Bundle branch reentrant ventricular tachycardia

Articolo redatto e revisionato del Dott. Luca Moretti

Bundle branch reentrant ventricular tachycardia is a form of macroreentry in which the His-Purkinje system is an essential part of the circuit. In the classic configuration, the impulse travels down one bundle branch in the anterograde direction, reaches the ventricle, and returns through the other branch in the retrograde direction. The result is generally a rapid monomorphic tachycardia with a bundle branch block appearance. Recognition is particularly important because targeted interruption of one limb of the circuit can eliminate the arrhythmia.

The condition is often associated with disease of the conduction system and structural heart disease, especially dilated or valvular disease, but it may also occur with preserved ventricular function. Frequency in case series depends heavily on the population studied and on how systematically diagnostic maneuvers are performed during electrophysiologic study. An ECG resembling ordinary bundle branch block may lead to erroneous classification as supraventricular tachycardia with aberrancy. Similarity to the sinus-rhythm QRS does not exclude a ventricular mechanism.

Ablation can cure the circuit, but it may also compromise residual anterograde conduction and does not eliminate concomitant myocardial substrates. The pathway therefore requires three linked assessments: demonstration of the reentrant mechanism, selection of the branch to treat, and planning of protection after the procedure. The risk of atrioventricular block, the possible need for pacing, and the indication for a defibrillator must be considered together with ventricular function and the underlying disease.

Circuit anatomy and conditions required for maintenance

The right and left bundle branches are connected proximally through the His system and distally through the Purkinje network and ventricular myocardium. Under normal conditions, rapid conduction and refractoriness limit the possibility that an impulse can complete a sustained circuit. Sufficiently marked His-Purkinje slowing, however, allows one component to recover excitability before the impulse returns. The arrhythmia requires a combination of conduction times, refractoriness, and unidirectional block, not simply the presence of a wide QRS.

In the most common configuration, the right bundle branch conducts anterogradely and the left retrogradely. Initial right-sided ventricular activation produces a left bundle branch block morphology. The reverse configuration uses the left bundle branch in the anterograde direction and the right as the return pathway, producing a right bundle branch block appearance. Both may be present in the same patient. The terms clockwise or counterclockwise depend on the schematic representation and by themselves add no clinical information beyond the direction of conduction in the two branches.

Traditional nomenclature distinguishes a type A form with left bundle branch block morphology and a type C form with right bundle branch block morphology. Type B interfascicular reentry, by contrast, uses the left fascicles as the anterograde and retrograde pathways. Recognizing this difference is essential because the right bundle branch may be outside the interfascicular circuit. Right bundle branch ablation, effective for classic reentry, is therefore not a universal treatment for every tachycardia involving the conduction system.

The circuit may be facilitated by persistent anatomical abnormalities or by functional slowing that appears only at particular pacing intervals. A normal His-ventricular interval in sinus rhythm therefore does not exclude reentry. During induction, a critical delay may develop that permits circular propagation. A single reentrant beat induced in the laboratory does not, however, equal a sustained clinical tachycardia: maintenance and correspondence with the patient's events should be documented.

A bundle branch block pattern on the ECG does not necessarily mean complete absence of all conduction in that branch. Marked slowing, direction of propagation, and concealed penetration may produce the surface pattern while conduction useful to the circuit remains, particularly in the retrograde direction. This distinction explains why an apparently blocked branch may still participate in the tachycardia. Interpretation should be based on intracardiac recordings rather than solely on the name of the baseline conduction disorder.

The proximal conduction system and septal myocardium contribute to the architecture, whereas the atria and atrioventricular node are not ordinarily required to maintain classic reentry. Ventriculoatrial conduction, when present, may create the impression of a one-to-one relationship without making the atrium an essential part of the circuit. This property has a precise therapeutic consequence: atrioventricular node ablation does not cure bundle branch reentry because it does not necessarily interrupt the circular pathway below the node.

Associated heart diseases and clinical presentation

Dilated cardiomyopathy is an important setting because dilation and abnormalities of the conduction system may create the delays required for the circuit. The disease may be ischemic or nonischemic, but the mechanism is not the same as ordinary reentry within a ventricular scar. The two substrates may coexist. After elimination of bundle branch reentry, myocardial tachycardias of a different origin may therefore remain inducible or appear later.

Valvular disease and valve procedures deserve particular attention. A tachycardia with bundle branch block morphology appearing in the weeks after a procedure may be facilitated by changes in conduction. Ventricular function may be relatively preserved, making identification of a treatable circuit especially relevant. The temporal relationship with the procedure does not by itself prove the mechanism, but it should guide electrophysiologic study even when the initial hypothesis is a scar-related arrhythmia.

Reentry has also been described in congenital heart disease, arrhythmogenic disorders, neuromuscular conditions such as myotonic dystrophy, and genetically determined electrical or conduction diseases. Pathogenic variants in SCN5A or LMNA may be relevant in some patients with otherwise unexplained conduction disorders, but they should not be sought indiscriminately after every episode. Phenotype, age, family history, and other manifestations guide genetic counseling and interpretation of results.

The tachycardia is often rapid, with rates that may be on the order of 200-300 beats per minute, and may cause syncope, collapse, or cardiac arrest. Slower and better-tolerated episodes also occur, particularly in relation to conduction characteristics and therapies. Clinical presentation also depends on ventricular function and hemodynamic reserve. Tolerance of a single episode does not reduce the need to identify a circuit capable of supporting rapid recurrences.

In defibrillator recipients, the arrhythmia may be terminated by antitachycardia pacing or shock before a complete ECG is obtained. Stored electrograms therefore become valuable for comparing the spontaneous event with induced tachycardias. A report of appropriate therapy does not, however, identify the mechanism: rate, sequence, and available signals must be analyzed. Documentation of multiple morphologies prevents every device intervention from being attributed to a single circuit.

At baseline ECG, prolonged PR, a wide QRS, or other signs of conduction disease are common, but their absence does not exclude the condition. Symptoms of bradyarrhythmia may coexist with tachycardia-related symptoms and influence treatment planning. A history of syncope may derive from either component or from both. For this reason, the pathway does not end with tachycardia termination: it should include assessment of residual conduction and the possibility of progression.

Electrocardiographic and electrophysiologic diagnosis

The QRS during tachycardia often shows a typical bundle branch block pattern, with relatively rapid initial forces because activation uses the conduction system. This may make some criteria used to distinguish ventricular from supraventricular tachycardia less apparent. Atrioventricular dissociation is useful when present but may be hidden by retrograde conduction. A configuration almost identical to that in sinus rhythm does not exclude bundle branch reentry and may in fact be characteristic of the mechanism.

Diagnosis is demonstrated by His and bundle branch recordings, ideally with reconstruction of the activation sequence on both sides of the septum when necessary. In classic forms, conduction-system potentials precede the QRS in an order consistent with the anterograde branch. The His-ventricular interval during tachycardia is often equal to or longer than at baseline. This relationship supports the mechanism but is not sufficient in isolation because the His bundle may also be activated during other tachycardias.

An important criterion concerns cycle-length oscillations. When tachycardia cycle length varies, changes in His-His intervals or intervals between bundle potentials may precede the corresponding ventricular-ventricular changes. This indicates that the conduction system governs timing rather than merely being activated retrogradely by the myocardium. In myocardial reentry, ventricular variations instead tend to precede those of the His system. The sequence should be verified on reliable signals rather than inferred from a single measurement.

Induction often requires a critical conduction delay and may be facilitated by pacing sequences that produce unidirectional block. Ventricular pacing, and in some cases atrial pacing, allows these properties to be explored. Demonstration of a reentrant beat or induced tachycardia should be linked to the clinical morphology. A negative protocol does not automatically exclude the mechanism when the documentation is strong, but the diagnostic limitation must be acknowledged before an irreversible lesion is created.

Entrainment from the right ventricular apex may show fusion and a post-pacing interval close to the tachycardia cycle length, consistent with participation of the ventricle in the circuit. A short return interval is a clue, not independent proof, because myocardial reentry near the pacing site may also produce it. Potential sequence, response to extrastimuli, and behavior of oscillations are integrated. The diagnosis should be sufficiently secure before sacrificing a bundle branch that may be essential for normal conduction.

Termination of tachycardia in association with block in a critical bundle branch, together with inability to reinduce it after an effective lesion, provides verification of the circuit when interpreted in the complete context. The fact that a tachycardia stops during catheter contact alone is not enough because mechanical trauma may be transient. Recordings should distinguish true interruption of the pathway from other causes of termination. The procedural result completes the demonstration but should not replace a well-founded initial hypothesis.

Differential diagnosis and definition of the underlying disease

Supraventricular tachycardia with aberrancy may produce a QRS almost identical to that of bundle branch reentry. The relationship with the atrium and the response to electrophysiologic maneuvers allow verification of which structures are required for the circuit. A one-to-one atrioventricular relationship does not prove atrial dependence, whereas well-documented dissociation excludes many supraventricular hypotheses. A junctional origin must also be distinguished through sequence and response to pacing rather than relying solely on the timing of His and QRS signals.

In myocardial reentry, the conduction system may be activated passively during tachycardia. A His potential present in every cycle is therefore insufficient to show that it governs the rhythm. The relationship between oscillations, bundle branch sequence, and reset maneuvers clarifies this difference. The two tachycardias may coexist in the same patient, especially in scar-related cardiomyopathy. Diagnosis of a curable bundle branch form does not make investigation for other clinically relevant mechanisms unnecessary.

Interfascicular reentry may show a right bundle branch block morphology with axis deviation and a His activation pattern different from classic reentry. The circuit may be entirely distal to the main bifurcation of the left bundle branch; in this case neither the right bundle branch nor the proximal left bundle necessarily constitutes a critical part. The lesion must involve the participating fascicle. Confusing the two mechanisms may create a new conduction disorder without eliminating the arrhythmia.

Idiopathic verapamil-sensitive fascicular ventricular tachycardia involves slow-conducting components of the Purkinje network and has characteristics different from classic macroreentry through the main bundle branches. Tachycardias mediated by atriofascicular pathways may also have a left bundle branch block morphology and require assessment of the role of the atrium and accessory connection. Drug response may provide guidance but does not replace demonstration of the circuit. The target should be chosen according to the structure essential for maintenance rather than the external appearance of the QRS.

Echocardiography defines ventricular function, dimensions, and valvular disease, while cardiac magnetic resonance imaging may identify scar or cardiomyopathy not evident on echocardiography. Coronary assessment is guided by the clinical setting. In patients who have undergone surgery or intervention, procedural documentation and the chronology of conduction changes are particularly useful. Preserved function does not exclude His-Purkinje disease and does not automatically make a rapid reentrant tachycardia harmless.

Assessment of baseline conduction includes PR, QRS, intracardiac intervals, and behavior during relevant pacing maneuvers. The ability of the bundle branch that will remain after ablation to sustain anterograde conduction should be examined. Drugs, electrolytes, and systemic diseases may alter this capacity and should be considered before a definitive decision. When there are signs of inherited or neuromuscular disease, etiologic evaluation also affects family surveillance and the risk of progression.

Ablation of the circuit and management of residual conduction

The acute episode is treated according to hemodynamic stability and presence of a pulse. Cardioversion or defibrillation is used when necessary without delaying treatment for complete electrophysiologic characterization. Drugs may contribute in some situations, but chronic control of bundle branch reentry with antiarrhythmic drugs alone is often unsatisfactory. Modifying conduction may change rate and inducibility without eliminating the pathway. Once the mechanism has been demonstrated, ablation generally represents the most effective specific treatment.

The right bundle branch is frequently the preferred target because of accessibility and the possibility of interrupting the circuit with a localized lesion. Recording should identify the bundle potential and distinguish it from the His bundle, maintaining an adequate functional distance from proximal structures. The expected effect is block in the treated branch and loss of the ability to sustain reentry. With marked baseline abnormalities, the change in the surface QRS may be less obvious and require intracardiac confirmation.

Target selection must consider conduction in the contralateral bundle branch. If effective anterograde activation depends almost entirely on the right bundle branch, its ablation may cause complete atrioventricular block. In selected patients, a strategy targeting the left bundle branch or a distal portion of it may be considered to interrupt the circuit while preserving the best residual pathway. This choice requires precise definition of the mechanism and cannot be inferred solely from the presence of left bundle branch block on the ECG.

Before the procedure, the possibility of temporary or permanent pacing should be planned. A pre-existing advanced disturbance may become evident after interruption of the last functionally adequate pathway. Discussion with the patient should include this risk and the type of device that might be appropriate, considering ventricular function and the anticipated need for pacing. The objective of eliminating tachycardia should not be separated from the consequences of the new ventricular activation sequence.

In interfascicular reentry, the target is the fascicle participating in the circuit. Ablating the right bundle branch, if passive, does not cure the tachycardia; a proximal left bundle lesion may also be ineffective when the circuit is more distal. Axis change may be a consequence of fascicular injury. It is therefore essential to distinguish this variant before energy delivery and avoid using the same site for every tachycardia involving the Purkinje system.

Endpoints include stable interruption of critical conduction and noninducibility of reentry with an appropriate protocol. Transient mechanical block may temporarily reduce inducibility and simulate a complete result. Observation and verification of signals help distinguish this phenomenon from a durable lesion. Disappearance of the clinical tachycardia should be accompanied, when relevant, by assessment of other inducible arrhythmias because successful treatment of the bundle branch circuit does not automatically identify every substrate present.

Series report high acute efficacy and relatively uncommon specific recurrence after stable interruption of the circuit. The most characteristic risk is additional damage to conduction, with a variable need for pacemaker implantation according to the underlying disease and strategy. General procedural risks and, for left-sided approaches, risks associated with left-heart access are added. Percentages observed in small groups cannot be applied uniformly to patients with very different conduction reserve.

Prognosis, devices, and follow-up after ablation

Successful ablation separates two prognostic aspects: the probability of recurrence of bundle branch reentry and the overall risk of the underlying heart disease. The former may fall dramatically after circuit interruption; the latter may remain high because of dysfunction, scar, other tachycardias, or progression of heart failure. A patient with normal function and isolated conduction disease does not have the same course as a person with advanced cardiomyopathy. Prognosis should be related to the actual clinical picture, not merely technical success.

The indication for an implantable cardioverter-defibrillator is reassessed according to prior events, ventricular function, and residual substrates. In patients with significant structural heart disease, elimination of a curable circuit does not automatically remove an indication for protection. There are selected patients with preserved function and no other mechanisms in whom the balance may be different, but the evidence is more limited. The decision should state which risks have been removed and which remain, avoiding automatic assumptions in either direction.

If a high percentage of ventricular pacing is required, device selection should consider ventricular function, dyssynchrony, and the risk of deterioration. In appropriate settings, cardiac resynchronization or physiologic pacing strategies may be indicated, but feasibility also depends on lesion location and disease of the conduction system. Pacemakers, defibrillators, and resynchronization systems address different objectives. The device is selected by integrating these objectives rather than as an undifferentiated consequence of ablation.

Follow-up monitors progression of conduction disease, ventricular function, symptoms, and device recordings. Residual bundle branch block may have hemodynamic consequences, and the underlying disease may evolve even after an effective procedure. New syncope or presyncope requires distinction among bradyarrhythmia, circuit recurrence, and myocardial tachycardia. Mere absence of the initial morphology does not exclude another arrhythmic cause and does not make electrogram review unnecessary.

Treatment of the underlying heart disease and heart failure continues according to indications, with reassessment of tolerability, electrolytes, and drugs that affect conduction. In patients with neuromuscular or genetic disease, the pathway should consider disease-specific progression and extracardiac manifestations. Family evaluation is defined according to the diagnosis and any pathogenic variant, without using uncertain genetic findings as independent proof of disease in relatives.

Procedural documentation should preserve the diagnostic sequence, the treated bundle branch, residual conduction, and any other arrhythmias identified. This information is important if the patient is followed at another center or develops a tachycardia with a new morphology. A report stating only ventricular tachycardia ablation loses essential elements for interpreting future risk. Distinguishing a cured circuit from persistent substrate makes subsequent decisions more coherent.

For the patient, understanding that a rapid arrhythmia can be eliminated while conduction may still require protection helps place the procedure in the correct perspective. Quality of life may improve through fewer syncopal episodes and shocks, while follow-up and devices are adapted to residual risk. The best result comes from a demonstrated electrophysiologic diagnosis, a target chosen according to conduction reserve, and follow-up that continues to consider the overall heart disease.