Relationship of whole body oxygen consumption to perfusion flow rate during hypothermic cardiopulmonary bypass

Abstract

Whole body oxygen consumption (Vo2) and its relationship to randomly selected arterial perfusion flow rates (Q) during profoundly hypothermic (20 degrees C) cardiopulmonary bypass were determined in 17 adult patients undergoing routine coronary artery bypass operations. Vo2 falls progressively as Q decreases, from 33 +/- 8.2 ml . min(-1) . m(-2) at Q of 2.0 L . min(-1) . m(-2) to 28 +/- 5.8 at Q of 1.5, 25 +/- 5.7 at Q of 1.0, 20 +/- 4.1 at Q of 0.5, and 14 +/- 5.4 at Q of 0.25. This progressive decrease suggests shutdown of areas of the microcirculation. The upper 70% confidence limit overlaps the asymptote at Qs above 1.2. Percent oxygen extraction increases progressively as Q decreases, from 11 +/- 3.3% at Q of 2.0 to 45 +/- 9.6% at Q of 0.25, suggesting reduced reserves. Mixed venous Po2 and oxygen saturation fall linearly with decreasing Q below 1.2 (r = 0.78 and r = 0.89, p less than 0.0001, respectively), suggesting decreasing flow to perfused areas. Internal jugular venous Po2 and oxygen saturation (measured in 10 patients) fall linearly with decreasing Q below 1.8 (r = 0.72 and r = 0.88, p less than 0.0001, respectively), suggesting decreasing flow to perfused areas of the brain and a difference from the rest of the body in its behavior with decreasing Q. Thus, during cardiopulmonary bypass cerebral blood flow, autoregulation seems present at 20 degrees C. The data set indicates that flows of about 1.2 may be adequate despite limited reserves.