The dynamic response of brain temperature to localized heating.

Several mathematical descriptions of heat transport in perfused tissues have been proposed but have not been thoroughly tested under conditions of time-varying temperatures. Data was obtained by measuring the response of brain temperature to step changes in temperature of chronically implanted thermodes in conscious baboons. These responses were compared to numerical solutions of an equation expressing heat transport in terms of conduction in the tissue and convection due to capillary blood flow. Good agreement between experimental and theoretical curves was obtained for values of k (thermal diffusivity) of 0.0017-0.0021 cm(2)/sec and ø (blood flow per unit volume of tissue) of 0.3-0.7 cm(3)/cm(3)-min. The predicted temperature response at a given tissue location was not greatly affected either by changes in k and ø over the physiological range, or by small errors in describing experimental geometry. However, inaccuracies in describing boundary locations or failing to account for the relatively avascular scar tissue around the thermode changed the value of ø needed to fit the data by as much as 50%. Thus, we conclude that the model described in this paper can be used for a description of thermal gradients surrounding a thermode but extreme caution should be exercised if such a model is used to quantitatively evaluate blood flow.