EP study will be performed in the fasting state and after antiarrhythmic medications are discontinued for 5 half-lives (>60 days for amiodarone). Catheters are advanced to the right atrium (RA), coronary sinus, and transseptally to left atrium (LA). A 64-pole basket catheter (FIRMap, Topera, Menlo Park, CA) will map the LA. RA recordings are made using a second basket, simultaneously when possible. Heparin will be infused to maintain activated clotting time >350 seconds.