Brain temperature and limits on transcranial cooling in humans: quantitative modeling results.

Selective brain cooling (SBC) of varying strengths has been demonstrated in a number of mammals and appears to play a role in systemic thermoregulation. Although primates lack obvious specialization for SBC, the possibility of brain cooling in humans has been debated for many years. This paper reports on the use of mathematical modeling to explore whether surface cooling can control effectively the temperature of the human cerebrum. The brain was modeled as a hemisphere with a volume of 1.33 1 and overlying layers of cerebrospinal fluid, skull, and scalp. Each component was assigned appropriate dimensions, physical properties and physiological characteristics that were determined from the literature. The effects of blood flow and of thermal conduction were modeled using the steady-state form of the bio-heat equation. Input parameters included core (arterial) temperature: normal (37 degrees C) or hyperthermic (40 degrees C), air temperature: warm (30 degrees C) or hot (40 degrees C), and sweat evaporation rate: 0, 0.25, or 0.50 l x m(-2) x h(-1). The resulting skin temperatures of the model ranged from 31.8 degrees C to 40.2 degrees C, values which are consistent with data obtained from the literature. Cerebral temperatures were generally insensitive to surface conditions (air temperature and evaporation rate), which affected only the most superficial level of the cerebrum (< or =1.5 mm) The remaining parenchymal temperatures were 0.2-0.3 degrees C above arterial temperatures, regardless of surface conditions. This held true even for the worst-case conditions combining core hyperthermia in a hot environment with zero evaporative cooling. Modeling showed that the low surface-to-volume ratio, low tissue conductivity, and high rate of cerebral perfusion combine to minimize the potential impact of surface cooling, whether by transcranial venous flow or by conduction through intervening layers to the skin or mucosal surfaces. The dense capillary network in the brain assures that its temperature closely follows arterial temperature and is controlled through systemic thermoregulation independent of head surface temperature. A review of the literature reveals several independent lines of evidence which support these findings and indicate the absence of functionally significant transcranial venous flow in either direction. Given the fact that humans sometimes work under conditions which produce face and scalp temperatures that are above core temperature, a transcranial thermal link would not necessarily protect the brain, but might instead increase its vulnerability to environmentally induced thermal injury.