Estimation of trunk extracellular volume by bioimpedance

Bioimpedance analysis (BIA) of the trunk using spot electrodes yields extracellular volume (ECV) that is spuriously low. This discrepancy has been attributed to the small ratio of trunk length to trunk surface area. If current is injected into the trunk through the limbs such as with whole body BIA the electric field in the trunk is inhomogeneous and extracellular resistance (R) between equidistant and sagittal electrodes decreases with increasing distance (d) from the point of current injection. The relationship between R measured between electrodes arranged in a rectangular array on the trunk surface and d was measured in 3 subjects. The dependency of R on d was modeled by the power relation R=ad^-b. An equation was developed to calculate trunk ECV and to correct for inhomogeneous resistance measured on the trunk surface. The equation is based on the assumption of an equivalent volume segment with a homogeneous electric field. The cross-sectional area of this equivalent volume segment was determined to be one fourth of the cross-sectional area of the trunk. Thus, a factor (k) of four has to be used to calculate ECV from R measured by two spot electrodes placed in a sagittal plane close to the point of current injection