Continuous measurement of segmental and whole body bio-impedance

Measurement of whole body bioimpedance provides a non-invasive and practical approach to estimate extracellular volume (ECV) in man. However, recent data indicate that ECV cannot be treated as a homogeneous compartment without obtaining paradoxical results. Therefore, bioimpedance measurements should include measurements of body segments such as the arm, the trunk, and the leg. A digital switch was developed and used to consecutively select different body segments for the continuous measurement of whole body and segmental bioimpedance at a minimum sampling period of 3 seconds. Bioimpedance data were modeled to determine the resistance of the ECV in the segments and in the whole body for a period of 30 minutes where subjects were in supine body position followed by a 30 minute standing phase. During supine body position modeled extracellular resistance increased in the leg, the arm, and in the whole body but slightly increased in the trunk. Changes in extracellular resistance reversed with subjects assuming a standing position. These measurements reflect the shifts in ECV between segments to be expected with changes in body position. Simultaneous measurement of bioimpedance in different body segments utilizing this approach might be useful to analyze the nature of fluid shifts in clinical applications such as hemodialysis and ultrafiltration.