Onset of cardiovascular drift during progressive heat stress in young adults (PSU HEAT project)

With climate change, humans are at a greater risk for heat-related morbidity and mortality, often secondary to increased cardiovascular strain associated with an elevated core temperature (T_c). Critical environmental limits (i.e., the upper limits of compensable heat stress) have been established based on T_c responses for healthy, young individuals. However, specific environmental limits for the maintenance of cardiovascular homeostasis have not been investigated in the context of thermal strain during light activity. Therefore, the purposes of this study were to 1) identify the specific environmental conditions (combinations of ambient temperature and water vapor pressure) at which cardiovascular drift [i.e., a continuous rise in heart rate (HR)] began to occur and 2) compare those environments to the environmental limits for the maintenance of heat balance. Fifty-one subjects (27 F; 23 ± 4 yr) were exposed to progressive heat stress across a wide range of environmental conditions in an environmental chamber at two low metabolic rates reflecting minimal activity (MinAct; 159±34 W) or light ambulation (LightAmb; 260±55 W). Whether systematically increasing ambient temperature or humidity, the onset of cardiovascular drift occurred at lower environmental conditions compared with T_c inflection points at both intensities (P < 0.05). Furthermore, the time at which cardiovascular drift began preceded the time of T_c inflection (MinAct P ¼ 0.01; LightAmb P ¼ 0.0002), and the difference in time between HR and T_c inflection points did not differ (MinAct P ¼ 0.08; LightAmb P ¼ 0.06) across environmental conditions for either exercise intensity. These data suggest that even in young adults, increases in cardiovascular strain precede the point at which heat stress becomes uncompensable during light activity.