Time–frequency analysis of time-varying in vivo myocardial impedance

• A time–frequency analysis for time-varying myocardial impedance is performed.
• A limited number of harmonics from the model describe the impedance over time.
• The root mean square error over time is bounded to 1 Ohm and 0.2 degrees.
• The in-cycle myocardial impedance is simulated using the Harmonic Impedance Spectra.

The frequency response identification of a (bio-)system that is inherently nonlinear and time-varying, e.g. in vivo myocardial impedance, through electrical impedance spectroscopy (EIS) is still a task in progress today that deserves further research. In this work, the in-cycle time-varying behavior of in vivo myocardium is modeled by means of Fricke-Morse’s circuit model. Temporal changes in the in vivo myocardial impedance during the cardiac cycle provide valuable information on the heart physiological processes. The action potentials generated by the movement of ions through the transmembrane ion channels in the cardiomyocites produce the dominant time-periodic changes in a living heart. By performing then a time-based domain analysis, a periodic reconstruction of the experimental data is done and a periodically time-varying (PTV) electrical circuit model is extracted. Furthermore, it is shown that a limited number of harmonic components of the electrical circuit parameters, which corresponds to an integer number of the cardiac frequency, is sufficient to provide a realistic evolution of the myocardial frequency response over time. The root mean square error over time (RMSEoT) for the periodically reconstructed model is bounded to 1 ΩΩ and 0.20.2 degrees in the frequency band [100,103] kHz. Finally, based on the extracted PTV impedance model, we use the tool of harmonic impedance spectra (HIS) [1] and [2] to simulate and analyze in the bi-frequency domain the frequency response behavior of the in vivo myocardial during the cardiac cycle.