Impact of Age and Heat Acclimation on the Onset and Sensitivity of Cardiac Strain during Heat Stress

Introduction: Older adults are at greater risk of adverse cardiovascular outcomes during extreme heat events. Heat acclimation reduces cardiac strain during heat stress in younger adults, but few studies have explored this possibility in older adults. Objectives and hypotheses: The objective of this study was to determine the effect of age and heat acclimation on the threshold at which cardiac strain increases during heat stress, and its subsequent rate of rise. We hypothesized that: 1) the onset of cardiac strain during heat stress occurs at a lower change in mean body temperature in older adults; 2) the thermosensitivity of cardiac strain during heat stress is greater in older adults; 3) heat acclimation delays the onset and reduces the thermosensitivity of cardiac strain during heat exposure; 4) heat acclimation yield similar benefits in in young and older adults. Methods: Fifteen young (5 females/10 males, 27 ± 5 years) and fourteen older (10 females/4 males, 67 ± 5 years) healthy adults were exposed to passive heat stress with a water-perfused suit before (pre) and after (post) 7 consecutive days of controlled hyperthermia achieve by hot water immersion. During passive heat stress, esophageal temperature was increased 1.2°C above baseline values. Mean body temperature was calculated as 0.8 x esophageal temperature + 0.2 x mean skin temperature. Heart rate was measured continuously from an ECG. The mean body temperature onset threshold of cardiac strain and thermosensitivity were determined by segmented linear regression. Data analyses were performed using a mixed ANOVA. Data are presented as mean ± standard deviation or as mean difference with [95% confident intervals]. Results: The onset of cardiac strain occurred at a lower change in mean body temperature in older adults (0.65 ± 0.09°C) compared to young (0.83 ± 0.09°C) adults (mean difference: -0.18°C [-0.36; 0.00], p=0.047). There was no effect of acclimation (p=0.361), nor interaction (p=0.656) for the onset of cardiac strain. The thermosensitivity of cardiac strain was lower in older adults (23 ± 3 bpm/°C) compared to young (34 ± 3 bpm/°C) adults (mean difference: -11 bpm/°C [-16; -6], p<0.001). Heat acclimation decreased the thermosensitivity of cardiac strain (mean difference: 4 bpm/°C [1; 6], p=0.014) in older (4 ± 7 bpm/°C) and young (5 ± 8 bpm/°C) adults, without an interaction effect (p=0.755). Conclusion: These results demonstrate thatthe onset ofcardiac strain occurs at a lower change in mean body temperature in older adults compared to young adults, whereas older adults display a lower thermosensitivity of cardiac strain during passive heat stress. Heat acclimation does not change the onset but reduces the thermosensitivity of cardiac strain irrespective of age. These findings suggest that heat acclimation reduces the rate of rise in cardiac strain of older adults during passive heat stress. Funding: Natural Sciences and Engineering Research Council of Canada This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.