The separate and combined influences of heat strain and dehydration upon physiological regulation and cognitive function

The influences of thermal strain and dehydration upon the regulation of body temperature, body-fluid and blood pressure, and the implications for cognitive function, are generally thought to be well known. However, much of the past research has not determined the affects of these separate and independent influences, due to the fact that both states are often simultaneously elicited, particularly during exercise. Therefore, four experimental studies were designed to evaluate the influences of hyperthermia, dehydration and exercise upon these regulatory systems, first separately, and then collectively. In the first two studies, the independent influences of moderate hyperthermia (mean body temperature: 38oC) and 3% to 5% dehydration (body-mass loss) were established in eight healthy, resting males by utilising a novel methodological approach, in which open-loop states were applied via whole-body thermal and hydration state clamping. Moderate hyperthermia resulted in increased cardiovascular strain, evidenced by significant elevations in skin blood flow (P0.05). Furthermore, a significant amount of fluid and electrolytes (P0.05), skin blood flow (P>0.05) and mean arterial pressure (P>0.05). These two states were then combined and their interactive influences were determined at rest in study three. No interactions between temperature and hydration level were present for the measures of body-fluid and cardiovascular function (P Finally, in study four physiological function was assessed in exercising individuals at different hydration levels (0%, 3% and 5% body-mass loss) to examine the separate and combined influences of exercise and dehydration. This was achieved by employing a thermally controlled exercise condition (recumbent cycling at 40% of maximal work rate, while mean body temperature remained at a thermoneutral level: 36oC), as well as a thermoneutral resting and a thermally uncontrolled exercise condition (identical exercise stimulus, but mean body temperature tracked changes in metabolic heat production resulting in mild hyperthermia: 36.5oC). Exercise per se resulted in a reduction in plasma volume (P0.05 for each comparison). However, when exercise to volitional fatigue was performed immediately after these submaximal exercise conditions, dehydration-mediated changes in body-fluid homeostasis resulted in cardiovascular limitations, thereby reducing maximal exercise performance in proportion to the level of dehydration. For the cognitive aspect of this project, another novel methodological approach was utilised. That is, the difficulty level of a visual perceptual and working memory task were matched (P>0.05), so that thermal and dehydration influences could be evaluated as a function of these cognitive domains, while the visual perceptual task was administered at an easy and difficult level of difficulty (P Interestingly, moderate hyperthermia and 3% to 5% dehydration had no adverse influences upon either task performance or brain electrical activity. Moderate hyperthermia resulted in faster reaction times independent of cognitive domain (P0.05) or dehydration (P>0.05). Furthermore, changes in brain electrical activity were limited to a reduction in beta power during moderate hyperthermia (P0.05 for each comparison). Moreover, the combination of these two states did not elicit interactive affects on these measures (P>0.05 for each comparison), indicating that healthy, resting individuals were able to tolerate these levels of hyperthermia and dehydration without impairing cognitive processes.