Evaluation of Resynchronization Therapy for Heart Failure

Inclusion Criteria: * The patients are eligible if they undergo an ICD implantation or replacement * They have a documented LVEF ≤ 35% measured in the previous 6 months (without major clinical subsequent event, such as heart surgery, since the LVEF measurement) * If measured by echocardiography, the LV end-diastolic diameter must be ≥ 60 mm * The duration of the QRS is \< 120 ms * They are in sinus rhythm * They cannot walk more than 400 meters during the screening 6-minute walk test (the patients must be limited by heart failure symptoms) Exclusion Criteria: * Patients with an indication for permanent ventricular pacing or with chronotropic insufficiency defined as follow: * Any condition where the treating physician believes it would not be acceptable for the patient to have his device NOT programmed with the SENSOR at ON for the duration of the study * Second or third degree AV block, either persistent or intermittent * Patients with a pacemaker or an ICD who are paced in the ventricular chamber more than 5% of the time * Patients with LV dysfunction associated with a reversible cause such as post-partum cardiomyopathy, tachycardia induced cardiomyopathy, acute myocarditis or acute toxic cardiomyopathy (including acute alcoholic) * Patients who had a myocardial infarction within the past 6 weeks\* defined by 2 of the 3 following conditions: * Prolonged chest pain * ECG changes suggesting of AMI * Cardiac enzymes elevation more than twice the local upper limit of normal) * Patients who had cardiac surgery within the past 6 weeks\* * Patients with moderate or severe cardiac valve stenosis (aortic, mitral, pulmonary or tricuspid) * Patients with an inability or a limitation to walk for reasons other than heart failure symptoms (e.g., angina, intermittent claudication, severe lung condition or arthrosis) * Patients with severe coexisting illnesses making survival \> 6 months unlikely * Patients who are pregnant and/or nursing. * Patients with inability or unwillingness to consent or comply with follow-up requirements * Patients participating in another study The 6-week period is calculated prior to the beginning of the baseline evaluation and not the implant procedure itself. This difference comes from the fact that the protocol includes a delay of 2 to 8 weeks between the implant procedure and the actual beginning of the patient's evaluation.