Cerebral blood flow, frontal lobe oxygenation and intra-arterial blood pressure during sprint exercise in normoxia and severe acute hypoxia in humans.

Cerebral blood flow (CBF) is regulated to secure brain O ₂ delivery while simultaneously avoiding hyperperfusion; however, both requisites may conflict during sprint exercise. To determine whether brain O ₂ delivery or CBF is prioritized, young men performed sprint exercise in normoxia and hypoxia (P _I O ₂  = 73 mmHg). During the sprints, cardiac output increased to ∼22 L min ^-1 , mean arterial pressure to ∼131 mmHg and peak systolic blood pressure ranged between 200 and 304 mmHg. Middle-cerebral artery velocity (MCAv) increased to peak values (∼16%) after 7.5 s and decreased to pre-exercise values towards the end of the sprint. When the sprints in normoxia were preceded by a reduced P _ET CO ₂ , CBF and frontal lobe oxygenation decreased in parallel ( r = 0.93, P < 0.01). In hypoxia, MCAv was increased by 25%, due to a 26% greater vascular conductance, despite 4-6 mmHg lower PaCO ₂ in hypoxia than normoxia. This vasodilation fully accounted for the 22 % lower CaO ₂ in hypoxia, leading to a similar brain O ₂ delivery during the sprints regardless of P _I O ₂ . In conclusion, when a conflict exists between preserving brain O ₂ delivery or restraining CBF to avoid potential damage by an elevated perfusion pressure, the priority is given to brain O ₂ delivery.