Background-Endocardial mapping before sustained monom-orphic ventricular tachycardia(SMVT) induction may reduce mapping time during tachycardia and facilitate the ablation of unmappable VT. Methods and Results-Left ventricular electroanatomic voltage maps obtained during right ventricular apex pacing in 26 patients with chronic myocardial infarction referred for VT ablation were analyzed to identify conducting channels(CCs) inside the scar tissue. A CC was defined by the presence of a corridor of consecutive electrograms differentiated by higher voltage amplitude than the surrounding area. The effect of different levels of voltage scar definition, from 0.5 to 0.1mV,was analyzed. Twenty-three channelswere identified in 20 patients. The majority of CCs were identified when the voltage scar definition was ≤0.2 mV. Electrograms with ≥2 components were recorded more frequently at the inner than at the entrance of CCs(100%versus 75%, P≤0.01). The activation time of the latest component was longer at the inner than at the entrance of CCs(200±40 versus 164±53 ms, P≤0.001). Pacing from these CCs gave rise to a long-stimulus QRS interval (110±49 ms). Radiofrequency lesion applied to CCs suppressed the inducibility in 88%of CC-related tachycardias. During a follow-up of 17±11 months, 23%of the patients experienced a VT recurrence. Conclusions-CCs represent areas of slow conduction that can be identified in 75%of patients with SMVT. A tie red decreasing-voltage definition of the scar is critical for CC identification. Background-Endocardial mapping prior sustained monom-orphic ventricular tachycardia (SMVT) induction may reduce mapping time during tachycardia and facilitate the ablation of unmappable VT. Methods and Results-Left ventricular electroanatomic voltage maps obtained during right ventricular apex pacing in 26 patients with chronic myocardial infarction referred for VT ablation were analyzed to identify conducting channels (CCs) inside the scar tissue. A CC was defined by the presence of a corridor of consecutive electrograms differentiated by higher voltage amplitude than the surrounding area. The effect of different levels of voltage scar Twenty-three channelswere identified in 20 patients. The majority of CCs were identified when the voltage scar definition was ≤0.2 mV. Electrograms with ≥2 components were recorded more frequently at the inner than at the entrance of CCs (100% versus 75%, P <0.01). The activation time of the latest component was longer at the inner than at the entrance of CCs (200 ± 40 versus 164 ± 53 ms, P ≦ 0.001). Pacing from these CCs gave rise to a long-stimulus QRS interval (110 ± 49 ms). Radiofrequency lesion applied to CCs suppressed the inducibility in 88% of CC-related tachycardias. During a follow-up of 17 ± 11 months, 23% of the patients experienced a VT recurrence. Conclusions-CCs representations of slow conduction that can be identified in 75% of patients with SMVT. A tie red decreasing-voltage definition of the scar is critical for CC identification.