Brain temperature is maintained by CBF, which serves as a body-coupled heat exchanger system penetrating all brain structures. Data presented herein quantitatively validate the manner in which CBF protects against extracranial cooling via the temperature shielding effect. It is often assumed that cerebral metabolism is responsible for maintaining brain temperature. However, according to Eq. 2 cerebral metabolism can only change brain temperature, compared with the temperature of incoming arterial blood, by the quantity Tm. Accordingly, for hematocrit level of 40% and arterial blood oxygen saturation of 100%, the maximum effect of metabolism on brain temperature cannot exceed 0.9°C even if all delivered oxygen is consumed [oxygen extraction fraction (OEF) = 1]. In practice, OEF rarely exceeds 40%, which results in a 0.3–0.4°C increase in brain temperature compared with that of incoming blood (32). Experimental measurements of arterial-venous temperature difference in monkeys (9) and humans (19) agree with this prediction. Small animals normally have higher metabolic rates compared with large animals. But this does not significantly affect brain temperature because more efficient heat removal is provided by higher CBF, the dominant effect in temperature regulation. Thus cerebral metabolic activity does not markedly affect brain temperature, and, indeed, it has been reported that patients in whom cerebral metabolism was reduced to 55% of normal did not show a larger brain-rectal temperature difference (15). A corollary is that body temperature is the primary determinant of deep brain temperature. Our experimental data demonstrate that deep brain temperature in rats under our experimental conditions is slightly (<1°C) lower than the deep body temperature.
Has anybody reported a treatment making them more temperature tolerant, and not improved their ability to use their legs/back, either better, or longer?
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