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

Whole body oxygen consumption (Vo2) and its relationship to randomly selected arterial perfusion flow rates (QD) during profoundly hypothermic (20° C) cardiopulmonary bypass were determined in 17 adult patients undergoing routine coronary artery bypass operations. Vo2 falls progressively as QD decreases, from 33 ± 8.2 ml · min-1 · m-2 at QD of 2.0 L · min-1 · m-2 to 28 ± 5.8 at QD of 1.5, 25 ± 5.7 at QD of 1.0, 20 ± 4.1 at QD of 0.5, and 14 ± 5.4 at QD of 0.25. This progressive decrease suggests shutdown of areas of the microcirculation. The upper 70% confidence limit overlaps the asymptote at QDs above 1.2. Percent oxygen extraction increases progressively as QD decreases, from 11 ± 3.3% at QD of 2.0 to 45 ± 9.6% at QD of 0.25, suggesting reduced reserves. Mixed venous Po2 and oxygen saturation fall linearly with decreasing QD below 1.2 (r =0.78 and r =0.89, p