Dynamics of segmental extracellular volumes during changes in body position by bioimpedance analysis

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Abstract
Extracellular volume (ECV) of arms, trunk, and legs determined from segmental bioimpedance data in 11 healthy men (31.6 ± 7 yr) obtained at the end of a 30-min equilibration phase in the supine body position was compared with ECV determined from whole body measurements (ECVWB). ECV was calculated from extracellular resistance ( R ECV) identified from the bioimpedance spectrum for a range of 10 frequencies. Whole body R ECV (527.6 ± 55.6 Ω) was equal to the sum of R ECV in the arms, trunk, and legs (241.6 ± 36.3, 49.2 ± 5.1, and 236.3 ± 25.5 Ω, respectively). The sum of equilibrated ECV in arms (1.31 ± 0.25 liters), trunk (10.08 ± 1.65 liters), and legs (2.80 ± 0.82 liters) was smaller than ECVWB (20.90 ± 2.59 liters). In six subjects who changed from a standing to a supine body position, ECV decreased in arms (−2.59 ± 2.51%, P = NS) and legs (−10.96 ± 3.02%, P < 0.05) but increased in the trunk (+4.2 ± 3.2%, P< 0.05). ECVWB also decreased (−4.98 ± 1.41%, P < 0.05). However, the sum of segmental extracellular volumes remained unchanged (−0.06 ± 0.07%, P= NS). The sum of segmental ECVs is not sensitive to changes in body position, which otherwise interferes with the estimation of ECV in bioimpedance analysis when ECVWBis used.