Epidemiology

Sustained BBR VT usually occurs in patients with structural heart disease, especially dilated cardiomyopathy. Idiopathic dilated cardiomyopathy is the anatomical substrate for BBR VT in 45% of cases, and BBR VT accounts for up to 41% of all inducible sustained VTs in this population. BBR VT can also be associated with cardiomyopathy secondary to valvular or ischemic heart disease, and has been reported with Ebstein anomaly, hypertrophic cardiomyopathy, and even in patients without structural heart disease other than intraventricular conduction abnormalities.²

In patients with spontaneous sustained monomorphic VT, the incidence of inducible BBR VT ranges from 4.5% to 6% in patients with ischemic heart disease to 16.7% to 41% in patients with nonischemic cardiomyopathy. BBR VT accounts for up to 6% of all forms of induced sustained monomorphic VT. Importantly, in patients with BBR VT, additional myocardial VTs occur in 25%.³ Of note, BBR VT is more frequently found in patients with VT clusters (up to 12.5%) than in patients with less frequent episodes of VT.⁴

Clinical Presentation

Sustained BBR VT is typically unstable secondary to very rapid ventricular rates (often 200 to 300 beats/min) and poor underlying ventricular function; 75% of patients present with syncope or cardiac arrest.

Initial Evaluation

BBR VT should be suspected in the presence of typical electrocardiogram (ECG) QRS morphology during normal sinus rhythm (NSR) and VT (see later), especially in a patient with dilated cardiomyopathy. Echocardiographic examination and coronary arteriography are required in most patients to evaluate for structural heart disease.

Principles of Management

Pharmacological antiarrhythmic therapy is usually ineffective. Radiofrequency (RF) catheter ablation of a bundle branch (typically the RB) can cure BBR VT and is currently regarded as first-line therapy.

Associated myocardial VT occurs in approximately 25% of patients post ablation, and these patients continue to be at a high risk of sudden cardiac death. Therefore, implantable cardioverter-defibrillator (ICD) therapy is indicated for secondary prevention, and additional antiarrhythmic therapy is required for some patients. ICD implantation will also provide back-up pacing, which is frequently required post ablation secondary to the development of atrioventricular (AV) block or an excessively prolonged His bundle–ventricular (HV) interval. Implantation of a dual-chamber or biventricular ICD should be considered in these patients.

Because BBR VT has a limited response to antiarrhythmic drugs and can be an important cause of repetitive ICD therapies, catheter ablation of the arrhythmia should always be considered as an important adjunct to the device therapy.

EP testing should be considered in patients with repetitive episodes of VT and dilated cardiomyopathy, history of cardiac valve repair or replacement, or QRS morphology during VT similar to sinus rhythm QRS. If sustained BBR VT is inducible during programmed stimulation, catheter ablation is recommended.⁴

Baseline ECG

The baseline rhythm is usually NSR or atrial fibrillation (AF). Almost all patients with BBR VT demonstrate intraventricular conduction abnormalities. The most common ECG abnormality is nonspecific intraventricular conduction delay (IVCD) with an LBBB pattern and PR interval prolongation (Fig. 26-2). Complete RBBB is rare but does not preclude BBR as the mechanism of VT. Although total interruption of conduction in one of the bundle branches would theoretically prevent occurrence of BBR, an ECG pattern of complete BBB may not be an accurate marker of complete conduction block; a similar QRS configuration can be caused by conduction delay, rather than block, in the bundle branch. Occasionally, complete AV block may be observed.²

FIGURE 26-2 ECG of bundle branch reentrant (BBR) ventricular tachycardia (VT). A, Normal sinus rhythm baseline with intraventricular conduction delay resembling left bundle branch block. B, BBR VT. Note typical-appearing complete LBBB in this rapid VT.

ECG during Ventricular Tachycardia

Twelve-lead ECG documentation of BBR VT is usually unavailable because the VT is rapid and hemodynamically unstable. The VT rate is usually 180 to 300 beats/min. QRS morphology during VT is a typical BBB pattern and can be identical to that in NSR. BBR VT with an LBBB pattern is the most common VT morphology, and it usually has normal or left axis deviation (see Fig. 26-2). In contrast to VT of myocardial origin, BBR with an LBBB pattern characteristically shows a rapid intrinsicoid deflection in the right precordial leads, suggesting that initial ventricular activation occurs through the HPS and not ventricular muscle. BBR VT with an RBBB pattern usually has a leftward axis, but it can have a normal or rightward axis, depending on which fascicle is used for anterograde propagation.¹

Baseline Observations during Normal Sinus Rhythm

Conduction abnormalities in the HPS are almost invariably present and are a critical prerequisite for the development of sustained BBR, regardless of the underlying anatomical substrate (Fig. 26-3).⁵ The average HV interval is about 80 milliseconds (range, 60 to 110 milliseconds). Although some patients can have the HV interval in NSR within normal limits, functional HPS impairment in these patients manifests as HV interval prolongation or split HB potentials, commonly becoming evident during atrial programmed stimulation or burst pacing. Nonspecific IVCD with an LBBB pattern and PR interval prolongation are the most common abnormalities.²

FIGURE 26-3 Bundle branch reentrant (BBR) ventricular tachycardia (VT) versus normal sinus rhythm (NSR). Sinus and BBR VT with His, left bundle branch (LB), and right bundle branch (RB) recordings in a patient with a prior septal myocardial infarction. Dashed lines mark the onset of His deflection. During NSR, the His potential is followed first by the LB potential; RB activation is further delayed. The fact that left bundle branch block (LBBB) is present on the ECG suggests that, although the LB is activated prior to the RB, delay is encountered more distally in the left ventricular His-Purkinje system such that LBBB is evident on the ECG. During BBR VT, activation propagates in an LB-His-RB sequence. Retrograde LB activation is very delayed, most likely because of the same factors responsible for the ECG in NSR.

Induction of Tachycardia

VES from the RV apex is the usual method used to induce BBR with an LBBB pattern. Induction is consistently dependent on the achievement of a critical conduction delay in the HPS (i.e., critical ventricular–His bundle [VH] interval) following the VES.

During RV pacing at a constant cycle length (CL) and during introduction of VES at relatively long coupling intervals, retrograde conduction to the HB occurs via the RB. At shorter VES coupling intervals, retrograde delay and block occur in the RB when its relative and effective refractory periods are encountered, respectively. When retrograde block occurs in the RB, the impulse propagates across the septum and retrogradely up the LB to the HB, producing a long V₂-H₂ interval. The LB would still be capable of retrograde conduction because of its shorter refractoriness and because of the delay associated with transseptal propagation. Further shortening of the coupling intervals is associated with increasing delay in LB conduction (i.e., increasing V₂-H₂ interval). Within a certain range of coupling intervals, increasing retrograde LB delay allows for the recovery of anterograde conduction via the RB, and another ventricular activation ensues, displaying a wide QRS with an LBBB pattern. This beat is called the “BBR beat” or “V₃ phenomenon.”²

An inverse relationship exists between retrograde conduction delay in the LB (V₂-H₂ interval) and the time of anterograde conduction in the RB (H₂-V₃ interval). This is because the faster the impulse propagates transseptally and up the LB, the more likely it will reach the RB while it is still refractory from the previous retrograde activation (concealment) by the VES, resulting in slower anterograde conduction down the RB.

BBR is more likely to occur when the VES is delivered following pacing drives incorporating long to short CL changes as compared with constant CL drives, because of CL dependency of the HPS refractoriness. An abrupt change in CL (i.e., long to short) can result in a more distal site of retrograde block, and less concealment, along the myocardium-Purkinje-RB axis, which can allow sufficient recovery of excitability in the anterograde limb of the circuit (i.e., the RB-Purkinje-myocardium axis) for reentry to develop. In addition, earlier recovery of excitability along this axis, because of the more distal site of block and less concealment, is associated with a shorter H₂-V₃ interval in this reentrant beat.¹

Procainamide, which increases conduction time within the HPS, especially in the diseased HPS, and, potentially, isoproterenol can facilitate induction of sustained BBR. In some patients, the arrhythmia can be inducible only with atrial pacing.²

Tachycardia Features

BBR VT can only be diagnosed using intracardiac recording (Table 26-1). AV dissociation is typically present, during the tachycardia, but 1:1 ventriculoatrial (VA) conduction can occur. BBR VT is characterized by inscription of the His potential before the QRS complex, with the HV interval during BBR with an LBBB pattern generally being similar to or longer than that during baseline rhythm (the HV interval is usually 55 to 160 milliseconds; see Fig. 26-3). However, in rare cases, the HV interval during BBR VT can be slightly shorter (by less than 15 milliseconds) than the HV interval in NSR, because during BBR the HB is activated in the retrograde direction simultaneously (in parallel) with the proximal part of the bundle branch serving as the anterograde limb of the reentry circuit, whereas during NSR, activation of the HB and activation of the bundle branch occur in sequence.¹

TABLE 26-1 Diagnostic Criteria of Bundle Branch Reentrant Ventricular Tachycardia

BBR = bundle branch reentry; HB = His bundle; HPS = His–Purkinje system; HV = His bundle–ventricular; LB = left bundle branch; LBBB = left bundle branch block; LB-V = left bundle–ventricular; RB = right bundle branch; RBBB = right bundle branch block; RB-V = right bundle–ventricular.

The relative duration of the HV interval recorded during VT as compared with NSR would depend on two factors: the balance between anterograde and retrograde conduction times from the upper turnaround point of the reentry circuit, and the site of HB recording relative to the upper turnaround point (i.e., the HB catheter electrode positioned at the proximal versus distal HB). Conduction delay in the bundle branch used as the anterograde limb of the circuit tends to prolong the HV interval during VT, whereas retrograde conduction delay to the HB recording site, as well as the use of a relatively proximal HB recording site (far from the turnaround point), tend to shorten the HV interval. The right bundle–ventricular (RB-V) interval during VT with LBBB morphology must always be longer than that recorded in sinus rhythm, emphasizing the importance of recording the RB potential during VT.¹ The HV interval during BBR with an RBBB pattern can be significantly different from that during NSR (HV interval, 65 to 250 milliseconds). During NSR, the HV interval is usually determined by conduction over whichever bundle branch conducts most rapidly, whereas during BBR it is determined by conduction over the typically diseased LB.

In the common type of BBR VT (LBBB pattern), the activation wavefront propagates retrogradely up the LB to the HB and then anterogradely down the RB, with subsequent ventricular activation. This sequence is reversed in BBR with an RBBB pattern. The HV and RB-V (during VT with LBBB morphology) or left bundle–ventricular (LB-V) intervals (during VT with RBBB morphology) are relatively stable.

During BBR VT, spontaneous variations in the V-V intervals are preceded and dictated by similar changes in the H-H (and RB-RB or LB-LB) intervals. In other words, the VT CL is affected by variation in the V-H (V-RB or V-LB) intervals. These changes can occur spontaneously, most commonly immediately after induction of the VT, or be demonstrated by ventricular stimulation during VT. However, oscillations in the V-V intervals can occasionally precede those of the H-H intervals during BBR VT because of conduction variations in the anterograde, rather than the retrograde, conducting bundle branch.³

Recording from both sides of the septum can help to identify the BBR mechanism. Documentation of a typical H-RB-V-LB activation sequence (during VT with LBBB morphology), or of an H-LB-V-RB activation sequence (during VT with RBBB morphology), supports the diagnosis of BBR (see Fig. 26-3).¹ Unfortunately, the RB potentials, LB potentials, or both are not always recorded, so that the typical activation sequences (LB-H-RB-V or RB-H-LB-V) are not available for analysis. Even if either activation sequence is present, the HPS (usually the LB) could be activated passively in a retrograde fashion to produce an H-RB-V sequence during a VT with LBBB pattern without reentry requiring the LB. In these cases, other diagnostic criteria for BBR should be used. In addition, during VT with LBBB morphology, RV activation must precede the LV activation. The opposite is true for the VT with RBBB morphology.

BBR can be terminated by block in the HPS—spontaneous, pacing-induced, secondary to catheter trauma, or caused by ablation.

Diagnostic Maneuvers during Tachycardia

Pacing maneuvers can be extremely helpful to establish the diagnosis of BBR; however, application of the pacing maneuvers during BBR is often not feasible because of the hemodynamic compromise commonly associated with these VTs.

Exclusion of Other Arrhythmia Mechanisms

Target of Ablation

The ablation target is either the RB or the LB; however, RB ablation is easier and usually is the method of choice. In most patients with BBR VT, although diffuse conduction system disease is present, the conduction abnormality in the LB is more severe than that in the RB. Nonetheless, preserved slow conduction over the LB can be demonstrated in the majority of patients, and the LB can still maintain 1:1 AV conduction during NSR.⁶ For the occasional patients in whom anterograde conduction down the LB is known to be inadequate for maintaining AV conduction, ablation of the RB will commit the patient to a permanent pacemaker. Therefore, ablation of the LB in these patients is preferable because it can prevent BBR and still preserve anterograde conduction.

The mere presence of LBBB on the surface ECG does not mean complete block in the LB. Signs that can indicate that conduction down the LB may be inadequate in maintaining 1:1 AV conduction and favor ablation of the LB over the RB include (1) development of high-grade AV block below the HB on transient RBBB that can occur secondary to RV catheter manipulation, and (2) observation of an LB potential following the ventricular electrogram either intermittently or during every sinus beat.

Ablation Technique

Ablation of the Right Bundle Branch

The HB divides at the junction of the fibrous and muscular boundaries of the intraventricular septum into the RB and LB. The RB is an anatomically compact unit that travels as the extension of the HB after the origin of the LB. The RB courses down the right side of interventricular septum near the endocardium in its upper third, deeper in the muscular portion of the septum in the middle third, and then again near the endocardium in its lower third. The RB is a long, thin, discrete structure; it does not divide throughout most of its course, and begins to ramify as it approaches the base of the right anterior papillary muscle, with fascicles going to the septal and free walls of the RV.

A quadripolar catheter is positioned at the HB region and maintained as a reference. The ablation catheter is initially positioned in the HB region and the area of the septum at which the largest His potential is recorded. The catheter is then advanced gradually (in the right anterior oblique [RAO] view) superiorly and to the patient’s left side, with clockwise torque to ensure adequate catheter tip contact with the septum and RB and continuous adjustment of the catheter’s curvature until the RB potential is recorded. Attempts should be made to obtain the distal RB recording to ensure that the catheter tip is away from the HB and LB.

The RB potential can be distinguished from the HB potential by the absence of or minimal atrial electrogram on the recording and presence of a sharp deflection inscribed at least 15 to 20 milliseconds later than the His potential (Fig. 26-4). An RB-V interval value of less than 30 milliseconds may not be a reliable marker of the RB potential in these patients because disease of the HPS can cause prolongation of the RB-V conduction time.

FIGURE 26-4 Ablation of bundle branch reentry (BBR). Sinus and right ventricular apical paced complexes from before (left) and after (right) catheter ablation of the right bundle branch (RB) in a patient with BBR VT. Four bipolar recordings from a catheter at the His position and RB are shown. Prior to RB ablation, propagation along the His-RB axis is linear, anterograde during sinus rhythm (green arrow), and retrograde during RV pacing (purple arrow). After RB ablation, anterograde propagation is interrupted (green arrow, red line) and retrograde activation of the His electrograms occurs after the local ventricular electrogram (purple arrow), indicating block in the RB with transseptal propagation, then up the left bundle.

When there is RB conduction delay at baseline, the RB potential can become hidden within the ventricular electrogram, and it may be impossible to map during NSR, especially when the surface ECG shows complete or incomplete RBBB. However, the RB potential should be readily observed during BBR beats induced by RV stimulation or during BBR VT. In this setting, anatomically guided lesions or a linear ablation perpendicular to the axis of the RB distal to the HB recording may be effective.

A 4-mm-tip ablation catheter is typically used for RB ablation. RF application is usually started at low levels (5 W) and gradually increased every 10 seconds, targeting a temperature of 60°C. In general, RBBB develops at 15 to 20 W. Successful ablation will result in clear development of RBBB in lead V₁ (Fig. 26-5; and see Fig. 26-4). Occasionally, an accelerated rhythm from the RB is observed during ablation (analogous to accelerated junctional rhythm with HB ablation; see Fig. 26-5).

FIGURE 26-5 Right bundle branch (RB) ablation. During RB ablation, delivery of radiofrequency current causes accelerated rhythm from the RB until complete block occurs (note lead V₁). The His bundle–ventricular interval after ablation is 145 milliseconds (baseline, 80 milliseconds). NSR = normal sinus rhythm; RBBB = right bundle branch block.

Endpoints of Ablation

Endpoints of RB ablation include the development of an RBBB pattern (see Fig. 26-5), noninducibility of BBR, and reversal of the direction of HB and RB activation during RV pacing. Prior to ablation, HB electrodes are depolarized during RV pacing from distal to proximal (RB-HB); after RB ablation, HB activation is delayed and reversed (HB-RB; see Fig. 26-4).

Endpoints of LB ablation include elimination of retrograde conduction of VESs over the LB and noninducibility of BBR.

After ablation, aggressive ventricular stimulation should be performed to evaluate the inducibility of VT. Also, decremental atrial pacing should be performed to evaluate the conduction properties of the HPS and the propensity for infra-Hisian AV block. It is advisable to stress the HPS with intravenous procainamide and ensure that anterograde conduction is preserved.³

Outcome

Recurrence of BBR VT after successful ablation is extremely rare. The reported incidence of clinically significant conduction system impairment requiring implantation of a permanent pacemaker varies from 10% to 30%.²

Pacemaker implantation is indicated when infra-Hisian AV block is demonstrated with atrial pacing, or when the post-ablation HV interval is 100 milliseconds or greater. As noted, associated myocardial VT is observed in approximately 25% of patients post ablation, and these patients continue to be at a high risk of sudden cardiac death. Therefore, ICD implantation is indicated for secondary prevention, and additional antiarrhythmic therapy is required for some patients.