Frank‐Starling Responses during Heat Stress in Healthy Aged Humans

Abstract

Healthy aging markedly affects the cardiovascular adjustments to heat stress in humans. In particular, the increase in cardiac output can be attenuated by 50% in older individuals, which has previously been attributed to an inability of the aged to maintain stroke volume when cardiac filling pressures are reduced. In young individuals, a leftward shift in the operating point of the Frank‐Starling response is an important adjustment by which stroke volume is maintained during heat stress. It remains unknown how healthy aging affects the Frank‐Starling response during heat stress. We therefore tested the hypothesis that a leftward shift in the operating point of the Frank‐Starling response is absent in older individuals during whole‐body passive heat stress. Right heart catheterization was performed in 10 aged (A: 70 ± 3 y, 166 ± 9 cm, 68 ± 8 kg, 4 males/6 females) and 12 young (Y: 26 ± 5 y, 165 ± 6 cm, 64 ± 10 kg, 6 males/6 females) healthy individuals to measure pulmonary capillary wedge pressure, cardiac output (thermodilution) and pulmonary artery blood temperature. Heart rate was measured using an electrocardiogram and used to calculate stroke volume. These measurements were obtained at normothermic baseline, as well as at an increase in pulmonary artery blood temperature of 1.5°C achieved through whole‐body passive heat stress. Cardiac output increased in both groups during heat stress (A: 4.5 ± 0.7 to 7.0 ± 0.9 L/min vs. Y: 5.0 ± 0.7 to 9.5 ± 1.1 L/min), the magnitude of which was lower in the aged (P<0.01). Heart rate was similar between groups during heat stress (A: 91 ± 11 vs. Y: 98 ± 14 beats/min, p=0.17) however, the increase from baseline was attenuated in the aged (A: +29 ± 8 vs. Y: +41 ± 9 beats/min, P<0.01). Indicative of a leftward shift in the operating point of the Frank‐Starling response, stroke volume was maintained during heat stress in both groups (A: 74 ± 13 to 78 ± 14 mL/min vs. Y: 89 ± 23 to 99 ± 21 mL/min) despite reductions in pulmonary capillary wedge pressure (A: 9.0 ± 1.9 to 6.6 ± 1.4 mmHg vs. Y: 10.6 ± 1.6 to 6.6 ± 2.0 mmHg). Overall, these results suggest that stroke volume is maintained during whole‐body passive heat stress in healthy aged humans due to an appropriate leftward shift in the Frank‐Starling response. Thus, attenuated increases in cardiac output in the aged while heat stressed are primarily due to attenuated increases in heart rate.Support or Funding InformationFunded by the National Institutes of Health (GM‐068865), Department of Defense (W81XWH‐12‐1‐0152) and ACSM Foundation Research Endowment.