Influence of moderate hypothermia on renal and cerebral haemodynamics and oxygenation during experimental cardiopulmonary bypass in sheep

Aim: Cardiac surgery requiring cardiopulmonary bypass (CPB) can result in renal and cerebral injury. Intraoperative tissue hypoxia could contribute to such organ injury. Hypothermia, however, may alleviate organ hypoxia. Therefore, we tested whether moderate hypothermia (30°C) improves cerebral and renal tissue perfusion and oxygenation during ovine CPB. Methods: Ten sheep were studied while conscious, under stable anesthesia, and during 3 h of CPB. In a randomized within-animal cross-over design, five sheep commenced CPB at a target body temperature of 30°C (moderate hypothermia). After 90 min, the body temperature was increased to 36°C (standard procedure). The remaining five sheep were randomized to the opposite order of target body temperature. Results: Compared with the standard procedure, moderately hypothermic CPB reduced renal oxygen delivery (−34.8% ± 19.6%, P = 0.003) and renal oxygen consumption (−42.7% ± 35.2%, P = 0.04). Nevertheless, moderately hypothermic CPB did not significantly alter either renal cortical or medullary tissue PO₂. Moderately hypothermic CPB also did not significantly alter cerebral perfusion, cerebral tissue PO₂, or cerebral oxygen saturation compared with the standard procedure. Compared with the anesthetized state, the standard procedure reduced renal medullary PO₂ (−21.0 ± 13.8 mmHg, P = 0.014) and cerebral oxygen saturation (65.0% ± 7.0% to 55.4% ± 9.6%, P = 0.022) but did not significantly alter either renal cortical or cerebral PO₂. Conclusion: Ovine experimental CPB leads to renal medullary tissue hypoxia. Moderately hypothermic CPB did not improve cerebral or renal tissue oxygenation. In the kidney, this is probably because renal tissue oxygen consumption is matched by reduced renal oxygen delivery.