A heart stimulator for electrical stimulation of a heart chamber includes a sensing stage sensing excitation of the heart chamber via an electrode lead having an electrode for picking up heart chamber electric potentials, a stimulation pulse generator generating electric stimulation pulses for delivery to the heart chamber via a stimulation electrode, and a control unit connected to the sensing stage and the stimulation pulse generator and being adapted to trigger the stimulation pulses at a controlled stimulation rate. A monitoring stage is provided for preventing too high of a stimulation rate for too long of a period of time, with the monitoring stage being connected to the control unit and being adapted to monitor the controlled stimulation rate, and to override the controlled stimulation rate by a fixed stimulation rate for a predetermined period of time if the average controlled stimulation rate exceeds a predetermined maximum rate.