Implantable medical device assets such as pacemaker, defibrillator, cardioverter and / or multisite device has detection IMPROVED late atrial extrasystoles

This device comprises: atrial event detection means (EVTA); means for applying a first window forming atrial relative refractory period (DARRP) for eliminating high or medium premature atrial extrasystoles and management of the atrioventricular delay; and means responsive to a change in the sinus atrial rate. Further comprising: means for applying to atrial sensed events a second window (DET), distinct from the first window and of longer duration, for the elimination of atrial extrasystoles low prematurity, the duration of the second window is a period variable, defined as a fraction of the current mean atrial interval (IAM); and means for temporarily inhibiting said means reacting to a variation of the atrial sinus rhythm in case of detection of an atrial event inside the second window.