title: Heart rate variability and baroreceptor sensitivity following exercise-induced hyperthermia in endurance trained men.
abstract: We evaluated the effect of exercise-induced hyperthermia (EIH) on autonomic nervous system (ANS) function in the early (<80 min) and late (24 and 48 h) stages of recovery. Eight males underwent three repeated 6 min 70° head-up tilts (HUT1, HUT2 and HUT3), each separated by 10-min supine rest in a non-exercise/non-heat stress control state (NHS). On a separate day, three 6 min 70° HUT were performed following EIH (esophageal temperature ≥ 40°C) and repeated after 24 and 48 h of recovery. Heart rate, stroke volume (SV), mean arterial pressure and cardiac output ([Formula: see text]) were evaluated during the last min prior to a change in posture. Responses to 70° HUT were compared to the same challenge performed without prior exercise and under a NHS condition. Relative to NHS, [Formula: see text] was maintained during the repeated HUT's following EIH, despite significant reductions in SV and sustained elevations in esophageal temperature (p < 0.05). The preserved [Formula: see text] appears to be due to increased HR (HUT1: NRS = 76 ± 3 beats min(-1), EIH = 126 ± 6 beats min(-1)) stemming from modulation of the ANS toward sympathetic dominance. Parasympathetic withdrawal was evidenced by a reduction in root mean squared successive difference (i.e., HUT1: NHS = 66 ± 12 ms, EIH = 9 ± 1 ms) of heart rate variability and paralleled by a reduction in baroreceptor sensitivity for all HUT's following EIH (p < 0.05). Despite significant modulation in ANS activity, Q is maintained and participants do not become orthostatic intolerant/syncopal during the short-term recovery period following EIH. Normal ANS and cardiovascular function is restored following 24 h of recovery.
authors.affiliation: Human and Environmental Physiology Research Unit, Faculty of Health Sciences, School of Human Kinetics, University of Ottawa, 125 University, Montpetit Hall, Ottawa, ON, K1N 6N5, Canada.
authors.forename: Rachel G
authors.initials: RG
authors.lastname: Armstrong
authors.forename: Saif
authors.initials: S
authors.lastname: Ahmad
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Seely
authors.forename: Glen P
authors.initials: GP
authors.lastname: Kenny
mesh_terms: D017704:Baroreflex; D005080:Exercise Test / Q000009:adverse effects*; D005260:Female; D005334:Fever / Q000209:etiology* / Q000503:physiopathology*; D006339:Heart Rate; D006801:Humans; D008297:Male; D010807:Physical Endurance; D011311:Pressoreceptors / Q000503:physiopathology*; D055815:Young Adult