Ventricular Rate Regularization for Improved Quality of Life in Patients With CHF and AF

Inclusion Criteria: * Be between 18-85 years old * Be willing and able to give informed consent * Patient currently has a CRT-D or ICD with \> 20% atrial fibrillation burden as recorded by defibrillator follow up report or a history of atrial fibrillation prior to implant. * Patient has the ability to complete a Six minute walk test with the only limiting factors to be fatigue or shortness of breath. Exclusion Criteria: * Expected mortality less than 6 months due to non-cardiac causes. * Pregnant women. * Creatinine greater than or equal to 2.5 mg/dl. * Anemia (HCT less than 30) * COPD causing significant dyspnea * Orthopedic problems affecting 6 minute walk.