Rate-dependent pacing failure after pacemaker implantation: Novel insights into the mechanism of using adenosine

An 82-year-old woman received pacemaker implantation for sick sinus syndrome. Two days after the implantation, electrocardiography showed 2:1 atrial pacing failure, followed by a bradycardia-dependent increase in the atrial pacing threshold during a pacemaker examination. However, transient 1:1 atrial pacing capture recovered by adenosine triphosphate (ATP) administration, which was performed to evaluate the bradycardia-dependent pacing failure mechanism. We considered this phenomenon to be caused by Phase 4 depolarization and avoided replacing this atrial lead. Three weeks later, the atrial pacing threshold had improved. We report the potential role of Phase 4 depolarization in a bradycardia-dependent increase in pacing threshold by using ATP.