Hypothermia does not improve cerebral autoregulation after traumatic brain injury (TBI) in rats

Introduction: Mild hypotension worsens outcome after human TBI (1), perhaps because of unpaired cerebral pressure autoregulation (2). Hypothermia improved outcome after TBI in animals (3) and humans (4), but the mechanisms are unclear. We evaluated the effects of hypothermia on autoregulation after fluid percussion injury (FPI) in rats. Methods: In an approved protocol, 24 Sprague-Dawley rats were mechanically ventilated with isoflurane in air: O₂ and prepared for lateral FPI and cerebral laser Doppler flowmetry (LDF) (5). The abdominal aorta was cannulated and connected to a reservoir. PaCO₂ was maintained at 38mmHg. Under 1% isoflurane, autoregulation was assessed before or 30 and 60 min after FPI by measuring LDF at randomly ordered blood pressures of 100, 80, 60, and 40 mm Hg. After baseline measurements at 37°C (temporalis), animals were randomized to 37°C without FPI (Control 37; n=7), 37°C with FPI of 1.8 atm (FPI 37; n=7), 32°C without FPI (Control 32; n=7), 32°C with FPI (Pre-FPI 32; n=7), or 37°C with FPI and immediate cooling to 32°C (Post-FPI 32, n=7). Results: PaCO₂, PaO₂, pH, hematocrit, and temperature were controlled as planned. No significant difference in %CBF change was seen between trauma groups with normothermia or hypothermia at 30 and 60 min. (Figure Presented) Conclusions: Hypothermia before or after brain trauma does not improve autoregulation after FPI. Better outcomes after FPI with hypothermia appear to be related to mechanisms other than improved cerebral circulatory control.