In this study, subjects will be randomized to Cardiac Resynchronization Therapy-Defibrillation (CRT-D) or Cardiac Resynchronization Therapy-Pacing (CRT-P) vs Pacing Therapy AAI. Randomization will be stratified by indication. Optimal pharmacological therapy in both treatment arms. Length of follow-up for each subject will be 2y, since all subjects will be followed to a common study termination date.