Rectal temperature reflects tympanic temperature during mild induced hypothermia in nonintubated subjects

Introduction: Mild induced hypothermia holds promise as an effective neuroprotective strategy following acute stroke and cardiac arrest. Dependable noninvasive measurements of brain temperature are imperative for the investigation and clinical application of therapeutic hypothermia. Although the tympanic membrane temperature correlates best with brain temperature, it is a cumbersome location to record from continuously in the clinical setting. Data are lacking regarding the relationship between rectal and tympanic temperatures in nonintubated humans undergoing induced hypothermia via surface cooling. Methods: We induced mild hypothermia in healthy volunteers using a novel surface cooling method (Arctic Sun Temperature Management System, Medivance, Inc., Louisville, CO). Core temperatures were recorded at the tympanic membrane (T-tym) and rectum (T-rec). The gradient was defined as (T-tym - T-rec). Controlled hypothermia was maintained for up to 300 minutes with a target T-tym of 34degreesC to 35degreesC; subjects were then actively rewarmed to a target T-tym of 36degreesC over 1.5 to 3 hours. Results: Twenty-two volunteers (10 males and 12 females) 31 +/- 8 years of age were studied. Subjects showed a triphasic temperature response: induction, maintenance, and rewarming. The mean gradient at baseline was -0.1 +/- 0.3degreesC and the maximum gradient increased to -0.6 +/- 0.4degreesC at 105 minutes. During maintenance of hypothermia (from 150 to 300 minutes), the mean gradient was -0.3 +/- 1 0.5degreesC (95% confidence limits, -1.2degreesC to 0.6degreesC). Conclusions: Our data suggest that T-tym and T-rec are not related during the induction of hypothermia via surface cooling but correlate during the maintenance phase, with a -0.3degreesC gradient. These findings support the use of rectal temperature as a measure of tympanic and, therefore, brain temperature during maintenance of induced hypothermia in nonintubated humans.