title: Monitoring acute whole-body fluid redistribution by changes in leg and neck volumes.
abstract: BACKGROUND
  Acute fluid shifts initiate chronic cardiovascular acclimation to altered posture or gravity.
  
  
  OBJECTIVE
  We hypothesized that neck volume increases with acute tilt from vertical to horizontal and head-down positions, and that neck volume correlates negatively with leg volume during tilting.
  
  
  METHODS
  Strain gauges measured changes in calf and neck volumes in 9 subjects during the following tilt table protocol: 90 degrees (upright control), 54 degrees, 30 degrees, 12 degrees, 0 degree (horizontal supine), -6 degrees (head-down tilt), -12 degrees, -6 degrees, 0 degree, 12 degrees, 30 degrees, 54 degrees, and 90 degrees. Each position was held for 30 s.
  
  
  RESULTS
  Tilting from 90 degrees upright to 0 degree supine increased neck volume 3.09 +/- 0.37% (mean +/- SE); neck volume increased further above upright control to 4.26 +/- 0.39% at -12 degrees head-down tilt. In the calf, tilting produced significant volume decrements of 1.66 +/- 0.36% below 90 degrees control at 0 degree supine, and 2.03 +/- 0.50% below control at -12 degrees tilt. Neck volume elevation consistently exceeded the absolute magnitude of calf volume reduction at a given tilt angle by a factor of about 1.5, and the two were linearly correlated (r2 = 0.60).
  
  
  CONCLUSIONS
  Responses of body segment volumes to tilt were practically instantaneous, indicating that venous blood volume translocation accounted for the changes. We conclude that leg and neck volume changes provide a convenient, non-invasive, and sensitive means of assessing acute regional fluid shifts in humans.
authors.affiliation: Gravitational Research Branch, NASA Ames Research Center, Moffett Field, CA, USA.
authors.forename: D E
authors.initials: DE
authors.lastname: Watenpaugh
authors.forename: G A
authors.initials: GA
authors.lastname: Breit
authors.forename: R E
authors.initials: RE
authors.lastname: Ballard
authors.forename: A R
authors.initials: AR
authors.lastname: Hargens
mesh_terms: D000328:Adult; D005260:Female; D018495:Fluid Shifts / Q000502:physiology*; D018470:Gravity, Altered / Q000009:adverse effects*; D018475:Head-Down Tilt / Q000009:adverse effects*; D006801:Humans; D007866:Leg / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic / Q000379:methods; D009333:Neck / Q000502:physiology*; D010993:Plethysmography, Whole Body / Q000379:methods* / Q000592:standards; D011187:Posture / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity