Abstract: An implantable cardioverter-defibrillator and/or heart pacemaker has a programmable electrical circuit for producing series of stimulation impulses, wherein electrical circuit is able, on detection of one or more successive extrasystole(s) not observing the natural frequency fe, to produce a series of stimulation impulses (SI) of a higher frequency fs(t) in relation to the earlier natural frequency. The higher frequency (shorter interval) is programmable between 50% and 100% of the mean interval between two or more natural systoles. The interval is dependent on the prematurity of the extrasystole. The frequency than subsequently falls steadily below the natural frequency or ends with the natural frequency fe. There is an immediately decrease--linear or exponential--in frequency and no acceleration due to multiple extrasystoles in order to avoid a continuous higher heart rate.