JMIR Publications

ABSTRACT

Background:

The impact of summer heat on resting heart rate (RHR) in the elderly, the role of heat adaptation behaviors, and the utility of wearable devices in assessing these behaviors remain insufficiently explored.

Objective:

This study assessed the short-term impact of summer temperatures on RHR and the modifying effects of heat adaptation behaviors in elderly individuals.

Methods:

Eighty-three elderly participants (aged 65 and older) in Taipei City were monitored during the summer of 2021 (May-September) using Garmin® smartwatches. RHR was defined as the lowest 30-minute heart rate per day. Heat adaptation behaviors were collected in September, and a mixed-effect model analyzed the influence of ambient temperature on RHR. Subgroup analyses and interaction terms evaluated the modification effects of these behaviors.

Results:

For each 1°C rise in temperature over lag days 0 and 1, RHR increased by 0.11 beats per minute (95% CI 0.07-0.15, P<.0001). After mutually adjusting for heat adaptation behaviors, significant protective effects were found in reducing physical activity (B=-0.15, 95% CI -0.23~-0.06, P=0.001), drinking iced water (B=-0.24, 95% CI -0.35~-0.13, P<.0001), increasing naps or sleep (B=-0.28, 95% CI -0.46~-0.1, P=.003), consuming extra water (≥500ml)(B=-0.10, 95% CI -0.19~-0.01, P=.023), using air conditioning (AC) before and during sleep (B=-0.15, 95% CI -0.24~-0.05), P=.002; B=-0.13, 95% CI -0.22~-0.03), P=.007), and using fans at night (B=-0.12, 95% CI -0.22~-0.03, P=.012). Certain behaviors had additional benefits in subgroups: reduced activity for those with higher body mass index (B=-0.28 95% CI -0.41~-0.16, P<.0001), AC use before sleep and iced water for diabetics (B=-0.47, 95% CI -0.73~-0.21, P=.0004; B=-0.46, 95% CI -0.7~-0.23, P=.0001), more naps for hypertensives (B=-1.06, 95% CI -1.4~-0.72, P<.0001), and AC or fan use during sleep for women (B=-0.22, 95% CI -0.34~-0.11), P<.0001; B=-0.23, 95% CI -0.35~-0.11, P=.0002) and older participants (B=-0.27, 95% CI -0.39~-0.15, P<.0001; B=-0.3, 95% CI -0.42~-0.17, P<.0001).

Conclusions:

Summer heat increases RHR in the elderly, but various adaptation behaviors can help mitigate this effect. The integration of smartwatch monitoring with personal behavior tracking may offer a practical approach to evaluating individual heat responses and the effectiveness of adaptive strategies.