Effects of systemic hypothermia and selective brain cooling on ischemic brain damage and swelling

The present study investigates the neuroprotective effects of temporary mild systemic hypothermia and selective brain cooling against focal cerebral infarction in the rat and the changes of cortical blood flow, and compares these two treatment modalities. In permanent middle cerebral artery (MCA) model, the treatments were induced 15 min following the artery occlusion. The animals were kept at the desired rectal or brain temperature (about 32 degrees C) for 30 min; (each, n = 6) and for 1 hr (each, n = 6), and then allowed to rewarm spontaneously, whereas control animals were kept at normothermia throughout the experiment. The volumes of brain infarction and edema were assessed 24 hr post-occlusion. The blood flow of the dorsolateral cortex was monitored by Laser-Doppler flowmetry (LDF) in the other experiments. Hemispheric infarct volume was attenuated only in the animals treated for 1 hr with systemic hypothermia (49.2%, P < 0.001) and selective brain cooling (26.7%, P < 0.01). The volume of brain swelling was diminished only in the animals treated with systemic hypothermia for 1 hr (23.6%, P < 0.05). LDF examination revealed a sharp drop in blood flow upon MCA occlusion and maintaining in low blood flow throughout the experiment in the control and systemic hypothermia. However, in the selective brain cooling, the reduced blood flow increased from 40% to 70% of baseline value while the brain was rewarmed. The present study indicates that mild systemic hypothermia has much stronger protective effects against focal cerebral infarction and edema than selective brain cooling. The lack of protective effects of selective brain cooling may be caused by post-cooling cerebral hyperemia in the ischemia area.