Human and Environmental Physiology Research Unit, Department of Health Sciences, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada.
Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada.
Am J Physiol Regul Integr Comp Physiol. 2024 Sep 1;327(3):R369-R377. doi: 10.1152/ajpregu.00089.2024. Epub 2024 Aug 5.
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 yr (34 female) were exposed to simulated heat-wave conditions for 8-9 h. When assessed at the group level, it took rectal temperature 3.3 [bootstrap 95% confidence interval: 2.9-3.9] h to reach thermal equilibrium (<0.05°C/h 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°C-40°C, 9-45% RH), equilibrium times were longer, ranging from 4.4 [3.8-5.3] to 5.2 [4.9-5.4] h. Furthermore, rectal temperature equilibrium was delayed 0.9 [0.5-1.4] and 1.8 [0.9-2.7] h compared with whole body heat storage in young and older adults, respectively (only assessed in 40°C, 9% RH). Individual-level equilibrium times ranged from 1 to 8 h. These findings highlight the importance of ecologically relevant exposure durations in translational research assessing the physiological impacts of hot weather. Deep body (rectal) temperature took 3-5 h on average and up to 6-8 h at the individual level to reach thermal equilibrium in young and older adults resting in the heat. Furthermore, stable rectal temperatures were delayed by up to 2 h relative to the achievement of heat balance (0 kJ/min rate of heat storage). We provide the first quantification of the temporal profiles of thermal strain during extended rest in conditions simulating hot weather.
人们普遍认为，在可补偿的热应激后 30-90 分钟内可以达到稳态体温调节反应。然而，这种假设是基于运动期间全身热量交换的测量，这些交换比深部体温更快达到稳定（平衡），尤其是在休息状态下。为了支持具有生态相关性的热暴露研究的设计，我们量化了在热环境中休息的年轻和老年成年人的深部体温（以直肠温度为指标）达到平衡的时间。我们还评估了直肠温度平衡相对于全身热量储存（直接量热法）的滞后。在两项基于实验室的试验（NCT04353076 和 NCT04348630）中，我们使用了 83 名年龄在 19-80 岁（34 名女性）的成年人的数据来估计平衡时间，这些成年人暴露于模拟的热浪条件下 8-9 小时。当在群体水平上进行评估时，直肠温度需要 3.3 [bootstrap 95%置信区间：2.9-3.9] 小时才能达到热平衡（变化率<0.05°C/h），在暴露于 40°C、9%相对湿度（RH）的年轻成年人中。在老年人中，他们暴露于更大范围的条件（31°C-40°C、9-45% RH），平衡时间更长，范围从 4.4 [3.8-5.3] 到 5.2 [4.9-5.4] 小时。此外，直肠温度平衡相对于年轻和老年成年人的全身热量储存分别延迟了 0.9 [0.5-1.4] 和 1.8 [0.9-2.7] 小时（仅在 40°C、9% RH 下进行评估）。个体水平的平衡时间范围从 1 到 8 小时。这些发现强调了在评估热天气对生理影响的转化研究中，采用具有生态相关性的暴露时间的重要性。在热环境中休息的年轻和老年成年人中，深部体温（直肠温度）平均需要 3-5 小时，在个体水平上最长需要 6-8 小时才能达到热平衡。此外，与达到热平衡（0 kJ/min 热量储存率）相比，稳定的直肠温度延迟了 2 小时。我们提供了在模拟热天气条件下长时间休息期间热应激的时间分布的首次量化。
