Modeling the non-stationary behaviour of time-varying electrical bioimpedance

The electrical bioimpedance (EBI) measurement of varying biological systems Z(?,t) (e.g. the heart, the lungs,.) by means of electrical impedance spectroscopy (EIS) remains an open challenge today. Briefly stated, the bioimpedance is widely assumed to be time-invariant when it is measured with the frequency sweep EIS approach. Hence, time-varying changes are thus ignored or treated as a noise source. In this work, we attempt to model the time-variant effects and obtain a simple (periodically) time-varying [(P)TV)] electrical circuit model with (P)TV parameters from experimental in vivo EBI data using the model proposed by Fricke- Morse. The aim is then to illustrate that a limited number of harmonic components of the electrical circuit parameters, which corresponds to an integer number of the bio-system periodicity, can be used to have a realistic evolution of the bioimpedance over time as well as in frequency.