Discrimination of atrial fibrillations for an active implantable medical device, in particular a defibrillator/cardiovertor

An active implantable medical device, in particular a defibrillator/cardiovertor, having:

• means for delivering a therapy for treatment of tachycardia including at least one of defibrillation, cardioversion, ventricular antitachycardiac pacing and atrial antitachycardiac pacing,
• means for sensing the ventricular and atrial activity including R-waves and P-waves, and
• means for suspecting and confirming a presence of an episode of tachycardia in response to the sensed activity, said means including means for analyzing a stability of RR intervals and a stability of associated PR intervals,
• wherein the improvement comprises:
• means, operating in response to a detection of stable RR intervals and unstable PR intervals, for discriminating between atrial fibrillation with a fast ventricular rhythm and atrial fibrillation with a ventricular tachycardia, and for controlling delivery of a differentiated therapy according to said discrimination.