The purpose of this study is to demonstrate that biventricular pacing (BiV) and left univentricular (left ventricular [LV] only) pacing are safe and effective for cardiac resynchronization therapy in heart failure patients implanted with a cardiac resynchronization therapy defibrillator (CRT-D) device.

The hypothesis to be tested by this clinical investigation is that patients indicated for an ICD with cardiac resynchronization therapy respond as well to LV only pacing as to BiV pacing.

Inclusion Criteria:

• Have an approved indication for implantation of an ICD for treatment of a life-threatening ventricular tachyarrhythmia(s)
• Have advanced heart failure with a New York Heart Association (NYHA) classification of III or IV, despite receiving a minimum of 30 days of stable optimal pharmacological therapy
• Have a ventricular conduction delay manifested as a QRS width >= 130 ms
• Have a left ventricular end diastolic diameter (LVEDD) >= 55 mm
• Have a left ventricular ejection fraction (LVEF) <= 35%

Exclusion Criteria:

• Have a CRT device already implanted
• Have a standard indication for bradycardia pacing
• Have a history of chronic atrial fibrillation [AF] (continuous AF lasting > 1 month) within 1 year prior to enrollment or have undergone cardioversion for AF in the past month)
• Have the ability to walk > 450 meters during the 6-minute walk test
• Have uncorrected primary valvular disease
• Have had a recent myocardial infarction, unstable angina or cardiac revascularization (percutaneous transluminal coronary angioplasty [PTCA] or coronary artery bypass graft [CABG]) within 1 month of enrollment
• Have had cerebrovascular accident (CVA) or transient ischemic attack (TIA) within 3 months of enrollment
• Have a life expectancy of less than one year
• Are unable to comply with the follow-up schedule and tests
• Are minors (age below 18 years)
• Are pregnant or are planning for pregnancy in the next 6 months