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The present study examined the effect of hyperthermia on the middle cerebral artery mean blood velocity (MCA Vmean) during prolonged exercise. We hypothesized that the cerebral circulation would be impaired when hyperthermia is superimposed during exercise and assumed that this could be observed as a reduced MCA Vmean. Eight endurance trained men [VO2max 70±1 ml min-1 kg-1 (mean ± SE)] performed two exercise trials at 57% of VO2max on a cycle ergometer in a hot (40°C; hyperthermic trial) and in a thermoneutral environment (18°C; control trial). In the hyperthermic trial, the oesophageal temperature increased throughout the exercise period reaching a peak value of 40.0±0.1°C at exhaustion after 53±4 min of exercise. In the control trial, exercise was maintained for 1 h without any signs of fatigue and with core temperature stabilized at 37.8±0.1°C after ~ 15 min of exercise. Concomitant with the development of hyperthermia, MCA Vmean declined by 26±3% from 73±4 cm s-1 at the beginning of exercise to 54±4 cm s-1 at exhaustion (P<0.001). In contrast, MCA Vmean remained unchanged at 70-72 cm s-1 throughout the 1 h control trial (see the figure). When individually determined regression lines for MCA Vmean and arterial PCO2 obtained during preliminary exercise tests were used to ascribe for the differences in arterial PCO2 between the hyperthermic and control trial, it appeared that more than half of the reduction in MCA Vmean (56±8%; see the figure) was related to a hyperventilation-induced drop in arterial carbon dioxide pressure. Declining cardiac output and arterial blood pressure during the hyperthermic trial presumably accounted for the last part of the reduction in MCA Vmean. The present results clearly demonstrate that the development of hyperthermia during prolonged exercise is associated with a marked reduction in middle cerebral artery mean blood velocity.
