Valve Replacements

· Ball and Cage - The caged ball design is one of the early mechanical heart valves that use a small ball that is held in place by a welded metal cage. The ball in cage design was modeled after ball valves used in industry to limit the flow of fluids to a single direction. Natural heart valves allow blood to flow straight through the center of the valve. This property is known as central flow, which keeps the amount of work done by the heart to a minimum. With non-central flow, the heart must work harder to compensate for the momentum lost to the change of direction of the fluid. Caged-ball valves completely block central flow, therefore the blood requires more energy to flow around the central ball. In addition, the ball is notorious for causing damage to blood cells due to collisions. Damaged blood cells release blood clotting ingredients; hence the patients are required to take lifelong prescriptions of anticoagulants.
· Sinlge Leaflet - tilting disc to better mimic the natural patterns of blood flow. The tilting-disc valves have a polymer disc held in place by two welded struts. The disc floats between the two struts in such a way, as to close when the blood begins to travel backward and then reopens when blood begins to travel forward again. The tilting-disc valves are vastly superior to the ball-cage design. The titling-disc valves open at an angle of 60° and close shut completely at a rate of 70 times/minute. This tilting pattern provides improved central flow while still preventing backflow. The tilting-disc valves reduce mechanical damage to blood cells. This improved flow pattern reduced blood clotting and infection. However, the only problem with this design is its tendency for the outlet struts to fracture as a result of fatigue from the repeated ramming of the struts by the disc.
· BiLeaflet - two semicircular leaflets that pivot on hinges. The carbon leaflets exhibit high strength and excellent biocompatibility. The leaflets swing open completely, parallel to the direction of the blood flow. They do not close completely, which allows some backflow. Since backflow is one of the properties of defective valves, the bileaflet valves are still not ideal valves. The bileaflet valve constitutes the majority of modern valve designs. These valves are distinguished mainly for providing the closest approximation to central flow achieved in a natural heart valve.
· Animal Tissue Valves (porcine/bovine) - Both the porcine and bovine pericardial valves are stented valves. The metal stent in these valves takes up room which could be available for blood flow. Stentless valves are made by removing the entire aortic root and adjacent aorta as a block, usually from a pig. The coronary arteries are tied off, and the entire section is trimmed and then implanted into the patient. The St. Jude Toronto Stentless Porcine Valve (SPV) is one such valve. It appears to have excellent hemodynamics, and a significant decrease in the thickness of the heart has been observed after the valve is implanted. However, the valve is extremely difficult to implant, and it is still too new to have any valid data accounting for durability. The most common cause of bioprosthesis failure is stiffening of the tissue due to the build up calcium. Calcification can cause a restriction of blood flow through the valve (stenosis) or cause tears in the valve leaflets. Since younger patients have a greater calcium metabolism, bioprostheses tend to last best in senior citizens.
· Homograft - a valve that is transplanted from a deceased person to a recipient. A recipient has minimal problems with valve rejection and they do not require immunosuppressive therapy. A homograft that has been donated must be cryopreserved in liquid nitrogen until it is needed. In cases where the valve implants fit the dimensions of the patient correctly, homografts tend to have good hemodynamics and good durability. However, it is not clear whether homografts have better hemodynamics or durability than animal tissue valves.
· Autograft – The dysfunctional aortic valve is removed and the patient's pulmonic valve is then transplanted to the aortic position. A homograft pulmonic valve is usually used to replace the patient’s pulmonic valve. The Ross procedure allows the patient the advantage of receiving a living valve in the aortic position.. The tissues of the patients’ pulmonary valve have not shown a tendency to calcify, degenerate, perforate, or develop leakage.

I think the Pulmonary Autograft is the most effective type of valve replacement. It seems to last the longest with the least complications. The long term survival and freedom from complications for patients with aortic valve disease are better with the Ross Procedure than any other type of valve replacement. After 20 years, only 15% of patients require additional valve procedures. In cases where a human pulmonary artery homograft is used to replace the patients’ pulmonary valve, freedom from failure has been 94% after 5 years time, and 83% at 20 years. This procedure proves at the moment to be very successful.

http://cape.uwaterloo.ca/che100projects/heart/files/testing.htm#mech1