Effects of graded hypothermia on outcome from brain ischemia.

The effects of mild (34 degrees C) and moderate (31 degrees C) hypothermia on the electroencephalogram, cerebral blood flow and outcome from incomplete brain ischemia were compared to normothermia (37 degrees C) in the rat. Rats were anesthetized with fentanyl (25 mu g kg(-1) h(-1)) and 70 percent nitrous oxide in oxygen. Ischemia was produced by right carotid ligation combined with hemorrhagic hypotension to 25 mmHg for 30 min. CBF was measured in the right parietal cortex by laser Doppler. Neurologic outcome was measured daily for 3 days after ischemia and histopathology determined at the end of the study. Before ischemia, neither EEG nor CBF were changed by hypothermia compared to normothermic controls. Total EEG and beta EEG (12.5-32 Hz) were abolished and CBF decreased by 70 percent during ischemia. During a 2.5 h recovery period total EEG and beta EEG activity recovered better with moderate hypothermia compared to normothermia. Recovery of cerebral blood flow was not significantly different between the treatment groups. Neurologic and histopathologic outcome were improved in rats receiving moderate hypothermia compared to normothermic controls. These results show that during severe incomplete ischemia, hypothermia has a graded effect on outcome which is consistent with its effects on brain metabolism.