An active implantable medical device, in particular a pacemaker, defibrillator and/or cardiovertor and/or a multisite device, in which electrodes are to be placed in a plurality of respective distinct sites comprising at least one ventricular site and one atrial site on the same side of the heart, these electrodes being connected to at least one circuit of collection of cardiac signals to detect a depolarization potential, and to a stimulation circuit to apply stimulation pulses to at least certain of the aforesaid sites, said device comprising:
- (a) means for evaluating the cardiac flow by measurement of a trans-valvular bio-impedance, said means operating by injection of a current between an atrial site and a ventricular site, and collection of a differential potential between an atrial site and a ventricular site, wherein said measurement uses a tripolar configuration with one site common to the injection and the collection, one site dedicated for the injection, and one site dedicated for the collection, the common site being located in one of an atrial site and a ventricular site and the two dedicated sites being located in the other of the atrial site and the ventricular site;
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- (b) means for varying an atrio-ventricular delay in the application of atrial and ventricular stimulation pulses, said means operating in response to the measured trans-valvular bio-impedance in the direction of the improvement of the cardiac flow.