A heart stimulator for stimulating at least an atrium and a ventricle of a heart, the heart stimulator comprising: at least one sensing stage being connectable to an electrode lead comprising an electrode for picking up electric potentials inside at least one atrium and one ventricle of a heart, said sensing stage being adapted to sense an excitation or a contraction of a heart chamber, at least one stimulation pulse generator adapted to generate electric stimulation pulses and being connectable to an electrode lead comprising a stimulation electrode for delivering electric stimulation pulses to at least said atrium and said ventricle of the heart, a control unit, which is connected to said sensing stage and to said stimulation pulse generator and which is adapted to trigger stimulation pulses that are generated by the stimulation pulse generator and that are to be delivered via said electrode lead, to perform at least a DDI mode of pacing, wherein atrial and ventricular stimulation pulses are triggered in an atrium asynchronous manner when a respective atrial or ventricular escape interval times out because no intrinsic atrial or ventricular contraction is sensed during said atrial or ventricular escape interval, respectively, wherein triggering of an atrial or a ventricular stimulation pulse is inhibited, when an intrinsic atrial or ventricular contraction, respectively, is sensed prior to time out of the respective atrial or ventricular escape interval, and wherein the atrial and the ventricular escape interval correspond to an overdrive stimulation rate, that is thought to be higher than an intrinsic heart rate, characterized in that said ventricular escape interval and said atrial escape interval during said DDI mode of pacing are resynchronized with an intrinsic heart rhythm when an intrinsic atrial event As is sensed prior to time out of an atrial escape interval. at least one sensing stage being connectable to an electrode lead comprising an electrode for picking up electric potentials inside at least one atrium and one ventricle of a heart, said sensing stage being adapted to sense an excitation or a contraction of a heart chamber, at least one stimulation pulse generator adapted to generate electric stimulation pulses and being connectable to an electrode lead comprising a stimulation electrode for delivering electric stimulation pulses to at least said atrium and said ventricle of the heart, a control unit, which is connected to said sensing stage and to said stimulation pulse generator and which is adapted to trigger stimulation pulses that are generated by the stimulation pulse generator and that are to be delivered via said electrode lead, to perform at least a DDI mode of pacing, wherein atrial and ventricular stimulation pulses are triggered in an atrium asynchronous manner when a respective atrial or ventricular escape interval times out because no intrinsic atrial or ventricular contraction is sensed during said atrial or ventricular escape interval, respectively, wherein triggering of an atrial or a ventricular stimulation pulse is inhibited, when an intrinsic atrial or ventricular contraction, respectively, is sensed prior to time out of the respective atrial or ventricular escape interval, and wherein the atrial and the ventricular escape interval correspond to an overdrive stimulation rate, that is thought to be higher than an intrinsic heart rate, characterized in that said ventricular escape interval and said atrial escape interval during said DDI mode of pacing are resynchronized with an intrinsic heart rhythm when an intrinsic atrial event As is sensed prior to time out of an atrial escape interval.