The second group of patients is those children with SVT and no evidence of ventricular preexcitation (i.e., no WPW). These patients have a normal, midline ventricular activation pattern in sinus rhythm. Standard treatment for children afflicted with these variants of SVT would also include EPS and RFA. As part of the standard EPS, these patients receive intermittent atrial and ventricular pacing. The result of the transient ventricular pacing is presumably a dysynchronous activation pattern that could be evaluated with TSI. With pacing off, the normal activation pattern should be restored. Therefore, these patients will also serve as their own controls.