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Idiopathic fascicular reentrant ventricular tachycardia
Pathophysiology
The His-Purkinje system plays an important role in the genesis of cardiac arrhythmias. The mechanism of several types of monomorphic ventricular tachycardias (VTs) has been directly linked to the Purkinje system, including bundle branch reentrant VT, interfascicular reentrant VT, fascicular reentrant VT, and focal nonreentrant fascicular VT. A subset of polymorphic VTs and ventricular fibrillation have 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%–10%) in the region of the left anterior fascicle (LAF), and rarely arises 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 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 ventricular (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. Also, 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. 28.1 ). The estimated distance between the entrance and exit of the circuit is approximately 2 cm.
The LAF-VT circuit is similar to LPF-VT, the difference being in the fascicle involved. Of note, a close association of fascicular reentrant VT with intracavitary structures (such as a false tendon or a fibromuscular band) has been reported. In a subset of fascicular reentrant VTs, known as papillary muscle fascicular reentrant VTs , fibromuscular bands near the papillary muscles likely serve as part of the reentry circuit.
Anatomy of the left fascicular system
The bundle branches and fascicles consist of bundles of Purkinje cells insulated from the surrounding myocardium by a dense sheath of connective tissue. Insulation is lost distally as the Purkinje network connects the ends of the bundle branches to the ventricular myocardium.
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 ( Fig. 28.2 ). An estimated 65% of individuals have a third fascicle of the LB, the left median fascicle (LMF). Unlike the cord-like right bundle, the LB and its divisions are diffuse, fan-like structures that quickly arborize just beyond their origin with extensive interconnections. 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 mid portion 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. A combination of subendocardial and free-running Purkinje fibers (false tendons) form complex three-dimensional mesh-like networks on the endocardial surface of both ventricles and penetrate only the inner third of the myocardium.
False tendons are single or multiple, thin, fibrous, or fibromuscular structures that traverse the cavity of the LV and have no connection with the valvular cusps. False tendons may contain fibrous tissues, myocardial fibers, Purkinje fibers, and blood vessels.
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
Verapamil-sensitive fascicular reentrant VT is the most common form of idiopathic left-sided VT. Age at presentation is typically 15 to 40 years (unusual after 55 years). Males are more commonly affected (60%–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, can be 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 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 electrophysiological 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 electrophysiology 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 the management of patients with severe symptoms is often limited. Catheter ablation is highly effective (success rate of 85%–90%) and is recommended for patients in whom drug therapy is not successful, not tolerated, or not preferred.
Electrocardiographic features
ECG during normal sinus rhythm
The resting ECG is usually normal. Symmetrical inferolateral T wave inversion can be observed after the termination of the VT (cardiac memory).
ECG during ventricular tachycardia
Fascicular VT is characterized by a 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 (60–80 milliseconds). The shorter QRS duration reflects the proximal exit of fascicular VTs from the His-Purkinje system. Additionally, during fascicular VT, the interventricular septum is activated early with a left-to-right direction, resulting in initial “r” waves in lead V 1 and small “q” waves in leads I and aVL (similar to classic RBBB pattern). Also, the rapid activation of the LV via the Purkinje system results in unopposed late activation of the RV 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, 120–250 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 configuration and a superior axis (most common form); (2) LAF-VT (with the reentrant circuit exit in the anterolateral wall of the LV) exhibits an RBBB configuration with right axis deviation; and (3) upper septal fascicular VT with a narrow QRS configuration and normal axis or right axis deviation (rare form).
In the most common form (LPF-VT), the QRS during VT typically has RBBB with LAF block configuration ( Fig. 28.3 ). QRS duration and R/S ratio in leads I and V 6 can potentially predict LPF-VT exit. A proximal exit of LPF-VT is characterized by QRS duration ≤120 milliseconds, R/S ratio ≥1 in lead I, and R/S ratio ≥0.6 in lead V 6 . A distal exit is characterized by QRS duration ≥135 milliseconds, R/S ratio ≤0.5 in lead I, and R/S ratio ≤0.3 in lead V 6 . A rare variant of LPF-VT originates from Purkinje arborization around the posterior papillary muscle and is characterized by RBBB configuration and superior right axis deviation or horizontal axis.
LAF-VT is characterized by RBBB morphology and right axis deviation. A papillary-Purkinje subtype exhibits RBBB configuration and right axis deviation with a deep S wave in leads I, V 5 , and V 6 .
Upper septal fascicular VT is characterized by a QRS duration of less than 120 milliseconds and right axis deviation or normal axis (identical to sinus axis) with identical precordial QRS configuration to that during sinus rhythm or RBBB pattern.
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