These fascicular arrhythmias usually have a focal mechanism with the target of catheter ablation being the site of earliest electrical activation recorded with a presystolic fas- cicular potential. Catheter ablation is highly effective for intrafascicular and fascicular VA. Serious compli- cations are infrequent and include bleeding at the site of arterial or venous access and a small risk of bundle branch block or atrioventricular block. 2. Idiopathic LVT is based on reentrant mechanism involving tissue with slow conduction properties along the LV septum as the anterograde limb and the normal left posterior fascicle of the His- Purkinje system as the retrograde limb. The slow conduction zone is verapamil-sensitive.S8.3-3 S8.3-6 These arrhythmias typically have a right bundle- branch block morphology with superior axis, though reversal of the circuit may produce a rela- tively narrow QRS during VT. Verapamil typically terminates these arrhythmias in the anterograde slow conduction zone.S8.3-3 S8.3-6 3. Although no RCTs have been published, the chronic use of oral verapamil for verapamil-sen- sitive idiopathic LVT has been reported to control this tachycardia in many patients, including both adults and children.S8.3-5,S8.3-8 S8.3-10 8.4. Idiopathic Polymorphic VT/VF Recommendations for Idiopathic Polymorphic VT/VF References that support the recommendations are summarized in Online Data Supplement 49. COR LOE Recommendations I B-NR In young patients (<40 years of age) with unexplained SCA, unexplained near drowning, or recurrent exertional syncope, who do not have ischemic or other structural heart disease, further evaluation for genetic arrhythmia syndromes is I B-NR In patients resuscitated from SCA due to idiopathic polymorphic VT or VF, an ICD is recommended if meaningful survival greater than 1 year is expected.S8.4-9 S8.4-13 I B-NR For patients with recurrent episodes of idiopathic VF initiated by PVCs with a consistent QRS morphology, catheter ablation is useful.S8.4-11,S8.4-14 Recommendation-Specific Supportive Text 1. 