Novel Transitional Zone Index Allows More Accurate Differentiation between Idiopathic Right Ventricular Outflow Tract and Aortic Sinus Cusp Ventricular Arrhythmias

Background: Although several ECG algorithms have been proposed for differentiating the origins of outflow tract ventricular arrhythmia (OT-VA), their accuracy is still limited in cases with cardiac rotation.Objective: The purpose of this study was to assess whether a novel “cardiac rotation-corrected” transitional zone (TZ) index would be a useful marker for differentiating right ventricular outflow tract (RVOT) origin from aortic sinus cusp (ASC) origin.Methods: Surface ECGs of OT-VAs with left bundle branch block morphology and inferior axis in 112 patients who were successfully ablated in the RVOT (n=87) or the ASC (n=25) were analyzed. According to the site of R-wave transition, TZ score was graded with 0.5-point increments (e.g. V2=2-point, V3–V4=3.5-point, V5=5-point) and TZ index was defined as follows: TZ score of OT-VA minus TZ score of sinus beat.Results: The TZ index was significantly lower in the ASC origin than in the RVOT origin (−1.2±0.9 vs. 0.3±0.7, p<0.0001). A cutoff value of the TZ index <0 predicted the ASC origin with 88% sensitivity and 82% specificity. The previously reported R-wave duration index ≥50% had a high specificity of 85% but a low sensitivity of 44%, and R/S-wave amplitude index ≥30% had 68% sensitivity and 79% specificity. The area under the curve by receiver operating characteristic curve analysis was 0.90 for the TZ index, which was significantly higher than the R-wave duration index and R/S-wave amplitude index of 0.74 and 0.76, respectively.Conclusion: This novel TZ index can be a more useful marker for differentiating RVOT origin from ASC origin.