In our retrospective analysis of a single-center registry, the interval from V(RVOT)-V(HB) and interval from QRS-V(HB) during VA were found to be useful in the localization of the origin of OT VAs. A V(RVOT)-V(HB) interval of ≤ 50 ms and QRS-V(HB) interval of ≤ 15 ms were the best cutoff values for the prediction of LVOT VA origin with high sensitivity and negative predictive value. Although there were substantial overlaps in these parameters between VAs of RVOT and LVOT origin, the median values were significantly different, and the current cutoff values were clinically practical to rule out an LVOT origin of VA.
Ventricular OTs have a complex three-dimensional anatomical structure, which complicates the recognition of VAs originating from these regions. Knowledge of the anatomy is crucial to understanding the electrophysiological features during the mapping of OT VAs. The His bundle is consistently located in the membranous portion of the interventricular septum, which lies between the juncture of the right and noncoronary cusps on the left side and the tricuspid annulus on the right side [18, 19]. Only the proximal portion of the RVOT merges with the tricuspid annulus and inflow portion of the RV close to the membranous septum, and the subpulmonary infundibulum of the RVOT overlies the anterior wall of the aortic sinuses. Consequently, the plane of the pulmonary valve and RVOT is located more superiorly than that of the aortic valve and LVOT, and the RVOT region is more distant from the His bundle compared with the LVOT in general. We measured the intervals of electrical activation time between references of the QRS complex or RVOT signal and the local V signal at the His bundle region; distinct anatomical distances from the RVOT or LVOT to His bundle region are reflected in a different range of parameters of OT VAs originating from each side.
The 12-lead surface ECG during VA provides comprehensive cues to regionalizing the origin of OT VA and assists in planning a focused EPS and ablation procedure. However, the precise differentiation of the VA focus is often not possible by examining the surface QRS morphology alone, primarily because VAs originating from separate locations of the OT can produce overlapping surface ECG morphologies. In addition, factors such as rotation of the cardiac axis , body habitus of the patients , and deviation of electrode placements  can alter the spatial relationship of the heart and surface electrodes and impair the consistency of surface ECG results, which limit the accuracy of algorithms predicting the origin of OT VAs based on these signals. Furthermore, as the surface ECG-based algorithms become more able to localize VAs with high accuracy and detail, multiple parameters are required, which may increase the chance of measurement error and increase intra- and inter-observer variability . A number of surface ECG-based algorithms have been introduced and validated to distinguish the origin of OT VA [9,10,11,12,13], but each method has limitations. No single solution for every circumstance has been elucidated.
Although the noninvasive pre-procedural prediction of OT VA origin with surface ECG is helpful for planning ablation, differentiation of VA origin usually requires detailed mapping of both OTs. In this regard, parameters obtained during the EPS would supplement the initial localization of OT VA and contribute to an effective ablation procedure. The limited diagnostic accuracy of ECG-based algorithms has been concerning, especially in the case of OT VAs with precordial transition at lead V3 [12, 20]. In our cohort, six patients presented with left bundle branch block and inferior axis QRS morphology with precordial transition at lead V3 (RVOT VA, 5; LVOT VA, 1). Of these patients, both a V(RVOT)-V(HB) interval of ≤ 50 ms and QRS-V(HB) interval of ≤ 15 ms correctly classified the origin of VA with 100% accuracy.
The His bundle region and RVOT are routinely assessed with catheters during EPS for OT VAs, and hence, V(RVOT)-V(HB) and QRS-V(HB) target intervals could readily be measured without the additional introduction of catheters. Current data showed that V(RVOT)-V(HB) interval and QRS-V(HB) interval could reliably discriminate VAs of LVOT origin from those of RVOT origin. In particular, our cutoff values effectively discriminated an LVOT origin with a high negative predictive value. Based on these parameters, operators could avoid the unnecessary risk of LVOT exploration and focus on detailed mapping in the RVOT region with confidence.
The results of the present study should be interpreted in the context of several limitations. First, we performed a retrospective analysis with pre-acquired electrogram in established OT VA cases to construct the proposed criteria. A relatively small number of OT VA cases are included, especially in the LVOT VA group, which included only six cases. A prospective analysis with adequate sample size should be performed to validate and generalize our results. Second, the V(RVOT)-V(HB) interval and the QRS-V(HB) interval showed high sensitivity, but only moderate specificity, in discrimination of an LVOT origin of VA. Considerable RVOT VA cases were over the proposed cutoff points regarding these parameters, and the practical utility of our criteria is currently limited to excluding an LVOT origin of VA with high negative predictive values. Although our criteria do not contribute to identification of an LVOT origin of VA, unnecessary invasive procedures requiring access to left-sided cardiac chambers could possibly be avoided, as these procedures are accompanied by the risk of stroke or inadvertent coronary artery damage. Focused mapping of the RVOT can also be accomplished with our criteria.