Inadequate heat release from the human brain during prolonged exercise with hyperthermia

Abstract

Brain temperature appears to be an important factor affecting motor activity, but it is not known to what extent brain temperature increases during prolonged exercise in humans. Cerebral heat exchange was therefore evaluated in seven males during exercise with and without hyperthermia. Middle cerebral artery mean blood velocity (MCA V(mean)) was continuously monitored while global cerebral blood flow (CBF) and cerebral energy turnover were determined at the end of the two exercise trials in three subjects. The arterial to venous temperature difference across the brain (v-aD(temp)) was determined via thermocouples placed in the internal jugular vein and in the aorta. The jugular venous blood temperature was always higher than that of the arterial blood, demonstrating that heat was released via the CBF during the normothermic as well as the hyperthermic exercise condition. However, heat removal via the jugular venous blood was 30 +/- 6 % lower during hyperthermia compared to the control trial. The reduced heat removal from the brain was mainly a result of a 20 +/- 6 % lower CBF (22 +/- 9 % reduction in MCA V(mean)), because the v-aD(temp) was not significantly different in the hyperthermic (0.20 +/- 0.05 degrees C) compared to the control trial (0.22 +/- 0.05 degrees C). During hyperthermia, the impaired heat removal via the blood was combined with a 7 +/- 2 % higher heat production in the brain and heat was consequently stored in the brain at a rate of 0.20 +/- 0.06 J g(-1) min(-1). The present results indicate that the average brain temperature is at least 0.2 degrees C higher than that of the body core during exercise with or without hyperthermia.

Figure 1. Oesophageal (▵), tympanic (□), arterial (•) and jugular venous (♦) temperature responses during cycling

Results obtained with a normal core temperature response (A, control trial) and during a similar exercise bout with progressive hyperthermia (B). Values are means of 7 subjects. Standard deviations are omitted for simplicity, but the s.d. values of all temperatures were in the range of 0.1-0.3 °C.

Figure 2. Middle cerebral artery mean blood velocity (MCA V_mean) during exercise with and without hyperthermia and during the subsequent recovery period

•, hyperthermia; ○, control. Values are means ± s.d. of 6 subjects. * Significantly lower than control.

Figure 3. Rate of cerebral heat production, heat removal via the jugular venous blood and heat storage during the last 15 min of the control and hyperthermic exercise trials

The rate of cerebral heat production, which is represented by the total height of the bars, was calculated on the basis of the Kety-Schmidt-determined values for cerebral oxygen uptake and lactate release, while heat removal via the jugular venous blood was determined on the basis of the global CBF and the arterial to internal jugular venous temperature difference. Storage is the average rate of heat storage from 30- 45 min of exercise, as determined from the change in cerebral venous blood temperature, and ‘heat removal via other mechanisms’ (than convective removal via the jugular venous blood) is the difference between the rate of cerebral heat production and the computed rate of storage in the brain summed with the rate of heat removal by the jugular venous blood. The values represent means of three subjects.