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Does passive heat acclimation impact the athlete's heart continuum?

Differentiating physiological adaptation from cardiac pathology can prove challenging in athletes, as the nature and magnitude of cardiac remodelling in response to exercise are governed by several important factors. These factors include age, sex, anthropometry, ethnicity and sporting discipline. One element not routinely considered in the athlete’s heart continuum, is the impact of repeated exposure to hot environmental conditions. Several major sporting events take place in the summer months, often in hot ambient conditions (e.g. 2004 Athens, 2008 Beijing and 2016 Rio Olympics, 2015 Beijing and 2019 Doha IAAF World Championships). The 2020 Tokyo Olympics are predicted to be the hottest on record. Exercising in the heat induces thermoregulatory and physiological strain, which in turn lead to reductions in endurance exercise capacity. Athletes will accordingly train repeatedly in a hot environment prior to a competition as a method to offset these potential performance impairments. Most natural acclimatisation (or artificial acclimation) protocols require athletes to undertake daily exposure to the heat for up to 14 days, in an attempt to increase whole-body (core and skin) temperature, induce sweating and increase skin blood flow. Heat acclimation, also induced by passive heating, elicits cardiovascular adaptations allowing for a lowered heart rate, an increased stroke volume, a better sustained _ Q, a better defended blood pressure and an increased myocardial efficiency and compliance while exercising in the heat. However, the repercussions of repeated heat exposure on cardiac structure remain unknown. Accordingly, we aimed to isolate the influence of repeated heat exposure on the myocardium, without the confounding factor of exercise, by examining the impact of 12 days of passive heat acclimation (PHA) on the resting cardiac structure and function of welltrained male athletes. Twelve healthy male participants (aged 32.7 9.3 years, height 177.4 7.8 cm, body mass 74 7 kg, and regular physical activity 7 2.3 hours per week) undertook a resting two-dimensional transthoracic echocardiographic examination (i33 Philips, USA) in a temperate environment (24 C and 40% relative humidity (RH)) in a normothermic state, the day before and the day following a 12-day PHA protocol. The study was conducted at the end of the winter in order to avoid possible heat acclimatisation. Participants were instructed to maintain their normal training routines in the weeks prior to and during the study, in order to avoid excess increases and decreases in training load. All participants were asked to refrain from exercise and alcohol consumption at least 24 hours, and abstain from caffeine/stimulant containing drinks 6 hours prior to echocardiography. Both echocardiographic examinations were performed at the same time of day to avoid diurnal variations. After each echocardiographic examination, a 10ml venous blood sample was collected from an antecubital vein to ascertain haemoglobin and haematocrit concentrations (Coulter DxH 800), from which possible changes in blood volume or plasma volume following PHA were calculated using the Dill and Costill formula. PHA consisted of 12 daily sessions of 60 minutes of heat exposure in a seated position in an environmental chamber set to 50 C and 50% RH for the first 10 minutes and then adjusted to 48 C and 50% RH for the remaining 50 minutes. Participants reached a rectal (Ellab, Hilleroed, Denmark) temperature above 38.5 C in 97% of the sessions. Data are presented as mean standard deviation. All echocardiographic parameters were assessed for normality using the Shapiro–Wilk test prior to data analysis. Pre and post-PHA values of body mass, heart rate, haematocrit and all echocardiographic parameters were assessed using a paired samples t-test.