Pacing Threshold Increase in Nonthoracotomy Implantable Defibrillator Leads: Implications for Battery Longevity and Margin of Safety

Abstract

Just as a stable defibrillation threshold is re-quired for implantable defibrillators to maintain efficacyand a margin of safety for the conversion of life-threateningventricular arrhythmias, a stable pacing threshold is alsorequired to provide bradycardia support and pacing to ter-minate ventricular tachycardias. This article reports thetemporal course of pacing thresholds in patients treatedwith a tripolar, tined endocardial defibrillator lead capableof bipolar sensing and pacing, and defibrillation. Seventeenpatients who underwent implantation of an implantabledefibrillator system using an integrated bipolar pac-ing/sensing system were prospectively studied over 18months. There were 16 males and one female, with a meanage of 69 ± 5 years (range 61–75 years). At implantation,predischarge, and every 2 months thereafter, the pacingpulse-width threshold was tested at both 2.5 and 5.0 Vstimulus amplitudes. After a mean follow-up of 363 ± 173days (range 34–597 days), the pacing threshold increasedfrom 0.08 ± 0.08 ms to 0.5 ± 0.3 ms at the 2.5 V amplitude(p≤0.01, CI-0.57 to -0.27) and from 0.04 ± 0.02 ms to 0.256 0.14 ms at the 5.0 V amplitude (p≤0.01, CI -0.28 to-0.14). Eight of the 17 patients (47%) received spontaneousimplantable defibrillator shocks for clinically detected ar-rhythmias, and the total number of joules delivered via theleads did not correlate with the pacing threshold changes.We conclude that the pacing threshold for the nonthoraco-tomy implantable defibrillator lead system studied is notstable and increases with time. This finding has implica-tions for defibrillator battery life in patients who use im-plantable defibrillators for bradycardia pacing.