SPECIFIC AIM: To test the safety and feasibility of using low molecular weight heparin (LMWH, enoxaparin sodium; Lovenox, Sanofi-Aventis) in lieu of unfractionated heparin (UFH) as antithrombotic therapy for patients in atrial fibrillation undergoing transesophageal echocardiography (TEE) guided chemical or electrical cardioversion to sinus rhythm.