Doctors use patterns on heart rhythm tracings (ECGs) to predict where abnormal heart rhythms originate, but these prediction methods were developed in people with normal heart structure. Patients with repaired Tetralogy of Fallot have hearts that developed differently, and cardiologists do not know if these prediction methods work accurately for them. In this study, the investigators will test whether three commonly used prediction methods work in Tetralogy of Fallot patients by pacing the heart from known locations during an already-scheduled heart procedure and comparing the predicted location to the actual location. Participation adds approximately 15 minutes to the procedure and does not require additional visits. The results will help cardiologists understand whether current methods can be trusted when planning treatments for abnormal heart rhythms in this patient population, or whether new prediction methods need to be developed.

Inclusion Criteria:

• Adult patients >/= 18 years with repaired tetralogy of Fallot with pulmonary stenosis scheduled for clinically-indicated diagnostic electrophysiology (EP) study.

Exclusion Criteria:

• Dextrocardia or mesocardia.
• Double outlet right ventricle.
• Tetralogy of Fallot with pulmonary atresia.
• Contraindication to femoral arterial access.
• Mechanical aortic prosthesis.
• Inability to provide informed consent.