Thermal and Cardiovascular Responses During a Very Hot and Dry Heatwave Simulation in Young and Older Adults

Abstract

Heatwaves cause disproportionately high mortality rates in those aged ≥ 65 years. While age differences in the cardiovascular responses to heat stress have been investigated, previous studies used a water-perfused suit to elicit heat stress. Therefore, how aging affects the cardiovascular responses to prolonged environmental heat stress is not fully understood. Purpose: We tested the hypothesis that 3 hours of exposure to a very hot and dry heat wave condition would increase core temperature and left ventricular systolic contractility (s’) to a greater extent in older, compared with younger adults. Methods: Healthy young (n = 13, 6F/7M, 29±4 years, 1.82±0.13 m-2) and older (n = 19, 10F/9M, 70±4 years, 1.85±0.18 m-2) adults were exposed to 47°C, 15% humidity) for 3-hours, interspersed with seven 5-minute bouts of physical activity at 3 METS to replicate heat generation associated with activities of daily living. Core temperature, brachial blood pressure and heart rate were monitored continuously during heating. Echocardiograms were obtained at baseline (in a thermoneutral environment) and at three hours into heat stress (i.e., post-heat wave) to assess s’. Drinking water was provided throughout the protocol to minimize dehydration. The magnitude of the change in each variable from baseline to post-heatwave between age groups was compared using independent samples t-tests. The association between the change in core temperature and the change in s’ was assessed with linear regression. Results: From baseline to 3-hour heatwave exposure, there was a greater increase in core temperature (Δ1.36±0.28 vs Δ0.74±0.28 °C, p < 0.0001) and s’ (Δ3.3±2.2 vs Δ1.6±1.8 cm/s, p = 0.011) in older, compared with younger subjects, respectively. However, the magnitude of the change in core temperature was not associated with the increase in s’ in older of young subjects (R2 = 0.065 and 0.120, respectively). The older subjects showed a greater decrease in mean arterial pressure compared with younger subjects (-14±10 vs -4±7 mmHg, p = 0.004). Heart rate increased similarly in older and young subjects (27±13 vs 27±12 b/min, p = 0.412). Conclusions: Our findings suggest that an index of left ventricular systolic contractility (s’) increases to a greater extent in older, compared with younger subjects. The change in s’ was not associated with the change in core temperature in either age group. Supported by NIH R01AG069005 (CGC), F32HL154559 & K01HL160772 (JCW), and F32HL154565 (LNB). 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.