OBJECTIVES: We investigated the electrocardiographic(ECG) and electrophysiologic characteristics of ventricular tachycardia(VT) originating within the pulmonary artery(PA). BACKGROUND: Radiofrequency catheter ablation(RFCA) is routinely applied to the endocardial surface of the right ventricular outflow tract(RVOT) in patients with idiopathic VT of left bundle branch block morphology. It was recently reported that this arrhythmiamay originate within the PA.METHODS:Activation mapping and ECG analysis were performed in 24 patients whose VTs or ventricular premature contractions(VPCs) were successfully ablated within the PA(PA group) and in 48 patients whose VTs or VPCs were successfully ablated from the endocardial surface of the RVOT(RV-end-OT group). RESULTS: R-wave amplitudes on inferior ECG leads, aVL/aVR ratio of Q-wave amplitude, and R/S ratio on lead V2 were significantly larger in the PA group than in the RV-end-OT group. On intracardiac electrograms, atrial potentials were more frequently recorded in the PA group than in the RV-end-OT group(58%vs. 12%; p < 0.01). The amplitude of local ventricular potentials recorded during sinus rhythm within the PA was significantly lower than that recorded from the RV-end-OT(0.62±0.56 mV vs. 1.55±0.88 mV; p < 0.01). CONCLUSIONS: Ventricular tachycardia originating within the PA has different electrocardiographic and electrophysiologic characteristics from that originating from the RV-end-OT.When mapping the RVOT area, the catheter may be located within the PA if a low-voltage atrial or local ventricular potential of < 1-mV amplitude is recorded. Heightened attention must be paid if RFCA is required within the PA. OBJECTIVES: We investigated the electrocardiographic (ECG) and electrophysiologic characteristics of ventricular tachycardia (VT) originating within the pulmonary artery (PA). BACKGROUND: Radiofrequency catheter ablation (RFCA) is routinely applied to the endocardial surface of the right ventricular outflow tract ) was in patients with idiopathic VT of left bundle branch block morphology. It was recently reported that this arrhythmia originated within the PA. METHODS: Activation mapping and ECG analysis were performed in 24 patients whose VTs or ventricular premature contractions (VPCs) were successfully ablated within the PA (PA group) and in 48 patients whose VTs or VPCs were successfully ablated from the endocardial surface of the RVOT (RV-end-OT group). RESULTS: R-wave amplitudes on inferior ECG leads, aVL / aVR ratio of Q -wave amplitude, and R / S ratio on lead V2 were significantly larger in the PA group than in the RV-end-OT group. On intracardiac electrograms, atrial potentials were more frequently recorded in the PA group than in the RV-end-OT group (58% vs. 12%; p <0.01). The amplitude of local ventricular potentials recorded during sinus rhythm within the PA was significantly lower than that recorded from the RV -end-OT (0.62 ± 0.56 mV vs. 1.55 ± 0.88 mV; p <0.01). CONCLUSIONS: Ventricular tachycardia originating within the PA has different electrocardiographic and electrophysiologic characteristics from that originating from the RV-end-OT. Mapping of the RVOT area, the catheter may be located within the PA if a low-voltage atrial or local ventricular potential of <1-mV amplitude is recorded. Heightened attention must be paid if RFCA is required within the PA.