Continuous monitoring of mixed venous oxygen saturation in patients with acute myocardial infarction.

Continuous measurement of mixed venous oxygen saturation (SvO2) has been suggested as an adjunct in monitoring critically ill patients. We evaluated SvO2 monitoring in 24 patients suffering from complicated myocardial infarctions. Cardiac output and arterial lactate levels were measured when there were persistent 5 percent changes in SvO2, and otherwise, every 12 hours or as indicated clinically. Increases in SvO2 by 5 and 10 percent corresponded with an increase in cardiac index in 78.5 percent and 75 percent of measurements, respectively. Decreases in SvO2 by 5 and 10 percent corresponded with decreases in cardiac index in 45.5 percent and 61 percent of measurements. Twenty percent changes in cardiac index showed dissimilar directional changes with SvO2 in 62 percent of cases. Arterial blood lactate levels correlated poorly with SvO2. Survivors had significantly higher mean SvO2 and cardiac indices than nonsurvivors (p less than 0.01). The clinical management of patients with myocardial infarction may not be altered in view of the limitations of SvO2 in reflecting tissue hypoxia. We conclude that continuous monitoring of SvO2 may not be a sensitive measure of cardiac output after acute myocardial infarction.