Physiological responses to 9 hours of heat exposure in young and older adults. Part I: Body temperature and hemodynamic regulation

Abstract

Aging is associated with an elevated risk of heat-related mortality and morbidity, attributed, in part, to declines in thermoregulation. However, comparisons between young and older adults have been limited to brief exposures (1-4 hours), which may not adequately reflect the duration or severity of the heat stress experienced during heat waves. We therefore evaluated physiological responses in 20 young (19-31 years; 10 females) and 39 older adults (61-78 years; 11 females) during 9-hours of rest in 40°C and 9% relative humidity. Whole-body heat exchange and storage were measured with direct calorimetry during the first 3 hours and final 3 hours. Core temperature (rectal) was monitored continuously. The older adults stored 88 kJ [95%CI: 29, 147] more heat over the first 3 hours of exposure (P=0.006). While no between-group differences were observed after 3 hours (young: 37.6 (SD 0.2)°C vs. older: 37.7 (0.3)°C; P=0.216), core temperature was elevated 0.3°C [0.1, 0.4] (adjusted for baseline) in the older group at hour 6 (37.6 (0.2)°C vs. 37.9 (0.2)°C; P<0.001) and 0.2°C [0.0, 0.3] at hour 9 (37.7 (0.3)°C vs. 37.8 (0.3)°C), though the latter comparison was not significant after multiplicity correction (P=0.061). Our findings indicate that older adults sustain greater increases in heat storage and core temperature during daylong exposure to hot dry conditions compared to their younger counterparts. This study represents an important step in the use of ecologically relevant, prolonged exposures for translational research aimed at quantifying the physiological and health impacts of hot weather and heat waves on heat-vulnerable populations.

Effects of Daylong Exposure to Indoor Overheating on Thermal and Cardiovascular Strain in Older Adults: A Randomized Crossover Trial

“…Supporting this postulate, we recently observed that although type 2 diabetes is associated with reduced heat loss during moderate exercise in the heat, 96 physiological responses did not differ between older adults with or without well-controlled type 2 diabetes during a resting 3-h exposure to 44°C and 35% relative humidity 97 conditions exceeding those measured indoors during heat waves. 20 , 21 , 34 Likewise, we recently showed no differences in body temperature 98 or cellular strain 99 between older adults with and without well-controlled type 2 diabetes and/or hypertension during 9-h of exposure to 40°C, 9% relative humidity. Furthermore, although maintaining high levels of aerobic fitness has been shown to prevent the age-related decline in thermoregulatory function during exercise heat-stress, these effects are considerably reduced at lower levels of heat stress like those that might be experienced by vulnerable groups indoors during hot weather.…”