1. An active implantable medical device of the cardiac resynchronizer type, comprising:a pulse generator comprising a memory having instructions stored thereon and a processor configured to execute the instructions to produce pacing pulses intended to be delivered to at least a left ventricle of a patient carrying the active implantable medical device;
at least one detection electrode, the pulse generator configured to detect atrial and ventricular events via signals received from the at least one detection electrode; and
at least one stimulation electrode;
wherein the processor is further configured by the instructions to:
apply an atrioventricular delay (AVD) between a sensed or stimulated atrial event and a delivery of a ventricular pacing pulse;
quantify a degree of fusion between the delivery of a pacing pulse to a cavity, left or right, and a spontaneous contraction of another cavity which is opposite, respectively right or left, and calculate a fusion rate value between two extreme values respectively corresponding to a full capture situation and to a spontaneous depolarization situation of said cavity; and
modify a value of the AVD applied to the delivery of said ventricular pacing pulse, as a function of a comparison made between i) a current value of the fusion rate calculated by processor, and ii) a target value of the fusion rate.