Recurrent Symptomatic Regular Tachycardia

Abstract

We present a case of bundle-branch reentry ventricular tachycardia (BBRVT) in a patient with underlying dilated cardiomyopathy. This is a rare form of VT using a reentry circuit typically consisting of the right bundle branch serving as the antegrade limb and the left bundle branch as the retrograde limb. It often occurs in patients with advanced conduction system disease, like in our case, a complete left bundle-branch block (LBBB). In BBR-VT there is often an unchanged QRS pattern compared with sinus rhythm which makes diagnosis challenging. In our case, even the automated morphology algorithm of the patient’s implantable cardioverter-defibrillator, which compared the EGM morphology during normal rhythm and tachycardia, failed to correctly identify the ventricular origin of the patient’s tachycardia. Catheter ablation of the right bundle branch is the preferred treatment of BBR-VT and resolved the problem in our patient.

Case presentation

A 53-year-old patient with an implantable cardioverterdefibrillator (ICD) presented with recurrent symptomatic tachycardia episodes. He had undergone primary prophylactic VVI-ICD implantation for dilated cardiomyopathy with severely reduced left ventricular ejection fraction (LVEF) 5 years before. His resting ECG at presentation showed persistent atrial fibrillation (AF) and a preexisting left bundle branch block. According to the device’s morphology algorithm (Boston Scientific Teligen 100, Rhythm ID algorithm), the tachycardia episodes showed an intracardiac electrogram (EGM) morphology match (RID+) with the baseline EGM. Some of the episodes were successfully terminated by anti-tachycardia pacing (ATP) (Figure 1), while others were accelerated to a faster ventricular tachycardia (VT) by ATP and subsequently terminated by ICD shocks. The presence of relevant coronary heart disease was ruled out by coronary angiography. Betablocker therapy was increased and loading with amiodarone was performed, both of which failed to prevent further tachycardia episodes and ICD shocks.

What is the tachycardia mechanism and how should it be managed?

Differentiation between supraventricular tachycardia (SVT) and VT was the first important step in this patient. The tracing in figure 1 shows the EGMs during a tachycardia episode with the nearfield bipolar EGM (recorded from the electrode tip) displayed on top and the farfield EGM (recorded from the right ventricular [RV] coil to the can) displayed on the bottom. The tracing in the upper panel shows an irregular ventricular activation during atrial fibrillation in the first part. Then a regular tachycardia starts with a sudden onset. Once 8/10 beats are below the VT threshold, a ventricular episode is declared (“V-Epsd” at the beginning of the middle panel). After additional 5 seconds of ongoing VT (programmed detection time), the criteria for a durable ventricular episode are met (“V-Dur”). However, the “Rhythm-ID” algorithm is active in this ICD. It is used in Boston Scientific ICDs for automated comparison of the EGM morphology during normal rhythm (persistent atrial fibrillation in our patient) and tachycardia for enhanced discrimination between SVT and VT. As shown in the second part of the middle panel, the “Rhythm ID” algorithm indicates a morphology match (RID+), and as a consequence, VT therapy is withheld. In the lower part of the tracing, the ninth beat no longer fulfills the beat-to-beat morphology match criteria (RID–). Accordingly, ATP is appropriately applied and terminates the tachycardia. Following tachycardia termination, AF is still the underlying rhythm. The termination of a regular tachycardia by ATP in the context of ongoing AF speaks strongly in favour of VT. On the other hand, no change in EGM morphology during the tachycardia, as indicated by the Rhythm-ID morphology match, clearly speaks in favour of SVT. However, the occurrence of regular SVTs such as atrioventricular-nodal reentry tachycardia (AVNRT) or ortho dromic atrioventricular-reentrant tachycardia (AVRT) during ongoing atrial fibrillation is very unlikely, despite some rare case reports suggesting the opposite [1]. Another potential differential diagnosis would be an atrial flutter with 1:1 conduction under treatment with amiodarone. But given the fact that some of the episodes were terminated by ATP this is very unlikely. Alternatively, VTs with an EGM morphology similar or identical to the regular supra-ventricular rhythm are rare. They may occur due to a idiopathic or scar-related septal ventricular tachycardia exit that allows early access to the normal conduction system. Another possibility is bundle branch reentry VT (BBR-VT), a rare form of VT in patients with preexisting conduction system diseases and typically left bundle-branch block (LBBB), in which the QRS and EGM during tachycardia match the QRS and EGM during normal rhythm with LBBB.

An electrophysiological (EP) study was performed. At the time of EP study, the patient was in sinus rhythm (conversion from atrial fibrillation resulting from the most recent ICD shock) with an LBBB configuration (Figure 2A). QRS duration was 138 ms and the HV interval was prolonged to 74 ms. With programmed stimulation from the RV apex, a LBBB-VT with a cycle length of 290 ms could be induced. Every ventricular depolarisation was preceded by a His bundle deflection with an HV interval of 80 ms during tachycardia. Entrainment from the RV apex resulted in a postpacing interval (PPI) – tachycardia cycle length (TCL) of 20 ms. Accordingly, a diagnosis of BBR-VT could be made. Following ablation of the right bundle branch, the surface ECG morphology changed from an LBBB to a right bundlebranch block (RBBB) morphology (Figure 2B) and the tachycardia was no longer inducible. Following the ablation procedure, the patient has not had any more monomorphic VT in more than a year. Subsequent routine interrogation of his VVI-ICD indicated less than 1% V pacing and he continued in New York Heart Association class II, so that no upgrade to a CRT-ICD system has been performed yet.

BBR-VT is a rare form of VT which most frequently occurs in patients with advanced structural heart disease, in particular in patients with nonischaemic dilated cardiomyopathy, and with preexisting conduction system disease, most typically LBBB [2]. In the most common form of BBR-VT, the right bundle branch serves as the antegrade limb and the left bundle branch as the retrograde limb of the tachycardia circuit (Figure 3). Of note, the LBBB at baseline results from a conduction delay rather than a complete block in the left bundle branch, given that retrograde conduction over the left bundle is a critical part of the circuit. Another prerequisite for sustained BBR-VT is that the conduction times over each bundle are longer than the r efractory period of the other bundle. The exit of ventricular activation occurs at the terminal portion of the right bundle branch system, therefore mimicking the QRS and EGM morphology during normal rhythm with LBBB. Antiarrhythmic treatment is often not successful and might even increase the probability of BBR-VT by further slowing conduction velocity. Rather, catheter ablation is the curative treatment of choice [3,4]. Ablation usually targets the right bundle branch, because it is technically easier due to its superficial location. It often results in a change from LBBB to RBBB as in our patient, and often requires pacing afterwards due to a markedly prolonged HV interval or complete AV block. Even if ablation is successful, ICD implantation is warranted in most patients because of their underlying heart disease [2,3,4].

Disclosure statement

No financial support and no other potential conflict of interest relevant to this article was reported.