Muscle blood flow is reduced with dehydration during prolonged exercise inhumans

1. The present study examined whether the blood flow to exercising muscles becomes reduced when cardiac output and systemic vascular conductance decli ne with dehydration during prolonged exercise in the heat. A secondary aim was to determine whether the upward drift in oxygen consumption ((V) over d ot(O2)) during prolonged exercise is confined to the active muscles. 2. Seven euhydrated, endurance-trained cyclists performed two bicycle exerc ise trials in the heat (35 degrees C; 40-50% relative humidity; 61 +/- 2% o f maximal (V) over dot(O2)), separated by 1 week. During the first trial (d ehydration trial, DE), they bicycled until volitional exhaustion (135 +/- 4 min, mean +/- S.E.M.), while developing progressive dehydration and hypert hermia (3.9 +/- 0.3% body weight loss; 39.7 +/- 0.2 degrees C oesophageal t emperature, T-oes). In the second trial (control trial), they bicycled for the same period of time while maintaining euhydration by ingesting fluids a nd stabilizing T-oes at 38.2 +/- 0.1 degrees C after 30 min exercise. 3. In both trials, cardiac output, leg blood flow (LBF), vascular conductan ce and (V) over dot(O2) were similar after 20 min exercise. During the 20 m in-exhaustion period of DE, cardiac output, LBF and systemic vascular condu ctance declined significantly (8-14%; P < 0.05) yet muscle vascular conduct ance was unaltered. In contrast, during the same period of control, all the se cardiovascular variables tended to increase. After 135 +/- 4 min of DE, the 2.0 +/- 0.6 l min(-1) lower blood flow to the exercising legs accounted for approximately two-thirds of the reduction in cardiac output. Blood flo w to the skin also declined markedly as forearm blood flow was 39 +/- 8 % ( P < 0.05) lower in DE vs, control after 135 +/- 4 min. 4. In both trials, whole body (V) over dot(O2) and leg (V) over dot(O2) inc reased in parallel and were similar throughout exercise. The reduced leg bl ood flow in DE was accompanied by an even greater increase in femoral arter ial-venous O-2 (a-vO(2)) difference. 5. It is concluded that blood flow to the exercising muscles declines signi ficantly with dehydration, due to a lowering in perfusion pressure and syst emic blood flow rather than increased vasoconstriction. Furthermore, the pr ogressive increase in oxygen consumption during exercise is confined to the exercising skeletal muscles.