Fluid restriction during exercise in the heat reduces tolerance to progressive central hypovolaemia

Abstract

This study tested the hypothesis that dehydration induced via exercise in the heat impairs tolerance to central hypovolaemia. Eleven male subjects (32 ± 7 years old, 81.5 ± 11.1 kg) walked (O2 uptake 1.7 ± 0.4 l min−1) in a 40°C, 30% relative humidity environment on three occasions, as follows: (i) subjects walked for 90 min, drinking water to offset sweat loss (Hydrated, n =11); (ii) water intake was restricted, and exercise was terminated when intestinal temperature increased to the same level as in the Hydrated trial (Isothermic Dehydrated, n = 11); and (iii) water intake was restricted, and exercise duration was 90 min (Time Match Dehydrated, n = 9). For each trial, tolerance to central hypovolaemia was determined following exercise via progressive lower body negative pressure and quantified as time to presyncope. Increases in intestinal temperature prior to lower body negative pressure were not different (P = 0.91) between Hydrated (1.1 ± 0.4°C) and Isothermic Dehydrated trials (1.1 ± 0.4°C), but both were lower than in the Time Match Dehydrated trial (1.7 ± 0.5°C, P < 0.01). Prior to lower body negative pressure, body weight was unchanged in the Hydrated trial (−0.1 ± 0.2%), but was reduced in Isothermic Dehydrated (−0.9 ± 0.4%) and further so in Time Match Dehydrated trial (−1.9 ± 0.6%, all P < 0.01). Time to presyncope was greater in Hydrated (14.7 ± 3.2 min) compared with Isothermic Dehydrated (11.9 ± 3.3 min, P < 0.01) and Time Match Dehydrated trials (10.2 ± 1.6 min, P = 0.03), which were not different (P = 0.19). These data indicate that inadequate fluid intake during exercise in the heat reduces tolerance to central hypovolaemia independent of increases in body temperature.

Transition duration of ingested deuterium oxide to eccrine sweat during exercise in the heat

“…During these conditions, sweat evaporation from the skin provides a means by which heat can dissipate to the environment, attenuating hyperthermia (Adams et al, 1975;Buono and Wall, 2000). However, continuous sweat production, without adequate fluid ingestion, eventually leads to dehydration, which results in reduced sweat production, and ultimately a rise in core temperature (Schlader et al, 2015). As such, during exercise in the heat an individual must ingest water to replace fluid that was lost as sweat.…”

“…Second, water intake maintains sweat production at levels sufficient to support evaporative cooling, thus attenuating hyperthermia (Sawka et al, 1985;Buono and Wall, 2000). In support of these two statements, failure to consume adequate water to prevent dehydration during exercise results in hypovolemia, hemoconcentration, decreased sweat production, and hyperthermia (Schlader et al, 2015).…”

The Effect of Passive Heat Stress and Exercise-Induced Dehydration on the Compensatory Reserve During Simulated Hemorrhage

“…Prior to the current study, however, it remained unknown if the compensatory reserve index appropriately tracks reductions in the physiological reserve to compensate for central blood volume loss during conditions often encountered by individuals at relatively greater risk of hemorrhagic injury (16–18), such as military personnel, firefighters, miners, etc. We therefore examined the effect of two conditions that reduce tolerance to simulated hemorrhage and that are often encountered in a field setting; whole-body passive heat stress and exercise-induced dehydration (19, 20). Both conditions reduced compensatory reserve prior to the onset of simulated hemorrhage (Fig.…”

“…For protocol #1, a subset of the data investigating tissue oxygen saturation during simulated hemorrhage has been published (21), while a subset of the data investigating the effects of dehydration on tolerance to simulated hemorrhage has been published for protocol #2 (19). Herein, measurements of compensatory reserve obtained as part of these experimental protocols are presented.…”

“…Passive heat stress and exercise-induced dehydration are two conditions often encountered during military activities and in civilian settings (16–18). Both of these conditions reduce human tolerance to reductions in central blood volume (19, 20) and therefore represent ideal scenarios to test the ability of the compensatory reserve index to track compensatory reserve during stressors encountered in the field. The primary purpose of this study was to test the hypothesis that compensatory reserve is reduced by passive heat stress and exercise-induced dehydration, prior to further reductions in central blood volume.…”