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 ($\dot{Q}$) 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, $\dot{Q}$ 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 $\dot{Q}$ 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.