In which condition can atrial fibrillation can degenerate into ventricular fibrillation?

In case of AF, the AV node exhibits decremental propey and controls the ventricular rate. WPW syndrome: Patient has a bypass tract (bundle of Kent) --> the impulses race down without any hindrance. Eg. if in AF ectopic foci are firing at 400 times per minute, the same impulses will pass down bundle of Kent and result in development of ventricular fibrillation.