Different physiological and subjective responses to the hyperthermia between young and older adults: Basic study for thermal therapy in cardiovascular diseases

Background. Thermal therapy has been used as adjuvant therapy in patients with cardiovascular disease. However, little is known about responses to thermal stress in older adults. We examined the effects of thermal stress in younger and older healthy Japanese individuals. Methods: The study included 12 young (mean age, 22 years) and 12 older (mean age, 68 years) healthy adults and was performed under strict temperature and humidity control to minimize confounding. Participants lay supine throughout three consecutive 30-minute phases: Phase I (heating at 70°C in a dome-shaped sauna), Phase II (insulation in the sauna), and Phase III (cool down). Physiological parameters and subjective thermal sensations were compared within and between two age groups. Results: Mean skin temperature increased significantly in both age groups (Phase I) and after the first 10 minutes was higher among older adults (by 6.8°C vs 6.0°C among younger; p <. 01). Mean rectal temperature increased by 0.6°C in both groups (Phase II). Mean heart rate increased significantly in both age groups (Phase II) and was higher among younger adults (by 21.4 vs 11.3 beats/min among older adults; p <. 05). Both systolic (by 15.1 mmHg) and diastolic (by 10.5 mmHg) blood pressure dropped significantly among older adults (Phase I), returning to baseline in Phase III; no changes were noted among those younger. There was no between-group difference in fluid loss or thermal sensations. Conclusions: Compared with younger adults, older adults are more likely to drop blood pressure in response to thermal stress but had similar fluid loss and subjective responses.