Effects of hypothermia on traumatic brain injury in immature rats

Hypothermia is beneficial in adult models of traumatic brain injury (TBI), but it has not been evaluated in an immature animal model. We hypothesized that brief hypothermia applied after TBI would reduce cerebral edema and lesion volume in immature rats. Male Wistar rats (3-4 weeks of age, 90-140 g) were anesthetized, intubated, mechanically ventilated, and subjected to TBI by weight drop onto the exposed right parietal cortex. Hypothermic rats were then cooled to a brain temperature of 32.0 +/- 0.5 degrees C for 4 h, and control rats were maintained at a brain temperature of 37.0 +/- 0.5 degrees C. Cerebral edema (wet - dry weight method) was assessed at 5 days. At 4 h, a reduction of percent brain water in the traumatized hemisphere was observed in hypothermic versus normothermic rats (81.75 +/- 0.60 vs. 82.53 +/- 0.67%; p<0.05), but by 24 h posttrauma, the groups were similar (p = 0.82). Total lesion volume (47.2 +/- 8.5 vs. 44.4 +/- 10.0 mm3; p = 0.51) and necrotic volume (20.2 +/- 6.3 vs. 20.0 +/- 7.9 mm3; p = 0.95) were similar in the hypothermic and normothermic groups. We conclude that in this model, a transient (4-h) application of moderate (32 degrees C) hypothermia reduces the cerebral edema characteristically seen in immature rats at 4 h, but this reduction is not sustained at 24 h. Attenuating or delaying the development of cerebral edema could have important therapeutic relevance after TBI. Transient hypothermia, however, did not reduce lesion volume at 5 days posttrauma.