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Mapping and Catheter Ablation for Ventricular Tachycardia Associated with Hypertrophic Cardiomyopathy

Kyoko Soejima, MD; Akiko Ueda, MD; Masaomi Chinushi, MD, PhD

Introduction

Hypertrophic cardiomyopathy (HCM) is an autosomal-dominant genetic disorder, characterized by a left ventricular (LV) hypertrophy. Mutations in genes encoding sarcomere protein cause extensive myocyte hypertrophy, myocardial fibrosis, and disarray. Asymmetrical hypertrophy in the basal septum and subaortic region is the most common type and causes LV outflow tract (LVOT) obstruction. Other forms include apical hypertrophy and mid-ventricular obstruction with or without apical aneurysms. Some patients progress to a dilated-phase, the so-called “burned-out type‘’ of HCM that resembles dilated cardiomyopathy.

Although most of the patients are asymptomatic and their clinical courses are benign, HCM is one of the major causes of sudden cardiac death in young people. The mechanisms of sudden cardiac death and syncope in HCM are severe heart failure, thromboembolic events, LVOT obstruction, myocardial ischemia, and ventricular arrhythmias. It had been believed that ventricular fibrillation (VF) and polymorphic ventricular tachycardia (VT) were more common,¹ and sustained monomorphic VT was less frequent. However, implantable cardioverter-defibrillator (ICD) recordings showed that monomorphic VT is more frequent than previously estimated.^2,3 It might have been underestimated, as VT degenerates into VF in early phase.⁴ Furushima et al. reported that 28% of hospitalized HCM patients had monomorphic VTs, and most of the patients had mid-ventricular obstruction.⁵

Imaging Study

The mechanism of sustained monomorphic VTs is thought to be reentry. Myocyte disarray and interstitial fibrosis associated with HCM can create dispersion of both depolarization and repolarization, promoting reentrant tachycardia. Distribution and extent of myocyte disarray and fibrosis depend on types of gene mutations in HCM patients. There has been a report that sudden deaths in patients with severe myocyte disarray could be caused by ischemia due to the abnormal blood pressure response rather than arrhythmia.⁶ However, others report that the myocyte disarray and disorganization of intercellular junctions can serve as arrhythmogenic substrate.⁷

Cardiac magnetic resonance imaging (MRI) can provide very important structural information,⁸ such as the presence of aneurysm, global or regional systolic and diastolic function, degree of outflow obstruction, and mitral regurgitation. Delayed contrast-enhancement MRI can evaluate the location and distribution of the scar, and is useful for planning the ablation method. The extent of late gadolinium enhancement has good pathological correlation with distribution of collagen tissue.⁹ These areas can be arrhythmogenic,^10–12 and the usefulness of MRI as a predictor of malignant cardiac event and VT inducibility has been reported.^12,13

Mapping and Ablation Strategy

Patients with mid-ventricular obstruction-type HCM usually have a low-voltage area with unexcitable scar in the apical aneurysm, and required detailed mapping in the aneurysm for the ablation of VT.14–16 Characteristic ECG morphology of VT arising from the apical aneurysm is usually RBBB with superior axis. Advancing the mapping catheter via the narrow neck of the aneurysm can be challenging and needs special attention. LV thrombus should be ruled out prior to the procedure. Preprocedural LV angiography, CT, or MRI is useful for the mapping. Recently, effectiveness of epicardial ablation in these types of VT has been reported.^17,18

Patients with apical HCM also have arrhythmogenic substrates in LVA. Inada et al. reported 3 apical HCM patients had sustained monomorphic reentrant VT originating from the apical wall.¹⁹ Two of these patients required endocardial and epicardial ablations, and the other required transcoronary ethanol ablation for the deep intramural reentrant circuit.

In a large series of HCM patients with relatively low ejection fraction, Santangeli et al. reported the most common location of reentry circuits were RV–LV junctions at the level of basal (42%) or apical (18%) segments.¹⁷ They emphasized the efficacy of epicardial ablation with an open-irrigated catheter because most scars were located at deep intramural myocardium or epicardium. Their procedures eliminated VTs successfully in more than two-thirds of patients. However, VTs were not curable in some cases with extremely thick wall or with scars originating from the interventricular septum. In such cases, transcoronary chemical ablation might be considered.

Usually, an irrigation catheter is used to make larger and deeper lesions. A D-curve catheter is used frequently for VT ablation, but if the ventricle is dilated, an F-curve catheter provides the longer reach to the wall.

Alcohol Ablation

Although the open-irrigated catheter makes deeper lesions, VTs can originate from the deep intramural area in a thick LV wall. In this situation, an alternative ablation method, such as chemical ablation or surgical ablation, might be required. The EHRA/HRS consensus on catheter ablation of ventricular arrhythmias states that ethanol ablation should be considered only in unstable cases refractory to endocardial and epicardial ablation.²⁰ The mechanism of chemical ablation is cytotoxic myocardial damage and ischemic injury by ethanol. The target coronary vessel supplying blood to the arrhythmogenic area must be identified, and it traditionally has been based on the empirical use of cold saline or contrast injection that results in termination of VT. However, a significant limitation of this method was the inability to record electrograms (EGMs) from the target coronary artery, which precludes the use of entrainment and pace mapping to confirm that the site is a critical portion of the VT circuit. Recently, a novel method of recording the EGM and pacing from the guidewire was reported.²¹ The intracoronary guidewire is advanced into the vessel, and its proximal portion is covered by the intracoronary balloon except the distal several millimeters, and the distal tip of the guidewire can be used to perform pace mapping or entrainment mapping (unipolar pacing). It is important to use the noncoated wire to record the EGM. Cold saline can then be injected into the target coronary artery to demonstrate VT termination. After confirmation of the target vessel, highly concentrated ethanol (1–2 mL) is injected during VT or sinus rhythm. It is crucial to occlude proximal portion of the target artery to prevent the reflux of ethanol. The potential risks of alcohol ablation are complete AV block (common in septal lesion), pericarditis, unintended infarction of myocardium due to ethanol reflux, and worsening of heart failure.

Cases

Case 1

A 49-year-old male with mid-ventricular obstruction-type HCM presented with frequent VT episodes. Obstruction and the apical aneurysm can be demonstrated clearly with cardiac MRI and LV angiogram (Figure 51.1). Substrate for the VT with extensive scar tissue can also be identified with delayed enhancement at the apex (Figure 51.1C). Typical 12-lead ECG for the apical origin VT can be recorded, and an ablation catheter was carefully advanced into the aneurysm (Figure 51.2, panels A and B). In the aneurysm, the site with presystolic potential (Figure 51.2C) was identified during VT, and entrainment with concealed fusion was demonstrated (Figure 51.2D). Radiofrequency (RF) application at this site terminated VT immediately (Figure 51.2E).

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