(757.8) Cardiac Adjustments During a Very Hot and Dry Heatwave Simulation in Older Adults

Poster Board Number: E457

Joseph Watso (University of Texas Southwestern Medical Center, University of Texas Southwestern Medical Center), Luke Belval (University of Texas Southwestern Medical Center, University of Texas Southwestern Medical Center), Josh Foster (University of Texas Southwestern Medical Center, University of Texas Southwestern Medical Center), Frank Cimino III (Institute for Exercise and Environmental Medicine), Bonnie Orth (Institute for Exercise and Environmental Medicine), James MacNamara (University of Texas Southwestern Medical Center, University of Texas Southwestern Medical Center), Satyam Sarma (University of Texas Southwestern Medical Center, University of Texas Southwestern Medical Center), Craig Crandall (University of Texas Southwestern Medical Center, University of Texas Southwestern Medical Center)

Heatwaves are the leading cause of death among natural disasters in the United States and are predicted to increase in frequency, duration, and severity. During heatwaves, older adults (over 65 years old) have disproportionally greater morbidity and mortality, primarily from cardiovascular complications. However, it is unknown whether a very hot and dry heat wave simulation would impair cardiac function in older adults.

Purpose: We tested the hypothesis that three hours of exposure to a very hot and dry heat wave simulation would increase left-ventricular contractility (s’) in older adults.

Methods: We assessed internal body temperature (ingestible telemetric pill and/or rectal thermistor), mean skin temperature (thermocouples), cardiac function (echocardiography), limb blood flow (brachial artery sonography) in eight healthy older adults (5 Female/3 Male; 70 ± 4 years; 26 ± 3 kg•m-2) while they were exposed to a simulation of the 2018 Los Angeles heatwave for three hours. We measured hemodynamics in a temperate environment (~23 ºC) before repeating these measures after three hours of heat exposure in an environmental chamber (46 ± 1 ºC, 17 ± 1 % relative humidity). For ecological validity, participants performed seven bouts of exercise (3 METS for 5 minutes) dispersed across the three-hour-long heat exposure to simulate activities of daily living. Additionally, participants consumed 9 mL of 15 ºC (tap-temperature) water per kilogram of body mass dispersed throughout the trial. We compared values before (BASE) or at the start (START) of heating with values obtained at the end of heating (END) using a Wilcoxon test for propagation velocity, limb blood flow and vascular conductance, and paired, two-tailed t-tests for all other variables.

Results: Internal body temperature (START: 36.8 ± 0.4, END: 38.2 ± 0.5 ºC, plt;0.01) and mean skin temperature (START: 35.5 ± 1.3, END: 37.3 ± 0.7 ºC, p=0.02) increased during heating. Heart rate increased (BASE: 73 ± 19, END: 99 ± 40 bpm, p=0.04) and mean arterial pressure decreased (BASE: 90 ± 14, END: 77 ± 11 mmHg, p=0.03) during heating. Combined [septal amp; lateral] s’ (BASE: 8.2 ± 1.4, END: 11.3 ± 1.5 cm/s, plt;0.01) increased during heating, for which changes from BASE to END were moderately, but non-significantly, related to Δ internal body temperature (R2 = 0.25, p = 0.21). Indices of left ventricular diastolic function did not change during heating (pgt;0.24 for E, A, E/A, combined e’, E/combined e’, propagation velocity, amp; iso-volumetric relaxation time). Limb blood flow and vascular conductance increased ~four-fold during heating (plt;0.02 for both).

Conclusion: These preliminary data suggest that older adults increase left-ventricular contractility to maintain stroke volume, potentially to compensate for increased limb vascular conductance, during three hours of exposure to a very hot and dry heatwave simulation.

Supported by NIH R01AG069005 (CGC), F32HL154559 (JCW), and F32HL154565 (LNB).