Exclusion Criteria for the Phase II Component 1. Patients anticoagulated for > 7 days. 2. Patients who have had a cardioembolic event within the last 1-month. 3. Patients requiring urgent cardioversion due to hemodynamic instability. 4. Patients in whom placement of an ICE catheter for adequate atrial visualization is technically not feasible. 5. Patients with contraindications for Warfarin. 6. Women of child bearing potential, in whom pregnancy cannot be excluded. 7. Patients who need anticoagulation withdrawn due to an elective procedure 8. Patients with any medical condition or social circumstance, which in the opinion of the investigator would make the patient's successful completion of the study doubtful.