Brain blood flow and metabolism do not decrease at stable brain temperature during cardiopulmonary bypass in rabbits

Cerebral blood flow (CBF) during human hypothermic cardiopulmonary bypass has been reported to decrease with time, suggesting that progressive cerebral vasoconstriction or embolic obstruction may occur. We tested the hypotheses: 1) that observed CBF reductions were due to continued undetected brain cooling and 2) that CBF during cardiopulmonary bypass would be stable after achievement of constant brain temperature. Anesthetized New Zealand White rabbits underwent cardiopulmonary bypass (membrane oxygenator, centrifugal pump, bifemoral arterial perfusion) and were assigned to one of three bypass management groups based on perfusate temperature and Pa(CO₂) management: group 1 (37° C, n = 8); group 2 (27° C, pH-stat, n = 9); and group 3 (27° C, α-stat, n = 8). Systemic hemodynamics, and cerebral cortical, esophageal, and arterial perfusate temperatures were recorded every 10 min for the first hour of bypass and again at 90 min. CBF and masseter blood flow (radiolabeled microspheres) were determined at 30, 60, and 90 min of bypass, while the cerebral metabolic rate for oxygen (CMR(O₂)) was determined at 60 and 90 min. Groups were comparable with respect to mean arterial pressure, central venous pressure, hematocrit, and arterial oxygen content throughout bypass. Cortical temperature was stable in normothermic (group 1) animals, and there was no significant change in CBF between 30 and 90 min of bypass: 68 ± 18 versus 73 ± 20 ml · 100 g^-1 · min^-1 (mean ± SD). In the hypothermic groups (2 and 3), cortical temperature equilibration (95% of the total change) required 41 ± 6 min. In agreement with our hypothesis, once brain temperature had stabilized in hypothermic animals, we found no significant change in CBF between 60 and 90 min of bypass - 37 ± 6 versus 37 ± 7 ml · 100 g^-1 · min^-1 (pH-stat) and 31 ± 6 versus 30 ± 3 ml · 100 g^-1 · min^-1 (α-stat). There was no significant change in CMR(O₂) between 60 and 90 min of bypass in any group - 3.8 ± 1.4 versus 3.4 ± 1.1 ml · 100 g^-1 · min^-1 (37°C); 1.4 ± 0.4 versus 1.2 ± 0.3 ml · 100 g^-1 · min^-1 (pH-stat); and 1.3 ± 0.4 versus 1.3 ± 0.5 ml · 100 g^-1 · min^-1 (α- stat). We conclude that at constant brain temperature, CBF and CMR(O₂) are stable during cardiopulmonary bypass, and if progressive cerebral vasoconstriction and/or embolic obstruction occur, such events occur either during the early phase of bypass or, if later, at levels below the resolution of this study.