Abstract
There is considerable heterogeneity in the thermal and cardiovascular responses to heat stress, even among older adults within a similar age range; hence, age alone may not predict heat-related risks. This variability may be in part explained by differences in cardiorespiratory fitness. We aimed to identify whether left ventricular end-diastolic volume (LVEDV), a characteristic of cardiorespiratory fitness, was associated with core temperatures and heart rate responses in older adults exposed to 3 h of very hot and dry heat stress. We hypothesized that individuals with smaller LVEDV would exhibit higher ending core temperature and heart rates following heat stress. Twenty-two older adults (73 ± 5 yr; 10 females) were exposed to 3 h of very hot and dry ambient heat stress (47°C, 15% relative humidity) with accompanying activities of daily living. We assessed thermal and cardiovascular responses at baseline and end-heating. End core temperature ranged from 37.4°C to 38.9°C, whereas end heart rate ranged from 53 to 113 beats/min. Baseline LVEDV was associated with end-heating core temperature (β = -0.018, SE = 0.004; P = 0.003) after controlling for body surface area and baseline core temperature. Likewise, LVEDV was associated with end-heating heart rate (β = -0.595, SE = 0.126; P < 0.001), after controlling for body surface area. However, LVEDV was not associated with whole body sweat rate (P = 0.100) or the change in forearm blood flow (P = 0.331) indexed to changes in core temperature. These findings suggest that individuals with small LVEDV may be susceptible to hyperthermia and tachycardia associated with environmental heat stress.NEW & NOTEWORTHY There is considerable heterogeneity in the thermal and cardiovascular responses to heat stress, even among older adults within a similar age range; hence, age alone may not predict heat-related risks. This variability may be in part explained by differences in physical fitness. Our findings show that individuals with low left ventricular end-diastolic volumes have greater core temperature and heart rate responses to environmental heat stress.