Mixed venous oxygen saturation during mobilization after cardiac surgery: are reflectance oximetry catheters reliable?

Background: Oximetry catheters immediately reflect changes in mixed venous oxygen saturation (S(v) over bar O-2). We have used the Baxter 2-SAT system to register changes in S(v) over bar O-2, during early mobilizations after cardiac surgery. To assess catheter reliability, readings were compared to blood gases. Methods: A total of 352 paired catheter and bench haemoximetry measurements were obtained at the expected highest and lowest levels of S(v) over bar O -2, during the mobilization procedures. The agreement between methods was e xplored by a Bland-Altman plot. The influence of haemoglobin (Hgb), pH, car diac output (CO), posture, catheter identity and catheter calibration on ag reement was assessed through analysis of covariance. Results: Data included a substantial number of low S(v) over bar O-2 values , 95 paired means of S(v) over bar O-2 less than or equal to 50% and 37 pai red means less than or equal to 40%. Mean oxygen saturation difference betw een catheter and haemoximeter readings was -1.6 +/- 5.7% (SD). Agreement be tween the methods depended upon the level of S(v) over bar O-2. At S(v) ove r bar O-2 of 65%, the two methods were virtually identical. Below 65%, the catheters increasingly underestimated the corresponding haemoximetric value s by 1.5% for every 10% reduction in S(v) over bar O-2 Agreement was to som e degree dependent on individual calibrations and catheter identity, but to a lesser extent on Hgb, CO and posture. Conclusion: The two methods are interchangeable for most clinical purposes. Catheter readings are, however, substantially lower than the corresponding haemoximetric measurements at low S(v) over bar O-2 values. Careful interp retation of the absolute values resulting from catheter measurements is rec ommended, especially when S(v) over bar O-2 readings are low.