Cardiac responses to environmental heat exposure in young and older adults

Abstract

Older individuals are at a greater risk for adverse cardiovascular events during extreme heat exposure. However, detailed characterization of their cardiac responses to environmental heat exposure is lacking. In 20 young (18-39 yr) and 20 older (>65 yr) adults (50% male in both groups), we document the echocardiography-assessed left ventricular responses to a very hot and dry [DRY, 47°C and 15% relative humidity (RH)] and hot humid (HUMID, 41°C and 40% RH) 3-h heat exposure, with intermittent bouts of light physical activity throughout. In both climates and in both age groups, heat stress 1) increased cardiac output by ∼0.7 ± 0.8 L/min, 2) decreased stroke volume by ∼7 ± 10 mL, and 3) augmented diastolic function through increased atrial contribution to filling by 5 ± 5%. In the DRY climate, mitral annular systolic velocity (s') increased to a greater extent in older subjects (Δ 3.3 ± 2.1 vs. Δ 1.5 ± 1.5 cm/s, P = 0.002), with less difference in HUMID (Δ 2.1 ± 1.3 cm/s vs. 1.4 ± 1.3 cm/s, P = 0.096). Despite these adjustments, systolic blood pressure was only maintained in the younger group and fell consistently in older individuals (0 ± 8 mmHg) in DRY (Δ -11 ± 14 mmHg vs. 1 ± 8 mmHg, P = 0.001) and HUMID (Δ -9 ± 15 mmHg vs. -1 ± 8 mmHg, P = 0.030). In summary, older adults rely on a greater augmentation of systolic function during extreme heat exposure, but the magnitude depends on the heat stress severity.NEW & NOTEWORTHY Comparing healthy young and older adults, we assessed left ventricular cardiac function (using echocardiography) during two separate 3-h extreme heat exposures in a very hot and dry or hot humid climate type. Although the augmentation of diastolic function and cardiac output were similar between age groups, older adults showed a greater increase in mitral annular systolic velocity with heat exposure, indicating a stronger reliance on systolic mechanisms to maintain stroke volume.

Keywords: age; cardiac; cardiovascular; heart; heat.

Figure 1.. Experimental protocol.

Healthy young (n = 20) and older (n = 20) participants were exposed to two 3-h extreme heat exposures on different days: 1) DRY (47°C and 15% humidity) and 2) HUMID (41°C and 40% humidity) in a randomised order. To mimic heat generation comparable with activities of daily living, participants performed seven 5 min bouts of recumbent cycling (∼3 METs) dispersed throughout the heat exposure. Participants consumed 3 mL/kg body mass of tap temperature water (~15°C) every hour during the heat exposures. At baseline and ~20 min prior to the end of heat exposure, ultrasound was used to assess left ventricular systolic and diastolic function, cardiac output, stroke volume, and brachial artery blood flow. Venous blood samples were taken at baseline, at the end of heat exposure, and 2-hours following recovery in a thermoneutral environment. Figure generated using Biorender.

Figure 2.. Stroke volume, cardiac output, systolic blood pressure, and percentage age-predicted maximum heart rate before and 3-hour into DRY (47°C, 15% RH, left) and HUMID (41°C, 40% RH, right) heat exposure in young (circles) and older (squares) adults.

Females shown with hatched symbols. Arrows indicate difference in average response from pre to post in each age group (dashed arrows used in older group). Significant age group (exact p values) and time differences (shown with a #) are shown in the event of a significant interaction term (p < 0.05).

Figure 3.. Indices of diastolic function. before and 3-hour into DRY (47°C, 15% RH, left) and HUMID (41°C, 40% RH, right) heat exposure in young (circles) and older (squares) adults.

Females shown with hatched symbols. Arrows indicate difference in average response from pre to post in each age group (dashed arrows used in older group). Significant age group (exact p values) and time differences (shown with a #) are shown in the event of a significant interaction term (p < 0.05).

Figure 4.. Indices of left ventricular systolic function (panel a) before and 3-hour into DRY (47°C, 15% RH, left) and HUMID (41°C, 40% RH, right) heat exposure in young (circles) and older (squares) adults. Plasma cardiac troponin I (panel b) was also measured 2-h into recovery in a thermoneutral environment.

Females shown with hatched symbols. Arrows indicate difference in average response from pre to post in each age group (dashed arrows used in older group). Significant age group (exact p values) and time differences (shown with a #) are shown in the event of a significant interaction term (p < 0.05).

Figure 5.. Regression analysis between multiple variables at the end of the DRY heat exposure.

High core temperature is indirectly related to end diastolic volume, a marker of left ventricular filling pressure. The prevailing end diastolic volume is associated with heart rate. That rise in heart rate is then linked to the augmentation of diastolic function. No such links were found which help explain the augmentation of systolic function. Hatched symbols show the female subjects. Linear regression p < 0.001.