Cardiac autonomic function is impaired in older adults during daylong exposure to simulated indoor overheating: a randomized crossover trial
Authors: Meade Robert , Akerman Ashley , Notley Sean , Kirby Nathalie , Kenny Glen 
Published: May 2023

Most heat-related deaths occur due to major adverse cardiovascular events. While recent studies have evaluated resting cardiovascular responses to peak outdoor conditions experienced during heat waves, heat-related deaths occur most often in the home, where conditions can differ substantially from the outdoors. In this randomized trial (ClinicalTrials.gov: NCT04348630) we assessed cardiac autonomic regulation of blood pressure (BP) in older adults during daylong exposures to conditions experienced indoors during heat waves – from an air-conditioned environment to a poorly insulated and ventilated domicile. We hypothesized that cardiac autonomic function would be impaired with increasing indoor heat stress, both at rest and during response tests emulating activities of daily living.Sixteen older adults (6 women, 71 (SD 3) yrs) completed 4 randomized indoor overheating simulations. In each, air temperature was held at 22°C, 26°C, 31°C, or 36°C for 8 hours (45% relative humidity). Participants completed a series of cardiac autonomic response tests during hour 7, but otherwise remained seated. Rectal temperature was monitored continuously. Beat-to-beat electrocardiogram and arterial BP (Penaz method) were measured in the response tests. Heart rate variability (RMSSD) was derived during paced breathing (15 breaths/min). Cardiac autonomic (30/15 ratio; ratio of longest and shortest RR intervals ~30 and ~15 heartbeats after standing) and systolic BP responses to standing (supine BP minus the lowest BP measured 1 and 2 min after standing) were evaluated in two supine-to-standing tests. Baroreflex sensitivity was determined during cyclical perturbations in BP (5 min × 6 squats/min). Mixed-effect model estimated marginal means were compared between the 22°C condition and the other conditions. Post-hoc linear trend analyses were also performed.Rectal temperature increased 0.07°C [95% CI: 0.06, 0.09] per 1°C rise in air temperature and was 0.9°C [0.8, 1.1] greater after 8-hours in 36°C than in 22°C (P<0.001). Compared to 22°C, RMSSD, the cardiac response to standing, and standing systolic BP were reduced in both 31°C (RMSSD: -5 ms [-8, -2]; 30/15 ratio: -0.12 [-0.18, -0.07]; standing BP: -22 mm Hg [-31, -13]; P<0.001) and 36°C (RMSSD: -10 ms [-13, -7]; 30/15 ratio: -0.16 [-0.22, -0.11]; standing BP: -36 mm Hg [-45, -26]; P<0.001). The supine-to-standing reduction in systolic BP increased with air temperature (linear trend: -0.8 mm Hg [-1.2, -0.3] per 1°C; P=0.024) but no between-condition differences were detected (P≥0.239). Baroreflex sensitivity was not different between conditions (P>0.999). Three participants experienced light headedness or an inability to remain standing during standing tests in 36°C.Daylong exposure to simulated indoor overheating impaired cardiac autonomic function in older adults, resulting in blunted blood pressure regulation and symptoms of orthostatic intolerance during postural changes consistent with activities of daily living. Health Canada and Canadian Institutes of Health Research (grant # 399434) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.