|31||Epicardial Ventricular Tachycardia Mapping and Ablation in Arrhythmogenic Right Ventricular Cardiomyopathy|
|Cory M. Tschabrunn, PhD; Francis E. Marchlinski, MD|
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a complex and frequently underrecognized disease that has challenged clinicians and scientists since it was first described in 1728.1 Several hypotheses have been proposed for the pathogenesis of ARVC. A primary triggering event acting alone or in combination with a genetically determined desmosomal cardiomyopathy appears to be most consistent with the clinical observations related to the diagnosis. It is primarily believed to be an inherited cardiomyopathy resulting from gene mutations that encode desmosomal proteins that are responsible for normal cell-cell adhesion. Alternatively, genetically determined desmosomal dysfunction may lead to inadequate cell adhesion and subsequent myocyte detachment and apoptosis.2,3 The resulting accumulation of fibrous and adipose tissue predominantly affects the right ventricular (RV) free wall and typically extends inward from the epicardium toward the endocardial surface.4,5 Although this underlying process of right ventricular scarring is unique to ARVC, the arrhythmogenic consequences that result from the development of fibrosis are similar to other nonischemic cardiomyopathies. The extensive RV fibrosis results in inhomogeneous conduction with slow and discontinuous electrical propagation in sinus rhythm that serves as the substrate for ventricular arrhythmias.
Much of our understanding of the electrophysiologic and electroanatomic substrate underlying ventricular tachycardia (VT) in patients with ARVC was derived from studies performed in the electrophysiology laboratory. Through this experience, much has been learned about the arrhythmia mechanisms and strategies required to facilitate successful catheter ablation. The ability to localize and define the associated abnormalities essential for VT enhanced the effectiveness of catheter ablation procedures. What was once considered a treatment of last resort has now become the preferred therapy for most patients with documented ventricular arrhythmias.6 In addition, assessment of the anatomic substrate during electrophysiology procedures has shed important light on controversies pertaining to disease pathogenesis.
This chapter will summarize these seminal insights derived from the electrophysiology laboratory and the lessons learned. We will also describe a methodical approach that can be utilized to successfully perform VT ablation in patients with ARVC, which frequently requires a combined endocardial and epicardial mapping and ablation approach.
Patients are typically diagnosed with ARVC after manifesting signs or symptoms during the second to fifth decade of life. The diagnosis is made if Task Force Criteria are met.7 Although implantable cardioverter-defibrillator (ICD) therapy is routine, management of recurrent VT with frequent device therapy can be difficult. Antiarrhythmic medications are often poorly tolerated and may only provide incomplete VT control. Antitachycardia pacing may be ineffective or accelerate VT, leading to ICD shocks. Inadequate arrhythmia control and the use of multiple antiarrhythmic medications is particularly debilitating for these young and physically very active patients.
Although techniques used in catheter ablation of VT in patients with ARVC have evolved over the last decade, outcomes are still inconsistent, ranging from 50–90%.8,9 This is likely the result of a number of different mapping and ablation strategies with variable endpoints, follow-up assessment, and operator experience.10–15 In our experience at the University of Pennsylvania, a comprehensive ablation strategy that targets the endocardial and, if VT remains inducible, the epicardial substrate with elimination of abnormal electrograms and all inducible VT provides long-term drug-free arrhythmia control in a large majority of patients. For this reason, we recommend catheter ablation to all patients with recurrent VT refractory or intolerant to medical therapy and offer catheter ablation as an appropriate consideration in patients who prefer not to take chronic antiarrhythmic drug therapy to prevent VT recurrence after a detailed discussion of risks and benefits.
PROCEDURAL APPROACH: ENDOCARDIAL MAPPING AND ABLATION
Endocardial Substrate Characterization
Advances in 3D electroanatomic mapping enabled a more thorough understanding of the complex electrophysiologic substrate in patients with ARVC and VT. Abnormal RV endocardial regions can be localized with electroanatomic mapping by identifying regions demonstrating low bipolar RV endocardial voltage (< 1.5 mV) and long-duration, low-amplitude, fractionated potentials. These regions of bipolar signal abnormality have been correlated to relevant histopathologic findings (myocyte loss with fibrofatty replacement) and critical VT circuits confirming the involvement of these areas in the arrhythmogenic mechanism.16 The endocardial distribution of electroanatomic scar in patients with VT and ARVC typically extends from the tricuspid valve and/or the pulmonary valve and converges over the RV free wall. Low-voltage abnormalities can also extend to the septal aspect of the perivalvular region(s), but free wall abnormalities predominate (Figure 31.1). Only rarely does VT originate and scar involve the RV apex (Figure 31.2).17
Figure 31.1 Right ventricle endocardial bipolar voltage map (0.5–1.5 mV) and electrogram (EGM) abnormalities in a patient with arrhythmogenic right ventricular cardiomyopathy (ARVC) and ventricular tachycardia (VT). Low-voltage and abnormal bipolar electrograms involving both peritricuspid valve (TV) and peripulmonic valve (PV) regions extending to the inferior free wall and RV outflow tract region, respectively, are shown. Consistent with a majority of ARVC-VT cases, the RV apex is spared.
Figure 31.2 Example of rare case of right ventricular endocardial apical scar and ventricular tachycardia in patient with ARVC. Low-voltage and abnormal bipolar electrograms (0.5–1.5 mV) involving peritricuspid and peripulmonic valve regions extending to the free wall, RV outflow tract region, and with RV apical involvement. In this case, two induced VTs were localized and successfully ablated from the RV apex. RV apical involvement with significant substrate abnormality and as the source of VT has been extremely uncommon in our clinical experience.
Evaluation in the electrophysiology laboratory typically begins with patients under conscious sedation to maximize the likelihood that induced VTs will be hemodynamically tolerated. A detailed RV endocardial voltage map is created in sinus rhythm using the standard 0.5–1.5 mV voltage cutoffs to define the endocardial substrate as previously discussed.15,18 Special attention is focused on the periannular area and any identified low-voltage areas to ensure adequate sampling has occurred.19–21 Occasionally, it can be technically challenging to perform catheter manipulation in the periannular tricuspid valve region. It is imperative to ensure adequate catheter contact during mapping to confirm low-voltage areas are due to abnormal substrate and not inadequate catheter–tissue contact. This process can be facilitated by (1) using a sheath that extends transvenously to the tricuspid valve and provides stability; and (2) looping the mapping catheter in the RV to facilitate acquisition of detailed recording along the free wall adjacent to the annulus. Colored tags are placed on the electroanatomic map when fractionated signals and/or isolated late potentials are identified to keep track of their location.22,23 Pace mapping is performed at sites of interest with late potentials and other multicomponent electrograms and are carefully analyzed. A match of the pace-map QRS morphology of the VT coupled with a long stimulus to QRS interval will identify additional sites of interest, which are given their own unique color tag. A sudden transition in paced QRS morphology coupled with changes in the stimulus to QRS interval may define anatomic boundaries of the isthmus, or if a long stimulus to QRS mimicking VT is identified that transitions to a paced QRS with a very poor match with VT, it will identify a critical isthmus of conduction that will need to be tagged and ultimately targeted for ablation.24
Although ARVC is known to primarily involve the RV, involvement of the left ventricle (LV) is more frequent than previously recognized. LV abnormalities have been documented with electroanatomic mapping and typically involve the basal perivalvular region, which is characteristic of other noninfarct related cardiomyopathies (Figure 31.3). Consideration of endocardial LV involvement is of particular importance if right bundle branch block VTs with positive R waves in all the precordial leads are seen as this suggests an LV basal lateral VT exit site.
Figure 31.3 Example of endocardial right ventricular and left ventricular bipolar voltage map in patient with ARVC. Electroanatomic abnormalities include the pulmonic (PV), tricuspid (TV), and mitral valves (MV). Region of LV bipolar low voltage demonstrated fractioned electrograms without any inducible LV VTs. Panel A: Bipolar RV and LV endocardial voltage maps (0.5–1.5 mV) in the left lateral (LL) projection demonstrating RV low-voltage area involving the pulmonic valve (PV). Panel B: Bipolar RV and LV endocardial voltage maps (0.5–1.5 mV) in the right lateral (RL) projection demonstrating RV and LV low-voltage areas surrounding the TV and MV. Fractioned EGM from MV region consistent with underlying tissue abnormalities, but no LV VTs induced during aggressive RV and LV stimulation is shown.
Endocardial VT Localization and Ablation Strategy
After completing the endocardial RV sinus rhythm substrate map and detailed pace mapping, programmed ventricular stimulation is performed. ICD electrograms are also recorded when VT is initiated. Induced VT ECG morphology and ICD electrograms are compared to previously captured clinical arrhythmias in addition to the pace map morphologies that were obtained during sinus rhythm mapping. Assessment of ICD electrograms may be especially useful to help identify the morphology of the most commonly occurring clinical VT. In selected patients in whom VT is not inducible at the time of the ablation procedure, the ICD electrogram may serve as a valuable reference and localizing tool during pace mapping at sites demonstrating late potentials and other electrogram abnormalities.25
For hemodynamically tolerated VT, the endocardial ablation strategy is guided primarily by activation and entrainment mapping. It is not uncommon for unstable VTs to also be induced. In these cases, ablation is guided by pace mapping and detailed substrate assessment and ablation targeting late, split, and/or fractionated electrograms. Ablation lesions are usually applied with an irrigated tip catheter for a minimum of 90 seconds. Power delivery begins at 20–25 W depending on the degree of contact and is typically titrated to a maximum of 40 W to obtain a 12- to 18-ohm impedance drop or an approximately 10–12% decrease from the baseline impedance.
Any VT that can be mapped and successfully ablated from the endocardium is targeted initially. When appropriate, the next step in the mapping and ablation procedure is to consider the necessity of intrapericardial access and epicardial mapping.26 We routinely perform detailed endocardial mapping and ablation before proceeding with epicardial access. Endocardial ablation achieving an endpoint of noninducibility is associated with ~50% long-term VT control (unpublished observations). More recently, Berreuzo and colleagues have suggested that patients with advanced forms of the disease may have only extensive epicardial dense scar and no active arrhythmogenic substrate.27 They suggest that most VT in this setting originates and can be eliminated from the endocardium. Endocardial mapping and ablation prior to epicardial access also has the advantage of eliminating all endocardial VTs and characterizing the remaining VT morphologies along with the corresponding endocardial bipolar and unipolar electrograms. Furthermore, if a complication occurs and surgery is required related to bleeding/laceration occurring with epicardial access or dense adhesions precluding percutaneous access, epicardial cryoablation guided by VT ECG and the distribution of endocardial unipolar electrograms can be performed with anticipated elimination of VT (Figure 31.4).
PROCEDURAL APPROACH: EPICARDIAL MAPPING AND ABLATION
Despite periprocedural advances with irrigated ablation catheter technology and improved techniques to identify RV endocardial bipolar electroanatomic voltage abnormalities and ablation targets, epicardial ablation is frequently required to eliminate VT. The epicardial-to-endocardial scarring process associated with ARVC often results in a more extensive abnormal epicardial substrate that may not be amendable to endocardial ablation alone. This is consistent with the pathologic description of predominantly epicardial abnormalities as part of the disease process.4 In patients with persistent inducible VT after endocardial ablation, a more extensive substrate abnormality will be found on the epicardial RV free wall. In patients that have failed endocardial ablation, ablation targeting the epicardial circuits and substrate was associated with superior long-term success rates.12