An implantable stimulator for treating arrhythmic function disturbances of a heart, comprising; a stimulation electrode for transmitting stimulation pulses to the heart, a pulse generator that is connected to the stimulation electrode for generating the stimulation pulses, a control unit connected to the pulse generator for controlling the pulse emissions, a hemodynamic sensor for controlling the pulse emission as a function of the cardiac pumping performance, the hemodynamic sensor including; two measuring electrodes located in or in electrical contact with a blood vessel, for measuring the blood impedance dependent on the blood throughput, a signal generator that generates an AC voltage for generating a test signal for impedance measurement, an electrical measuring device connected to the twvo measuring electrodes for determining the blood throughput as a function of the measured electrical signal; and a blood throughput signal-analysis unit for recognizing an arrhythmic cardiac finction disturbance using the measured throughput quantity.