Improvements in Orthostatic Tolerance with Exercise Are Augmented by Heat Acclimation: A Randomised Controlled Trial

Abstract

Introduction Heat adaptation is protective against heat illness however its role in heat syncope, due to reflex mechanisms, has not been conclusively established. The aim of this study was to evaluate if heat acclimation (HA) was protective against heat syncope and to ascertain underlying physiological mechanisms. Methods 20 (15 males, 5 females) endurance trained athletes were randomised to either 8 days of mixed active and passive HA (HEAT) or climatically temperate exercise (CONTROL). Prior to, and following, the interventions participants underwent a head up tilt (HUT) with graded lower body negative pressure (LBNP), in a thermal chamber (32.0 ± 0.3 °C), continued until presyncope with measurement of cardiovascular parameters. Heat stress tests (HST) were performed to determine physiological and perceptual measures of HA. Results There was a significant increase in orthostatic tolerance (OT), as measured by HUT/LBNP, in the HEAT group (pre-intervention; 28 ± 9 mins, post-intervention; 40 ± 7 mins) compared to CONTROL (pre-intervention; 30 ± 8 mins, post-intervention; 33 ± 5 mins) (p = 0.01). HA resulted in a significantly reduced peak and mean rectal and skin temperature (p < 0.01), peak heat rate (p < 0.003), thermal comfort (p < 0.04) and rating of perceived exertion (p < 0.02) during HST. There was a significantly increased plasma volume (PV) in the HEAT group in comparison to CONTROL (p = 0.03). Conclusions HA causes improvements in OT and is likely to be beneficial in patients with heat exacerbated reflex syncope. HA mediated PV expansion is a potential physiological mechanism underlying improved OT.