AUTHOR=Kampmann Bernhard , Bröde Peter 

TITLE=Heat Acclimation Does Not Modify Q10 and Thermal Cardiac Reactivity

JOURNAL=Frontiers in Physiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.01524

DOI=10.3389/fphys.2019.01524

ISSN=1664-042X

ABSTRACT=<p>Heat acclimation (HA) is an essential modifier of physiological strain when working or exercising in the heat. It is unknown whether HA influences the increase of energy expenditure (<italic>Q</italic><sub>10</sub> effect) or heart rate (thermal cardiac reactivity TCR) due to increased body temperature. Therefore, we studied these effects using a heat strain database of climatic chamber experiments performed by five semi-nude young males in either non-acclimated or acclimated state. Measured oxygen consumption rate (VO<sub>2</sub>), heart rate (HR), and rectal temperature (<italic>T</italic><sub>re</sub>) averaged over the third hour of exposure were obtained from 273 trials in total. While workload (walking 4 km/h on level) was constant, heat stress conditions varied widely with air temperature 25–55°C, vapor pressure 0.5–5.3 kPa, and air velocity 0.3–2 m/s. HA was induced by repeated heat exposures over a minimum of 3 weeks. Non-acclimated experiments took place in wintertime with a maximum of two exposures per week. The influence of <italic>T</italic><sub>re</sub> and HA on VO<sub>2</sub> and HR was analyzed separately with mixed model ANCOVA. Rising <italic>T</italic><sub>re</sub> significantly (<italic>p</italic> &lt; 0.01) increased both VO<sub>2</sub> (by about 7% per degree increase of <italic>T</italic><sub>re</sub>) and HR (by 39–41 bpm per degree <italic>T</italic><sub>re</sub>); neither slope nor intercept depended significantly on HA (<italic>p</italic> &gt; 0.4). The effects of <italic>T</italic><sub>re</sub> in this study agree with former outcomes for VO<sub>2</sub> (7%/°C increase corresponding to <italic>Q</italic><sub>10</sub> = 2) and for HR (TCR of 33 bpm/°C in ISO 9886). Our results indicate that both relations are independent of HA with implications for heat stress assessment at workplaces and for modeling heat balance.</p>