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An artificial heart valve is a one-way valve implanted into a person's heart to replace a heart valve that is not functioning properly (valvular heart disease). Artificial heart valves can be separated into three broad classes: mechanical heart valves, bioprosthetic tissue valves and engineered tissue valves.
The human heart contains four valves: tricuspid valve, pulmonary valve, mitral valve and aortic valve. Their main purpose is to keep blood flowing in the proper direction through the heart, and from the heart into the major blood vessels connected to it (the pulmonary artery and the aorta). Heart valves can malfunction for a variety of reasons, which can impede the flow of blood through the valve (stenosis) and/or let blood flow backwards through the valve (regurgitation). Both processes put strain on the heart and may lead to serious problems, including heart failure. While some dysfunctional valves can be treated with drugs or repaired, others need to be replaced with an artificial valve.
There are many potential causes of heart valve damage, such as birth defects, age related changes, and effects from other disorders, such as rheumatic fever and infections causing endocarditis. High blood pressure and heart failure which can enlarge the heart and arteries, and scar tissue can form after a heart attack or injury.
The three main types of artificial heart valves are mechanical, biological (bioprosthetic/tissue), and tissue-engineered valves. In the US, UK and the European Union, the most common type of artificial heart valve is the bioprosthetic valve. Mechanical valves are more commonly used in Asia and Latin America. Companies that manufacture heart valves include Edwards Lifesciences, Medtronic, Abbott (St. Jude Medical), CryoLife, and LifeNet Health.
'Lowell' Edwards is recognized as the first to invent a truly successful heart valve. His design relied on a patented, caged-ball check valve. Edwards' design was surgically implanted by Albert Starr for the first time in 1960 and was successfully used to save heart patients around the world for the next 47 years. The design consisted of a silicone ball enclosed in a methyl metacrylate cage welded to a ring. Edward's invention is known today as the Starr-Edwards valve, which continues to provide life-saving service for many heart patients treated before 2007. The Star-Edwards valve set a record for providing a patient 48 years of service before requiring replacement. Mechanical heart valves, such as the Star-Edwards Valve, are strongly associated with blood clot formation and require a high dose of anticoagulation, usually with a target INR of 3.0–4.5. In 2007 the Starr-Edwards Valve was retired and replaced by Edwards Lifesciences with the Edwards Myxo ETlogix annuloplasty ring.
Many of the complications associated with mechanical heart valves can be explained through fluid mechanics. For example, blood clot formation is a side effect of high created by the design of the valves. From an engineering perspective, an ideal heart valve would produce minimal pressure drops, have small regurgitation volumes, minimize turbulence, reduce prevalence of high stresses, and not create flow separations in the vicinity of the valve.
Implanted mechanical valves can cause foreign body rejection. The blood may coagulate and eventually result in a hemostasis. The usage of anticoagulation drugs will be interminable to prevent thrombosis.
Alternatives to animal tissue valves are sometimes used, where valves are used from human donors, as in aortic and pulmonary . An aortic homograft is an aortic valve from a human donor, retrieved either after their death or from a heart that is removed to be replaced during a heart transplant. A pulmonary autograft, also known as the Ross procedure, is where the aortic valve is removed and replaced with the patient's own pulmonary valve (the valve between the right ventricle and the pulmonary artery). A pulmonary homograft (a pulmonary valve taken from a cadaver) is then used to replace the patient's own pulmonary valve. This procedure was first performed in 1967 and is used primarily in children, as it allows the patient's own pulmonary valve (now in the aortic position) to grow with the child.
In recent years, scientists have developed a new tissue preservation technology, with the aim of improving the durability of bioprosthetic valves. In sheep and rabbit studies, tissue preserved using this new technology had less calcification than control tissue. A valve containing this tissue is now marketed, but long-term durability data in patients are not yet available.
Current bioprosthetic valves lack longevity, and will calcify over time. When a valve calcifies, the valve cusps become stiff and thick and cannot close completely. Moreover, bioprosthetic valves can't grow with or adapt to the patient: if a child has bioprosthetic valves they will need to get the valves replaced several times to fit their physical growth.
Tissue engineered heart valves can be person-specific and 3D modeled to fit an individual recipient 3D printing is used because of its high accuracy and precision of dealing with different biomaterials. Cells that are used for tissue engineered heart valves are expected to secrete the extracellular matrix (ECM). Extracellular matrix provides support to maintain the shape of the valves and determines the cell activities. (1991). 9781461366805 ISBN 9781461366805
Scientists can follow the structure of heart valves to produce something that looks similar to them, but since tissue engineered valves lack the natural cellular basis, they either fail to perform their functions like natural heart valves, or function when they are implanted but gradually degrade over time. An ideal tissue engineered heart valve would be non-thrombogenic, biocompatible, durable, resistant to calcification, grow with the surrounding heart, and exhibit a physiological hemodynamic profile. To achieve these goals, the scaffold should be carefully chosen—there are three main candidates: decellularized ECM (xenografts or homografts), natural polymers, and synthetic polymers.
Mechanical valves can be a better choice for younger people and people at risk of valve deterioration due to its durability. It is also preferable for people who are already taking blood thinners and people who would be unlikely to tolerate another valve replacement operation.
Tissue valves are better for older age groups as another valve replacement operation may not be needed in their lifetime. Due to the risk of forming blood clots for mechanical valves and severe bleeding as a major side effect of taking blood-thinning medications, people who have a risk of blood bleeding and are not willing to take warfarin may also consider tissue valves. Other patients who may be more suitable for tissue valves are people who have other planned surgeries and unable to take blood-thinning medications. People who plan to become pregnant may also consider tissue valves as warfarin causes risks in pregnancy.
The performance of an artificial heart valve can be tested in vitro before clinical use by means of a pulse duplicator.
The most common problems with artificial heart valves are various forms of degeneration, including gross billowing of leaflets, ischemic mitral valve pathology, and minor chordal lengthening. The repairing process of the artificial heart valve regurgitation and stenosis usually requires an open-heart surgery, and a repair or partial replacement of regurgitant valves is usually preferred.
Researchers are investigating catheter-based surgery that allows repair of an artificial heart valve without large incisions.
Researchers are investigating Interchangeable Prosthetic Heart Valve that allows redo and fast-track repair of an artificial heart valve.
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