiency of oxygenation is controversial. The theoretical relationships between six measures of oxygenation (PaO2, SaO2, P(A-a)O2, PaO2/FIO2, P(A-a)O2/PaO2, PaO2/PAO2) and venous admixture were analyzed with special reference to the influence of the concentration of inspired oxygen (FIO2). In addition, the predictability of Qva/Qt was evaluated on the basis of 100 authentic sets of arterial and mixed venous blood gas data. Of the above six indices, PaO2/FIO2 was most accurate (r = 0.88), but was far from independent of the FIO2 used. However, a direct Estimate of Venous Admixture (EVA) - based on arterial blood analysis and with C(a-v)O2 assigned a value of 50 ml.l-1 - predicted Qva/Qt still better (r = 0.95). It is concluded that when mixed venous blood gas data are lacking, EVA is the preferable measure of oxygenation.