Heat acclimation with controlled heart rate: the effect of hydration on adaptation, cardiac function and exercise performance

Abstract

The effects of hydration status during heat acclimation on adaptation, and its influence on acute thermoregulatory and cardiovascular responses to exercise in heat acclimated individuals remains contentious. The aims of this thesis were to 1) characterise the responses to heat acclimation with controlled heart rate and the effect hydration status had on adaptive responses, 2) investigate the effectiveness of these interventions on exercise performance and, 3) determine the acute effects of maintaining euhydration or allowing progressive dehydration on central haemodynamics and thermoregulation during prolonged submaximal exercise. Chapter 4 reported responses to heat acclimation with both maintained euhydration and matched levels of dehydration in a counterbalanced cross-over study. Euhydrated acclimation increased sweat rate, lowered skin temperature and improved cycling time trial performance in the heat. These responses were not observed with dehydrated acclimation. Neither intervention lowered core temperature or increased plasma volume at rest or increased maximal aerobic cycling capacity in a temperate environment. Chapters 5 and 6 explored the haematological, thermal and haemodynamic responses to prolonged submaximal exercise following euhydrated and dehydrated heat acclimation, respectively. Responses were compared to pre-acclimation euhydrated and dehydrated trials with matched body mass deficits. Prior to both interventions, dehydration resulted in a reduction in cardiac output and mean arterial pressure. This was associated with hyperthermia, a decline in blood volume and increased heart rate impairing ventricular filling as end diastolic and stroke volumes were significantly lower than euhydration whilst end systolic volume remained similar. With acclimation, in the absence of an increased plasma or blood volume, stroke volume was not augmented by either intervention when euhydrated. Furthermore, with matched progressive dehydration, neither acclimation intervention altered the responses seen in pre-acclimation trials. These findings highlight the persistent effect of dehydration on the development of thermal and cardiovascular strain during exercise in the heat despite acclimation.