A cardiac stimulator having at least one stimulation unit which is connected or connectable to one or more stimulation electrodes, and is configured to deliver at least sub-threshold stimulation pulses for cardiac contraction modulation therapy, an impedance detection unit which is connectable to one or more electrodes, and is configured to detect a voltage or current intensity that occurs as the result of a particular sub-threshold stimulation pulse, and to determine a particular impedance value, an impedance evaluation unit, configured to determine at least one value based on ventricular volume, and/or a value based on minute ventilation, and a control unit connected to the stimulation unit and the cardiac rhythm detection unit, and is configured to control a delivery of a stimulation pulse via the stimulation unit such that the cardiac stimulator can deliver sub-threshold stimulation pulses for cardiac contraction modulation therapy.