In the present study we examined the effect of hyperthermia on the middle cerebral artery mean blood velocity (MCA V_mean) during prolonged exercise. We predicted that the cerebral circulation would be impaired when hyperthermia is present during exercise and assumed that this could be observed as a reduced MCA V_mean.

Eight endurance trained men (maximum oxygen uptake (V̇_O2,max) 70 ± 1 ml min⁻¹ kg⁻¹ (mean ±s.e.m.)) performed two exercise trials at 57 % of V̇_O2,max 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 stabilised at 37.8 ± 0.1 °C after ≈15 min of exercise.

Concomitant with the development of hyperthermia, MCA V_mean 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 V_mean remained unchanged at 70‐72 cm s⁻¹ throughout the 1 h control trial.

When individually determined regression lines for MCA V_mean and arterial carbon dioxide pressure (P_a,CO2) obtained during preliminary exercise tests were used to account for the differences in P_a,CO2 between the hyperthermic and control trial, it appeared that more than half of the reduction in MCA V_mean (56 ± 8 %) was related to a hyperventilation‐induced drop in P_a,CO2. Declining cardiac output and arterial blood pressure accounted for the remaining part of the hyperthermia‐induced reduction in MCA V_mean.

The present results demonstrate that the development of hyperthermia during prolonged exercise is associated with a marked reduction in MCA V_mean.