A heart stimulator having an electric input that is connected or can be connected to a plurality of electrodes is disclosed. The heat stimulator comprises a stimulation control unit, one or more stimulation units, an impedance determination unit and an impedance evaluation unit. The one or more stimulation units are configured to generate stimulation pulses and to deliver a stimulation pulse when triggered by said stimulation control unit. The stimulation control unit is operatively connected to one or more stimulation units to control pacing parameters such as pacing site and/or the timing of stimulation pulses to be delivered by said one or more stimulation units. The impedance determination unit is configured to determine impedance values reflecting intracardiac impedance, and the impedance evaluation unit is configured to evaluate the impedance values and to provide an evaluated impedance signal to the stimulation control unit. The impedance determination unit is configured to determine a plurality of impedance values between different pairs of electrodes that are connected to the electric input during operation of the heart stimulator. The impedance evaluation unit is configured to generate a resultant impedance signal via a weighted sum of impedance values between different pairs of electrodes connected to said electric input during operation of the heart stimulator or the geometric combination of said impedance signals. The stimulation control unit is configured to determine respective pacing parameters such as pacing sites and/or timing of stimulation pulses depending on the resultant impedance signal.
