An implantable device for improving the pump function of the heart of a human patient by applying an external force on the heart muscle. The device comprises at least one pump device comprising a heart contacting organ, an operating device for operating the heart contacting organ, and an implantable pressurized fluid system. The fluid system comprises a first implantable chamber adapted to hold a pressurized fluid, wherein said first chamber is adapted to hold a fluid having a high pressure and a second implantable chamber adapted to hold a pressurized fluid, wherein said second chamber is adapted to hold a fluid having a lower pressure. The movement of the heart contacting organ assists the pump function of the heart.