Narrow QRS Tachycardia with 1:1 VA Conduction and Irregular Cycle Length:  What Is the Mechanism?

Siaplaouras, Stephanos; Buob, Axel; Neuberger, H.R.; Mewis, Christian

doi:10.1046/j.1540-8167.2005.40741.x

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“…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 although alternans is usually due to alternating conduction times in the anterograde limb of the macroreentrant circuit, it has also been reported in the retrograde limb 2–6 . Several mechanisms have been proposed to explain retrograde alternans in circus movement tachycardias using the native conduction system as the retrograde limb.…”

Section: Discussionmentioning

confidence: 84%

“…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 although alternans is usually due to alternating conduction times in the anterograde limb of the macroreentrant circuit, it has also been reported in the retrograde limb. [2][3][4][5][6] Several mechanisms have been proposed to explain retrograde alternans in circus movement tachycardias using the native conduction system as the retrograde limb. Josephson suggested that rate changes during antidromic AVRT are most commonly due to changing retrograde conduction over different fascicles.…”

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confidence: 99%

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Cycle Length Alternation During Pacing

Jastrzębski

2006

Cardiovasc electrophysiol

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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

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Section: Discussionmentioning

confidence: 84%

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confidence: 99%