Cerebral oxygenation is reduced during hyperthermic exercise in humans

Aim: Cerebral mitochondrial oxygen tension (PmitoO2) is elevated during moderate exercise, while it is reduced when exercise becomes strenuous, reflecting an elevated cerebral metabolic rate for oxygen (CMRO2) combined with hyperventilation-induced attenuation of cerebral blood flow (CBF). Heat stress challenges exercise capacity as expressed by increased rating of perceived exertion (RPE). Methods: This study evaluated the effect of heat stress during exercise on PmitoO2 calculated based on a Kety-Schmidt-determined CBF and the arterial-to-jugular venous oxygen differences in eight males [27 +/- 6 years (mean +/- SD) and maximal oxygen uptake (VO2max) 63 +/- 6 mL kg-1 min-1]. Results: The CBF, CMRO2 and PmitoO2 remained stable during 1 h of moderate cycling (170 +/- 11 W, similar to 50% of VO2max, RPE 9-12) in normothermia (core temperature of 37.8 +/- 0.4 degrees C). In contrast, when hyperthermia was provoked by dressing the subjects in watertight clothing during exercise (core temperature 39.5 +/- 0.2 degrees C), PmitoO2 declined by 4.8 +/- 3.8 mmHg (P < 0.05 compared to normothermia) because CMRO2 increased by 8 +/- 7% at the same time as CBF was reduced by 15 +/- 13% (P < 0.05). During exercise with heat stress, RPE increased to 19 (19-20; P < 0.05); the RPE correlated inversely with PmitoO2 (r2 = 0.42, P < 0.05). Conclusion: These data indicate that strenuous exercise in the heat lowers cerebral PmitoO2, and that exercise capacity in this condition may be dependent on maintained cerebral oxygenation.