PEGASUS CRT is a multicenter trial that will assess the effect of a cardiac resynchronization therapy defibrillator (CRT-D) device programmed to DDD-70 or DDDR-40 compared to a CRT device programmed to DDD-40 in heart failure patients. The effect of atrial support pacing in heart failure patients will be assessed using a clinical composite rating. A sub-study will evaluate the effect pacing mode has on exercise capacity in this heart failure population.