On the Effect of Body Capacitance to Ground in Tetrapolar Bioimpedance Measurements

Tetrapolar bioimpedance measurements on subjects have long been suspected of being affected by stray capacitance between the subjects' body and ground. This paper provides a circuit model to analyze that effect in the frequency range from 100 Hz to 1 MHz in order to identify the relevant parameters when impedance is measured by applying a voltage and measuring both the resulting current and the potential difference between two points on the surface of the volume conductor. The proposed model includes the impedance of each electrode and the input impedance of the differential voltage amplifier. When common values for the circuit parameters are assumed, the simplified model predicts: 1) a frequency-independent gain (scale factor) error; 2) inductive artifacts, that is, the measured impedance increases with increasing frequency and may include positive angle phases; and 3) resonance that can affect well below 1 MHz. In addition to the stray capacitance to ground, relevant parameters that determine those errors are the capacitance of the “low-current” electrode and the input capacitance of the differential voltage amplifier. Experimental results confirm those theoretical predictions and show effects from several additional resonances above 1 MHz that also depend on body capacitance to ground.

I. Introduction

Tetrapolar bioimpedance measurements theoretically exclude the contribution from electrode polarization impedance to the measurement result [1]. Typically, current is injected into the sample tissue or volume conductor by two electrodes and the potential difference between two points is measured by a separate electrode pair. The unknown impedance is then calculated by dividing this voltage drop by the current injected. Hence, if part of the injected current follows a path other than that between the potential electrodes, the result will be wrong. Stray capacitance from the body to (earth) ground offers such a possible separate path, particularly when measuring large volume conductors at frequencies higher than about 100 kHz.