Central Venous Oxygen Saturation Cannot Replace Mixed Venous Saturation in Patients Undergoing Cardiac Surgery

ObjectiveIt has been argued that venous oxygen saturation from a central venous catheter (ScvO2) could be an inexpensive alternative to mixed venous oxygen saturation (SvO2). The aim was to evaluate whether ScvO2 measurements could replace SvO2 readings after cardiac surgery and to analyze factors influencing any differences found.DesignA prospective observational study.SettingA university hospital.ParticipantsTwenty patients scheduled for elective cardiac surgery.InterventionsPatients were followed postoperatively with corresponding ScvO2/SvO2 measurements.Measurements and Main ResultsThe overall bias between ScvO2 and SvO2 was 1.9. In coronary artery bypass graft (CABG) patients, the bias was 0.6 compared with 6.4 in procedures involving aortic valve replacement. In situations with peripheral saturation (SAT) <92%, the bias was 10.7 compared with 0.8 when SAT was ≥99%. In 25.5% of measurements, the ScvO2 was more than 10% different from SvO2, and in only 50% the difference was less than 5%.ConclusionsThe ScvO2 and SvO2 measurements are not interchangeable, and, especially in patients undergoing aortic valve surgery, this lack of agreement is crucial. However, the present data indicate that ScvO2 may be used in CABG patients, although it is not completely accurate in terms of absolute venous saturations. A low SAT, low hemoglobin, or low cardiac index increased the venous gap.