Continuous monitoring of mixed venous oxygen saturation in cardiac surgery

Serial measurement of mixed venous oxygen saturation is useful in the care of critically ill patients. It is an index of cardiac output and overall tissue perfusion. Previously, lack of refinement of the technology for continuous monitoring of mixed venous oxygen saturation deterred its clinical application. The authors evaluated the Oximetrix ShawTM catheter oximeter system between May 1980 and April 1981 in 84 high-risk and moderately high-risk patients. Fifty-four had undergone only myocardial revascularization while 30 had undergone valvular or combined procedures. In 20 patients with compromised left ventricular function (mean ejection fraction of less than 40%) continuous mixed venous oxygen saturation was compared to hemodynamic parameters in an intraoperative and early postoperative study. The results indicated that satisfactory mixed venous oxygen saturation (more than 65%) correlated with normal hemodynamic measurements including cardiac output and cardiac index. In general, a fall in mixed venous oxygen saturation of more than 10% was noted before the mean blood pressure, heart rate or pulmonary capillary wedge pressure changed. Cardiac output, cardiac index, systemic vascular resistance and left ventricular stroke work index were found to change in association with a change in mixed venous oxygen saturation. A fall (mixed venous oxygen saturation less than 65%) can be related to: (a) abnormal hemodynamic status--reduced cardiac output, hypotension, elevated systemic vascular resistance and arrhythmias, (b) abnormal oxygen demand--shivering, suctioning, positioning and pyrexia and (c) abnormal oxygen supply--anemia, airway obstruction and altered diffusion of oxygen at the alveolar capillary membrane. The Oximetrix system proved reliable. Mixed venous oxygen saturation is a nonspecific indicator of hemodynamic status. Continuous monitoring of the mixed venous oxygen saturation facilitates optimal patient management by immediately alerting intensive care personnel to the development of inadequate tissue perfusion.