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

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 preexercise euhydrated body mass (EUH; -0.6 ± 0.4%) or becoming progressively dehydrated (DEH; -3.6 ± 0.7%). Rectal (T _re ) and skin (T _sk ) temperatures, heart rate (HR), LV volumes and function, systemic hemodynamics and BV were measured at rest and during bouts of semirecumbent cycling (55% V̇o _2max ) at 20, 100 and 180 min, interspersed by periods of upright exercise. T _re , 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 T _re (0.6 ± 0.3°C) and HR (16 ± 7 beats/min) and lower end-diastolic volume (29 ± 16 mL), stroke volume (26 ± 16 mL), and Q̇ (2.1 ± 0.8 L/min) compared with 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. NEW & NOTEWORTHY This study demonstrates that 10 days of exercise heat acclimation has minimal effects on left ventricular volumes, intrinsic cardiac function, and systemic hemodynamics during prolonged, repeated semirecumbent exercise in moderate heat, where heart rate and blood volume are similar to preacclimation levels. However, progressive dehydration is consistently associated with similar degrees of hyperthermia and tachycardia and reductions in blood volume, diastolic filling of the left ventricle, stroke volume, and cardiac output, regardless of acclimation state.