Exercise heat acclimation has minimal effects on left ventricular volumes, function and systemic hemodynamics in euhydrated and dehydrated trained humans

Abstract

Heat acclimation (HA) may improve the regulation of cardiac output (Q̇) through increased blood volume (BV) and left ventricular (LV) diastolic filling, and attenuate reductions in Q̇ during exercise-induced dehydration; however, these hypotheses have never been directly tested. Before and following 10-days exercise HA, eight males completed two trials of submaximal exercise in 33°C and 50% relative humidity while maintaining pre-exercise euhydrated body mass (EUH; -0.6±0.4%) or becoming progressively dehydrated (DEH; -3.6±0.7%). Rectal (Tre) and skin (Tsk) temperatures, heart rate (HR), LV volumes and function, systemic hemodynamics and BV were measured at rest and during bouts of semi-recumbent cycling (55% V̇O2max) at 20, 100 and 180 min, interspersed by periods of upright exercise. Tre, BV, HR, LV volumes, LV systolic and diastolic function and systemic hemodynamics were similar between trials at rest and during the first 20 min of exercise (all P>0.05). These responses were largely unaffected by HA at 180 min in either hydration state. However, DEH induced higher Tre (0.6±0.3°C) and HR (16±7 beats.min-1) and lower stroke volume (26±16 ml), end-diastolic volume (29±16 ml) and Q̇ (2.1±0.8 L.min-1) compared to EUH at 180 min (all P<0.05), yet LV twist and untwisting rate were increased or maintained (P=0.028 and 0.52, respectively). Findings indicate HA has minimal effects on LV volumes, LV mechanical function and systemic hemodynamics during submaximal exercise in moderate heat where HR and BV are similar. In contrast, DEH evokes greater hyperthermia and tachycardia, reduces BV, and impairs diastolic LV filling, lowering Q̇, regardless of HA state.

Exercise Heat Acclimation With Dehydration Does Not Affect Vascular and Cardiac Volumes or Systemic Hemodynamics During Endurance Exercise

“…As we have demonstrated previously, LV diastolic or systolic function at rest and during exercise-induced dehydration >3% were not altered after 10 days of euhydrated exercise HA ( Travers et al, 2020a ). Instead, LV diastolic filling was impaired alongside similar reductions in BV and elevations in HR both pre- and post-HA.…”

“…Therefore, data are presented for the seven participants who completed both HA interventions ( Figure 1 ). Data from the participants that completed the euhydrated HA intervention have been published as part of a larger cohort ( Travers et al, 2020a ). Participants were all trained cyclists and triathletes regularly undertaking >5 h of endurance training per week and were non-native residents of Qatar.…”

“…Together with a decreased HR, a greater PV and therefore total blood volume (BV) is proposed to support the diastolic filling of the left ventricle (LV) (Wyndham et al, 1968;Senay, 1986), such that stroke volume (SV) may be increased during exercise in the heat (Rowell et al, 1967;Wyndham et al, 1968;Nielsen et al, 1993). Recently, however, our own work and that of others has demonstrated that both passive HA (Trachsel et al, 2020) and exercise HA with maintained euhydration (Travers et al, 2020a) had minimal effects on LV volumes, function and systemic hemodynamics when PV and HR were similar pre-to post-HA.…”

Effect of Exercise-Induced Reductions in Blood Volume on Cardiac Output and Oxygen Transport Capacity

“…This view is supported by Kanstrup and Ekblom (1982), who demonstrated that an increase in PV by a 700 mL dextran infusion augments SV at rest and throughout exercise and thereforeQ. In terms of favorable changes inVO 2max following heat training, there is still a lack of evidence in the literature (Travers et al, 2020;Baranauskas et al, 2021).…”

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

“…plasma and erythrocyte) that occurs with an increase in fitness following aerobic training, which is considered an important contributor to improvements in performance via larger stroke volume and greater ventricular filling (44,407). Recent studies however, have reported that 5-day passive (980) and 10-day exercise-heat (987) acclimation have minimal effects on left-ventricular volumes, function and systemic hemodynamics at rest and during exercise. It must be noted that post-acclimation echocardiography measurements in these studies were undertaken at similar core temperatures and heart rates as pre-acclimation, along with a similar plasma volume (1-3% expansion).…”