Endless loop tachycardia started by an atrial premature complex in a patient with a dual chamber pacemaker.

In a patient with a dual chamber pacemaker that senses in both the atrium and ventricle (VDD, DDD), a ventricular depolarization temporally displaced from a P wave can cause retrograde atrial activation and initiate an endless loop pacemaker-mediated tachycardia. A case in which an endless loop tachycardia was initiated by an end-diastolic atrial premature complex is reviewed. Retrograde conduction occurred because of the change in the temporal relation of atrial sensing and atrioventricular (AV) node depolarization. The implanted pacemaker did not have the capability of atrial refractory programmability. Atrial refractory interval extension, which occurs in this model after a ventricular premature complex to protect against a retrograde P wave, was not invoked since the tachycardia was begun by an atrial rather than a ventricular premature complex. The tachycardia was controlled by shortening the programmable AV delay. The mechanism of tachycardia induction and its management are outlined. Atrial refractory programmability is required in all VDD or DDD pacemakers.