Atrial pacing on alternate beats in a single-chamber atrial pacemaker: What is the mechanism?

A 60-year-old male with a single-chamber AAI pacemaker implanted for symptomatic sinus node dysfunction presented for routine follow-up. ECG showed an alternating pattern of atrial sensed (As) and atrial paced (Ap) beats. Analysis demonstrated a shorter As-Ap interval (1000 ms) compared to a prolonged Ap-As interval (∼1200 ms), inconsistent with hysteresis. The delayed atrial pacing suggested oversensing. Further evaluation identified far-field R-wave sensing following atrial paced beats, which reset the lower rate interval, leading to suppression of timely atrial pacing. This phenomenon occurred selectively after paced beats because the subsequent QRS complex fell outside the atrial refractory period, unlike after sensed beats. Device interrogation confirmed far-field R-wave oversensing and a longer AV interval following paced events. Management included prolongation of the atrial refractory period and adjustment of the lower rate interval, which resolved the alternating pacing pattern. This case highlights the importance of recognizing far-field R-wave oversensing as a cause of atypical pacing behavior and underscores the need for a thorough understanding of pacemaker timing cycles.