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

Area of Science:

• Exercise Physiology
• Environmental Medicine
• Cardiovascular Physiology

Background:

• Heat adaptation is known to protect against heat illness.
• The protective role of heat acclimation against heat syncope, a condition linked to reflex mechanisms, requires further investigation.

Purpose of the Study:

• To evaluate the protective effects of heat acclimation (HA) against heat syncope.
• To identify the underlying physiological mechanisms responsible for any observed protective effects.

Main Methods:

• Twenty endurance-trained athletes were randomized into a heat acclimation (HEAT) group or a control (CONTROL) group for 8 days.
• Participants underwent head-up tilt (HUT) with lower body negative pressure (LBNP) and heat stress tests (HST) to assess orthostatic tolerance and physiological responses before and after interventions.

Main Results:

• The HEAT group showed a significant increase in orthostatic tolerance (OT) compared to the CONTROL group (40 ± 7 min vs. 33 ± 5 min).
• Heat acclimation led to reduced rectal and skin temperatures, heart rate, and perceived exertion during heat stress.
• Significantly increased plasma volume (PV) was observed in the HEAT group compared to the CONTROL group.

Conclusions:

• Heat acclimation enhances orthostatic tolerance, indicating its potential benefit for individuals experiencing heat-exacerbated reflex syncope.
• Heat acclimation-induced plasma volume expansion is a likely physiological mechanism contributing to improved orthostatic tolerance.