<P> Patients with leaky aortic valves (aortic insufficiency) often tolerate even severe degrees of insufficiency for a relatively long time before symptoms develop . Surgery is indicated for symptoms such as shortness of breath, and in cases where the heart has begun to enlarge (dilate) from pumping the increased volume of blood that leaks back through the valve . </P> <P> There are two basic types of artificial heart valve: mechanical valves and tissue valves . </P> <P> Tissue heart valves are usually made from animal tissue, either animal heart valve tissue or animal pericardial tissue . The tissue is treated to prevent rejection and calcification . </P> <P> There are alternatives to animal tissue valves . In some cases a homograft - a human aortic valve--can be implanted . Homograft valves are donated by patients and recovered after the patient dies . The durability of homograft valves is comparable to porcine and bovine tissue valves . Another procedure for aortic valve replacement is the Ross procedure (or pulmonary autograft). In a Ross procedure, the aortic valve is removed and replaced with the patient's own pulmonary valve . A pulmonary homograft (pulmonary valve taken from a cadaver) is then used to replace the patient's own pulmonary valve . This procedure was first used in 1967 and is used primarily in children, as the procedure allows the patient's own pulmonary valve (now in the aortic position) to grow with the child . </P>

During valve replacement surgery a valve that comes from an animal is known as