Mitral Heart valve replacement surgery is conducted under general anesthesia and typically lasts about 3 to 5 hours. A large incision is made in the chest and the patient is placed on a heart-lung machine during the surgery. Blood is circulated outside of the body and oxygen is added to it using a heart-lung (cardiopulmonary bypass) machine. The damaged mitral valve is removed and replaced with a plastic or metal mechanical valve, or a bioprosthetic valve, which is usually made from pig tissue.