Monitoring acute whole-body fluid redistribution by changes in leg and neck volumes.
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.
Watenpaugh
D E
DE
Gravitational Research Branch, NASA Ames Research Center, Moffett Field, CA, USA.
Breit
G A
GA
Ballard
R E
RE
Hargens
A R
AR
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