A medical system has an implantable heart stimulator with sensing and stimulating pairs of electrodes, with an electric field through the heart being generated by the respective pairs by the application of alternating voltages at a preset frequency to the respective pairs. A signal receiver receives a signal representing the voltage potential difference between the voltage potential at one of the electrodes in the pair, and a reference electrode. The detected voltage is related to the generated electric field, and the signal receiver generates a potential different signal that is supplied to a control unit to determine parameters therefrom representing cardiac activity.