Electrode-tissue interface polarization

Whenever electricity flows across a circuit, there is a resistance to flow encountered by the electrons. For pacing systems, the resistance is determined by the complex interaction of multiple components. Because some of these components are also characterized by the ability to retain charge or capacitance, the term impedance is preferred. At the time of lead implantation, it is this complicated series of resistance and capacitance factors that are measured and are referred to as system impedance. For a pacing circuit, the system impedance has five basic components a low, purely resistive conductor impedance, a high cathode electrode impedance, complex polarization effects at the electrode-tissue interface, a low tissue impedance, and the anode electrode impedance (Fig. 1.3). 

Fig. 1.4 Polarization effect at the electrode-tissue interface. Within the electrode, current flow is due to movement of electrons (e ). At the electrode-tissue interface, the current flow becomes ionic. The negatively charged ions (Cl", OH") flow into the tissues toward the anode leaving behind oppositely charged particles attracted by the emerging electrons. It is this capacitance effect at the electrode-tissue interface, that is the basis of polarization.

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