Certain embodiments of the present invention disclose an implantable electrotherapy device comprising a housing in which are arranged an activity sensor, an impedance or conductivity measuring unit and an evaluation unit, wherein the evaluation unit is connected to the impedance or conductivity measuring unit and to the activity sensor and is adapted to evaluate the impedance or conductivity signal produced by the impedance or conductivity measuring unit and the respectively time-associated activity level signal and for producing and outputting a contractility signal, in such a way that the contractility signal is derived from the impedance or conductivity signal and the activity level signal and reflects the respective contractile state of a heart associated with an activity level signal value.