Heart rate responses during progressive heat stress in young, healthy adults (PSU HEAT Project)
Authors: Cottle Rachel , Fisher Kathleen , Wolf S. Tony , Kenney W. Larry 
Published: May 2023

As a consequence of climate change, humans are at greater risk for heat-related morbidity and mortality, often secondary to increased cardiovascular strain associated with heat stress. Critical environmental limits (i.e., upper limits for compensable heat stress) have been established based on core temperature (Tc) responses for healthy, young individuals during progressive heat stress at metabolic rates approximating those of activities of daily living. However, it is crucial to understand the specific combinations of temperature and humidity above which cardiovascular strain increases, placing individuals at elevated risk. These upper limits for the maintenance of cardiovascular homeostasis, and how they compare to the upper limits for the maintenance of heat balance, have not been investigated. Therefore, the purposes of this study were to (1) identify the specific environmental conditions (ambient temperature and humidity) at which elevated cardiovascular strain (i.e., a continuous rise in heart rate) increases and (2) compare the environments at which increases in cardiovascular strain occur relative to those at which a continuous rise in Tc is observed. Forty-five subjects (26 W; 23±4 yrs) were exposed to progressive heat stress in an environmental chamber at two low metabolic rates reflecting minimal activity (MinAct; 159 ± 34 W) or light ambulation (LightAmb; 260 ± 55 W) in warm-humid (~35°C, 50-80%RH) and/or hot-dry (38°C-52°C, <30%RH) environments. Each subject completed 1-4 experimental trials randomized for metabolic/environmental conditions. The points at which heart rate and Tc equilibrium could no longer be maintained were identified. In warm-humid environments, increases in cardiovascular strain significantly preceded the Tc inflection point for MinAct (35°C, 65±7% rh vs.35°C, 73±8% rh; p<0.001) and LightAmb (35°C, 52±7% rh vs.35°C, 58±7% rh; p<0.001). Similarly, in hot-dry environments, increases in cardiovascular strain significantly preceded the Tc inflection point for MinAct (45±3°C, 19% rh vs.48±3°C, 17% rh; p<0.001) and LightAmb (40±2°C, 25% rh vs.43±2°C, 22% rh; p<0.001). Together, these data suggest that even in healthy young adults, increases in cardiovascular strain precede the point at which heat stress becomes uncompensable at metabolic intensities reflecting activities of daily living and light ambulation. Additionally, the results presented herein provide critical environmental limit data that can be used in policy decisions, evidence-based alert communications, and safety guidelines to mitigate cardiovascular-related morbidity and mortality during extreme heat events. This research was supported by the National Institute on Aging Grant T32 AG049676 to the Pennsylvania State University (to R.M.C.) and National Institutes of Health Grant R01 AG067471 (to W.L.K.) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.