The most common mecha- nism involves antegrade conduction over the right bundle branch and retrograde conduction over the left bundle branch, thereby produc- ing left bundle-branch block QRS morphology during VT, which is often rapid and poorly tol- erated. Catheter ablation of the right or left bundle branch interrupts the circuit and is usually After ablation, severely impaired atrioventricular conduction can be present, requir- ing permanent pacing, which can have hemody- Many patients have other inducible scar related VTs or meet eligibil- ity for an ICD due to severity of associated heart disease. 2. Endocardial catheter ablation failure can be due to location of the arrhythmia substrate in the midmyocardium or epicardium, and this is more likely in patients with nonischemic rather than ischemic cardiomyopathy, and in arrhythmogenic right ventricular cardiomyopathy.S12-7 S12-9 In the CLINICAL STATEMENTS AND GUIDELINES epicardial ablation was required in 30% of patients with VT related to NICM com- pared with 1.2% of patients with ischemic car- diomyopathy. A wide QRS with marked slurring of the initial portion of the QRS and a QS com- plex in the lateral or inferior leads during VT sug- gests an epicardial circuit in NICM, but the ECG does not reliably predict epicardial VT locations in patients with prior MI. Preprocedural cardiac MRI and intraprocedural electroanatomic mapping are useful tools to guide the localization of epi- cardial scar that may be the source of reentrant Pericardial adhesions prevent percu- taneous access in some patients, notably many with prior cardiac surgery. Percutaneous pericar- dial access for mapping and ablation is associated with a serious complication rate of approximately 5% and tamponade from RV puncture or lac- eration that can require emergent surgery or be fatal, coronary artery injury and phrenic nerve injury can occur.S12-11,S12-12 Reported experience is from tertiary referral centers. 13. 