Cerebral oxygenation is reduced during hyperthermic exercise in humans

Aim: Cerebral mitochondrial oxygen tension (P_mitoO₂) is elevated during moderate exercise, while it is reduced when exercise becomes strenuous, reflecting an elevated cerebral metabolic rate for oxygen (CMRO ₂) 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 P_mitoO₂ 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 (VO_2max) 63 ± 6 mL kg^-1 min^-1]. Results: The CBF, CMRO₂ and P_mitoO ₂ remained stable during 1 h of moderate cycling (170 ± 11 W, ∼50% of VO_2max, RPE 9-12) in normothermia (core temperature of 37.8 ± 0.4 °C). In contrast, when hyperthermia was provoked by dressing the subjects in watertight clothing during exercise (core temperature 39.5 ± 0.2 °C), P_mitoO₂ declined by 4.8 ± 3.8 mmHg (P < 0.05 compared to normothermia) because CMRO ₂ 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 P _mitoO₂ (r² = 0.42, P < 0.05). Conclusion: These data indicate that strenuous exercise in the heat lowers cerebral P _mitoO₂, and that exercise capacity in this condition may be dependent on maintained cerebral oxygenation.