The first step forapproaching the catheter ablation of VAs is todetermine the likely site of the VA origin byevaluating the surface electrocardiograms (ECGs).This process will guide one to the designfor an ablation strategy and can improve thesuccess rate and safety of the procedure. ECGalgorithms are very helpful for localizing thesites of VA origins, especially of idiopathicVAs, for several reasons. First, ECG algorithmsare more accurate for idiopathic VAs than forscar-related VAs because during idiopathic VAsthe sequence of ventricular activation usuallypropagates away from the site of initiationthrough the healthy ventricular myocardium ina predictable manner. Second, the mechanismsunderlying idiopathic VAs are simple, with mostbased on focal mechanisms (triggered activity,abnormal automaticity, and micro-reentry) andless commonly macro-reentrant mechanisms (leftfascicular ventricular tachycardias).