Implantable medical device with an impedance determination unit with constant current/voltage source having current feed terminals connected to electrodes for intracorporal placement which generates measuring current pulses having constant current/voltage, for causing a current through a body via intracorporally placed electrodes, a measuring unit for measuring voltage/current strength of voltage/current fed through body, an impedance value determination unit connected to the current/voltage source and adapted to determine an impedance value for each measuring current pulse, and an impedance measuring control and evaluation unit connected to the impedance determination unit which controls the unit and evaluates a sequence of consecutive impedance values, the impedance determination unit further adapted to determine at least intrathoracic and intracardiac impedance values for same period of time, the intrathoracic values sampled with a lower sampling rate than the intracardiac values.