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.

HYPOTHESIS: Early cardioversion from atrial fibrillation can be safely performed using a short-term anticoagulation strategy of low molecular weight heparin (Lovenox, Sanofi-Aventis) compared to unfractionated heparin, accompanied by a TEE examination prior to cardioversion. The use of LMWH with TEE will result in a safe, cost-effective, and possible efficacious approach to cardioversion of atrial fibrillation compared to UFH with TEE.

Inclusion Criteria:

• Patients with chronic or paroxysmal atrial fibrillation of > 2 days duration who are candidates for early chemical or electrical cardioversion
• Patients with atrial flutter with a documented history (electrocardiography) of atrial fibrillation
• Males and females 18 years of age or older
• Patients have available results of routine clinical labs for standard clinical chemistry and complete blood count within the previous 14 days

Exclusion Criteria:

• An INR > 1.4 in patients who have received warfarin prior to enrollment.
• Use of IV heparin for more than 72 hours immediately prior to randomization.
• Patients with contraindications to TEE, such as dysphagia, or esophageal strictures.
• Patients who will need anticoagulation discontinued because of elective intervening procedure, such as cardiac catheterization or surgery.
• Patients with contraindications to warfarin or heparin
• Patients who require concomitant therapy during the study period with any drug known to affect coagulation or platelet function (i.e. ASA, NSAID, antiplatelet drugs)
• Women of childbearing potential, unless pregnancy can be excluded by medical history incompatible with pregnancy or by serum or urine beta HCG levels.
• Patients who are hemodynamically unstable and thus may require immediate cardioversion.
• Weight less than 40 kg (88 pounds) or more than 125 kg (275 pounds)
• History of gastrointestinal bleeding disorder and/or endoscopically verified ulcer disease within the last year
• History of intracranial or retinal bleeding, or other known disorders with an increased risk of bleeding
• Ischemic stroke in the previous three months
• Uncontrolled hypertension (systolic blood pressure greater than 180 mm Hg or diastolic blood pressure greater than 100 mm Hg)
• Malignancy currently under active treatment, including melanoma
• Patients with renal insufficiency (creatinine > 2.0 mg/dL) or are renal transplant subjects
• Patients with anemia (Hgb less than 10 gm/dL)
• Patients with thrombocytopenia (platelet count less than 100 x 10^9/L)
• Positive fecal hemoglobin test
• Life expectancy of less than 6 months
• History of drug and/or alcohol abuse within the last two years
• Patients unable or unwilling to give informed consent
• Patients unable or unwilling to return for follow-up
• Prisoners or wards of the state
• Patients with psychological problems that may decrease compliance with the protocol
• Not willing to complete the Quality of Life Questionnaires x 3
• Participating in another clinical trial and/or taking an investigational medication in the past 30 days
• Patient language, learning skills, or home environment unconducive to self-management