The purpose of this study is to evaluate the effect on distance covered in a 6 minute walking test of Rate Responsive pacing in patients with permanent atrial fibrillation and reduced left ventricular ejection fraction treated with atrioventricular junction ablation and biventricular pacing.

Patients with permanent atrial fibrillation and reduced left ventricular ejection fraction treated with atrioventricular junction ablation and biventricular pacing will be enrolled. Two 6' walking test (6WT), one during VVI 70/min biventricular pacing (RR off) and the other during VVIR 70-120/min biventricular pacing (RR on), will be performed one week apart; patients will be randomly and blindly assigned with a 1:1 ratio to group A (first 6WT RR off) or B (first 6WT RR on). The difference between the two tests will be recorded.

Inclusion Criteria:

• Permanent, refractory atrial fibrillation treated with atrio-ventricular node ablation and biventricular pacing
• Left ventricular systolic dysfunction
• Ability to perform a 6-minute walking test
• Completion of the 18th year of age

Exclusion Criteria:

• Contraindications to 6 minute walking test
• IV NYHA Class