Effects of delayed, prolonged hypothermia on the pial vascular response after traumatic brain injury in rats

Object. In the experimental setting, hypothermia has been demonstrated to attenuate the damaging consequences of stroke and traumatic brain injury (TBI). Laboratory studies of TBI have focused primarily on the use of early hypothermic intervention, with little consideration of the potential efficacy of more delayed but prolonged hypothermia, which would constitute a more clinically relevant approach. In this investigation, the authors evaluated whether delayed, prolonged hypothermia after TBI protected the cerebral microcirculation. Methods. Male Sprague-Dawley rats were equipped with cranial windows for direct visualization of the pial arterial circulation and then subjected to impact-acceleration brain injury. The rats were randomly divided into four experimental groups: Group I consisted of normothermic animals; in Group 2 the rats received a I-hour period of hypothermia (32degreesC) 30 minutes posttrauma, followed by slow rewarming (32-37degreesC/90 minutes); and in Groups 3 and 4 the rats received a more delayed induction (at I hour postinjury) of either I hour (Group 3) or 2 hours (Group 4) of hypothermia, followed by the slow rewarming. The pial arteriolar responses to acetylcholine (ACh) or hypercapnia were measured until up to 6 hours postinjury. With this approach the authors found that the normothermic group demonstrated severely impaired vasoreactivity in terms of ACh-dependent dilation and CO2 reactivity in comparison to baseline values (p < 0.001). In contrast, hypothermia of short duration that was initiated early (30 minutes postinjury) conferred significant cerebrovascular protection (p < 0.001), yet this protection was reduced when the onset of this I-hour hypothermic period was postponed to I hour postinjury. Nevertheless, reduced protection could be significantly improved (p < 0.001) with prolongation of the hypothermic period to 2 hours. Conclusions. The results of this study show that early as well as delayed but prolonged hypothermia attenuate the impaired vascular responsiveness seen after TBI, indicating the potential clinical usefulness of this treatment.