Isorhythmic interaction between a dual-chamber pacemaker and an intrinsic rhythm: pacemaker malfunctioning or not?

A patient underwent cardiac surgery (including a biatrial maze procedure) for congenital heart disease and paroxysmal atrial tachycardia. Previously, a dual-chamber pacemaker was inserted for bradycardia. Postoperative ECG showed a narrow complex rhythm interspersed with pacing stimuli that appeared premature and unable to capture. The phenomenon was due to isorhythmic interaction between the pacemaker and the patient's own intrinsic rhythm, which is compatible with normal pacemaker functioning. The different patterns possible for the phenomenon and the necessary conditions for their occurrence are elucidated.