Inclusion Criteria:  The patient must indicate their acceptance to participate in the study by signing an informed consent document. The patient must be ≥ 18 years of age. Left bundle branch block, QRS ≥130 and LVEF <=35%. No indication of stimulation for AV block. Non-left bundle branch block, QRS ≥150 and LVEF <=35%. Patients with indication of resynchronization therapy for ventricular dysfunction (LVEF <40%) and indication of cardiac pacing for AV block. LVEF <=35% in NYHA class III or IV if they are in atrial fibrillation and have intrinsic QRS >=130 ms, provided a strategy to ensure biventricular capture is in place.  Exclusion Criteria:  Myocardial infarction, unstable angina or cardiac revascularization during the previous 3 months. Pregnancy. Participating currently in a clinical investigation that includes an active treatment.