ObjectivesTo determine whether transient, moderate hypothermia is beneficial after traumatic brain injury in fentanyl-anesthetized rats.DesignProspective, randomized study.SettingUniversity-based animal research facility.SubjectsAdult male Sprague-Dawley rats.InterventionsAll rats were intubated, mechanically ventilated, and anesthetized with fentanyl (10 &mgr;g/kg intravenous bolus and then 50 &mgr;g·kg−1·hr−1infusion). Controlled cortical impact was performed to the left parietal cortex, followed immediately by 1 hr of either normothermia (brain temperature 37 ± 0.5°C) or hypothermia (brain temperature 32 ± 0.5°C). Hypothermic rats were rewarmed gradually over 1 hr. Fentanyl anesthesia and mechanical ventilation were continued in both groups until the end of rewarming (2 hrs after traumatic brain injury).Measurements and Main ResultsHistologic assessment performed 72 hrs after traumatic brain injury was the primary outcome variable. Secondary outcome variables were physiologic variables monitored during the first 2 hrs after traumatic brain injury and plasma catecholamine and serum fentanyl concentrations measured at the end of both hypothermia and rewarming (1 and 2 hrs after traumatic brain injury). Contusion volume was larger in hypothermic vs. normothermic rats (44.3 ± 4.2 vs. 28.6 ± 4.0 mm,p< .05), but hippocampal neuronal survival did not differ between groups. Physiologic variables did not differ between groups. Plasma dopamine and norepinephrine concentrations were increased at the end of hypothermia in hypothermic (vs. normothermic) rats (p< .05), indicating that hypothermia augmented the systemic stress response. Similarly, serum fentanyl concentrations were higher in hypothermic (vs. normothermic) rats at the end of both hypothermia and rewarming (p< .05), demonstrating that hypothermia reduced the clearance and/or metabolism of fentanyl.ConclusionsModerate hypothermia was detrimental after experimental traumatic brain injury in fentanyl-anesthetized rats. Since treatment with hypothermia has provided reliable benefit in experimental traumatic brain injury with inhalational anesthetics, these results indicate that the choice of anesthesia/analgesia after traumatic brain injury may dramatically influence response to other therapeutic interventions, such as hypothermia. Given that narcotics commonly are administered to patients after severe traumatic brain injury, this study may have clinical implications.