An active implantable medical device such as a cardiac prosthesis for stimulation, resynchronization and/or defribrillation, comprising: - means for collecting ventricular depolarizations; - means for collecting left atrial depolarizations; - means for delivering left atrial stimulation pulses (StimOG); and - means for controlling the atrial stimulation, adapted to prematurely and selectively deliver said left atrial stimulation pulses, by applying an abridged left interatrial delay, that is shorter than the sinusal rhythm coupling interval, the atrial stimulation control means comprising: - means adapted to deliver, after a first cardiac cycle with no left atrial stimulation (Cycle 2), a premature left atrial stimulation pulse during a second, immediately consecutive cardiac cycle (Cycle 3), with application of an abridged left interatrial delay, that is shorter than the sinusal rhythm coupling interval; characterized in that the atrial stimulation control means further comprise: - means adapted to deliver a non-premature left atrial stimulation pulse during a third cardiac cycle (Cycle 4) that is immediately consecutive to said second cardiac cycle (Cycle 3), with application of a left interatrial delay corresponding to the sinusal rhythm coupling interval; and - means adapted to: • evaluate the atrioventricular interval separating the left atrial depolarization from the ventricular depolarization at said second and third cardiac cycles, or the interventricular delay separating two successive ventricular depolarizations; • comparing the so-evaluated atrioventricular interval to the atrioventricular interval in sinusal rhythm, or the so-evaluated interventricular delay to the interventricular delay sinusal rhythm; and • modifying, or not, at least one parameter of the atrial stimulation control means as a function of the comparison result. - means for collecting ventricular depolarizations; - means for collecting left atrial depolarizations; - means for delivering left atrial stimulation pulses (StimOG); and - means for controlling the atrial stimulation, adapted to prematurely and selectively deliver said left atrial stimulation pulses, by applying an abridged left interatrial delay, that is shorter than the sinusal rhythm coupling interval, - means adapted to deliver, after a first cardiac cycle with no left atrial stimulation (Cycle 2), a premature left atrial stimulation pulse during a second, immediately consecutive cardiac cycle (Cycle 3), with application of an abridged left interatrial delay, that is shorter than the sinusal rhythm coupling interval; - means adapted to deliver a non-premature left atrial stimulation pulse during a third cardiac cycle (Cycle 4) that is immediately consecutive to said second cardiac cycle (Cycle 3), with application of a left interatrial delay corresponding to the sinusal rhythm coupling interval; and - means adapted to: • evaluate the atrioventricular interval separating the left atrial depolarization from the ventricular depolarization at said second and third cardiac cycles, or the interventricular delay separating two successive ventricular depolarizations; • comparing the so-evaluated atrioventricular interval to the atrioventricular interval in sinusal rhythm, or the so-evaluated interventricular delay to the interventricular delay sinusal rhythm; and • modifying, or not, at least one parameter of the atrial stimulation control means as a function of the comparison result. • evaluate the atrioventricular interval separating the left atrial depolarization from the ventricular depolarization at said second and third cardiac cycles, or the interventricular delay separating two successive ventricular depolarizations; • comparing the so-evaluated atrioventricular interval to the atrioventricular interval in sinusal rhythm, or the so-evaluated interventricular delay to the interventricular delay sinusal rhythm; and • modifying, or not, at least one parameter of the atrial stimulation control means as a function of the comparison result.