ORIGINAL ARTICLE Heart rate variability and baroreceptor sensitivity following exercise-induced hyperthermia in endurance trained men Rachel G. Armstrong • Saif Ahmad • Andrew J. Seely • Glen P. Kenny Received: 27 September 2010 / Accepted: 28 April 2011 Ó Springer-Verlag 2011 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 C40°C) and repeated after 24 and 48 h of recovery. Heart rate, stroke volume (SV), mean arterial pressure and cardiac output ( _ 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, _ 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 _ 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. Parasympa- thetic 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 mod- ulation 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 car- diovascular function is restored following 24 h of recovery. Keywords Postural stress Á Exercise Á Heart rate variability Á Heat stress Introduction It is well documented that the autonomic nervous system (ANS) plays a key role in the regulation of cardiovascular function during and following a dynamic bout of exercise (Brenner et al. 1997; Iellamo 2001; Parekh and Lee 2005). Heart rate variability (HRV), which represents the time difference between repeated heart beats (Flouris and Che- ung 2009), and baroreceptor sensitivity (BRS), which represents the amount of change in heart rate attributable to changes in systolic blood pressure (Ogoh et al. 2005), can be used as a reflection of the cardiorespiratory control system providing information regarding the balance between sympathetic and parasympathetic function of the ANS (van Ravenswaaij-Arts et al. 1993). By performing an intermittent orthostatic challenge of 70° head-up tilt we induced changes in baroreceptor loading status, invoking a compensatory cardiovascular reflex response (Rowell et al. 1973) isolating the effects of a severe hyperthermic episode on ANS function. Independent factors such as exercise (Armstrong et al. 2010; Brenner et al. 1997; Parekh and Lee 2005), passive heat exposure (Brenner et al. 1997, 1998) and/or orthostatic Communicated by Narihiko Kondo. R. G. Armstrong Á G. P. Kenny (&) 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 e-mail: gkenny@uottawa.ca S. Ahmad Á A. J. Seely Divisions of Thoracic Surgery and Critical Care Medicine, Ottawa Health Research Institute, Ottawa Hospital-General Campus, Ottawa, ON K1H 8L6, Canada 123 Eur J Appl Physiol DOI 10.1007/s00421-011-1989-x