LAUSR

Idiopathic ventricular arrhythmias (VAs) arising from the outflow tract (OT) region have long been perceived to be “easy” targets for catheter ablation, to originate primarily from the right ventricular OT (RVOT), and to be successfully eliminated with a single procedure more than 90% of the time. However, OT VAs have been increasingly recognized to exit from structures immediately anatomically adjacent to the RVOT, specifically the left ventricular (LV) ostium, which includes the sinus of Valsalva, the LVOT immediately beneath the aortic valve, and the epicardium and so‐called LV summit. In fact, these LV ostial sites are targeted for ablation in as many as 37% of patients with idiopathic OT premature ventricular contractions (PVCs). The success of ablation, not to mention procedural complexity, may vary substantially based on the presumed site of origin (SOO), ranging from 67% among epicardial PVCs to 93% among RVOT PVCs. A number of clues derived from painstaking analyses of 12‐lead electrocardiogram (ECG) OT VA morphologies have been reported in an effort to better predict whether the SOO may be better accessible from the RVOT versus LVOT, and to facilitate preprocedural planning for ablation. While the baseline ECG provides an important starting ground, VAs originating from the LV ostium pose additional challenges during mapping and ablation, which may further confound interpretation and decisions about the ablation approach. A common mistake is to assume that the SOO is relatively superficial and is determined by the site of successful ablation, that is, the last location where ablation energy was applied and VA termination or elimination was affected. Conversely, it is also often erroneously assumed that failure to suppress the VA at one site means that it is not the true SOO and that ablation at that site is not only ineffective but, possibly, unnecessary. Both of these presumptions are too binary to be compatible with reality, especially when one considers the complexity of intertwining conductive tissue and three‐dimensionality of the OT region. OT VAs that have challenged these misconceptions the most, as well as have contributed to acute ablation failures, are those that are actually midmyocardial, originating from a source so deep within cardiac tissue that ablation applied from one or even multiple sites surrounding it may be ineffective. Along these lines, the rules that apply to successful RVOT ablations, in which the sites with the earliest activation during VA mapping correlate with excellent pacemap sites and successful ablation, do not apply well to LVOT sites. With the VAs arising from the LVOT, poor pacemaps are often obtained at the sites of earliest activation and successful ablation; this is one of several manifestations of preferential conduction that have been demonstrated within the septal LVOT and throughout the LV ostial region. Another important example is the appearance of multiple PVC/VT morphological changes with ablation, likely representative of intramural foci with altered exits. In this issue of Journal of Cardiovascular Electrophysiology, Dr Shirai et al at the University of Pennsylvania further quantified and characterized this concept of preferential conduction revealed by ablation within the LV ostium. Importantly, recognition of this phenomenon ultimately led to successful ablation in most patients, and with a single procedure. In their series of 446 patients with OT VAs undergoing ablation over a 5‐year period, 17 (4%) were identified to have an acute change in PVC/VT QRS morphology during ablation, compared with the morphology just before ablation. All of these patients had only a single VA morphology observed before ablation, and the indication for ablation was frequent PVCs (13%‐40% burden) in all of them; one also had sustained VT. In each of these cases, ablation at the initial site of the earliest activation within the OT, which included the septal RVOT (one case) and sites