A Model of Impedance of Cardiac Pacemaker Electrodes

Abstract

Cardiac electrostimulation uses an implanted pace maker passed through a cardioelectrode. The contact of the electrode with cardiac tissue gives rise to a electric double layer (Fig. 1a). The electric double layer contains capacitance C p , resistance R p , and cardiac tissue resist ance R т representing electrode polarization [1]. The chain is characterized by impedance Z . 1 A telemetric unit of the pacemaker monitors electrode insulation damage, and this also provides electrode service live prognosis.The electrode is connected to the pacemaker output via dividing capacitor C d . Voltage pulse U(t) induces a decrease in the electrode current I(t) due to the capacitor C d charge and polarization capacitance C р (Fig. 1, b and c). The charge Q = ∫I(t)dt decreases within pulse time t p . This increases the required pulse amplitude U and decreases service life. The value of capacitance C d determines energy loses [2]. The losses in C р can be compensated by appro priate electrode material and coating technology [3].Results of clinical testing of electrodes are reported in the literature [4,5]. New domestic pacemaker models provide impedance monitoring [6]. The goal of this work was to construct a model of impedance for cardiac pace maker electrodes in vitro and in vivo.Pacemaker manuals contain a mathematical (calcu lation) and a metrological (experimental) definition of impedance. Voltage pulse trailing front δ U is measured in the pacemaker telemetric unit [7] (Fig. 1b). Impedance Z is calculated assuming Z contains only an active compo nent. Some pacemaker models suggest that δ U decay is linear:Other pacemaker models suggest that δ U decay is exponential:In some cases the impedance is determined from current I(0) from a time of a few microseconds using volt age pulse U(0):Impedance measurement in 67 patients using three models of impedance meter and five models of pacemak er telemetric unit were reported in [7]. The Z value varia tion was 18 34% and detection error ε ex was ≈20%. This error was due to physiological in vivo factor, parameters of C d capacitor, Eqs. (1) (3), and electrode contact polarizaThe impedance of the electrode of pacemaker telemetry units was measured to diagnose damage to the conduc tor and/or isolation of the electrode, as well as to predict the service life of an implanted pacemaker. However, different pacemaker models use different methods for measuring the impedance. The procedure of physical and mathematical modeling allows in vivo impedance to be compared with in vitro impedance values. A physical model was determined using active resistance and polarization capacitance and resistance. The mathematical model of the impedance was determined using Orcad Capture by comparing different definitions of impedance. It is shown that for electrodes without excessive polarization the difference between the methods of measuring impedance pacemaker telemetry units did not exceed the declared measurement error of 20%. A method for measuring and calculating the impedance in vitro is suggested. Th...