Complicated Holter tracing with an incidence of inappropriate mode switch due to sensing abnormalities in a patient with dual-chamber pacemaker

24-hour Holter monitoring of a 59 year-old man with DDDR pacemaker (programmed mode: DDD) implanted for sick sinus syndrome with paroxysmal atrial fibrillation was performed one month after implantation, due to palpitations. Several episodes of rapid pacing of decreasing rate were detected. Intracardiac recording stored in the pacemaker memory had shown episodes of atrial lead oversensing, which led to the mode switch, resulting in DDIR mode. Signals which caused oversensing were not seen in Holter tracing.