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