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Bs Ngô Thị Cẩm Hoa Khoa Phẫu Thuật Tim- BV 115 CONTENTS 1. Types of heart valve prostheses 2. Mechanical valves 2.1 Types of mechanical heart valves 2.2 Durability 2.3 Blood damage 3. Biological valves 4. Selection of an artificial valve. 5. Antithrombotic therapy in MHV Types of heart valve prostheses There are two main types of artificial heart valves: - The mechanical valves. - The biological valves. Types of heart valve prostheses - Mechanical heart valves Ball and cage Tilting disk Bi-leaflet Tri-leaflet - Biological heart valves Allograft/ autograft Xenograft Mechanical valves - The first mechanical prosthetic heart valve was implanted in 1952. Over the years, 30 different mechanical designs have originated worldwide. - These valves have progressed from simple caged ball valves, to modern bileaflet valves. - Heart valves are designed to fit the peculiar requirements of blood flow through the specific chambers of the heart, with emphasis on producing more central flow and reducing blood clots. Mechanical valves Caged ball design Tilting-disc valves Bileaflet valves The caged ball valve - - It is one of the early mechanical heart valves. It uses a silicone ball that is held in place by a welded metal cage. When blood pressure in the chamber of the heart exceeds that of the pressure on the outside of the chamber the ball is pushed against the cage and allows blood to flow. At the completion of the heart's contraction, the pressure inside the chamber drops and is lower than beyond the valve, so the ball moves back against the base of the valve forming a seal. The caged ball valve Disadvantages Caged-ball valves completely block central flow. Therefore the blood requires more energy to flow around the central ball. The ball is notorious for causing damage to blood cells due to collisions. They have a high tendency to forming blood clots, so the patient must have a high degree of anti-coagulation, usually with a target INR of 2.5-3.5. Edwards Lifesciences discontinued production of the Starr-Edwards valve in 2007. The tilting-disc valves - - - In the mid-1960s, a new class of prosthetic valves were designed that used a tilting disc to better mimic the natural patterns of blood flow. The tilting-disc valves have a polymer disc (pyrolytic carbon disc) held in place by two welded struts (metal 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 titling-disc valves open at an angle of 60° and close shut completely at a rate of 70 times/minute. The tilting-disc valves Advantages - This tilting pattern provides improved central flow while still preventing backflow. - The tilting-disc valves reduce mechanical damage to blood cells. Disadvantages - 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. The Medtronic-Hall; Bjork-Shiley; Harken valve;… Bileaflet valves In 1979, a new mechanical heart valve were known as bileaflet valves and consisted of 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. Bileaflet valves Advantages -These valves are distinguished mainly for providing the closest approximation to central flow achieved in a natural heart valve. -These bileaflet valves have the advantage that they have a greater effective opening area (2.4-3.2 square cm c.f. 1.5-2.1 for the single-leaflet valves). Also, they are the least thrombogenic of the artificial valves. Disadvantages - 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. Durability - Mechanical heart valves are today very reliable and allow the patient to live a normal life. - Most mechanical valves last for at least 20 to 30 years Blood damage Clots formed by red blood cell (RBC) and platelet damage can block up blood vessels and occurs in one of three basic pathways: - Tissue factor exposure. - Platelet activation. - Contact activation by foreign materials. Blood damage All MHV models are vulnerable to thrombus formation due to: - High shear stress. - Stagnation. - Flow separation. Blood damage The caged-ball designs: Experience high stresses at the walls that can damage cells. Flow separation due to high-velocity reverse flow surrounded by stagnant flow. Tilting-disc valves: Flow separation behind the valve struts and disc as a result of a combination of high velocity and stagnant flows. The bileaflet models: - High stresses during forward and leakage flows as well as adjacent stagnant flow in the hinge area. The hinge area is the most critical part of bileaflets and is where the thrombus formation is usually prevalent. Biological valves There are two groups: - Human tissue valves (Homografts, Autografts) - Animal tissue valves (xenografts, pericardial sac): Biological valves Human tissue valves: Homografts: within 24h of donor death, sterilized with antibiotics and preserved at -196C. They are inserted directly, usually in the aortic position. Autografts: ROSS Procedure. Animal tissue valves: Porcine heterografts ( the Hancock valve, The CarpentierEdwards, The Medtronic Intact valve). Stentless Porcine Xenografts: St. Jude Medical valve, The Edwards stentless valve, the Medtronic Freestyle valve). Pericardial aortic valves: Bovine pericardial valves. Biological valves - They do not require long-term anticoagulats. - They have better hemodynamics. - They do not cause damage to blood cells. - They do not suffer from many of the structural problems experienced by the mechanical heart valves. Biological valves Selection of an artificial valve The following groups of patients should receive bioprosthetic valves : - Patients with coexisting disease who are prone to hemorrhage and who therefore tolerate antocoagulants poorly: bleeding disorders, instertinal polyposis, angiodysplasia - Noncompliant with permanent anticoagulants treatment, who are unwilling to take anticoagulants on a regular basis or who live in developing nations and cannot be monitored - Patients older than 65 years old in whom bioprosthetic valves deteriorate slowly and because of their age may also be at greater risk hemorrhage while taking anticoagulants - Patients with a small annulus in whom an unstented bioprosthetic graft may provide superior hemodynamics - Woman patients (< 40 years of age) Conclusion - There are two main types of artificial heart valves:the mechanical valves,the biological valves. - The most used heart valves in the US and EU are those utilizing tissue leaflets. Mechanical valves are more commonly used in Asia and Latin America. - The following companies manufacture tissue heart valves: Edwards Lifesciences, Medtronic, St. Jude, Sorin, ATS Medical, 3F, and CryoLife. - InThe Future, A new generation of mechanical valves made of materials with improved blood contact properties that do not cause clotting in the blood stream. better wear characteristics and resistance to infection are under development.