VA in Patients With No Apparent Structural Heart Disease See Section 8. VA that are not associated with underlying structur- al heart disease or a genetic arrhythmia syndrome are commonly referred to as idiopathic. Most idiopathic VA are monomorphic and based on a focal mecha- nism of triggered activity or abnormal automaticity; a few are due to reentry. For patients who are symp- tomatic, and in whom antiarrhythmic medications are ineffective, not tolerated, or not desired by the patient, catheter ablation is a treatment option. The ablation strategy is to identify the site of origin mani- fested by the earliest site of electrical activation, or when this is not practical, by pace mapping. Catheter ablation of idiopathic VA is usually accomplished with endocardial catheterization, though an epicardial ap- proach through the coronary venous circulation or a subxiphoid pericardial puncture may occasionally be required. Ablation failure for idiopathic VA is often due to inability to provoke the arrhythmia to allow mapping in the electrophysiological laboratory or ori- gin from an inaccessible region. 5.4.3. Scar-Related VT See Section 8. For most patients with structural heart disease, sustained monomorphic VT is due to reentry through regions of surviving myocardial fibers associated with areas of fibrous scar. The ablation strategy for these re- entry circuits is to identify and eliminate channels of surviving myocardium within the scar that are often as- sociated with slow conduction facilitating reentry. For most VTs that are related to prior MI, the substrate is on the subendocardial surface of the left ventricle. In NICM, the reentrant circuits are more variable in loca- tion, often involve the epicardial surface of either ven- tricle and frequently extending into the midmyocar- dium where ablation may be difficult to achieve from either surface. 