Dynamics of segmental extracellular volumes during changes in body position by bioimpedance analysis.
Level 4 - case-series / case-control
Small single-group repeated-measures physiological study without a randomized control group
PubMed 9688726 · doi:10.1152/jappl.1998.85.2.497
What was done
Segmental bioimpedance spectroscopy across 10 frequencies was measured in 11 healthy men (mean age 31.6 ± 7 years) after 30 minutes in the supine position to calculate extracellular volume (ECV) and resistance (RECV) for the arms, trunk, and legs versus whole-body ECV (ECVWB). In a subgroup of 6 participants, segmental and whole-body ECV changes were monitored while transitioning from standing to supine positions.
What was found
Whole-body RECV (527.6 ± 55.6 Ω) matched the sum of segmental resistances (arms: 241.6 ± 36.3 Ω; trunk: 49.2 ± 5.1 Ω; legs: 236.3 ± 25.5 Ω). The sum of segmental ECVs (arms: 1.31 ± 0.25 L; trunk: 10.08 ± 1.65 L; legs: 2.80 ± 0.82 L) was smaller than ECVWB (20.90 ± 2.59 L). Upon moving from standing to supine, ECV decreased in the arms (-2.59 ± 2.51%, P = NS) and legs (-10.96 ± 3.02%, P < 0.05) and increased in the trunk (+4.2 ± 3.2%, P < 0.05), leading to a significant drop in ECVWB (-4.98 ± 1.41%, P < 0.05). Conversely, the sum of segmental ECVs remained unchanged (-0.06 ± 0.07%, P = NS).
Why it matters
Segmental bioimpedance eliminates body position artifacts that distort whole-body bioimpedance calculations during acute postural fluid redistribution.
Limits
The sample size was very small (n = 11 overall, n = 6 for postural transitions) and limited strictly to young healthy men. The abstract does not report comparison against reference dilution standards to confirm absolute volumetric accuracy.
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