A cardiac pacemaker has a pulse generator which emits stimulation pulses which
are respectively separated by stimulation intervals and which collectively have
an average duration. A modulation device alternatingly shortens and lengthens the
stimulation intervals, without changing the average duration. An evaluation unit
analyzes signals detected after each stimulation pulse and determines the electric
restitution of the heart at the average stimulation interval duration on the basis
of a measurement of the duration of the action potential. Changes in a measuring
variable, associated with the duration of the action potential, caused by the modulation
of the stimulation intervals is determined in a relationship to the average duration
of the stimulation interval. This relationship is compared with at least one predetermined
value, and the average duration of the stimulation interval is controlled on the
basis of this comparison.
