ECG measurement protocol
The Carto 3 Version 6 electrophysiological mapping system was used to measure the parameters of the QRS morphology during sinus rhythm (SR) and VA. The lead gain was uniform with a paper speed of 100 mm/s. Several parameters were obtained similar to previous studies: (1) R-wave and QRS complex amplitudes in lead V1–V3 during SR and VA, measured from the peak of the QRS complex to the isoelectric line and the nadir of the QRS complex; (2) R-wave duration in leads V1–V2 during VA, measured from the onset of the QRS complex to the transection point between the R-wave and isoelectric line; (3) total QRS duration during VA, measured from the earliest onset of the QRS complex to the time of the latest activation in any lead; (4) the precordial transition lead was defined as the position in which the amplitudes of the R- and S-waves were equal. The precordial transition zone (TZ) score was defined as the lead in which the R/S-wave amplitude ratio was 0.9–1.1. If the ratio was >1.1, the TZ score was graded in decrements of 0.5 points. On the other hand, if it was <0.9, the TZ score was graded in increments of 0.5 points; (5) R-wave deflection interval, measured from the initiation to the peak of the QRS complex in leads V2 and V3 during VA; and (6) amplitude of the QRS complex within the initial 40 ms in lead V2. The ECG parameters[3–11] proposed to predict the origin of the VA from the RVOT or LVOT are presented in Table 1. For all cases, quantitative measurements were performed by two observers. If there was apparent discordance between the two observers, Dr. Jiang was consulted. In our validation study, all 100 patients were selected from two different hospitals, and ECG measurements were performed by observers different from the previous study. Calculation of the parameters and statistical analysis were performed by the Tianjin Institute of Cardiovascular Diseases; they were blinded to the procedure. Our study protocol was approved by the ethics committee of the hospital. Owing to the retrospective observational nature of the study, it was not registered on ClinicalTrials.gov.