INTRODUCTION

Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiomyopathy with autosomal dominant inheritance and a variable clinical penetrance.¹ Sarcomere gene mutation results in extensive myocyte hypertrophy, myocyte disarray, and fibrosis. As a result, asymmetric basal septal hypertrophy ensues near the confluence of the anterior septum and anterior left ventricular (LV) wall. Other variants include apical HCM and HCM with apical aneurysms. Some patients progress to a late-phase, ‘burnt out’ type of HCM that resembles dilated cardiomyopathy.

Hypertrophic cardiomyopathy is the leading cause of sudden cardiac death (SCD) in the United States, accounting for annual mortality rate of 6%, predominantly from polymorphic ventricular tachycardia (VT) or ventricular fibrillation (VF).² Sustained monomorphic VT is less common, typically occurring in HCM patients with mid-ventricular obstruction or LV apical aneurysms. The recurrence of VT is high in HCM patients (approximately 56%) with low reproducibility of sustained monomorphic VT induction (polymorphic VT/VF are often induced) on an electrophysiology study.

Hypertrophic cardiomyopathy with apical aneurysms represents an uncommon and often underrecognized subgroup (2% of HCM patients) with unfavorable prognosis (10.5% adverse events per year as compared to general HCM population).³ Aneurysmal development is thought to occur due to increased wall stress induced by dynamic midventricular obstruction, increased LV cavitary systolic pressure, and intramural coronary artery compression resulting in microvascular ischemia and subsequently replacement fibrosis.⁴

The management of ventricular arrhythmias in HCM patients includes antiarrhythmic therapy (which has limited efficacy), implanted cardioverter defibrillator (ICD) therapy, and more recently, catheter-based radiofrequency ablation. VT circuits in HCM patients are classically epicardial along with an endocardial substrate (just as in any other nonischemic substrate). In addition, because of increased LV thickness, and endocardial-only approach for catheter ablation for VT may have limited efficacy. Therefore, a successful VT ablation approach in these patients often involves targeting broad and deep regions both epicardially and endocardially.

MECHANISMS OF VENTRICULAR ARRHYTHMIAS

The fundamental difference in the underlying arrhythmogenic substrate between ischemic and nonischemic cardiomyopathy patients eventually decides the procedural challenges and success rates of catheter ablation. Although the arrhythmogenic substrate in HCM patients has not been completely defined, the hypertrophied and distorted cardiomyocytes result in a chaotic, disorganized myocyte pattern (instead of parallel cellular arrangement), forming spirals around the connective tissue foci. This disorganized pattern is not specific to HCM and also occurs in hypertensive heart disease, congenital heart disease, aortic stenosis, Noonan disease, and Friedreich ataxia. Nonetheless, myocyte disarray is more pronounced in HCM than other disease states.

The slow/distorted conduction and increased electrical dispersion from myocyte disarray, and diffuse interstitial myocardial fibrosis/scarring have been proposed as factors contributing to ventricular arrhythmogenesis.⁵ However, in one report, SCD in patients with severe myocyte disarray was probably due to ischemia from abnormal blood pressure response than arrhythmia itself.6 Autopsy examination of HCM patients with SCD has demonstrated macroscopic fibrotic myocardium, with remarkable association of myocardial fibrosis and VT occurrence on cardiac magnetic resonance imaging (MRI). Studies have shown that the extent and distribution of fibrosis on cardiac MRI is an important predictor of arrhythmic events in HCM patients.^7–9 In fact, the degree of late gadolinium enhancement on cardiac MRI correlates with collagen tissue distribution histologically.¹⁰

The pattern of myocardial scar/fibrosis detected in HCM patients with ventricular arrhythmia is strikingly different from that observed in ischemic cardiomyopathy (which typically follows the coronary artery distribution, progressing from subendocardial to transmural), and other dilated cardiomyopathy patients (characterized by patchy distribution, with midmyocardial and epicardial involvement). In a small subset of HCM patients with burnt-out ventricles characterized by LV ejection fraction < 50%, LV wall thinning, and LV dilatation, transmural scar can be seen—thereby promoting reentrant VT.^11,12

PREABLATION EVALUATION

Patients with HCM (as in other nonischemic cardiomyopathies) typically have intramural and epicardial scar—portending a poor success rate with an endocardialonly approach for VT ablation. The decision for an epicardial approach to VT ablation in HCM requires several factors to be taken into consideration: (1) prior unsuccessful endocardial ablation; (2) electrocardiographic characteristics; (3) scar localization by cardiac MRI or cardiac CT; and (4) prior cardiac surgery. Preprocedural scar registration (via cardiac MRI or cardiac CT) permits real-time incorporation of the putative arrhythmogenic substrate into the electroanatomic maps.

Poor or paradoxical blood flow in a dyskinetic or akinetic aneurysmal apex can serve as a nidus for endocardial thrombus formation. Therefore, all patients in our center undergo transthoracic echocardiogram with ultrasonic contrast prior to the procedure to assess for LV thrombus; cardiac MRI/CT can also help identify apical thrombus. Special attention must also be paid to hemodynamic parameters—right ventricular (RV) and LV function, as well as fluid status optimization in order to minimize the incidence of intraprocedural complications related to pump failure. On a case-by-case basis, one should consider whether to admit the patient a day prior to ablation for heart failure optimization.

ECG Characteristics of VT in HCM Patients with Apical Aneurysm

During VT, the QRS complex typically has a northwest axis, with QS complexes in V4–V6 in VT suggesting an apical exit. Determining septal versus lateral exit largely depends on QRS morphology in lead V1 during VT. Right bundle branch morphology in lead V1 suggests lateral exit (Figure 33.1), while left bundle branch morphology in lead V1 suggests septal exit.

Figure 33.1 12-lead electrocardiograms of ventricular tachycardia. Right bundle branch morphology, QS complex V3–V6, right-superior axis suggestive of apical lateral exit.

ABLATION PROCEDURE

Access

During epicardial ventricular tachycardia ablation, procedural planning plays a very important role: (1) prior history of cardiac surgery or pericarditis; (2) normal baseline coagulation parameters; (3) withholding systemic anticoagulation and antiplatelet agents (5–7 days prior); (4) blood type and screen; (5) baseline transthoracic echocardiogram; and (6) surgical backup, if needed.

Following initial transvenous accesses, intracardiac echocardiogram (ICE) is performed to assess for preexisting pericardial effusion so as to establish a baseline and avoid any confusion or unwarranted intervention during or after the procedure. No systemic anticoagulation is initiated until pericardial access is obtained. The puncture is performed under general anesthesia; beyond the pain that is otherwise caused by pericardial manipulation, anesthesia also allows transient apnea during puncture to minimize diaphragmatic excursion (and in turn cardiac motion), thereby decreasing the risk of inadvertent RV puncture during the epicardial access. Several epicardial access techniques have been developed over years: (1) the initially-described approach using a 17-gauge blunt epicardial needle (Tuohy or Pajunk needle); (2) a micropuncture needle epicardial access approach; (3) a needle-in-needle technique; and (4) the EpiAccess fiberoptic needle system (EpiEP Inc., New Haven, CT). Discussing the details of these different procedural techniques and their pitfalls is beyond the scope of this chapter.

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