The effect of hypothermic cardiopulmonary bypass and total circulatory arrest on cerebral metabolism in neonates, infants, and children.

Cardiopulmonary bypass management in neonates, infants, and children often requires the use of deep hypothermia at 18 degrees C with occasional periods of circulatory arrest and represents marked physiologic extremes of temperature and perfusion. The safety of these techniques is largely dependent on the reduction of metabolism, particularly cerebral metabolism. We studied the effect of hypothermia on cerebral metabolism during cardiac surgery and quantified the changes. Cerebral metabolism was measured before, during, and after hypothermic cardiopulmonary bypass in 46 pediatric patients, aged 1 day to 14 years. Patients were grouped on the basis of the different bypass techniques commonly used in children: group A--moderate hypothermic bypass at 28 degrees C; group B--deep hypothermic bypass at 18 degrees to 20 degrees C with maintenance of continuous flow; and group C--deep hypothermic circulatory arrest at 18 degrees C. Cerebral metabolism significantly decreased under hypothermic conditions in all groups compared with control levels at normothermia, the data demonstrating an exponential relationship between temperature and cerebral metabolism and an average temperature coefficient of 3.65. There was no significant difference in the rate of metabolism reduction (temperature coefficient) in patients cooled to 28 degrees and 18 degrees C. From these data we were able to derive an equation that numerically expresses a hypothermic metabolic index, which quantitates duration of brain protection provided by reduction of cerebral metabolism owing to hypothermic bypass over any temperature range. Based on this index, patients cooled to 28 degrees C have a predicted ischemic tolerance of 11 to 19 minutes. The predicted duration that the brain can tolerate ischemia ("safe" period of deep hypothermic circulatory arrest) in patients cooled to 18 degrees C, based on our metabolic index, is 39 to 65 minutes, similar to the safe period of deep hypothermic circulatory arrest known to be tolerated clinically. In groups A and B (no circulatory arrest), cerebral metabolism returned to control in the rewarming phase of bypass and after bypass. In group C (circulatory arrest), cerebral metabolism and oxygen extraction remained significantly reduced during rewarming and after bypass, suggesting disordered cerebral metabolism and oxygen utilization after deep hypothermic circulatory arrest. The results of this study suggest that cerebral metabolism is exponentially related to temperature during hypothermic bypass with a temperature coefficient of 3.65 in neonates infants and children. Deep hypothermic circulatory arrest changes cerebral metabolism and blood flow after the arrest period despite adequate hypothermic suppression of metabolism.