Muscle blood flow is reduced with dehydration during prolonged exercise in humans

Abstract

1. The present study examined whether the blood flow to exercising muscles becomes reduced when cardiac output and systemic vascular conductance decline with dehydration during prolonged exercise in the heat. A secondary aim was to determine whether the upward drift in oxygen consumption (VO2) during prolonged exercise is confined to the active muscles. 2. Seven euhydrated, endurance-trained cyclists performed two bicycle exercise trials in the heat (35 C; 40-50 % relative humidity; 61 +/- 2 % of maximal VO2), separated by 1 week. During the first trial (dehydration trial, DE), they bicycled until volitional exhaustion (135 +/- 4 min, mean +/- s.e.m.), while developing progressive dehydration and hyperthermia (3.9 +/- 0.3 % body weight loss; 39.7 +/- 0.2 C oesophageal temperature, Toes). In the second trial (control trial), they bicycled for the same period of time while maintaining euhydration by ingesting fluids and stabilizing Toes at 38.2 +/- 0.1 C after 30 min exercise. 3. In both trials, cardiac output, leg blood flow (LBF), vascular conductance and VO2 were similar after 20 min exercise. During the 20 min-exhaustion period of DE, cardiac output, LBF and systemic vascular conductance declined significantly (8-14 %; P < 0.05) yet muscle vascular conductance was unaltered. In contrast, during the same period of control, all these 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 flow 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 VO2 and leg VO2 increased in parallel and were similar throughout exercise. The reduced leg blood flow in DE was accompanied by an even greater increase in femoral arterial-venous O2 (a-vO2) difference. 5. It is concluded that blood flow to the exercising muscles declines significantly with dehydration, due to a lowering in perfusion pressure and systemic blood flow rather than increased vasoconstriction. Furthermore, the progressive increase in oxygen consumption during exercise is confined to the exercising skeletal muscles.

Figure 1. Cardiac output and blood flows during the dehydration and control trials

A, cardiac output; B, 2-legged blood flow; C, non-exercising tissue blood flow; D, forearm blood flow. Here and in subsequent figures: ▪, dehydration; □, control. * Significantly lower than 20 min value (P < 0.05). † Significantly lower than control (P < 0.05).

Figure 2. Mean arterial blood pressure and vascular conductances during the dehydration and control trials

A, mean arterial blood pressure; B, systemic vascular conductance; C, 2-legged vascular conductance; D, non-exercising tissues vascular conductance; E, forearm vascular conductance. Conductance units in B, C, D and E were multiplied by 10 (e.g. 10 l min⁻¹ mmHg⁻¹). * Significantly lower than 20 min value (P < 0.05). † Significantly lower than control (P < 0.05).

Figure 3. Arterial noradrenaline and adrenaline concentrations during the dehydration and control trials

A, arterial noradrenaline concentration; B, arterial adrenaline concentration. * Significantly higher than 20 min value (P < 0.05). † Significantly higher than control (P < 0.05).

Figure 4. Oxygen consumption during the dehydration and control trials

A, pulmonary oxygen consumption; B, 2-legged oxygen consumption; C, non-exercising tissues oxygen consumption. * Significantly higher than 20 min value (P < 0.05).

Figure 5. Oxygen parameters measured during the dehydration and control trials

A, leg femoral arterial-to-venous oxygen difference (Leg a-vO₂diff); B, arterial oxygen volume; C, femoral venous oxygen volume; D, leg oxygen delivery; E, leg oxygen extraction. * Significantly different from 20 min value during dehydration trial (P < 0.05). Femoral venous oxygen volume also declines significantly over time during control. † Significantly different from control (P < 0.05).