Narrow QRS Tachycardia With Varying VA and RR Intervals: What Is the Mechanism?

Abstract

AV node reentry, catheter ablation, retrograde conduction, Wenkebach blockA 45-year-old woman was referred for radiofrequency catheter ablation of narrow QRS tachycardia that was terminated with intravenous adenosine. Twelve-lead electrocardiography (ECG) was normal during sinus rhythm. She was taking calcium channel blockers and that was stopped 5 half-lives prior to the procedure. The electrophysiological study showed a normal atrio-Hisian (AH) interval of 104 milliseconds and His-ventricular (HV) interval of 42 milliseconds during sinus rhythm. Retrograde conduction was central and decremental. Anterograde study demonstrated dual atrioventricular (AV) nodal physiology. Atrial pacing protocols easily and reproducibly induced narrow QRS tachycardia. What is the mechanism? CommentaryThe tachycardia represents a slightly irregular supraventricular tachycardia without the requirement for the atrium, ruling out atrial tachycardia and atrioventricular reentry. This leaves only junctional tachycardia (JT), AV nodal reentry, and the much less commonly observed nodoventricular reentry. JT is unlikely by the method of induction and the slight irregularity. The third and seventh cycles of the tachycardia showed prolongation of RR interval as shown in Figures 1 and 2. The intracardiac recordings (Fig. 2) showed 3:2 ventriculoatrial (VA) response for the first 3 beats followed by 4:3 VA response and 3:2 VA response for the last 3 beats. A retrograde VA Wenkebach pattern is noted as evidenced by the progressive prolongation of the VA interval till it stops conducting to atrium. The atrial activation J Cardiovasc Electrophysiol, Vol. 00, pp. 1-2, xxxx 2016. Figure 2. The AH and the His-His (HH) intervals are essentially constant except for the cycle following the retrograde block, during which they are significantly prolonged. Retrograde conduction to atrium resumes after the prolonged AH and HH intervals and it is followed by retrograde HA Wenkebach.The tachycardia represents slow-slow type of AV nodal reentrant tachycardia with retrograde HA Wenkebach and abrupt prolongation of the anterograde AV nodal conduction time after the HA block, as evidenced by the increase in the HH interval, with subsequent resumption of retrograde HA conduction. Prolongation of the anterograde conduction time allowed restoration of retrograde conduction of the subsequent beat, suggesting a dependency and relationship of retrograde conduction on the anterograde AV nodal conduction time by an undefined mechanism. The underlying morphological substrate behind this phenomenon involves 2 anatomical or functional AV nodal or intranodal circuits in the anterograde limb with distinct conduction properties, which is manifested after the retrograde block and a slow conducting AV nodal circuit as the retrograde limb.The patient has undergone a successful slow pathway ablation and neither dual AV node physiology nor tachycardia could be demonstrated after that.

An irregular supraventricular tachycardia: What is the mechanism?

“…Indeed, it would be only speculative to make a concise correlation with anatomy since the AH change over a slow pathway might be both due to functional decremental conduction or perhaps more than one distinct anatomical slow pathway. Either way, the prolongation of the anterograde conduction time allowed restoration of retrograde conduction of the fast pathway . Therefore, we speculated that there are two distinct atrial sequences and this is most compatible with two routes of conduction to the atria with the switch to the shorter VA facilitated by the AH prolongation of the previous cycle.…”

“…Either way, the prolongation of the anterograde conduction time allowed restoration of retrograde conduction of the fast pathway. 12 Therefore, we speculated that there are two distinct atrial sequences and this is most compatible with two routes of conduction to the atria with the switch to the shorter VA facilitated by the AH prolongation of the previous cycle. Whereas the shorter VA can only be AVNRT, the longer one progressively shortens and this would be most unlikely to be an accessory pathway.…”

“…It is also important to be alert to the possibility of double tachycardia during ablation procedure as the switch from one tachycardia to another creates confusion and may be misdiagnosed as a recurrence despite successful ablation of first tachycardia . Finally, the differential diagnosis of the irregular NQTs includes the irregular AT, irregular AVNRT with variable block to the atrium and/or ventricle or double tachycardias …”

“…Spontaneous decrease of the VA interval with parallel to increase in AH interval is most likely explained by the transition from an atypical AVNRT to a typical AVNRT by a spontaneous AH jump due to block in the antegrade and decremental slow pathway . Indeed, it would be only speculative to make a concise correlation with anatomy since the AH change over a slow pathway might be both due to functional decremental conduction or perhaps more than one distinct anatomical slow pathway.…”

“…7,8 Finally, the differen-tial diagnosis of the irregular NQTs includes the irregular AT, irregular AVNRT with variable block to the atrium and/or ventricle or double tachycardias. 2,[9][10][11][12] Although the presence of dual AV nodal pathways and initiation of the tachycardia by AH jump favors AVNRT (first AH jump, Figure 2), it is also possible finding in patients with AT or atrioventricular reentrant tachycardia (AVRT). Spontaneous changes in the measured intracardiac intervals during tachycardia (wobble) are useful in determining the mechanism of arrhythmia.…”