Abstract
Idiopathic fascicular ventricular tachycardia (VT) is a reentrant tachycardia involving the left fascicular Purkinje system. The reentry circuit is most commonly (90%) located in the territory of the left posterior fascicle, infrequently (5%–10%) in the region of the left anterior fascicle, and rarely arise from fascicular locations high in the septum. Fascicular VT is also referred to as verapamil-sensitive VT, given its tendency to slow or terminate with intravenous verapamil.
Fascicular VT accounts for 10% to 15% of all idiopathic VTs. The clinical course is generally benign, and the prognosis is excellent. The diagnosis of fascicular VT is based on (1) VT morphology on the surface ECG (right bundle branch block with left or, less commonly, right axis deviation); (2) VT sensitivity to verapamil; and (3) absence of structural heart disease. Invasive electrophysiological testing is required to confirm the diagnosis.
Oral verapamil or diltiazem is useful in mild cases; however, long-term efficacy is variable and the benefit of these drugs in management of patients with severe symptoms is often limited. Catheter ablation is highly effective (success rate of 85% to 90%) and is recommended for patients in whom drug therapy is not successful, not tolerated, or not preferred.
Keywords
idiopathic ventricular tachycardia, fascicular ventricular tachycardia, verapamil-sensitive ventricular tachycardia, His-Purkinje system, wide-complex tachycardia
Outline
Pathophysiology, 858
Epidemiology, 859
Clinical Presentation, 859
Initial Evaluation, 859
Principles of Management, 860
Electrocardiographic Features, 860
Electrocardiogram During Normal Sinus Rhythm, 860
Electrocardiogram During Ventricular Tachycardia, 860
Electrophysiological Testing, 860
Induction of Tachycardia, 860
Diagnostic Maneuvers During Tachycardia, 861
Exclusion of Other Arrhythmia Mechanisms, 861
Mapping, 861
Activation Mapping, 861
Entrainment Mapping, 863
Pace Mapping, 863
Electroanatomic Mapping, 864
Noncontact Mapping, 864
Ablation, 864
Target of Ablation, 864
Ablation Technique, 866
Endpoints of Ablation, 867
Outcome, 867
Empirical Ablation of Noninducible Ventricular Tachycardia, 867
Pathophysiology
The His-Purkinje system plays an important role in the genesis of cardiac arrhythmias. The mechanisms of several types of monomorphic ventricular tachycardias (VTs) have been directly linked to the Purkinje system, including bundle branch reentrant VT, interfascicular reentrant VT, fascicular VT, and focal Purkinje VT. A subset of polymorphic VTs and ventricular fibrillation has also been linked to triggers originating from the Purkinje system. Most of those arrhythmias occur in patients with underlying structural heart disease; however, fascicular VT and a subset of focal Purkinje VTs are idiopathic, occurring in the absence of apparent cardiac disease.
Idiopathic fascicular VT is a reentrant tachycardia involving the left fascicular Purkinje system. The reentry circuit is most commonly (90%) located in the territory of the left posterior fascicle (LPF), infrequently (5% to 10%) in the region of the left anterior fascicle (LAF), and rarely arise from fascicular locations high in the septum. Fascicular VT is also referred to as “verapamil-sensitive” VT, given its tendency to slow or terminate with intravenous verapamil.
Tachycardia Circuit
In the most common form (LPF-VT), the tachycardia circuit incorporates the LPF serving as one limb and abnormal Purkinje tissue with slow, decremental conduction serving as the other limb. The anterograde limb may be associated with longitudinal dissociation within the LPF or contiguous tissue that is directly coupled to the LPF (such as a false tendon) or, alternatively, has ventricular myocardium interposed. The zone of slow conduction appears to depend on the slow inward calcium current because the degree of slowing of tachycardia cycle length (TCL) in response to verapamil is entirely attributed to its negative dromotropic effects on the area of slow conduction.
The entrance site to the slow conduction zone is thought to be located closer to the base of the left ventricle (LV) septum. From there, activation propagates anterogradely (from basal to apical segments along the LV septum) over the abnormal Purkinje tissue with decremental conduction properties and verapamil sensitivity, which serves as the anterograde limb of the circuit and appears to be insulated from the nearby ventricular myocardium. The lower turnaround point of the reentrant circuit is located in the lower third of the septum, where the wavefront captures the fast conduction Purkinje tissue from or contiguous to the LPF, and retrograde activation occurs over the LPF from the apical to basal septum forming the retrograde limb of the reentrant circuit. In addition, at the lower turnaround point, anterograde activation occurs down the septum to break through (at the exit of the tachycardia circuit) in the posterior septal myocardium. The upper turnaround point of the reentrant circuit occurs over a zone of slow conduction located close to the main trunk of the left bundle branch (LB) in the basal interventricular septum ( Fig. 24.1 ). The estimated distance between the entrance and exit of the circuit is approximately 2 cm.
Anatomy of the Left Fascicular System
The LB arises from the His bundle (HB) as numerous fine, intermingling fascicles that leave the left margin of the branching HB through most of its course along the crest of the muscular ventricular septum. The predivisional portion of the LB penetrates the membranous portion of the interventricular septum under the aortic ring and then divides under the septal endocardium into two branches: the LAF and the LPF. An estimated 65% of individuals have a third fascicle of the LB, the left median fascicle (LMF). The fascicles cascade down the LV septum in a fan-like configuration with extensive interconnections. Unlike the cord-like right bundle branch, the LB and its divisions are diffuse, fan-like structures that quickly arborize just beyond their origin. The LAF represents the superior (anterior) division of the LB, the LPF represents the inferior (posterior) division, and the LMF represents the septal (median) division.
The LB subdivisions extend to the midportion of the septum before they detach from the underlying endocardium and form free-running false tendons that traverse the ventricular chamber, projecting predominantly toward the papillary muscles. The fascicles become ramified in the ventricular apex and extend back along the ventricular walls toward the cardiac base.
The thin LAF crosses the anterobasal LV region toward the anterolateral papillary muscle and terminates in the Purkinje system of the anterolateral LV wall. The LPF appears as an extension of the main LB and is broad in its initial course. It then fans out extensively toward the posterior papillary muscle and terminates in the Purkinje system of the posteroinferior LV wall. The LMF runs to the interventricular septum; it arises in most cases from the LPF, less frequently from the LAF, or from both fascicles, and in a few cases it has an independent origin from the central part of the main LB at the site of its bifurcation.
Epidemiology
Fascicular VT accounts for 10% to 15% of all idiopathic VTs. Age at presentation is typically 15 to 40 years (unusual after 55 years). Males are more commonly affected (60% to 80%). The clinical course is generally benign, and the prognosis is excellent. Sudden cardiac death is very rare. Tachycardia-induced cardiomyopathy precipitated by incessant VT can be observed in 6% of patients. Spontaneous remission of the VT can occur with time.
Clinical Presentation
Most patients present with mild to moderate symptoms of palpitations and lightheadedness. Occasionally, symptoms are debilitating and include fatigue, dyspnea, and presyncope. Syncope and cardiac arrest are rare. The VT is typically paroxysmal and can last for minutes to hours. Although fascicular VT can occur at rest, it is sensitive to catecholamines and often occurs during physical or emotional stress. Rarely, the VT can become incessant, sustained for a long period (days) and does not revert spontaneously to normal sinus rhythm (NSR). When incessant, fascicular VT can precipitate tachycardia-induced cardiomyopathy and heart failure.
Initial Evaluation
The diagnosis of fascicular VT is based on: (1) VT morphology on the surface electrocardiogram (ECG) (right bundle branch block [RBBB] with left or, less commonly, right axis deviation); (2) VT sensitivity to verapamil; and (3) absence of structural heart disease. Invasive electrophysiology (EP) testing is required to confirm the diagnosis. Evaluation to exclude structural heart disease is necessary and typically includes echocardiographic examination (that may show one or more prominent false tendons), stress testing, and coronary arteriography, depending on patient age and risk factors.
Principles of Management
Acute Management
Electrical cardioversion is required for VT termination in hemodynamically unstable patients. For stable patients with an established diagnosis of verapamil-sensitive fascicular VT, intravenous verapamil is the first-line treatment and is very successful in acutely terminating the VT. Intravenous verapamil slows the rate of VT progressively and then terminates it. Diltiazem is equally effective. Nonsustained VT may continue to occur for a while after termination.
Response of VT to lidocaine, procainamide, amiodarone, sotalol, and beta-blockers is less consistent, and these drugs are usually ineffective. Carotid sinus massage and Valsalva maneuvers have no effect on the VT. Fascicular VT is generally unresponsive to adenosine; however, when catecholamine stimulation (isoproterenol infusion) is required for the initiation of VT (in the EP laboratory), the VT can become adenosine sensitive.
Chronic Management
Oral verapamil or diltiazem is useful in mild cases; however, long-term efficacy is variable and the benefit of these drugs in management of patients with severe symptoms is often limited. Catheter ablation is highly effective (success rate of 85% to 90%) and is recommended for patients in whom drug therapy is not successful, not tolerated, or not preferred.
Electrocardiographic Features
Electrocardiogram During Normal Sinus Rhythm
The resting ECG is usually normal. Symmetrical inferolateral T wave inversion can be observed after termination of the VT (cardiac memory).
Electrocardiogram During Ventricular Tachycardia
Fascicular VT is characterized by relatively narrow QRS duration (127 ± 11 milliseconds) and short RS interval (the duration from the beginning of the QRS to the nadir of the S wave) in the precordial leads (60 to 80 milliseconds). The shorter QRS duration reflects the proximal exit of fascicular VTs from the His-Purkinje system (HPS). In addition, during fascicular VT, the interventricular septum is activated early with a left-to-right direction, resulting in an initial “r” waves in lead V 1 and small “q” waves in leads I and aV L (similar to classic RBBB pattern). In addition, the rapid activation of the LV via the Purkinje system results in unopposed late activation of the right ventricular outflow tract (RVOT), leading to a large R′ amplitude in lead V 1 (and r < R′).
The VT rate is approximately 150 to 200 beats/min (range, 120 to 250 beats/min). Alternans in the TCL is frequently noted; otherwise, the VT rate is stable.
According to the fascicular territory involved and, consequently, the QRS morphology, fascicular VTs are classified into three subtypes: (1) LPF VT (with the reentrant circuit exit in the inferoposterior septum) exhibits an RBBB and superior axis configuration (most common form); (2) LAF VT (with the reentrant circuit exit in the anterolateral wall of the LV) exhibits RBBB and right-axis deviation configuration; and (3) upper septal fascicular VT with a narrow QRS and normal axis configuration (rare form). In the most common form (LPF-VT), the QRS during VT typically has RBBB with LAF block configuration. The R/S ratio is less than 1 in leads V 5 and V 6 ( Fig. 24.2 ). VTs arising more toward the middle at the region of the posterior papillary muscle have a left superior axis and RS in leads V 5 and V 6 , whereas those arising closer to the apex have a right superior axis with a small “r” and deep S (or even QS) in leads V 5 and V 6 .
Electrophysiological Testing
Induction of Tachycardia
Given the reentrant mechanism of fascicular VT, the arrhythmia is usually inducible by programmed electrical stimulation. In fact, fascicular VT is characterized by its reproducible initiation and termination not just by ventricular stimulation but also by atrial pacing. Often, isoproterenol infusion facilitates VT induction. Consistent with a reentry mechanism, an inverse relationship is observed between the coupling interval of the initiating ventricular extrastimulation (VES) or ventricular pacing cycle length (PCL) and the first VT beat.
Diagnostic Maneuvers During Tachycardia
Entrainment
Entrainment of fascicular VT can usually be demonstrated with ventricular pacing at a PCL approximately 10 to 30 milliseconds shorter than the TCL. Manifest entrainment is more frequently achieved when pacing is performed from the RVOT because the RVOT is closer to the entrance site of the area of slow conduction in the reentrant circuit, located near the base of the LV septum. On the other hand, pacing from the RV apex is less likely to demonstrate entrainment and, when it does, it is unlikely to demonstrate manifest fusion because of the larger distance from the entrance site of the circuit and because of the narrow excitable gap of the reentrant circuit.
Resetting
Fascicular VT can be reset by VES, with an increasing or mixed resetting response (characteristic of reentrant circuit with an excitable gap).
Termination
VT can be reproducibly terminated with programmed atrial or ventricular stimulation.
Exclusion of Other Arrhythmia Mechanisms
The differential diagnosis of fascicular VT includes interfascicular VT, supraventricular tachycardia (SVT) with aberrant conduction, and VT originating from the LV papillary muscles.
Interfascicular Ventricular Tachycardia
Interfascicular VT has several characteristic features ( see Chapter 26 ): (1) bifascicular block QRS morphology during VT, which is identical to that during NSR; (2) reversal of activation sequence of the HB and LB during VT; and (3) spontaneous oscillations in the TCL caused by changes in the LB-LB interval that precede and drive the TCL. Interfascicular VT terminates with VES or radiofrequency (RF) ablation that produces block in LAF or LPF.
Supraventricular Tachycardia
When fascicular VT is associated with a 1 : 1 VA conduction and because of its responsiveness to verapamil and inducibility by atrial pacing, it can be misdiagnosed as SVT with bifascicular block aberrancy. During SVT (similar to NSR), the HB is activated in an anterograde direction with sequential activation of the HB and ventricles ( see Chapter 21 ). In contrast, during fascicular VT, the HB is activated retrogradely, with parallel activation of the HB and ventricle. Thus the HV interval during SVT with aberrancy is equal to or slightly longer than that during NSR. On the other hand, the HV interval during fascicular VT is frequently negative (i.e., His potential is recorded after onset of the QRS). The His potential can also precede the QRS onset during fascicular VT (especially during upper septal fascicular VT); however, the HV interval during VT will be shorter than that during NSR. In addition, unlike fascicular VT, aberrantly conducted SVTs are typically responsive to adenosine, beta-blockers, and Valsalva maneuvers.
Papillary Muscle Ventricular Tachycardia
Papillary muscle VTs typically manifest as premature ventricular complexes (PVCs) rather than sustained monomorphic VT and are more likely to occur in older patients with structural heart disease. VTs arising from the LV papillary muscles exhibit a QRS morphology that can mimic fascicular VT. However, compared with fascicular VT, papillary muscle VTs have a broader QRS complex (150 ± 15 milliseconds vs. 127 ± 11 milliseconds). In addition, VTs originating from the papillary muscles (which are farther away from the septum) lack the rsR′ pattern in lead V 1 ; rather, those VTs often have a qR pattern (or, less commonly, a monophasic R wave) in lead V 1 . Furthermore, spontaneous variations in QRS morphology occur relatively frequently during VTs originating from the LV papillary muscles, a feature that can help distinguish these VTs from LV fascicular VT; the latter being a reentrant tachycardia with a consistent QRS morphology. Unlike patients with fascicular VAs, papillary muscle VAs are typically not inducible with programmed electrical stimulation, which is consistent with a nonreentrant mechanism of papillary muscle VTs.
Focal Purkinje Ventricular Tachycardia
Idiopathic focal VTs can arise from the Purkinje system in either ventricle and can present as PVCs, accelerated idioventricular rhythm, or VT. Focal Purkinje VTs arising from the left Purkinje network exhibit an RBBB pattern with either left- or right-axis deviation and can be difficult to distinguish from fascicular VT. In contrast to the reentrant idiopathic fascicular VT, focal Purkinje VTs are most likely related to abnormal automaticity. These VTs are sensitive to autonomic tone and frequently display chronotropic properties. Focal Purkinje VTs are typically induced by exercise and catecholamines and slowed or terminated by beta-blockers (and hence classified as “propranolol-sensitive” VTs) and lidocaine. Unlike fascicular VT, focal Purkinje VTs are not responsive to verapamil and cannot be induced or terminated by programmed electrical stimulation. In addition, these VTs are transiently suppressed by adenosine and with overdrive pacing.
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