After induction of anesthesia, a standard baseline programmed stimulation electrophysiological protocol is performed using transvenous endocardial electrodes that allow calculating the following parameters: right atrial refractory period, atrioventricular node effective refractory period, accessory pathway effective refractory period, and right ventricular effective refractory period. Decremental pacing of the right atrium or ventricle allows determining the shortest cycle length during antegrade and retrograde conduction, which is also called the "Wenckebach cycle length," with a 1:1 conduction over the normal atrioventricular node and accessory pathway. Right atrial or ventricular extrastimulus testing involves pacing at a standard drive cycle length of 200, 400 or 600 ms, followed by an extrastimulus coupled (steps of 20 ms from 600-300 ms and steps of 10 ms less than 300 ms) to every eighth drive beat. Baseline intervals measured during sinus rhythm include intraatrial conduction time (PA interval), and atrial-His interval. During induced reciprocating tachycardia, conduction times of each component of the reentrant circuit is measured and included cycle length and atrial-His, His-ventricular, and ventriculoatrial intervals. All electrophysiological measurements is recorded using a standard digitizing tablet at a paper speed of 100 mm/s. Programmed stimulation is performed using a Mennen stimulator, and data is displayed, recorded, and analyzed using a standard computerized multichannel system. If the clinically suspected SVT will not induced during the baseline protocol, isoproterenol (0.03-0.07 µg • kg- 1 • min-1) will be infused, and the protocol repeated until SVT will be induced. After conclusion of the diagnostic study, ablation of the abnormal substrate(s) will be performed using a commercial radiofrequency generator (ATKAR). If needed, the muscle relaxation will be reestablished using pancuronium 0.05mg/kg to allow the ablation be performed during apnea to avoid respiration-related intrathoracic movement. This approach permits application of radiofrequency energy near vital structures, such as the atrioventricular node, with reduced concern of potentially dangerous catheter dislodgement. The final diagnostic electrophysiological study will be repeated at baseline and with isoproterenol (0.03-0.07 µg • kg- 1 • min- 1). Extubation will be performed after the patient awake. Post procedural care will be provided in the recovery room, and patients will be transferred to a ward when hemodynamically stable, fully conscious and able to protect the airway, without pain and severe nausea or active vomiting.