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

Abstract

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 (V̇O2) during prolonged exercise is confined to the active muscles. Seven euhydrated, endurance‐trained cyclists performed two bicycle exercise trials in the heat (35 °C; 40–50% relative humidity; 61 ± 2% of maximal V̇O2), 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. In both trials, cardiac output, leg blood flow (LBF), vascular conductance and V̇O2 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. In both trials, whole body V̇O2 and leg V̇O2 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. 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.

Induction and decay of short-term heat acclimation

“…Heatand-exercise induced hypohydration manifests itself as hyperosmotic hypovolaemia (Nadel et al 1980), which impairs cutaneous blood flow and sweat rate, and raises heart rate, core temperature, glycogenolysis, perceived exertion and permeability of tight-membranes (Mack and Nadel 1996;Gonzalez-Alonso et al 1998;Gonzalez-Alonso et al 1999;Blatteis 2000;Gonzalez-Alonso and Calbert 2003;Maughan 2003;Watson et al 2005). The consequence is impaired exercise tolerance and performance in temperate conditions which has previously been debated (Sawka and Noakes 2007).…”

The Different Effects of Fluid with and without Carbohydrate Ingestion on Subjective Responses of Untrained Men during Prolonged Exercise in a Hot Environment

“…However, Gonzalez-Alonso et al suggested that despite cardiovascular drift being markedly incurred at the 120th minute of exercise, corresponding to ~60% V O2max in a dehydration trial, the increase in V O2 for exercising and nonexercising muscles of the whole body was similar to the euhydration trial (2). Additionally, Vallier et al observed that VO 2 was not different in trials with or without fluid ingestion; however, VO 2 significantly increased without fluid ingestion in a trial involving 3 h of cycling at 60% VO 2max in a hot environment (4).…”

“…Fluid ingestion, especially in a hot environment, attenuates hyperthermia and cardiovascular drift (1)(2)(3)(4), and carbohydrate ingestion reduces the rating of perceived exertion (RPE) during prolonged exercise in endurance-trained subjects (5)(6)(7)(8)(9)(10). However, there is no available data about the physiological and subjective benefits of maintaining body water by ingesting fluid with or without carbohydrate supplementation during prolonged submaximal exercise in untrained healthy subjects.…”

“…Gonzalez-Alonso et al suggested that despite a marked cardiovascular drift at the 120 min of exercise, corresponding to ~60% maximal oxygen consumption (V O 2 max ) in a dehydration trial, the increase in V O 2 for exercising and nonexercising muscle of the whole body was similar to a euhydration trial ( 2 ). Additionally, it has been demonstrated that during prolonged exercise an increase in carbohydrate availability by carbohydrate supplementation attenuated RPE ( 8 , 9 , 11 ) while fluid ingestion did not attenuate the RPE ( 4 , 12 ).…”

“…Maintaining euhydration by fluid ingestion attenuates cardiovascular strain induced by both hyperthermia and dehydration during prolonged submaximal exercise in the heat (2)(3)(4). Gonzalez-Alonso et al suggested that despite a marked cardiovascular drift at the 120 min of exercise, corresponding to ~60% maximal oxygen consumption (V O 2 max ) in a dehydration trial, the increase in V O 2 for exercising and nonexercising muscle of the whole body was similar to a euhydration trial ( 2 ).…”