Cardiac resynchronization therapy (CRT) is used to treat dilated heart failure with discoordinate contraction. Dyssynchrony typically stems from electrical delay leading to mechanical delay between the septal and lateral walls. Right ventricular apical pacing might be associated with long-term adverse effects on left ventricular function, and alternative pacing sites such as high septal or RVOT has been suggested. Previous studies have however been conflicting. The aim of this study is to determine if bi-ventricular pacing from RVOT + LV is better than RVapex+LV. To avoid influence from possible atrial delay only patients with chronic atrial fibrillation are included. All patients will receive a bi-ventricular pacemaker with 3 leads placed in RVapex, RVOT and via coronary sinus to pace the left ventricle. After a run in period (to determine if AV-junction ablation is necessary) the patients are randomised to one of the following pacing configurations: RVOT+LV or RVapex+LV with cross-over after 3 month.

Inclusion Criteria:

• Heart failure NYHA III-IV
• LVEF<35%
• QRS duration>150 ms
• Chronic atrial fibrillation.

Exclusion Criteria:

• Heart failure not related to systolic function
• Unstable angina pectoris, AMI, PCI or CABG within 2 month
• Inclusion in other studies