title: Effects of aging on cardiovascular responses to gravity-related fluid shift in humans.
abstract: BACKGROUND
  Fluid shift induced by postural change causes autonomic 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 autonomic 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 degrees 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 autonomic nervous system to gravity-related fluid shift is reduced in elderly people, and this may cause blood pressure instability.
authors.affiliation: Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Japan.
authors.forename: C
authors.initials: C
authors.lastname: Miwa
authors.forename: Y
authors.initials: Y
authors.lastname: Sugiyama
authors.forename: T
authors.initials: T
authors.lastname: Mano
authors.forename: T
authors.initials: T
authors.lastname: Matsukawa
authors.forename: S
authors.initials: S
authors.lastname: Iwase
authors.forename: T
authors.initials: T
authors.lastname: Watanabe
authors.forename: F
authors.initials: F
authors.lastname: Kobayashi
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D001341:Autonomic Nervous System / Q000502:physiology*; D017704:Baroreflex / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology; D002320:Cardiovascular Physiological Phenomena; D018495:Fluid Shifts; D006112:Gravitation; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D013318:Stroke Volume / Q000502:physiology