Toward a Better Understanding of the Thermal and Cardiovascular Strain Experienced by Older Adults During Extreme Heat Events

This thesis evaluated physiological responses of young and older adults during extreme heat events and the extent to which commonly recommended heat-health guidelines (indoor temperature limits) and heat mitigation strategies (cooling centres) are effective at limiting hyperthermia and cardiovascular burden. A multidisciplinary narrative review and three experimental studies were conducted. In the review, the mechanisms by which aging impairs the regulation of body temperature and hemodynamic stability, and how they may contribute to the increased risk of heat-related mortality and morbidity in older adults, were summarized. A lack of ecologically minded study designs in previous research evaluating the physiological responses supporting homeostasis and health during heat stress (i.e., body temperature regulation and cardiovascular stability) was also identified. The three experiments were therefore designed as day-long (8-9 hour) extreme heat simulations to 1) evaluate age-related alterations in thermoregulatory and cardiovascular function during peak heat conditions; 2) assess how these responses translate to indoor environments; and 3) quantify the effectiveness of cooling centers, a widely recommended heat mitigation strategy, for limiting hyperthermia and cardiovascular burden. In the first study, healthy older adults (age: 64-78 years; n=19) stored 87 kJ [95% confidence limits: 33, 141] more heat than their younger (age: 19-31 years; n=20) counterparts (328 [71] kJ vs. 241 [SD: 87]; P<0.001) during the first three hours of a 9-hour exposure to extreme heat (40°C and 15% relative humidity). This resulted in a 0.4°C [0.2, 0.6] greater increase in body core temperature in the older adults that was maintained throughout exposure (1.0 [0.3] vs 0.6 [0.3]°C; P<0.001). These findings were extended in the second study, wherein it was demonstrated that healthy older adults (age: 66-78 years, n=8) exhibit progressive elevations in body temperatures (P<0.001) and attenuations in cardiovascular autonomic function (P<0.001) during 8 hours of rest in conditions representative of those experienced indoors during extreme heat events. These ranged from an actively cooled environment (22°C), through indoor temperature thresholds recommended by Toronto Public Health (26°C) and the World Health Organization (31°C), to poorly insulated and ventilated homes and/or dwellings without access to air conditioning (36°C; 45% relative humidity in all conditions). In the third study, it was shown that short-term exposure to a cool environment midway through (hours 5-6) a day-long (9 hour) simulated heat event reduced core temperature in a group of healthy older adults (age: 67-78 years; n=8) by 0.8°C [0.6, 1.0] compared to an age-matched group not removed from the heat (from study 1). Despite this, core temperature rose rapidly upon return to the heat and was statistically equivalent in both groups by the end of exposure (37.8 [0.3] vs 37.9 [0.3]°C; P=0.011). The findings of this thesis indicate that even healthy older adults experience sustained elevations in body temperature and cardiovascular burden during extreme heat events and that commonly recommended heat-health guidelines (indoor temperature limits) and mitigation strategies (cooling centres) may not provide adequate protection. Collectively, this work represents a considerable advance in our understanding of the physiological burden experienced by older adults during hot weather and extreme heat events.

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