Overcoming the Impedance Range Limitations of Portable Bioelectrical Impedance Spectroscopy Clinical Devices

Several portable commercial bioimpedance spectroscopy (BIS) devices are used in clinical research, but are limited by their reduced impedance measuring range to less demanding four-electrode configuration measurements. Some of these devices provide raw bioimpedance data for research purposes. The SFB7 device from ImpediMed (R) is a typical portable clinical device which perform 256 measurement points from 3 kHz to 1 MHz, providing for each point the resistance R and the reactance X. Its main drawback is its impedance range, limited to only 1100 Omega, making it less suitable for more demanding, higher impedance, two-electrode measurements which are potentially of great clinical interest. Some benchtop reference devices have larger frequency and impedance ranges than the SFB7 but are not portable and are not designed specifically for BIS measurement on humans. This article proposes a method to overcome the impedance range limitation of the SFB7 by connecting in parallel to the investigated bioimpedance a fixed and known impedance, whose value (and hence that of the total measured impedance) is within the device's range. The investigated impedance is then calculated by eliminating the contribution of the known impedance via a post processing of the data. This concept has been successfully tested on a physical model, and an in vivo example has validated the practicality of performing demanding two-electrode measurements on individuals using a SFB7, with relative errors of R and X inferior to 2.5% and 7% respectively.