Time to reach equilibrium deep body temperatures in young and older adults resting in the heat: A descriptive secondary analysis.

ABSTRACT

It is commonly thought that steady-state thermoregulatory responses are achieved within 30-90 min of compensable heat stress. However, this assumption is based on measurements of whole-body heat exchange during exercise, which stabilize (equilibrate) more rapidly than deep body temperatures, especially under resting conditions. To support the design of ecologically relevant heat exposure studies, we quantified equilibrium times for deep body temperature, as indexed by rectal temperature, in young and older adults resting in the heat. We also evaluated the lag in rectal temperature equilibrium relative to whole-body heat storage (direct calorimetry). Equilibrium times were estimated with data from two laboratory-based trials (NCT04353076 and NCT04348630) in which 83 adults aged 19-80 years (34 female) were exposed to simulated heat wave conditions for 8-9 hours. When assessed on the group level, it took rectal temperature 3.3 [bootstrap 95%CI 2.9 to 3.9] hours to reach thermal equilibrium (<0.05°C/hour rate of change) in young adults exposed to 40°C, 9% relative humidity (RH). In older adults, who were exposed to a greater range of conditions (31-40°C, 9-45% RH), equilibrium times were longer, ranging from 4.4 [3.8 to 5.3] to 5.2 [4.9 to 5.4] hours. Furthermore, rectal temperature equilibrium was delayed 0.9 [0.5 to 1.4] and 1.8 [0.9 to 2.7] hours compared to whole-body heat storage in young and older adults, respectively (only assessed in 40°C, 9% RH). Individual level equilibrium times ranged from 1-8 hours. These findings highlight the importance of ecologically relevant exposure durations in translational research assessing the physiological impacts of hot weather.