Baseline QRS Axis Distinguishes Para-Hisian PVC From Outflow Tract PVCs.

BACKGROUND: Para-Hisian premature ventricular contraction (PVC) is challenging and needs to be distinguished pre-procedurally from other outflow tract PVCs (OT-PVCs) based on ECG morphology. OBJECTIVE: To investigate whether para-Hisian PVCs can be distinguished from right and left ventricular outflow tract (RVOT and LVOT) by comparing QRS axis of PVC and sinus rhythm (SR). METHODS: We retrospectively analyzed 160 idiopathic PVC cases without conduction abnormalities. The site of origin was defined by the successful ablation site. Baseline SR and PVC axes were calculated using the net QRS amplitudes in leads I and III. Differences between SR and PVC axes were compared among para-Hisian, RVOT, and LVOT PVCs. RESULTS: Seventeen para-Hisian, 84 RVOT, and 59 LVOT PVCs were identified. The mean differences between SR and PVC axes were -4.9±26.1°, 42.1±21.0°, and 61.2±22.9°, with para-Hisian PVCs showing the smallest deviation from baseline SR axis. An axis difference of ≤18° strongly supports a para-Hisian PVC, rather than other OT-PVCs (area under curve=0.94; sensitivity=0.94; specificity=0.82). As SR axis deviated leftward, positive component in lead I became more prominent in RVOT or LVOT PVCs, resulting in some cases exhibiting QRS morphologies mimicking para-Hisian PVCs (n=14/84 and n=4/59, respectively). However, these PVCs exhibited substantial difference between SR and PVC axes. In contrast, para-Hisian PVCs consistently showed similarity between SR and PVC axes, regardless of baseline axis deviation. CONCLUSIONS: The axis of idiopathic OT-PVC can be influenced by baseline SR axis deviation. Comparison of SR and PVC axes is essential to distinguish true para-Hisian PVCs from other OT-PVCs.