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

Abstract

Oximetry catheters immediately reflect changes in mixed venous oxygen saturation (SvO2). We have used the Baxter 2-SAT system to register changes in SvO2 during early mobilizations after cardiac surgery. To assess catheter reliability, readings were compared to blood gases. A total of 352 paired catheter and bench haemoximetry measurements were obtained at the expected highest and lowest levels of SvO2 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. Data included a substantial number of low SvO2 values, 95 paired means of SvO2 < or = 50% and 37 paired means < or = 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 SvO2. At SvO2 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 SvO2. Agreement was to some degree dependent on individual calibrations and catheter identity, but to a lesser extent on Hgb, CO and posture. The two methods are interchangeable for most clinical purposes. Catheter readings are, however, substantially lower than the corresponding haemoximetric measurements at low SvO2 values. Careful interpretation of the absolute values resulting from catheter measurements is recommended, especially when SvO2 readings are low.