Background

Idiopathic ventricular tachycardia in patients with an anatomically normal heart is a distinct entity whose management and prognosis differs from ventricular tachycardia associated with structural heart disease.

The tachycardia's QRS morphology on surface electrocardiogram (ECG) predicts the site of origin and is commonly classified as right ventricular tachycardia or left ventricular tachycardia. Patients generally tolerate the tachycardia and sudden cardiac death is rare in this patient population. Treatment options include pharmacotherapy or catheter ablation. The prognosis of these patients remains excellent.

Observed Ventricular tachycardias (VT) are usually related to structural heart disease. However in 10% of patients with VT, no structural heart disease, metabolic/electrolyte abnormalities or long QT syndrome can be found. These arrhythmias have been called idiopathic VT. They consist of various subtypes defined by their clinical presentation (repetitive monomorphic tachycardias, exercise-induced sustained ventricular arrhythmias) and/or their underlying mechanism (adenosine sensitive triggered arrhythmias, intrafascicular or interfascicular re-entrant arrhythmias).

These arrhythmias have certain anatomic locations within the heart and manifest specific electrocardiographic (ECG) patterns which help to identify their site of origin. A characteristic common to most cases of idiopathic ventricular tachycardia is good prognosis, although patients should continue to have periodic cardiac follow-ups to rule out latent progressive heart disease such as arrhythmogenic right ventricular dysplasia or other forms of cardiomyopathies.

This review summarises common forms of idiopathic ventricular tachycardias that the general cardiologist should know (Table 1).

I -  Outflow tract ventricular tachycardias

Outflow tract ventricualr tachycardias (OT VT) comprise a subgroup of idiopathic VT that are predominantly localised in and around the right and left ventricular outflow tracts. OT VT are the most common form of idiopathic VTs and originate, in more than 80-90% of cases, from the right ventricular outflow tract. They manifest at a relatively early age (30-50 years, range, 6 to 80 years) with equal distribution between sexes in left ventricular outflow tract VT (LVOT VT) whereas right ventricular outflow tract VT (RVOT VT) is more common in females.

The typical presentation of these arrhythmias consists of:

1. Nonsustained, repetitive, monomorphic VT.  This is the most common form (60-90%). It is characterised by frequent ventricular ectopy, right ventricular couplets and salvos of non sustained ventricular tachycardia (NSVT) with left bundle branch block morphology and inferior QRS axis. These extrasystoles occur more often during the day than at night, at rest or following a period of exercise and are transiently suppressed by sinus tachycardia. They may diminish or disappear with exercise during stress testing.
2. Paroxysmal, exercise-induced sustained VT. This VT may be initiated during exercise or recovery. Exercise stress testing is frequently uses to initiate and evaluate RVOT VT, but is not clinically helpful in most cases.

Clinical presentation,  most patients present with palpitations, less frequently with dizziness and a minority of patients present syncope.
Initial evaluation must include,

• Structurally normal hearts, ECG and echocardiogram are usually normal, but MRI may show abnormalities of the RV in up to 70% of patients, including focal thinning, diminished systolic wall thickening and abnormal wall motion.
• Origin in the RVOT/LVOT (common embryonic origin)
  RVOT VT should be distinguished from ARVD. VT in ARVD may have morphologic features similar to RVOT VT but does not terminate with adenosine. In ARVD, the resting 12 lead ECG typically shows inverted T waves in right precordial leads and when present, RV conduction delay with an epsilon wave, best seen in leads V1-V2. The differential diagnosis of RVOT VT also includes tachycardias associated with atriofascicular fibers (Mahain fibers) and VT occurring in patients after repair of tetralogy of Fallot.  

ECG recognition. RVOT VT is associated with a characteristic ECG morphology of LBBB with inferior axis (Figure 1a). Anterior sites in the RVOT shows a dominant Q-wave or a qR complex in lead I and a QS complex in aVL. Pacing at the posterior sites produce a dominant R-wave in lead I, QS  or R-wave in aVL and an early precordial transition (R/S = 1 by V3)1.

LVOT VT is suggested by LBBB morphology with inferior axis with small R-waves in V1 and early precordial transition (R/S = 1 by V2 or V3) or RBBB morphology with inferior axis2-3 and presence of S-wave in V64 (Figure 1b).
Aortic sinus cusp origin is sometimes difficult to differentiate from RVOT VT because both are so close to each other. Coronary cusp origin has to be though when we fail an ablation in the RVOT , ECG shows a LBBB inferior axis morphology with taller monophasic R-waves in inferior leads and an early precordial R-wave transition by V2-V3. Ouyang et al5 evaluated the ECG differences between RVOT/aortic sinus cusp VT origin. They found that a broader R-wave duration and a taller R/S wave amplitude in V1-V2 favored VT arising from the aortic cusp.

II - Management

The decision to treat patients with OT VT depends on frequency and severity of symptoms. Treatment options include medical therapy vs catheter ablation.

1. Acute termination of RVOT VT, can be achieved by vagal maneuver or adenosine (6 mg until 24 mg). Intravenous verapamil (10 mg given over 1 min) is an alternative if the patient has adequate blood pressure. These drugs may suppress triggered rhythms. Cases of hemodynamic instability warrant emergent cardioversion.
2. Chronic management. Long term treatment options include medical therapy and catheter ablation. Medical therapy may be indicated in patients with mild to moderate symptoms. They include beta-blockers, verapamil, diltiazem (rate of efficacy of 20 to 50%)6,7. Alternative therapy include class IA, IC and III agents. Radiofrecuency ablation has cure rates of 90%8 with a recurrence rate of 5% (mainly in the first year). It is the treatment of choice for patients with symptomatic, drug refractory VT, drug intolerance or do not desire long-term drug therapy and it should be strongly considered for the following patients with a potentially malignant form of  OT VT: a) a history of syncope; b) very fast VT; c) ventricular premature beats with a short coupling interval.

III - Idiopathic left ventricular tachycardia or fascicular VT

This form of idiopathic VT was first described by Zipes et al9  in 1979 with the following characteristics: induction with atrial pacing, RBBB morphology with left axis deviation and occurrence in patients without structural heart disease. In 1981, Belhassen et al10 showed that this form of VT could be terminated by verapamil, the fourth identifying featured.

This tachycardia typically occurs in patients between the ages of 15 to 40 years 11-12. Most of the affected patients are males (60 to 70%).
Clinical presentation. Symptoms include palpitations, fatigue, dyspnea, dizziness and presyncope. Syncope and sudden death are very rare. Most of the episodes occur at rest, although can be triggered by exercise and emotional stress. Tachycardia-induced cardiomyopathy due to incessant tachycardia has been described (11).
The most likely mechanism of idiopathic left ventricular tachycardia is reentry with an excitable gap and a zone of slow conduction since can be initiated and terminated with programmed stimulation as well as the demonstration of entrainment of the tachycardia with rapid pacing (13-14).

ECG recognition. The baseline 12-lead ECG is normal in most patients or it may show transient T-wave inversions related to T-wave memory shortly after a tachycardia episode terminates. The ECG during tachycardia is characterized by a right bundle branch block QRS configuration with a left superior axis, suggesting an exit site from the infero-posterior ventricular septum (Figure 2). The QRS duration in fascicular VT varies from 140 ms to 150 ms and the duration from the beginning of the QRS onset to the nadir of the S-wave in the precordial leads is 60 to 80 ms. This makes it difficult to differentiate this tachycardia from supraventricular tachycardia with aberrancy using the criteria based on QRS morphology and RS interval15. However a careful analysis of the surface ECG can demonstrate VA dissociation and rapid atrial pacing during tachycardia can demonstrate AV dissociation and favors the diagnosis of fascicular VT.

IV - Management

The long-term prognosis of patients with fascicular VT without structural heart disease is very good. Arrhythmias in patients with sporadic, well-tolerated episodes of idiopathic left ventricular tachycardia may not progress despite absence of pharmacologic therapy16. Patients with moderate symptoms can be treated with oral verapamil (120 to 480 mg/day).

Radiofrequency catheter ablation is an appropriate management strategy for patients with severe symptoms or those intolerant or resistant to antiarrythmic therapy. It could been performed successfully by targeting the earliest high-frequency Purkinje potencial during VT17,18. Long-term success after catheter ablation is more than 92% with rare complications that include mitral regurgitation due to catheter entrapment in the chordae of the mitral valve leaflet and aortic regurgitation due to damage to the aortic valve using a retrograde aortic approach19.

Figure 1a/1b. RVOT non-sustained monomorphic ventricular tachycardia (on the left side) and LVOT ventricular bigeminy (on the right side).


 
Figure 2.  Fascicular VT with RBBB morphology and left anterior fascicular block pattern.