Pacemaker-mediated endless loop tachycardia at rates below the upper rate limit

Abstract

Pacemaker-mediated tachycardias are a well-recognized complication of atrial synchronized dual chamber pacing systems. 1 They result from the sensing of retrograde P waves or ectopic atrial activity by the atrial sensing circuit with consequent triggering of ventricular pacing. Most pacemaker-mediated tachycardias are initiated by ventricular ectopic beats with retrograde atrial activation. Other initiating mechanisms have been reported 2,3 including tracking of a sinus P wave. 4 When these arrhythmias are sustained by consistent retrograde atrial activity following each paced ventricular beat which then triggers the following beat, they have been termed “endless loop tachycardias.” 5 This category of pacemaker-mediated tachycardia persists until either sensing of retrograde P waves ceases or ventriculoatrial conduction terminates. Endless loop tachycardias are generally maintained at the programmed upper rate limit of the pacemaker. 2,6 We recently saw a patient with an endless loop tachycardia that had persisted for several days and was ended by magnet application. The tachycardia persisted despite an algorithm in the generator that stops atrial tracking after 15 responses at the upper rate limit. Because the pacemaker was normal, it was postulated that the tachycardia rate was below the upper rate limit. In such a tachycardia, the sum of the retrograde ventriculoatrial conduction time and the atrioventricular (AV) interval is greater than the cycle length of the programmed upper rate limit. We detail the interrelations between the ventriculoatrial conduction time and AV interval in several patients who manifested endless loop tachycardias below the programmed upper rate limit.