Brain and central haemodynamics and oxygenation during maximal exercise in humans

Abstract

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 approximately 22 and approximately 115%, respectively. Thereafter, brain extraction of O(2), glucose and lactate increased by approximately 45, approximately 55 and approximately 95%, respectively, while frontal cortex tissue oxygenation, MCA V(mean) and cardiac output declined approximately 40, approximately 15 and approximately 10%, respectively. At exhaustion in both trials, systemic VO(2) 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.

Figures

Figure 1. Left and right middle cerebral artery blood flow velocity (MCA Vmean) and near-infrared spectroscopy-determined cerebral tissue oxygenation at rest, during submaximal and maximal exercise and during 10 min of recovery in heat stress and control trials

MCA Vmean data are means ±s.e.m. for 6 subjects. Tissue oxygenation data are from a representative subject. Higher than value at start of maximal exercise, P < 0.05. † Lower than the peak value during maximal exercise, P < 0.05.

Figure 2. Arterial concentrations and arterial–internal jugular venous differences (a-v diff) for O2, lactate and glucose at rest, during submaximal and maximal exercise and during 10 min of recovery in heat stress and control trials

Data are means ±s.e.m. for 13 subjects. Different from the start of maximal exercise, P < 0.05. † Different from control, P < 0.05.

Figure 3. Cardiac output, heart rate, stroke volume, central venous pressure, mean arterial pressure, systemic vascular conductance, O2delivery, a-v O2diff, O2 extraction and O2uptake at rest, during submaximal and maximal exercise and during 10 min of recovery in heat stress and control trials

Data are means ±s.e.m. for 6 subjects. Lower than peak value during maximal exercise, P < 0.05.

Figure 4. Plasma ATP concentration at rest, during maximal exercise and during 10 min of recovery in heat stress and control trials

Data are means ±s.e.m. for 4–6 subjects. Different from value at the start of maximal exercise, P < 0.05.

Figure 5. Oxygen extraction across the brain, the systemic circulation and the exercising legs during maximal exercise in heat stress and control trials

Data on the exercising legs and brain were obtained in the same subjects in a previous study (González-Alonso & Calbet, 2003) and the present Study 1. Note the similarity of the average performance times measured in these subjects at two different time points separated by a year. Data on the systemic circulation were obtained in 6 different subjects for Study 2. Different from value at 0 min, P < 0.05.