Cycle Length Alternation During Pacing

Abstract

A 79-year-old male with a complete atrioventricular (AV) block, most likely at the His-Purkinje level as there was a slow escape rhythm with broad QRS complexes, was implanted with a VDD pacemaker (Biotronik, Actros SLR, Berlin, Germany) with the following parameters: lower rate = 60 ppm, upper rate = 160 ppm, and rate-variable AV delay (180 msec at basic rate, 120 msec at a rate of 111-130 beats/minute and 100 msec at a rate >130 beats/minute). Several hours after implantation the patient developed recurring episodes of pacemaker-mediated tachycardia that were initially attributed to tracking of atrial fibrillation because of their irregularity (Fig. 1). However, some of the episodes were regular with clear retrograde P waves. Both regular and irregular episodes of arrhythmia were easily interrupted using magnet application and were finally completely eliminated by extending the total atrial refractory period from 425 msec to 525 msec. What is the mechanism of the arrhythmia and cycle length alternans shown in Figure 1? DiscussionMost of the pacemaker-mediated tachycardia episodes in the present case were irregular, showing alternation of two Figure 1. Spontaneous initiation of irregular pacemaker-mediated tachycardia.cycle lengths: 405 msec and 520 msec. Alternans was usually present at the beginning and end of the tachycardia and was mainly due to two distinct retrograde conduction times (approximately 265 msec and 360 msec, Fig. 1) and was slightly augmented by rate-variable AV delay (120 msec in longer cycles and 100 msec in shorter cycles). The same two distinct retrograde conduction times were present during ventricular pacing at 60 ppm and episodes of regular pacemakermediated tachycardia with a cycle length of 520 msec (Fig. 2). Therefore, the initial diagnosis of ventricular tracking of atrial fibrillation was dismissed, and the arrhythmia was considered to be an endless loop tachycardia (ELT).Pacemaker ELT is a recognized complication of DDD and VDD pacing. ELT in a patient with complete AV block is not unusual as approximately 30% of such patients have preserved retrograde conduction, a prerequisite for ELT development. However, irregularity due to cycle length alternans during ELT is unusual. Conduction alternans may be caused by alternation of conduction velocity, propagation pathways, or refractoriness. The cycle length of ELT is determined by the speed of conduction in all parts of the macroreentrant circuit: the atrium, AV node, ventricle, and the pacemaker circuitry. From an electrophysiological viewpoint, in ELT the pacemaker acts as an electronic accessory pathway conducting in an orthodromic fashion. Therefore, ELT should present similar features as spontaneous AV reentrant tachycardia (AVRT) observed in preexcitation syndromes, and indeed many such electrophysiological similarities have been previously reported. 1 Cycle length alternans generally occurs at the initiation or termination of the AVRT or following infusion of drugs that alter AV node refractoriness, and

Pacemaker Malfunction Due to Atrial and Ventricular Leads Switched in the Header: Two Faces of the Same Mistake?

“…This bears similarity to o‐AVRT where, if reentrant retrograde atrial depolarization conducts too quickly the ventricle it can meet a still refractory accessory pathway and break the tachycardia. Many other similarities between reentrant atrioventricular tachycardias using either the accessory pathway or a pacemaker as one limb of the circuit have been reported, and it appears important to understand the macro‐reentrant pacemaker tachycardia mechanisms of initiation, perpetuation and termination 4,5 …”

Intermittent Irregular Pacemaker‐Mediated Tachycardia

“…The cycle length reflected the sum of the VA time as measured from the markers (i.e., RVP/LVP to AS) of about 480 ms and the sensed AV delay of 130 ms. ELT of variable cycle lengths have previously been reported. The irregular cycle lengths may be mediated by changes in conduction time in retrograde or anterograde limbs of the ELT circuit or change in the mechanism of ELT. Jastrzebski described a case of ELT that was initiated by ventricular oversensing and triggered pacing in a patient with biventricular defibrillator).…”

Pacemaker‐Mediated Bigeminy: What Is the Mechanism?

“…This explanation is also supported by the observation that the ventriculoatrial interval during the retrograde conduction test was 184 ms rather than the 230 ms observed during bigeminy. Although the author is not aware of any other reports of pacemaker‐mediated bigeminy, interplay between ELT and dual AV nodal physiology has been described 4 . Moreover, interactions between dual AV nodal physiology and ELT should be expected, as, mechanistically, ELT closely resembles atrioventricular reentrant tachycardia (AVRT) known to be influenced in several ways by dual AV nodal physiology 5 .…”

“…Although the author is not aware of any other reports of pacemakermediated bigeminy, interplay between ELT and dual AV nodal physiology has been described. 4 Moreover, interactions between dual AV nodal physiology and ELT should be expected, as, mechanistically, ELT closely resembles atrioventricular reentrant tachycardia (AVRT) known to be influenced in several ways by dual AV nodal physiology. 5 The present data may also suggest that some ELTs are in fact pacemaker-mediated AV nodal reentrant tachycardias.…”