Effects of aging on cardiovascular responses to gravity-related fluid shift in humans

Abstract

Background. Fluid shift induced by postural change causes autonomie neural responses of the cardiovascular system that buffer blood pressure fluctuation. The aim of the study was to clarify the effects of aging on cardiovascular autonomie functions in response to gravity-related fluid shift that unloads or loads the baroreceptors in human subjects. Methods. A chest electrocardiogram, blood pressure by Finapres, and stroke volume by impedance method were measured in healthy young men (23-31 years old) and healthy elderly men (74-80 years old) during supine rest, at 90° head-up tilt and thermoneutral head-out water immersion. Spectral analysis was applied to the time series data of the R-R intervals (heart rate variability [HRV]) and systolic blood pressure (blood pressure variability [BPV]). The arterial baroreflex gain for heart rate was estimated using frequency transfer function analysis. Results. The young subjects had stable blood pressure, despite the larger amount of fluid shift induced by both tilt and immersion, and had marked changes in HRV and BPV. The elderly subjects failed to maintain stable blood pressure during these perturbations, despite less fluid shift and no significant changes in HRV and BPV. The arterial baroreflex gain for heart rate was not changed in the elderly subjects, whereas the gain decreased with upright in the young subjects and showed an increasing tendency during immersion compared with upright posture. Conclusions. These findings suggest that the adaptivity of the autonomie nervous system to gravity-related fluid shift is reduced in elderly people, and this may cause blood pressure instability. Copyright 2000 by The Gerontological Society of America.