The present invention relates to a system of multiple implantable medical devices for an impedance measurement comprising a first implantable medical device, at least a second implantable medical device distinct from the first implantable medical device and; an analysis module comprising at least one amplifier and one envelope detector, one of the first implantable medical device or the second implantable medical device being a subcutaneous implantable cardioverter defibrillator or a subcutaneous loop recorder, and the other of the first implantable medical device or the second implantable medical device being an implantable endocardial device.