INTRODUCTION

Ventricular arrhythmias arising from the left ventricular (LV) summit are increasingly recognized and referred for catheter ablation. Symptoms include palpitations, but the indication for catheter ablation is strongest in the setting of drug-refractory PVC-induced cardiomyopathy.

The LV summit is defined as the epicardial cap of the basal anterior LV ostium, delineated by the left anterior descending artery as the septal boundary and the left circumflex artery as the lateral boundary.¹ The proximity to the coronary arteries provides both a method to access this region through the coronary venous system (accessible) but also anatomic limitations for mapping and ablation due to perivascular fat and concern for coronary artery injury.

While PVCs are more common in idiopathic substrates without structural heart disease, sustained VT is less commonly seen as the presenting arrhythmia. Unlike arrhythmias that arise from the right ventricular (RV) outflow tract due to excessive intracellular calcium (cAMP-mediated afterdepolarization), the mechanism and rationale behind the proclivity for a ventricular arrhythmia to originate from the LV ostium and LV summit is unknown. In this chapter, we describe the rationale and approach to the ablating ventricular arrhythmias refractory to traditional approaches using a novel, totally endoscopic robotic approach with standard electrophysiologic (EP) catheters for mapping and ablation.

RATIONALE

The two most common reasons for failure during endocardial ablation are sites of origin that are midmyocardial or epicardial. By anatomic definition, the LV summit is an epicardial structure, and arrhythmias that have activation from the left coronary cusp or aorto-ventricular junction earlier than the epicardium are not technically LV summit ventricular arrhythmias. Ablation, however, can be effective from an anatomic approach at sites are that are closest in proximity from the best endocardial vantage point.² Sites that are typically sampled that best approximate the corresponding endocardial adjacent regions of the LV summit include the left coronary cusp, the basal anterior LV anterior to the aorto-mitral commissure, and the most anterior and leftward portion of the RV outflow tract, including the pulmonary artery (Figure 11.1). High power, long duration application of radiofrequency may have higher attendant risks but allow for deeper penetration from the endocardium. Lower ionic irrigation provides a higher impedance to decreases dispersion of current and may be an effective strategy.³

The coronary venous system provides a convenient method accessible via the right atrium to sample the epicardial activation of a ventricular arrhythmia by recording local electrograms from within the great cardiac vein and anterior interventricular vein.^4–6 The course of the great cardiac vein as it turns into the anterior interventricular vein in the interventricular septum is variable, and the inaccessible region of the LV summit has been described as the space between the great cardiac vein–anterior interventricular vein junction and the left main bifurcation ( Figure 11.2).⁷ It is important to note that the earliest recorded activation timing does not necessarily denote the earliest site of activation in the heart. Sites with early activation within the coronary venous system may be even earlier on the adjacent subepicardium. For this reason, strategies to address the region of the inaccessible portion (via coronary venous approach) of the LV summit are necessary in select patients when other first-line approaches are unsuccessful.

The percutaneous epicardial approach pioneered by Sosa et al.8 has been demonstrated to be a paradigm shift to characterize the epicardial components of arrhythmogenic substrate in ischemic cardiomyopathy, nonischemic cardiomyopathy, and arrhythmogenic right ventricular cardiomyopathy. In cases of idiopathic arrhythmias, the yield is much lower as the sites of earliest activation are typically adjacent to the coronaries.^9,10 In general, the 5-mm safety margin from the coronary arteries that is most likely to prevent injury during radiofrequency application is violated in this region. The complexity of this region is compounded by the presence of large intervening ramus intermedius branches that bisect the LV summit triangle. More important, epicardial fat up to 1 cm in thickness has been reported, which impairs contact for accurate mapping and the ability to delivery effective radiofrequency energy (Figure 11.3). Cryoablation has been shown to be a potential alternate energy source, although epicardial fat also provides a similar barrier to create an effective lesion. For this reason, the percutaneous approach should be discouraged as a general principle, as the coronary venous system, which is typically underneath or embedded within the epicardial fat, provides a closer vantage point to the subepicardium.

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