Circulatory arrest at 20° C is used during open heart surgery in infants. It has been stated that significant brain damage does not occur. Piglets between two and six weeks of age were cooled to 20° C using extracorporeal circulation and a membrane oxygenator. After one hour of circulatory arrest the perfusion system was used to rewarm the animals and restore normal circulation. Electroencephalogram was monitored throughout perfusion and surgery, and repeated on surviving animals on the third,fi/th, seventh and tenth postoperative days. On the tenth day the animals were killed by injection of pentobarbitone. Other groups were subjected to Continuous per fusion at 20° C, Continuous per fusion at 37° C, Thoracotomy and cannulation, Ischaemia, and Hypoxia. The return of E.E.C. activity was delayed after circulatory arrest compared with those continuously per/used. Lesions were found in the cerebral cortex in all the animals which had circulatory arrest and those subjected to ischaemia and hypoxia. The brains of animals of the other groups were indistinguishable from those killed without any experimental procedure. Despite apparent recovery, brain damage following hypothermic arrest during open heart surgery remains possible.

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