MONITORING ACUTE WHOLE-BODY FLUID REDISTRIBUTION BY CHANGES IN LEG AND NECK VOLUMES

Background: Acute fluid shifts initiate chronic cardiovascular acclima tion to altered posture or gravity. Hypothesis: We hypothesized that n eck volume increases with acute lilt from vertical to horizontal and h ead-down positions, and that neck volume correlates negatively with le g volume during tilling. Methods: Strain gauges measured changes in ca lf and neck volumes in 9 subjects during the following tilt table prot ocol: 90 degrees (upright control), 54 degrees, 30 degrees, 12 degrees , 0 degrees (horizontal supine), -6 degrees (head-down tilt), -12 degr ees, -6 degrees, 0 degrees, 12 degrees, 30 degrees, 54 degrees, and 90 degrees. Each position was held for 30 s. Results: Tilting from 90 de grees upright to 0 degrees 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 pr oduced significant volume decrements of 1.66 +/- 0.36% below 90 degree s control at 0 degrees supine, and 2.03 +/- 0.50% below control at -12 degrees tilt. Neck volume elevation consistently exceeded the absolut e magnitude of calf volume reduction at a given tilt angle by a factor of about 1.5, and the two were linearly correlated (r(2) = 0.60). Con clusions: Responses of body segment volumes to till. were practically instantaneous, indicating that venous blood volume translocation accou nted for the changes. We conclude that leg and neck volume changes pro vide a convenient, non-invasive, and sensitive means of assessing acut e regional fluid shifts in humans.