VT in structurally normal heart

Ventricular tachycardia (VT) in structurally normal heart

VT in structurally normal heart constitutes about 10% of patients with ventricular tachycardia. Echocardiogram and coronary angiograms are normal in these cases, but MRI may show subtle abnormalities. Localized sympathetic denervation may be seen in some of them. Baseline ECG is normal in many situations.

Following are the main types of ventricular tachycardia (VT) with structurally normal heart:

1. Right ventricular outflow tract (RVOT) VT
2. Left ventricular outflow tract (LVOT) VT
3. Idiopathic left ventricular tachycardia (ILVT)
4. Catecholaminergic polymorphic VT (CPVT)
5. Ventricular tachycardia in Brugada syndrome
6. Ventricular tachycardia in Long QT syndrome
7. Ventricular tachycardia in Short QT syndrome

The first three are monomorphic VT while the latter three are polymorphic in nature.

Right ventricular outflow tract (RVOT) VT

Right ventricular outflow tract (RVOT) VT is a wide QRS tachycardia with LBBB pattern and inferior axis. It occurs in third to fifth decade and constitutes about 90% of outflow VTs. There are two types – Non-sustained, repetitive variety and Paroxysmal, exercise-induced, sustained variety. Both are terminated by adenosine, in contrast from VT in arrhythmogenic right ventricular dysplasia (ARVD).

Exercise stress testing to is used to initiate and evaluate RVOT VT. Initiation depends on a critical heart rate which differs in each patient. MRI may show abnormalities of right ventricle in up to 70%, which include focal thinning, diminished systolic wall thickening, and abnormal wall motion.

Differential Diagnosis of RVOT VT

Differential diagnosis of RVOT VT include ARVD, Mahaim fiber tachycardia, AVRT using a right-sided accessory pathway and VT in patients after repair of tetralogy of Fallot.

Mechanism of RVOT VT

Intracellular calcium overload is thought to be the mechanism which enhances function of Na+/Ca++ exchanger, thereby increasing inward Na+ current and delayed afterdepolarization which initiate tachycardia.

Role of adenosine, beta blockers and verapamil in RVOT VT

cAMP regulates intracellular calcium. Increased levels of cAMP will increase intracellular calcium levels.

Adenosine acts by lowering cAMP concentration. Beta-blockers act by inhibiting adenylate cyclase which mediates the synthesis of cAMP. Verapamil has its action by inhibiting L-type Ca++ channels.

Treatment of RVOT VT

Beta-blockers, Verapamil or diltiazem can control RVOT VT with about 25% to 50% efficacy. Class IA, IC, III including amiodarone have been tried in the treatment of RVOT VT. Radiofrequency catheter ablation has cure rates of 90% and is the preferable option, given the young age of patients with RVOT VT.

Left ventricular outflow tract (LVOT) VT

Left ventricular outflow tract (LVOT) VT is characterized by S waves in lead I, and R-wave transition in V1/ V2 and constitutes about 10% of outflow VT. There are two varieties of LVOT VT -supravalvular and infravalvular. Absence of S wave in V5/V6 is suggestive of supravalvular origin, while the presence of S wave in V5/V6 indicates infravalvular origin.

Ablation of LVOT VT

There is a risk of left main coronary artery (LMCA) occlusion while ablating LVOT VT. Hence coronary angiography before, during and after ablation is recommended. The ablation catheter tip has to be kept 1 cm away from the ostia of the coronary arteries.

Idiopathic left ventricular tachycardia

Idiopathic left ventricular tachycardias (ILVT) are verapamil-sensitive intrafascicular tachycardias. Three types are described: RBBB, left-axis pattern – originating from left posterior fascicle (90 – 95%); RBBB, right-axis pattern – originating from left anterior fascicle and left septal fascicular tachycardia with normal axis. It is seen in 2nd – 4th decade and more in men (60%-80%).

Treatment of ILVT

ILVT can be terminated with intravenous verapamil. Long-term therapy with verapamil is also feasible.

Radiofrequency catheter ablation is highly effective (85%-90%) in those with severe symptoms.

Identifying the focus of ablation can be achieved by the recognition of Purkinje potential, late diastolic potential, or earliest ventricular activation.

Purkinje potential

Purkinje potentials are high frequency, short duration, potentials preceding the QRS complex. They are also called P potential and diastolic potential. Purkinje potentials can be recorded both in sinus rhythm and during VT. Pacing at sites of earliest P potential produces QRS identical to that of the clinical tachycardia. They occur 30 to 40 ms before the VT QRS complex.

Primary ablation of ILVT

Primary ablation of ILVT has been suggested by some authors because fascicular VT is sometimes difficult to induce despite pharmacological provocation. Primary ablation has a higher success rates, lesser procedure time, lower fluoroscopy time and requires lesser number of RF energy deliveries [1].

Catecholaminergic Polymorphic VT

Catecholaminergic Polymorphic VT (CPVT) is a bidirectional polymorphic VT, which is induced by exercise or catecholamine infusion. Family history of premature SCD / stress-related syncope is obtained in one third of the patients. Exercise or acute emotion triggers the syncope in CPVT. Symptoms typically manifest in childhood. CPVT has a genetic basis with 5 genetic types described so far. Ryanodine receptor 2 (RyR2) mutation is transmitted as an autosomal dominant trait, while calsequestrin (CASQ2) mutation is transmitted as autosomal recessive variety [2]. These two are designated as CPVT 1 and CPVT2. Three other genetic types – CPVT3, CPVT4 and CPVT5 have also been documented.

Treatment of catecholaminergic polymorphic VT

Beta-blockers are the preferred therapy in CPVT. ICD may be required in 30% of patients, but a word of caution is needed since there is a risk of electrical storm with ICD discharge which can cause an emotional stress and a vicious cycle of CPVT and shocks.

Ventricular tachycardia in Brugada syndrome

Brugada syndrome is characterised by an apparent RBBB pattern with ST elevation in V1 to V3, associated with life-threatening cardiac arrhythmias, typically polymorphic VT. There is a tendency for familial occurrence and it is associated with SCN5A mutation and several other mutations. Loss of the action potential dome in the epicardium but not endocardium causes the right precordial ST elevation in Brugada syndrome. Implantation of a cardioverter defibrillator is the only effective treatment, though quinidine has been suggested in addition [3].

Ventricular tachycardia in Long QT syndrome

There are at least 16 genotypes of long QT syndrome (LQT 1 to LQT 16) as per OMIM database. Phenotypically LQT1 has broad-based T waves with indistinct onset while LQT2 has bifid T waves and LQT3 a long isoelectric ST segment

Treatment of LQTS

First and foremost in the treatment of long QT syndrome is the exclusion of acquired LQTS, which is much more common. Avoidance of QT prolonging drugs is essential. Beta-blockers are the most useful therapy in LQTS. ICD placement, along with beta-blocker therapy is the best option for secondary prevention in a case of long QT syndrome with history of arrhythmic syncope. Sodium channel blocking drugs like ranolazine, mexiletine and flecainide have been used in the treatment of LQT3 which is due sodium channel SCN5A mutation [4].

References

1. Gupta AK, Kumar AV, Lokhandwala YY, Vora AM, Maheshwari A, Thakur RK. Primary radiofrequency ablation for incessant idiopathic ventricular tachycardia. Pacing Clin Electrophysiol. 2002 Nov;25(11):1555-60.
2. Francis J, Sankar V, Nair VK, Priori SG. Catecholaminergic polymorphic ventricular tachycardia. Heart Rhythm. 2005;2:550-4.
3. Mazzanti A et al. Efficacy and Limitations of Quinidine in Patients With Brugada Syndrome. Circulation: Arrhythmia and Electrophysiology. 2019; 12(5). https://doi.org/10.1161/CIRCEP.118.007143.
4. Blich M, Khoury A, Suleiman M, Lorber A, Gepstein L, Boulous M. Specific Therapy Based on the Genotype in a Malignant Form of Long QT3, Carrying the V411M Mutation. Int Heart J. 2019 Jul 27;60(4):979-982.