Road to the Summit May Follow an Eccentric Path.

The left ventricular (LV) ostium or LV summit is the most superior aspect of the LV outflow tract (LVOT). Ventricular arrhythmias (VAs) arising from this region represent one of the most common sites of origin of idiopathic VAs.1 The LV summit is anatomically defined as the epicardial LV surface bounded by an arc from the left anterior descending coronary artery superior to the first septal perforating branch anterior to the left circumflex coronary artery laterally.2 The LV summit is transected laterally by the great cardiac vein (GCV) at its junction with the anterior interventricular vein, dividing the LV summit into what has traditionally been called the basal (inaccessible) and apical (accessible) segments.2–4 Not surprisingly, arrhythmias originating from the basal (inaccessible) LV summit have a significantly lower rate of ablation success (≈50%) compared with those originating from the apical (accessible) LV summit (≈100%).4 Catheter ablation in this region is challenging because of the complex and variable anatomy in close proximity to critical structures as well as intramural foci that are often encountered.5–7 To aid in determining the site of origin of VAs in the LV summit, several ECG and electrophysiological characteristics have been described, and various ablation strategies have been proposed.3,4,8,9 ECG findings are consistent with the more septal and superior location of the basal LV summit in relationship to the apical LV summit. Compared with the apical segment, arrhythmias originating from the basal segment typically have a left bundle branch block pattern, shorter QRS duration (≤175 ms), greater R-wave amplitude in the inferior leads, smaller R-wave ratio in III/II and Q wave ratio in aVL/aVR, and a later precordial transition.4 A direct ablation approach through the cardiac venous system is usually recommended when an early ventricular activation is recorded within the distal GCV.4,10 This approach can be problematic secondary to difficulty in passing the ablation catheter to the site of interest, inability to achieve adequate power, and proximity to coronary arteries. Alternatively, an anatomic approach from the adjacent endocardial site closest to the LV summit arrhythmia origin can be used, such as from the aortomitral continuity, LVOT, or coronary cusps.4 Predictors of successful ablation using an anatomic approach include a Q-wave ratio of <1.45 in leads aVL/aVR and a close anatomic distance <13.5 mm from the earliest activation site in the GCV.8 Importantly, anatomic proximity and not activation timing dictates the success of an anatomic approach.11 Finally, a percutaneous epicardial approach can be considered when a direct or anatomic ablation approach is unsuccessful. Unfortunately, this approach is only successful in a minority of patients (14%–17%), limited by proximity to major coronary arteries, the left atrial appendage, and poor energy delivery due to the presence of thick epicardial fat.3,4,9 EDITORIAL