Brain and central haemodynamics and oxygenation during maximal exercise in humans

During maximal exercise in humans, fatigue is preceded by reductions in systemic and skeletal muscle blood flow, O-2 delivery and uptake. Here, we examined whether the uptake Of O-2 and substrates by the human brain is compromised and whether the fall in stroke volume of the heart underlying the decline in systemic O-2 delivery is related to declining venous return. We measured brain and central haemodynamics and oxygenation in healthy males (n = 13 in 2 studies) performing intense cycling exercise (360 +/- 10 W; mean +/- s.e.m.) to exhaustion starting with either high (H) or normal (control, C) body temperature. Time to exhaustion was shorter in H than in C (5.8 +/- 0.2 versus 7.5 +/- 0.4 min, P < 0.05), despite heart rate reaching similar maximal values. During the first 90 s of both trials, frontal cortex tissue oxygenation and the arterial-internal jugular venous differences (a-v diff) for O-2 and glucose did not change, whereas middle cerebral artery mean flow velocity (MCA V-mean) and cardiac output increased by similar to22 and similar to115%, respectively. Thereafter, brain extraction Of O-2, glucose and lactate increased by similar to45,similar to55 and similar to95%, respectively, while frontal cortex tissue oxygenation, MCA V-mean and cardiac output declined similar to40, similar to15 and similar to10%, respectively. At exhaustion in both trials, systemic V-O2 declined in parallel with a similar fall in stroke volume and central venous pressure; yet the brain uptake Of O-2, glucose and lactate increased. In conclusion, the reduction in stroke volume, which underlies the fall in systemic O-2 delivery and uptake before exhaustion, is partly related to reductions in venous return to the heart. Furthermore, fatigue during maximal exercise, with or without heat stress, in healthy humans is associated with an enhanced rather than impaired brain uptake Of O-2 and substrates.