A study of electrode-tissue impedances encountered in cardiac pacing

Any study of the electrical properties of electrodeelectrolyte systems is rendered difficult by their dependance on, amongst other things, electrode history, electrode surface finish, electrode geometry, electrode area, electrolyte composition and the frequency and amplitude of the applied signal. Distinguished scientists in this field have recently concluded that "there appears to be, so far, no model available which would account for both the linear and non-linear behaviour of the electrodes in the frequency and time domain" (Fischler and Schwan, 1981). Such a model has, however, been developed in this thesis, which, it is believed, successfully interprets and models the wealth of empirical observations reported in the literature. The source of the observed frequency dependence of the inter-electrode impedance is traced to the electrodeelectrolyte interface impedance and is approximated by a "constant phase angle impedance", ZCpA. It is established that the form of this impedance is largely due to electrode surface effects and to a lesser degree by those of specific adsorption. It is demonstrated that the observed non-linearities of the electrode system are largely due to a charge transfer "resistance", RCT, which shunts ZcpA . Empirical expressions of the system's non-linearity found in the literature are physically interpreted and similar expressions are derived based on the model. The changes in inter-electrode impedance due to electrode failures are investigated, modelled and interpreted. Such alterations in loading are found to give rise to changes in pacemaker output and current response waveform. For certain pacers these externally observable parameters can enable the detection and classification of electrode failure, sometimes in a latent stage before failure to pace.