Inclusion Criteria:

• The patient (male or non-pregnant female) must be >18 years of age
• Have ECG documented AF at the time of enrollment into the study
• Require (be scheduled for) electrical cardioversion
• Female patients of childbearing potential must have a documented negative pregnancy test during the index hospitalization.
• The patient or legally authorized representative willing to give written informed consent, prior to the procedure, using the ICF approved by the UCR Institutional Review Board. Surrogate consent must be given per CV Research SOP.
• Have a serum potassium level between 3.5 and 5.5 meq/L
• If diabetic, have a hemoglobin A1C level <11%
• NYHA I-II can be enrolled if no hospitalizations for heart failure within 6 months or current, known EF<40%.

Exclusion Criteria:

• The patient is unable or unwilling to cooperate with the study follow-up procedures.
• Pregnant and/or lactating women, and women of child bearing potential not using acceptable means of contraception. Women of childbearing potential must be using adequate measures of contraception (as determined by the Investigator) to avoid pregnancy and should be highly unlikely to conceive during the study period. Women of childbearing potential must have a negative pregnancy test at screen.
• Participation in any other clinical trials involving investigational or marketed products within 30 days prior to entry in the study.
• Current diagnosis of angina pectoris (no events within a period of 3 months prior to Visit 1 (V1)
• History or current diagnosis of symptomatic heart failure (NYHA classes III-IV) or
• documented ejection fraction of < 40 %. Myocardial Infarction within 3 months; coronary revascularization (PCI or CABG), unstable angina within 1 month of V1.
• Stroke, deep vein thrombosis (DVT), pulmonary embolism (PE) or transient ischemic attack (TIA) within 3 months of V1.
• Substance or alcohol abuse within 6 months of V1
• Known allergy to any of the drugs administered in the study (angiotensin receptor blocker (ARB) or an angiotensin converting enzyme inhibitor (ACE-I), Valsartan, amiodarone, flecainide or propafenone)
• Any other contraindication listed in the labeling of warfarin or acenocoumarol.
• Patients treated with ARBs or ACE-I within 1 month of V-1
• Impaired renal function defined as a serum creatinine > 2.5 mg/dL
• Evidence of hepatic disease evidenced by an AST or ALT value > 2 times the upper limit of the institution's normal values.
• Significant non-cardiovascular illness or condition likely to result in severe incapacitation or death prior to study completion.