Brain damage after deep hypothermia in dogs.

Histopathological and succinic acid dehydrogenase (SAD) activity changes, and the electron spin resonance (ESR) were examined in the brains of dogs exposed to various hypothermic conditions. A simple hypothermic cardiac arrest of 5 min duration occurred in the group subjected to 28 degrees C, and normal SAD activity was noted in dogs exposed 1-hour extracorporeal circulation under normothermia. In cases of a hypothermia below 28 degrees C, a decrease in enzyme activity, marked histopathological changes and the disappearance of the ESR signal were observed. In dogs killed immediately after induction of 20 degrees C hypothermia, the histopathological changes were primarily ischemic in nature, while in dogs killed at 1-12 month intervals after the experiments, irreversible gliosis was dominant. Our findings suggest that, to avoid brain damage under conditions of deep hypothermic circulatory arrest, the critical range is between 26-28 degrees C for rectal, and approximately 28-30 degrees C for brain temperature.