Assessment of Cardioversion Using Transesophageal Echocardiography II (ACUTE II)

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