RESUMEN
INTRODUCTION: Differentiation of right versus left ventricular outflow tract (RVOT vs. LVOT) arrhythmia origin with left bundle branch block right inferior axis (LBRI) morphology is relevant to ablation planning and risk discussion. Our aim was to determine if lead I R-wave amplitude is useful for differentiation of RVOT from LVOT arrhythmias with LBRI morphology. METHODS: The R-wave amplitude in lead I was measured in a retrospective cohort of 75 consecutive patients with LBRI pattern ventricular arrhythmias (VAs) successfully ablated from the RVOT (n = 54), LVOT (n = 16), or the anterior interventricular vein (AIV; n = 5). The optimal R-wave threshold was identified and diagnostic indices were compared with the previously reported transitional zone (TZ) index and V2S/V3R index. RESULTS: An R-wave amplitude greater than or equal to 0.1 mV predicted LVOT origin with 75% sensitivity and 98.2% specificity. In comparison, the TZ and V2S/V3R indices had 50% and 68.8% sensitivity, and 75.9% and 88.9% specificity, respectively, for predicting LVOT origin. The area under the curve (AUC) was 0.85 for lead I R-wave amplitude, 0.87 for V2S/V3R, and 0.72 for the TZ index. Of 36 cases with QS in lead I, 30 (83.3%) were from the anterior RVOT, three (8.3%) from the LVOT, and three (8.3%) from the AIV. CONCLUSION: The presence of R-wave amplitude in lead I (≥0.1 mV) is a simple and useful criterion to identify LVOT cusp or endocardium focus in LBRI arrhythmias. A QS pattern in lead I suggests an origin in the anterior RVOT, or less commonly the adjacent LV summit.