Effects of hyperthermia on cerebral blood flow and metabolism during prolonged exercise in humans

Abstract

The development of hyperthermia during prolonged exercise in humans is associated with various changes in the brain, but it is not known whether the cerebral metabolism or the global cerebral blood flow (gCBF) is affected. Eight endurance-trained subjects completed two exercise bouts on a cycle ergometer. The gCBF and cerebral metabolic rates of oxygen, glucose, and lactate were determined with the Kety-Schmidt technique after 15 min of exercise when core temperature was similar across trials, and at the end of exercise, either when subjects remained normothermic (core temperature = 37.9 degrees C; control) or when severe hyperthermia had developed (core temperature = 39.5 degrees C; hyperthermia). The gCBF was similar after 15 min in the two trials, and it remained stable throughout control. In contrast, during hyperthermia gCBF decreased by 18% and was therefore lower in hyperthermia compared with control at the end of exercise (43 +/- 4 vs. 51 +/- 4 ml. 100 g(-1). min(-1); P < 0.05). Concomitant with the reduction in gCBF, there was a proportionally larger increase in the arteriovenous differences for oxygen and glucose, and the cerebral metabolic rate was therefore higher at the end of the hyperthermic trial compared with control. The hyperthermia-induced lowering of gCBF did not alter cerebral lactate release. The hyperthermia-induced reduction in exercise cerebral blood flow seems to relate to a concomitant 18% lowering of arterial carbon dioxide tension, whereas the higher cerebral metabolic rate of oxygen may be ascribed to a Q(10) (temperature) effect and/or the level of cerebral neuronal activity associated with increased exertion.

Cerebrovascular responsiveness to steady-state changes in end-tidal CO2 during passive heat stress

“…Rasmussen et al (30) suggested that the observed increase in CO 2 reactivity, coupled with decreases in PET CO 2 , provides a mechanistic basis for previously observed reductions in cerebral blood flow when subjects exercised in the heat (25,26). The most obvious reason for conflicting findings with respect to cerebral vascular CO 2 reactivity between the present study and that of Rasmussen et al (30) is the absence of exercise in the present study.…”

Neurohumoral responses during prolonged exercise in humans

“…Significantly different from * control and † rest (P Ͻ 0.05). (24,46), and the associated systemic dopamine response could be affected by sensory feedback arising secondary to the increased body temperature. Thus dopamine in the sympathetic ganglia is secreted mainly from the sensory neurons (3).…”

“…The subjects were successfully rehydrated between trials, as indicated by similar hemoglobin concentrations at the onset of the trials (mean range between bouts: 135-137 g/l). Also, in these trained subjects, 1 h of rest is sufficient for the cerebral metabolism, the core and brain temperatures, blood lactate, and plasma osmolarity to recover from the first exercise bout, independent of whether or not hyperthermia is superimposed (46,49). In the present study, fatigue was considered as increased difficulty in retaining the required power output, and during exercise the subjects expressed their ratings of perceived exertion (RPE) on the Borg scale (9), allowing for an evaluation of their perception of effort.…”

“…Global CBF was measured by the Kety-Schmidt technique in the desaturation mode with 133 Xe as the tracer (32,39,46). Cerebral plasma flow was calculated on the basis of the CBF and the corresponding hematocrit, and the cerebral uptake or release of a given substance was calculated as the cerebral plasma flow multiplied by the a-v difference of the substance.…”

Development of a perceptual hyperthermia index to evaluate heat strain during treadmill exercise

“…Although there is a reduction in cerebral blood flow, cerebral oxygen delivery does not appear to be jeopardized during exercise and thermally stressful conditions. 59 One apparent consequence of reduced cerebral blood flow was a reduction of total body heat removal as there was not temperature gradient between the brain and the rest of the body due to increased core temperature.…”