Conduction System Pacing vs. Biventricular Pacing for Cardiac Resynchronization Therapy

Inclusion Criteria: * ≥18 years of age, * LVEF ≤ 35%, * NYHA Class II-IV (IV only outpatients), * optimized in medical treatment (OMT) AND one of the following: . LBBB according to AHA/ACC/HRS Scientific Statement from 2009 and ≥130ms or * LBBB-like intraventricular conduction delay (IVCD) \> 150ms or RV paced QRS and indication for upgrade to CRT (\> 40% RV pacing) OR * ≥18 years of age, * LVEF ≤ 40 % * pacing indicated by AV-block and, thus, expected large percentage of ventricular pacing. Exclusion Criteria: * recent acute myocardial infarction (AMI) * coronary artery bypass graft (CABG) (\<3 months) * life expectancy \<2 years, patients in hemodialysis * treatment with a cardiac implantable electronic device (CIED) is contraindicated. * Patients are excluded with regards to the MRI sub study if eGFR \> 35 ml/min, in case or contrast allergy or certain metal implants.