An implantable medical device has an impedance or admittance determination unit, an alternating current or voltage source, a measuring unit, and an analysis unit which is connected to the alternating current or AC voltage source and the measuring unit to calculate an impedance value or an admittance value at different times. The impedance or admittance determination unit may generate measuring current having two different frequencies (preferably below 100 kHz), and the analysis unit may calculate pairs of impedance or admittance values which are chronologically assigned to one another for different frequencies of the measuring current, and calculate a value for a blood impedance or blood admittance component which is independent of the impedance of the body tissue surrounding a particular blood vessel, The analysis unit may also determine trends in this value over time as an indicator for a change of blood hematocrit value.