to which an atrial pacing sensing lead 24 and a ventricular pacing sensing lead 26 are connected by way of the connector 22 the atrial pacing sensing lead 24 is connected to both the atrial sensing stage 14 and the atrial stimulation stage 16 likewise the ventricular pacing sensing lead 26 is connected to the ventricular sensing stage 18 and the ventricular stimulation stage 20 via the connector 22 both the atrial stimulation stage 16 and the ventricular stimulation stage 20 are designed to deliver a pacing pulse to the respective lead if triggered triggering of the atrial stimulation stage 16 and or the ventricular stimulation stage 20 is caused by the control unit 12 and in particular by an atrial timer vei av timer 30 and or a ventricular timer vei timer 40 respectively being part of the control unit 12 e g as software modules all elements shown within the control unit 12 may be implemented as software modules for example in accordance with an embodiment of the present invention the ventricular timer vei timer 40 triggers a ventricular stimulation pulse at the end of a ventricular escape interval vei unless a ventricular contraction is sensed by the ventricular sensing stage 18 prior to expiration of the ventricular escape.