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

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 (ECV(WB)). ECV was calculated from extracellular resistance (R(ECV)) identified from the bioimpedance spectrum for a range of 10 frequencies. Whole bodyR(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 ECV(WB) (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). ECV(WB) 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 ECV(WB) is used.