Polarization properties of small-surface-area pacemaker electrodes--implications on reliability of sensing and pacing

The polarization characteristics of Pt-Ir and Elgiloy small-surface-area (10-12.5 mm2) pacemaker electrodes were studied at AC linear (sensing) and DC non-linear (pacing) conditions. The electrodes' AC polarization impedance was approximately equal to the demand pacemaker's input impedance, which causes waveform distortions ofthe sensed R-wave potentials. The pacemaker's coupling capacitor adds to the distortion's effect. As a result of amplitude attenuation (up to approximately 50%) and slew rate changes, the pacemaker may fail to recognize the ventricular complexes, reverting to hazardous competitive pacing. The impact of the DC polarization elements and of the coupling capacitor on the effectiveness of pacing was examined. The deficiency of small area electrodes was pointed out, this being counter-balanced to some extent by their lower pacing thresholds. The necessity to ensure a sufficient safety margin between the pacing pulse and the stimulation threshold, to avoid possible reasons for creation of exit blocks, was stressed. In reducing the electrode surface, it is advisable not to step down below a 15-20 mm2 effective area for Pt-Ir, and a 40-50 mm2 area for Elgiloy.