This paper deals with the control of a mechanical cardiac compression assist device. The device acts outside of the heart, squeezing it to pump the blood out. Unlike invasive assist devices like the left ventricle assist devices or the total artificial hearts, cardiac compression avoids direct contact with the patients' blood and thus blood damage and immune reactions are greatly reduced. Requirements of an assist device are discussed, potential controllers are briefly assessed and an impedance controller is selected. Variance of the stiffness parameter within the impedance filter is considered as a means to provide variable assistance to the heart.