Case Study #1: Incessant Supraventricular Tachycardia (SVT) A two-month-old, 4.5 kilogram infant presented with incessant SVT and tachycardia-mediated cardiomyopathy. The patient required intubation and mechanical ventilation due to respiratory failure. Previous treatments, including medical therapy with esmolol, amiodarone, and procainamide, were unsuccessful. Resting ECG exhibited WPW-A, indicating a left-sided accessory pathway. Due to hemodynamic deterioration, the patient was taken for ablation. Procedure. Single R IJV access was obtained using the Seldinger micropuncture technique, and a 3 French (Fr) sheath was placed. A 16-pole microcatheter (PATHFINDER ®, Cardima, Inc. Fremont, California) was then inserted without a guiding catheter. The tip of the microcatheter was placed in the RV outflow tract/main pulmonary artery. Excellent RA and RV potentials were recorded. Pacing RA or RV was also easily accomplished via the single multipole catheter. The patient was then ablated with a 5 Fr, 4-mm radiofrequency catheter (Marinr, Medtronic, Inc.). Outcome. Following ablation, there were occasional episodes of ventricular premature beats, but no further episodes of SVT, and the sinus rhythm ECG showed no delta wave. The tachycardia-mediated myopathy resolved, and the patient was discharged home 11 days post ablation. Amiodarone therapy was maintained empirically for several weeks following discharge, then discontinued. Over four months of follow up, the ventricular extrasystoles have persisted, but there have been no further episodes of SVT and no evidence of preexcitation. Case Study #2: Idiopathic Dilated Cardiomyopathy A three-month-old infant presented with nonischemic idiopathic dilated cardiomyopathy. History included left bundle branch block and persistent heart failure despite maximal medical therapy. The patient was referred for pre-transplant evaluation. Procedure. RIJ and R Femoral access were obtained. Through a trans-septal approach with a 5 Fr sheath, an octapolar microcatheter (PATHFINDER ®, Cardima Inc.) was inserted into the LV. A quad catheter was placed in the RV apex, and a 5 Fr bipolar esophageal lead was placed behind the LA. After obtaining baseline hemodynamics and RV and LV activation patterns, a trial of resynchronization therapy was performed. The patient s hemodynamic status instantaneously improved with CRT pacing. Discussion The cases presented here illustrate some of the unique challenges in pediatric electrophysiology. Although ablation and resynchronization therapy are both standard care in adults, there remains a significant and often justifiable barrier to implementation of these therapies in infants and young children. In accordance with guidelines developed by the Pediatric Electrophysiology Society, we have typically deferred elective ablation therapy for symptomatic arrhythmias until age 5 years and/or weight greater than 15 kilograms. However, for refractory cases unresponsive to medical therapy, ablation can be performed effectively, even in small infants. Cardiac resynchronization therapy in pediatrics is in the very early stages of development. We have advocated a trial of CRT in the cardiac cath lab to assess responsiveness prior to subjecting the patients to the morbidity of a surgical implant procedure. The PATHFINDER ® microcatheter (Cardima, Inc.) has been a successful tool to use in pediatric cases. These catheters have a unique advantage their small size (1.5-2.5 Fr) and atraumatic design allow easy placement in even the smallest infants. They are easily maneuverable without the use of bulky guide catheters, and allow for performance of complex procedures with minimal vascular access, therefore decreasing trauma to the vessels. At Saint Louis Children s Hospital, the PATHFINDER ® microcatheter has application in a variety of cases, including diagnostic EPS, intracardiac mapping during ablation, temporary endocardial pacing, and trial of multisite ventricular pacing for CRT.