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 (PIO2=73mmHg). During the sprints, cardiac output increased to similar to 22 Lmin(-1), mean arterial pressure to similar to 131mmHg and peak systolic blood pressure ranged between 200 and 304mmHg. Middle-cerebral artery velocity (MCAv) increased to peak values (similar to 16%) after 7.5s and decreased to pre-exercise values towards the end of the sprint. When the sprints in normoxia were preceded by a reduced PETCO2, 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 4-6mmHg lower PaCO2 in hypoxia than normoxia. This vasodilation fully accounted for the 22 % lower CaO2 in hypoxia, leading to a similar brain O-2 delivery during the sprints regardless of PIO2. 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.