small pumps, often implantable, designed for temporarily assisting the heart, usually the left ventricle, to pump blood; they consist of a pumping chamber and a power source, which may be partially or totally external to the body and activated by electromagnetic motors; the devices are used after myocardial infarction or to wean the repaired heart from the heart-lung machine after open-heart surgery; do not confuse with the heart lung machine, for this use CIRCULATORY ASSIST and HEART LUNG BYPASS.