Atrial Fibrillation Associated With Heart Failure Treated by BIOTRONIK's CRT-DX System

Inclusion Criteria: * Standard CRT-D indication according to current guidelines * Patient has documented history of paroxysmal, persistent, or long-standing persistent atrial fibrillation * De novo implant or upgrade from a DX implantable cardioverter-defibrillator (ICD) system * Implant planned to occur within 30 days of consent * Patient is able to understand English or Spanish * Patient is able to understand the nature of the study and provide informed consent * Patient is willing and able to complete all routine study visits at the investigational site for up to 12 months of follow-up * Patient is willing to utilize BIOTRONIK Home Monitoring® via CardioMessenger * Patient age is greater than or equal to 18 years After consent has been signed, additional inclusion criteria must be fulfilled for study participation: * Baseline subject assessment is evaluated as New York Heart Association class II, III or ambulatory IV heart failure at study Enrollment Visit * Baseline subject assessment of six-minute walk test is completed at study Enrollment Visit and walk distance ≤ 450 meters (1,476 feet) Exclusion Criteria: * Contraindication to CRT-D/CRT-DX * Patient has current or previous atrial pacing need * Patient is considered for a His Bundle Pacing system * Patient has current or previous pacemaker, non-DX ICD implant, or bi-ventricular pacing system prior to enrolling * Patient is currently planned for a pulmonary vein isolation catheter ablation procedure within 3 months of consent * Patient life expectancy is less than 1 year * Patient is expected to receive heart transplantation or ventricular assist device within 1 year after implant * Patient is expected to have a cardiac surgical procedure, such as coronary artery bypass graft or valve transcatheter replacement/repair or surgery, planned to occur within 6-months after implant (excludes atrioventricular node ablation procedures) * Patient is currently on dialysis, or is expected to receive dialysis within 1 year of implant * Patient is enrolled in any investigational cardiac device or drug trial that might significantly affect the studied outcomes * Any condition (e.g. severe arthritis, wheelchair bound, etc.) preventing the patient from performing the required six-minute walk test * Conditions that prohibit placement of any of the system leads * Patient reports pregnancy at the time of consent