Oxygenation strategy and neurologic damage after deep hypothermic circulatory arrest. II. Hypoxic versus free radical injury

Objectives: Laboratory studies suggest that myocardial reperfusion injury is exacerbated by free radicals when pure oxygen is used during cardiopulmonary bypass. In phase I of this study we demonstrated that normoxic perfusion during cardiopulmonary bypass does not increase the risk of microembolic brain injury so long as a membrane oxygenator with an arterial filter is used. In phase II of this study we studied the hypothesis that normoxic perfusion increases the risk of hypoxic brain injury after deep hypothermia with circulatory arrest. Methods: With membrane oxygenators with arterial filters, 10 piglets (8-10 kg) underwent 120 minutes of deep hypothermia and circulatory arrest at 15^oC, were rewarmed to 37^oC, and were weaned from bypass. In 5 piglets normoxia (PaO"2 64-181 mm Hg) was used during cardiopulmonary bypass and in 5 hyperoxia (PaO"2 400-900 mm Hg) was used. After 6 hours of reperfusion the brain was fixed for histologic evaluation. Near-infrared spectroscopy was used to monitor cerebral oxyhemoglobin and oxidized cytochrome a,a"3 concentrations. Results: Histologic examination revealed a significant increase in brain damage in the normoxia group (score 12.4 versus 8.6, P = .01), especially in the neocortex and hippocampal regions. Cytochrome a,a "3 and oxyhemoglobin concentrations tended to be lower during deep hypothermia and circulatory arrest in the normoxia group (P = .16). Conclusions: In the setting of prolonged deep hypothermia and circulatory arrest with membrane oxygenators, normoxic cardiopulmonary bypass significantly increases histologically graded brain damage with respect to hyperoxic cardiopulmonary bypass. Near-infrared spectroscopy suggests that the mechanism is hypoxic injury, which presumably overwhelms any injury caused by increased oxygen free radicals. (J Thorac Cardiovasc Surg 1999;117:1172-9)