When atrial fibrillation is present, the QRS complex occurs at random intervals and the biventricular pacing device does not know when to initiate atrial pacing and may not be able to initiate biventricular pacing (if the native QRS complex comes earlier than expected). This results in less beneficial effects on cardiac output and thus symptoms.

Therefore, it is recommended that any patient with permanent atrial fibrillation undergoing biventricular pacing also have AV nodal ablation performed, thus eliminating the unpredictability of the onset of the QRS complex allowing for near 100% biventricular pacing.