Low-flow hypothermic cardiopulmonary bypass protects the brain

Cerebral protection during surgical procedures necessitating circulatory arrest or low flow remains the factor that most limits the critical time for repair of lesions. In vivo phosphorus-31 nuclear magnetic resonance spectroscopy was used to assess the metabolic state of the brain during circulatory arrest by measuring the concentration of high-energy phosphate compounds and the intracellular pH. The degree of cerebral protection during deep hypothermic cardiopulmonary bypass at low flow rates was compared with that obtained with a period of circulatory arrest interrupted by intermittent systemic perfusion. Sheep were instrumented with cannulas for cardiopulmonary bypass, and a radiofrequency coil was positioned on the skull. Animals were placed in the bore of a 4.7 Tesla magnet, cooled with the aid of cardiopulmonary bypass to 15° C, and had either circulatory arrest (n = 5) or continuous low flow rates of 5 ml/kg/min (n = 6) or 10 ml/kg/min (n = 7) for 2 hours. A fourth group (n = 5) underwent 1 hour of circulatory arrest, systemic reperfusion for 30 minutes, then another hour of circulatory arrest. Both circulatory arrest and a flow rate of 5 ml/kg/min resulted in severe intracellular acidosis and depletion of high-energy phosphates. A flow of 10 ml/kg/min preserved high-energy phosphates and intracellular pH. Therefore deep hypothermia with cardiopulmonary bypass flows as low as 10 ml/kg/min can maintain brain high-energy phosphate concentrations and intracellular pH for 2 hours in sheep, whereas flows of 5 ml/kg/min or intermittent full-flow systemic perfusion between periods of circulatory arrest offers less protection. Previous studies from our laboratory have shown that improvement in nuclear magnetic resonance parameters positively correlates with improved survival and preservation of neurologic function.