Adequate flow during cardiopulmonary bypass is best indicated by:

C. Mixed venous hemoglobin saturation of 78%.

DISCUSSION: The purpose of cardiopulmonary bypass is to provide adequate circulation of blood to sustain aerobic metabolism. Oxygen consumption during bypass depends on bypass flow until a critical flow is attained. With higher flows there is no further increase in oxygen consumption (i.e., oxygen consumption becomes flow independent), and the mixed venous hemoglobin saturation increases. A mixed venous hemoglobin saturation of 78% indicates that bypass flow is above the critical level and that flow is adequate. The other variables do not ensure adequate bypass flow.