subcutaneous implantable cardioverter-defibrillators are implanted in the left side of the chest over the sixth rib between the left midaxillary and left anterior axillary lines. ICDs with epicardial sensing and pacing leads are still being im- planted in some patients especially those with certain forms of congenital heart disease. The transvenous ICD has been in clinical use for >3 decades, and robust data from high-quality RCTs support its use in various patient populations includ- ing survivors of cardiac arrest, patients with VT and structural heart disease, and patients with significant LV dysfunction. 5.4. Catheter Ablation 5.4.1. General Considerations Catheter ablation is an important treatment option for patients with VA when antiarrhythmic medications are ineffective, not tolerated, or not desired by the patient. Monomorphic VA usually have an origin or substrate that can be targeted for ablation. Ablation is an op- tion for selected patients with polymorphic VT/VF only if an initiating PVC focus or substrate can be identified. The ablation strategy, risks and outcomes are related to the mechanism and location of the VA. Most VA origi- nate close to the subendocardium and are approached through a transvenous (for the right ventricle) or trans- aortic/transeptal (for the left ventricle) catheterization. Some diseases give rise to VA from the subepicardium, which may be approached by epicardial mapping and ablation. Pericardial access is usually achieved by a per- cutaneous subxiphoid puncture. The catheter ablation procedure usually involves attempts to induce VT by programmed electrical stimulation to confirm the di- agnosis and guide ablation. Problems limiting success include inability to induce an arrhythmia for mapping (common with idiopathic VA), or origin of the arrhyth- mia from an inaccessible location in the myocardium (common in some cardiomyopathies). CLINICAL STATEMENTS AND GUIDELINES 5.4.2. VA in Patients With No Apparent Structural Heart Disease See Section 8. 