Abstract

The influence of cardiac output on the correlation between central venous oxygen saturation and mixed venous oxygen saturation was assessed in 51 patients who had both a pulmonary artery catheter and separate central venous catheter in situ. Seventy-six paired samples were taken from the catheters and oxygen saturation measured immediately in a Ciba Corning 2500 Co-oximeter. Cardiac output was measured using a standard thermodilution technique. The data were separated into groups with low cardiac index (< 2.5 litre min-1 m-2; n = 20), medium cardiac index (2.5-4.0 litre min-1 m-2; n = 36) and high cardiac index (> 4.0 litre min-1 m-2; n = 20). The correlation coefficients of the three groups were: low cardiac index 0.95, medium cardiac index 0.88 and high cardiac index 0.95 (P < .001 for all three groups). All measurements were made before any x-ray and necessary repositioning of the central venous catheter. These results suggest that central venous oxygen saturation is a useful estimate of mixed venous oxygen saturation and that the influence of cardiac output on that estimate is minimal.