Abstract: A cardiac stimulation apparatus is provided which includes an implantable cardiac pacer 10 having an antitachycardia circuit 12 and an implantable defibrillator circuit 58. The patient's electrical heart rate is sensed and if the heart rate exceeds a predetermined threshold, the antitachycardia circuit 12 is activated. If the antitachycardia circuit 12 fails to lower the heart rate to below a selected rate, the defibrillator circuit 58 is activated. A coded pulse train is transmitted between the pacer 10 and the defibrillator circuit 58 for isolating the pacer 10 from the defibrillator circuit 58 during operation of the defibrillator circuit 58.