How the body controls brain temperature: the temperature shielding effect of cerebral blood flow

Abstract

. How the body controls brain temperature: the temperature shielding effect of cerebral blood flow. J Appl Physiol 101: [1481][1482][1483][1484][1485][1486][1487][1488] 2006. First published July 13, 2006; doi:10.1152/japplphysiol.00319.2006.-Normal brain functioning largely depends on maintaining brain temperature. However, the mechanisms protecting brain against a cooler environment are poorly understood. Reported herein is the first detailed measurement of the brain-temperature profile. It is found to be exponential, defined by a characteristic temperature shielding length, with cooler peripheral areas and a warmer brain core approaching body temperature. Direct cerebral blood flow (CBF) measurements with microspheres show that the characteristic temperature shielding length is inversely proportional to the square root of CBF in excellent agreement with a theoretical model. This "temperature shielding effect" quantifies the means by which CBF prevents "extracranial cold" from penetrating deep brain structures. The effect is crucial for research and clinical applications; the relationship between brain, body, and extracranial temperatures can now be quantitatively predicted.brain temperature regulation; cerebral metabolism; hypothermia; bioheat equation BLOOD FLOW SERVES VARIOUS roles in brain functioning. It delivers nutrients, removes waste products and, importantly, supports brain temperature regulation. Indeed, the temperature of incoming arterial blood is the main determinant of the brain temperature (9). Although this is correct for deep brain structures, it is not clear how the temperature distribution in superficial brain structures depends on the extracranial temperature. How deeply an external "cold assault" penetrates into the functioning brain remains an open question. This issue is especially important because of numerous current attempts to use mild hypothermia for treatment of stroke, multiple sclerosis, and other brain injuries (see, for example, Refs. 6,[10][11][12]17,23,24). Therapeutic results have been conflicting, in large part because quantifying the resultant cooling of human brain in vivo is beyond current technology. Therefore, developing a biophysical framework describing the determinants of heat flow, thus temperature regulation, in human brain would be a substantial achievement. Within such a framework one could predict the brain temperature response, or lack thereof, to external devices designed to impose mild hypothermia. Herein we report experimental measurements of the brain temperature profile that together with theoretical considerations quantitatively establish a general phenomenon: the temperature shielding effect of blood flow, which is responsible for brain protection against external cooling.Major mechanisms responsible for body temperature regulation in mammals are well known (see, for example, Ref. 28). In our experiments, the body temperature was kept constant by circulating warm water as described in MATERIALS AND METHODS. Blood flow acts as a heat exchanger with pip...

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