During heat stress, stroke volume is maintained in young adults despite reductions in cardiac filling pressures. This is achieved by a general augmentation of cardiac function, highlighted by a left and upward shift of the Frank-Starling relation. In contrast, healthy aged adults are unable to maintain stroke volume during heat stress. We hypothesized this would be associated with a lack of shift in the Frank-Starling relation. Frank-Starling relations were examined in eleven aged (69 ± 4 y, 4 males/7 females) and twelve young (26 ± 5 y, 6 males/6 females) adults during normothermic and heat stress (1.5°C increase in core temperature) conditions. During heat stress, increases in cardiac output were attenuated in aged adults (+2.5 ± 0.3 vs. young: +4.5 ± 0.5 L/min, P<0.01) due to an attenuated chronotropic response (+30 ± 4 vs. young: +42 ± 5 beats/min, P<0.01). In contrast to our hypothesis, a leftward shift of the Frank-Starling relation maintained stroke volume during heat stress in aged adults (76 ± 8 vs. normothermic: 74 ± 8 mL, P=0.38) despite reductions in cardiac filling pressure (6.6 ± 1.0 vs. normothermic: 8.9 ± 1.1 mmHg, P<0.01). In a subset of participants, volume loading was used to return cardiac filling pressure during heat stress to normothermic values, which resulted in a greater stroke volume for a given cardiac filling pressure in both groups. These results demonstrate that the Frank-Starling relation shifts during heat stress in healthy young and aged adults, thereby preserving stroke volume despite reductions in cardiac filling pressures.
- Cardiac output
- Heart rate
- Stroke volume
- Copyright © 2016, Journal of Applied Physiology