Limited benefit of slow rewarming after cerebral hypothermia for global cerebral ischemia in near-term fetal sheep.

The optimal rate of rewarming after therapeutic hypothermia for neonatal hypoxic-ischemic encephalopathy is unknown, although it is widely suggested that slow rewarming is beneficial. Some preclinical studies suggest better outcomes with slower rewarming, but did not control for the duration of hypothermia. In this study, near-term fetal sheep (0.85 gestation) received 30 min cerebral ischemia followed by normothermia, 48 h hypothermia with rapid rewarming over 1 h, 48-h hypothermia with slow rewarming over 24 h, or 72-h hypothermia with rapid rewarming. Slow rewarming after 48 h of hypothermia improved recovery of EEG power compared with rapid rewarming (p < 0.05), but was not different from rapid rewarming after 72 h of hypothermia. At seven days recovery, neuronal survival was partially improved by both fast and slow rewarming after 48-h hypothermia, but less than 72-h hypothermia in the cortex and CA4 (p < 0.05). In conclusion, although electrographic recovery was partially improved by slow rewarming over 24 h following cerebral hypothermia for 48 h, optimal neuroprotection was seen with hypothermia for 72 h with rapid rewarming, suggesting that the overall duration of cooling was the critical determinant of outcomes after therapeutic hypothermia.