Influence of cardiac output on the correlation between mixed venous and central venous oxygen saturation

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.