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Fibrosis represents an open issue for mid- to long-term active implants, like pacemakers, given that this biological tissue surrounds the stimulation electrodes and can impact or modify the performances of the system. For this reason, we present a strategy for the continuous sensing of fibrosis induced by cardiac implants, based on the use of the same set of electrodes involved in the implant stimulation process and whose implementation can be integrated into the pacing and sensing circuitry of pacemakers. To do this, the proposed measurement system complies with certain requirements for its integration, such as rapid measurement time, flexibility, low power consumption, and low use of resources. This was achieved through the use of an orthogonal multitone stimulation signal and the design of an Orthogonal Frequency Division Multiplexing (OFDM) architecture that are the bases of the system. As a proof of concept, we implemented this technique within a FPGA. Initial tests of this system have been performed through in vitro measurements of cell cultures related to fibrosis, which, once validated, have allowed us to advance to ex vivo measurements of inhibited and perfused cardiac tissue; these are the conditions that offer a first view of in vivo measurements. This article describes the measurement system implemented and also discusses the results of its validation and of the in vitro and ex vivo measurements, comparing them with results obtained by a reference instrument.

Orthogonal Multitone Electrical Impedance Spectroscopy (OMEIS) for the Study of Fibrosis Induced by Active Cardiac Implants