This study is a randomized, double-blinded, crossover, multi-center, prospective trial. The study will consist of up to 122 subjects who require treatment of advanced heart failure through cardiac resynchronization therapy (CRT) with back-up defibrillation capabilities. Eligible patients will have a successfully implanted BIOTRONIK Lumax HF-T CRT-D system and have received simultaneous biventricular pacing for a minimum of 90 days prior to enrollment. The 90-day period is being required to allow the treatment effect of CRT therapy with SIM to be complete and to ensure the patient is receiving a stable and optimal CHF medical regimen. The patients will have the interventricular delay feature programmed after a standardized optimization procedure. Patients, along with study personnel evaluating the study endpoint measures, will be blinded to the type of CRT therapy delivered during the study follow-up period.