A cardiovascular device (11), adapted to be fitted into a cardiac ventricular cavity (2) having a volume with blood flowing therethrough, which is bounded by walls (8) and has a larger longitudinal dimension (D1) and a smaller transverse dimension (D2). The device includes a diaphragm (16) that can be disposed in the ventricular cavity (2) substantially transverse to the larger longitudinal dimension (D1), in such an arrangement as to reduce ventricular volume, the diaphragm (16) having a peripheral edge (15) which can be sealingly engaged with the ventricle walls (8) and being adapted to be alternately driven between an active blood pushing displacement and an inactive return displacement.