Inclusion Criteria:

• Accepted indication for ICD implant
• Severe heart failure (NYHA Class III or IV) at the time of enrollment
• May have pre-existing ICD, provided subject is on stable, optimal medical regime
• Sinus rhythm with spontaneous QRS duration greater than or equal to 150 ms, or a QRS duration greater than or equal to 130 ms with an inter-ventricular mechanical delay (IVMD) greater than or equal to 40 ms
• Left-ventricular ejection fraction (LVEF) of 35% or less

Exclusion Criteria:

• Any generally accepted indication for standard cardiac pacing, or any contraindication for standard cardiac pacing
• Any contraindication for ICD therapy
• Currently implanted with a lead positioned in or through the coronary sinus
• Hypertrophic or obstructive cardiomyopathy
• Acute myocarditis
• Unstable coronary symptoms (unstable angina or myocardial infarction) within the last month
• Recent or planned cardiac revascularization or coronary angioplasty
• Correctable valvular disease that is the primary cause of heart failure
• Mechanical tricuspid valve
• Chronic atrial arrhythmia or cardioversion for atrial fibrillation within the past month, or paroxysmal atrial fibrillation requiring new pharmacologic therapy within the past month
• Systolic blood pressure consistently above 170 mmHg or consistently below 80 mmHg
• Supine resting heart rate exceeding 100 bpm
• Receiving continuous IV infusion of positive inotropic therapy or intermittent therapy (IV infusion) more than twice per week
• Heart transplant recipient
• Primary pulmonary disease of a severity that might limit the patient's ability to perform a treadmill test
• Serum creatinine above 3.0 mg/dL
• Serum hepatic functions at or above three times the upper normal limit
• Cerebrovascular event within the previous three months
• Inability to walk or other physical impediments which might prevent the patient from completing a maximum, symptom-limited treadmill test
• Age of less than 18 years
• Pregnancy