Duelling pacemakers: Unexpected pacemaker interaction resulting from a proprietary rate smoothing algorithm

Case Report An 89-year-old man had a left-sided dual-chamber pacemaker (Symphony DR; Sorin CRM, Clamart, France) implanted for syncope, second-degree Mobitz II heart block, and periodic third-degree heart block. He was found to have an abrupt increase in right ventricular lead impedance and high capture thresholds, consistent with an impending lead fracture. Owing to occlusion of the left innominate vein, the left-sided pacemaker was abandoned and a new dualchamber pacemaker was implanted on the right side (Reply DR; Sorin CRM) and programmed to DDD 60–130 ppm. Paced and sensed atrioventricular (AV) delays were 220 msec and 165 msec, respectively. Given that the patient was dependent on ventricular pacing (VP), the original left-sided device was set with a backup rate of VVI at 40 ppm in case of early dislodgement of the new right-sided lead. The patient was seen in follow-up 1 month later. The devices were interrogated and it was found that the original left-sided device was pacing approximately 75% of the time (Figures 1 and 2). Thus, despite the abandoned device’s being programmed at a backup rate slower than the lower