Dehydration affects cerebral blood flow but not its metabolic rate for oxygen during maximal exercise in trained humans

Intense exercise is associated with a reduction in cerebral blood flow (CBF), but regulation of CBF during strenuous exercise in the heat with dehydration is unclear. We assessed internal (ICA) and common carotid artery (CCA) haemodynamics (indicative of CBF and extra-cranial blood flow), middle cerebral artery velocity (MCA V-mean), arterial-venous differences and blood temperature in 10 trained males during incremental cycling to exhaustion in the heat (35 degrees C) in control, dehydrated and rehydrated states. Dehydration reduced body mass (75.8 +/- 3 vs. 78.2 +/- 3 kg), increased internal temperature (38.3 +/- 0.1 vs. 36.8 +/- 0.1 degrees C), impaired exercise capacity (269 +/- 11 vs. 336 +/- 14 W), and lowered ICA and MCA V-mean by 12-23% without compromising CCA blood flow. During euhydrated incremental exercise on a separate day, however, exercise capacity and ICA, MCA V-mean and CCA dynamics were preserved. The fast decline in cerebral perfusion with dehydration was accompanied by increased O-2 extraction (P < 0.05), resulting in a maintained cerebral metabolic rate for oxygen (CMRO2). In all conditions, reductions in ICA and MCA V-mean were associated with declining cerebral vascular conductance, increasing jugular venous noradrenaline, and falling arterial carbon dioxide tension (P-aCO2) (R-2 >= 0.41, P <= 0.01) whereas CCA flow and conductance were related to elevated blood temperature. In conclusion, dehydration accelerated the decline in CBF by decreasing P-aCO2 and enhancing vasoconstrictor activity. However, the circulatory strain on the human brain during maximal exercise does not compromise CMRO2 because of compensatory increases in O-2 extraction.