Age-Related Differences in the Onset of Cardiovascular Drift during Progressive Heat Stress (PSU HEAT Project)

Abstract

As a result of the increasing duration, frequency, and severity of heat waves, older men and women are at greater risk for heat-related morbidity and mortality, with the majority of excess heat-related deaths attributed to increased cardiovascular strain associated with heat stress. PURPOSE: To (1) identify the environmental conditions (ambient temperature (Tdb) and humidity (rh)) at which heart rate (HR) begins to increase from equilibrium (i.e., the onset of cardiovascular drift), and (2) compare the environments in which cardiovascular drift begins relative to those at which a continuous rise in core temperature (Tc) is observed in young (Y), middle (MA), and older (O) adults. Methods: 26 Y (10 F; 19-29 yrs), 27 MA (21F; 40-64 yrs), and 19 O (7 F; 65-76 yrs) subjects were exposed to progressive heat stress in an environmental chamber at a low metabolic rate reflecting activities of daily living (159 ± 34 W) in a warm-humid (WH, 34-36°C, 50-80% rh) and a hot-dry (HD, 38°C-52°C, <30% rh) environment. The environmental thresholds above which HR and Tc equilibrium could no longer be maintained were identified. Results: In WH environments, sustained increases in HR preceded the Tc inflection point in Y (73±5% rh vs. 81±5% rh; p<0.001), MA (67±11% rh vs. 73±12% rh; p<0.001), and O adults (53±11% rh vs. 57±12% rh; p=0.041). Similarly, in HD (15-20% rh) environments, the onset of cardiovascular drift preceded the Tc inflection point in Y (47±3°C vs. 49±3°C; p=0.004), MA (44±4°C vs. 46±4°C; p<0.001), and O adults (42±5°C vs. 44±4°C; p=0.007). Additionally, in WH environments, cardiovascular drift began at lower combinations of Tdb and rh in O vs. Y subjects (34°C, 53±11% rh vs. 34°C, 73±5% rh; p<0.001) and O vs. MA adults (34°C, 53±11% rh vs. 34°C, 67±11% rh; p=0.008). In HD environments, cardiovascular drift began at lower environments in MA vs. Y (44±4°C, 18% rh vs. 47±3°C, 15% rh; p=0.044) and O vs. Y (42±5°C, 20% rh vs. 47±3°C, 15% rh; p=0.006) CONCLUSION: These data suggest that cardiovascular drift, indicating increases in cardiovascular strain, occurs during compensable heat stress at metabolic rates reflecting activities of daily living in Y, MA, and O adults. Further, there is an age-associated shift in the onset of cardiovascular drift toward lower combinations of Tdb and rh. SIGNIFICANCE/NOVELTY: These results are the first to identify the specific environmental conditions at which increases in cardiovascular strain occur across the adult age spectrum. These findings provide concrete data for the development of safety guidelines and policy decisions to mitigate cardiovascular-related morbidity and mortality during impending heat events. Supported by NIA Grant T32 AG049676 (to R.M.C.) and NIH Grant R01 AG067471 (to W.L.K.). This is the full abstract presented at the American Physiology Summit 2024 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.