An implantable heart stimulating device has a stimulation pulse generator that emits stimulation pulses at an adjustable stimulation rate, an activity sensor that emits an activity signal in response to detected activity of the patient, and a physiological parameter sensor that generates a physiological sensor signal in response to a detected physiological parameter. The activity and physiological sensor signals are supplied to a control arrangement that sets the stimulation rate for the stimulation pulse generator by executing a stimulation rate algorithm dependent on those signals. In the stimulation rate algorithm, if the physiological signal indicates an emotional stress on the part of the patient, the stimulation rate is increased to an adjustable emotional stress rate level, and if no increase in the activity signal occurs during a predetermined time period following the stimulation rate increase, the stimulation rate is decreased.