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

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Abstract
Background: Oximetry catheters immediately reflect changes in mixed venous oxygen saturation (Sv¯O2). We have used the Baxter 2‐SAT system to register changes in Sv¯O2 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 Sv¯O2 during the mobilization procedures. The agreement between methods was explored by a Bland‐Altman plot. The influence of haemoglobin (Hgb), pH, cardiac output (CO), posture, catheter identity and catheter calibration on agreement was assessed through analysis of covariance. Results: Data included a substantial number of low Sv¯O2 values, 95 paired means of Sv¯O2≤50% and 37 paired means ≤40%. Mean oxygen saturation difference between catheter and haemoximeter readings was −1.6±5.7% (SD). Agreement between the methods depended upon the level of Sv¯O2. At Sv¯O2 of 65%, the two methods were virtually identical. Below 65%, the catheters increasingly underestimated the corresponding haemoximetric values by 1.5% for every 10% reduction in Sv¯O2. Agreement was to some 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 Sv¯O2 values. Careful interpretation of the absolute values resulting from catheter measurements is recommended, especially when Sv¯O2 readings are low.