Abstract: This invention is an apparatus for modifying cardiac output of the heart of a subject, including one or more sensors which sense signals responsive to cardiac activity, and a stimulation probe including one or more stimulation electrodes which apply non-excitatory stimulation pulses to a cardiac muscle segment. Signal generation circuitry is coupled to the one or more sensors and the stimulation probe, which circuitry receives the signals from the one or more sensors and generates the non-excitatory stimulation pulses responsive to the signals.