Artificial heart valve

Heart valves are flaps of tissue within the heart that open to allow blood toflow from one of the heart's four chambers to the next and then close to prevent any blood from leaking back. When any one of the heart's four valves becomes too diseased or damaged to function adequately, the only effective treatment is valve replacement. This was not possible until the advent of open-heart surgery in the 1950s. Researchers then set out to design a valve that could be easily implanted and tolerated by surrounding tissue, would not promoteclot formation, and would be durable. A precursor of the artificial heart valve was developed by American surgeon Charles A. Hufnagel (1916-), who inserted a tube-and-float device in a patient's descending aorta in 1952, toprevent aortic backflow. An artificial cardiac aortic valve (a caged-ball device) was implanted into a human being in March 1960 by Dwight Harken at PeterBent Brigham Hospital in Boston. This was followed shortly by a total artificial mitral valve replacement (a flexible-leaflet) performed by Nina Braunwald at the National Institutes of Health.

The first completely successful artificial heart valve, an invention that became, and remains, a standard in the field, was designed and implanted in a fifty-two-year-old man by surgeons Albert Starr (1926-) and M. L. Edwards in Portland, Oregon, in 1961. This Starr-Edwards valve consisted of a silicone-rubber ball enclosed in a stainless steel cage; today, the valve has a hollow metal ball, an alloy cage, and a Teflon base. Later in the 1960s, the tilting-disc valve was developed. Valve design was improved after the space program produced pyrolite carbon, a strong and durable new material. The St. Jude Medical pivoting bileaflet valve, consisting of two leaflets rotating within a ring, was introduced in 1976. Mechanical heart valves made the headlines in 1990when Shiley Laboratories recalled all its convexo-concave tilting-disk valves from the market because of the risk of strut fractures that could cause sudden cardiac failure; many patients with other types of Shiley valves wronglybelieved their implants were also likely to fail suddenly.

Because of the danger of blood clot formation, implantation of a mechanical heart valve requires that the patient take anticoagulation medication for life. Alternatives to artificial valves carry a much lower risk of blood clots and reduce the need for anticoagulation therapy. These include both human and animal valve transplants: Human tissue valves are either allografts (or homografts)--valves transplanted from a deceased person whose organshave been donated for transplantation, or autografts--in which the pulmonic valve is removed from the patient to replace the abnormal aortic valveand the pulmonic valve is then replaced with an allograft. Animal valve transplants are either porcine (pig) or bovine (cow) tissue, traditinoally held in place by a stent. In 1997, the United States Food and Drug Administration approved the "Stentless Toronto SPV Valve," a porcine tissue valve manufactured by St. Jude Medical, Inc, which is entirely supported by the patient's aorta. With no stent to take up space, a larger valve can be implanted, improving blood flow.

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