Heart monitoring system includes implantable medical device and service center. Implantable medical device includes stimulation pulse generator, ventricular sensing stage, activity sensor, impedance determination unit with a constant current or voltage source to generate sub-threshold measuring current pulses having constant current strength or constant voltage, measuring unit for measuring a voltage corresponding to a current fed through a body, impedance value determination unit connected with measuring unit adapted to determine impedance value for each measuring current pulse, and perform intrathoracic impedance measurement, a control unit adapted to collect data representing values of changes, and initiate data transmission, implant transceiver unit communicating with the service center with a data evaluation module including data trending of stored data with a user interface and said data evaluation module adapted to allow a physician to set for each data trend trigger criteria for decompensation detection and generate a decompensation indicator signal.