the sensing and pacing electrodes 11 at the atrium of the patient 39 s heart not shown for sensing atrial signals and administering pacing therapy to the atrium and a ventricular sensing and pacing lead 13 extending to a corresponding one of the sensing and pacing electrodes at the ventricle of the patient 39 s heart for sensing ventricular signals and administering pacing therapy to the ventricle when device 1 is a single chamber device the atrial sensing and pacing lead 12 and its associated sensing and pacing electrode are omitted from the device 1 in both cases the device 1 is further provided with one or more ventricular electrodes shown generally at 14 for delivering defibrillation therapy to the ventricle of the heart and suitable ventricular fibrillation leads 15 interconnecting the electrodes 14 and the system module 10 device 1 is further provided with sensing circuitry shown generally at 16 which includes an amplifier 17 for amplifying the cardiac signals sensed by electrodes 11 the amplified cardiac signals outputted by amplifier 17 are fed to an analog to digital converter 18 which converts the cardiac signals to digital form and outputs the digital cardiac signals to a microprocessor shown generally at 19 which in response to various inputs received from other components of the module 10 performs various operations and generates various control and data outputs to a control module shown generally at 20 which incorporates both pacemaking and defibrillating functions therein pacing therapy is delivered from the action module 20 to the atrial and ventricular sensing and pacing leads 11 and 12 by means of a bus shown generally at 21 a suitable power supply not shown for example a battery in the case of an implantable.