Cardiac responses to environmental heat exposure in young and older adults.

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 are lacking. In 20 young (18 to 39 years) and 20 older (> 65 years) 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% RH) and hot humid (HUMID, 41°C and 40% RH) 3-hour heat exposure, with intermittent bouts of light physical activity throughout. In both climates and in both age groups, heat stress increased cardiac output by ~0.7 ± 0.8 L/min, decreased stroke volume by ~7 ± 10 mL and augmented diastolic function through increased atrial contribution to filling by 5 ± 5%. In the DRY climate, mitral annual systolic velocity (s') increased to a greater extent in older subjects (Δ3.3 ± 2.1 vs Δ1.5 ± 1.5 cm/s, = 0.002), with less difference in HUMID (Δ2.1 ± 1.3 cm/s vs 1.4 ± 1.3 cm/s, = 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, = 0.001) and HUMID (Δ-9 ± 15 mmHg vs -1 ± 8 mmHg, = 0.030). In summary, older adults rely on a greater augmentation of systolic function during extreme heat exposure, but the magnitude of depends on the heat stress severity.