Design: This study will be a multicentre, prospective, randomized controlled trial. Patients with ischemic or nonischemic cardiomyopathy heart failure (NYHA II, III or ambulatory class IV), left ventricular dysfunction (EF ≤ 35%), prolonged intraventricular conduction (QRS duration ≥ 120ms), and persistent or permanent AF will be considered for the study. Persistent AF will be defined as patients where obtaining and maintaining sinus rhythm is deemed either not worthwhile, or to be ineffective in the long term, or where both the patient and the physician accept the presence of AF, where rhythm control intervention is, by definition, no longer pursued. Permanent AF is defined as AF where sinus rhythm cannot be restored.