Use of bioimpedance spectroscopy to determine extracellular fluid, intracellular fluid, total body water, and fat-free mass.

Abstract

The use of bioelectrical methods for estimating human body composition has been of interest since the 1960s. It is theoretically possible to measure the extracellular fluid (ECF) and intracellular fluid (ICF) volumes with bioimpedance spectroscopy. Fundamental to this is a basic understanding of the electrical properties of living tissue. Living tissues have distinct frequency dispersions which result from different mechanisms of structural relaxation. β dispersion, which occurs at low radio frequencies and represents the capacitive characteristics of the cell membrane, is of interest in the estimation of ECF and ICF. Due to high cell membrane capacitance (CM) at low frequencies there is little conduction occurring in the ICF compartment, thus, conductivity is governed by the properties of the ECF. High frequency currents show little effect due to cell membrane capacitance so current flows through both ECF and ICF more uniformly and proportionally to the relative conductivities and volumes of the compartments. Resistance to an alternating electrical current can be regarded as a vector with both direction and size and represents a point in a plane where the distance from the point to the origin is the absolute value of Z (impedance =√(R2 + X2) R = resistance X=reactance); arctan (X/R) is its phase angle (θ)1. When Z of a circuit is measured and the frequency is varied a series of values are derived which can be represented as points on a curve. The curve formed by these points can be translated into an equivalent electrical model where the values correspond to specific compositional elements2.