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Abstract

 

    The heart is a vital part of the human anatomy because it functions as a pump to circulate blood throughout the body. Heart valves allow the heart to pump blood to specific locations efficiently.  These valves are prone to disease and malfunction, and can be replaced by prosthetic heart valves. The two main types of prosthetic heart valves are mechanical and bioprosthetic.  The mechanical  valves are excellent in terms of durability, but are hindered by their tendency to coagulate the blood.  Bioprosthetic valves are less durable and must be replaced periodically.  All valve types must be durable, because the body is an extremely hostile environment for a foreign object, including prosthetic heart valves.  Today, chemical engineers are researching new designs of prostheticheart valves.  Many engineers believe the future lies within the regime of tissue engineering.  

 

Introduction

 

The Heart

 

    The heart consists of four chambers: the right atrium, the right ventricle, the left atrium, and the left ventricle. It's function is to pump oxygen-rich blood to the arteries where the blood can flow to the cells of the body to provide them with oxygen. The deoxygenated blood from the cells is circulated back to the heart to regain the oxygen that was lost. The deoxygenated blood from the body enters the right atrium, and once this chamber fills with blood, the atrium contracts, forcing the blood down through the tricuspid valve into right ventricle. Next, the ventricle contracts, pushing the blood to the lungs through the pulmonary valve to receive oxygen. The oxygen-rich blood returns to the left atrium of the heart and then it travels to the left ventricle through the mitral valve. From the left ventricle, the blood travels through the aortic valve to the large blood vessel called the aorta. The aorta then distributes blood to the rest of the body.

 

 

 

Heart Valves

 

   Heart valves are very important, as they prevent the backflow of blood, which ensures the proper direction of blood flow through the circulatory system. Without these valves, the heart would have to work much harder to push blood into adjacent chambers. The heart is composed of 4 valves:tricuspid, pulmonary, mitral, and aortic.

 

Heart Valve Problems

 

   There are numerous complications and diseases of the heart valves that prevent the proper flow of blood. Heart valve diseases fall into two categories, Stenosis and Incompetence. The stenotic heart valve prevents the valve from opening fully, due to stiffened valve tissue. Hence, there is more work required to push blood through the valve. Whereas, the incompetent valves cause inefficient blood circulation by  permitting backflow of blood in the heart.

 

 

 

Treatment Options

 

    On a large scale, medication is the best alternative, although in some cases defective valves have to be replaced with a prosthetic valve in order for the patient to live a normal life.  An enormous amount of research and development has proven to be most beneficial, as prosthetic heart valve technology has saved hundreds of thousands of lives. Engineers and scientists have done much work to design a valve that can withstand millions, if not billions, of cardiac cycles.

 

    The two main prosthetic valve designs include mechanical and bioprosthetic(tissue) heart valves, some of which are shown below.

 

 

 

 

 

MECHANICAL HEART VALVES

 

Evolution of Mechanical Heart 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.

 

    The caged ball design is one of the early mechanical heart valves, that uses 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.

 

    For a decade and a half, the caged ball valve remained the best design. 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 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.

 

    In 1979, a new mechanical heart valve was introduced. These valves 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. 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.

 

Materials

 

    Current research has been able to produce materials that do not cause clotting in the blood stream. However, they have yet to design an entire valve that will not induce coagulation.

 

    Most commonly used materials include:
    - stainless steel alloys
    - molybdenum alloys
    - pyrolitic carbon for the valve housings and leaflets
    - silicone, teflon®
    - polyester (Dacron®) for sewing rings

 

    A new generation of mechanical valves made of materials with improved blood contact properties, better wear characteristics and resistance to infection are under development.

 

Advantages

 

    The main advantages of mechanical valves are their high durability. Mechanical heart valvesare placed in young patients because they typically last for the lifetime of the patient.

 

Disadvantages

 

    The main problem with all mechanical valves is the increased risk of blood clotting. When blood clots of any kind occur in the heart, there is a high probability of a heart attack or stroke. As a result, to prevent blood clots, mechanical valve recipients must take anti-coagulant drugs (sodium warfarin) chronically, which effectively makes them borderline hemophiliacs. The anti-coagulant used causes birth defects in the first trimester of fetal development, rendering mechanical valves unsuitable for women of child-bearing age. Mechanical valves are suitable for people who do not want additional valve replacement surgery in the future.

 

The Future of Mechanical Heart Valves

 

    The new age tools that are being used to improve mechanical valve design include accelerated wear testing, advanced blood contact property testing, computer assisted design and manufacturing, coatings to reduce the chance of infection and improve healing and advanced polymer chemistry to develop the next generation of medical materials.

 

 

 

PROSTHETIC TISSUE VALVES

 

    Prosthetic tissue valves can be broken into two groups: human tissue valves, and animal tissue valves. Both types are often referred to as bioprosthetic valves, which hold many advantages over mechanical valves. The design of bioprosthetic valves are closer to the design of the natural valve.  Bioprosthetic valves do not require long-term anticoagulats, have better hemodynamics, do not cause damage to blood cells, and do not suffer from many of the structural problems experienced by the mechanical heart valves. 

 

Human Tissue Valves

 

    Human tissue valves fall into two categories: Homografts, which are valves that are transplanted from another human being, and Autografts, which are valves that are transplanted from one position to another within the same person.

 

    A homograft is 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.

 

    Autografts are valves taken from the same patient that they are implanted into. The most common autograft procedure is the Ross procedure, which is used in patients with diseased aortic valves. 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 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. The tissues of the patients' pulmonary valve have not shown a tendency to calcify, degenerate, perforate, or develop leakage.

 

 

 

    The Ross procedure requires a high level of technical skill on the part of the surgeon.  The pulmonic valve and the pulmonary homograft must be sculpted to fit the aortic root. Many patients have small amounts of aortic regurgitation, which in some cases is severe enough to merit a second operation for valve replacement. Other possible complications could include stenosis, right-sided endocarditis, as well as the usual complications of valve replacement.

 

Animal Tissue Valves

 

    Animal tissue valves are often referred to as heterograft or xenograft valves. These valves are most often heart tissues recovered from animals at the time of commercial meat processing. The leaflet valve tissue of the animals is inspected, and the highest quality leaflet tissues are then preserved. They are then stiffened by a tanning solution, most often glutaraldehyde. The most commonly used animal tissues are: porcine, which is valve tissue from a pig, and bovine pericardial tissue, which is from a cow.

 

    In Porcine valves, the valve tissue is sewn to a metal wire stent, often made from a cobalt-nickel alloy.   The wire is bent to form three U-shaped prongs. A Dacron cloth sewing skirt is attached to the base of the wire stent, and then the stents themselves are also covered with cloth. Porcine valves have good durability and usually last for ten to fifteen years.

 

    Bovine pericardial valves are similar to porcine valves in design. The major difference is the location of the small metal cylinder which joins the ends of the wire stents together. In the case of pericardial valves, the metal cylinder is located in the middle of one of the stent post loops. Pericardial valves have excellent hemodynamics and have durability equal to that of standard porcine valves after 10 years.

 

    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. Once a bioprosthesis is implanted, the valve itself does not require any type of anti-coagulant drugs. Its degeneration is simply a gradual process, as it grows with the body.

 

    The future for replacement heart valves lies in tissue engineering. The most ideal replacement would be formed from the patient's tissue, and tailored to the right shape and dimensions. Researchers have transplanted specifically tailored valves into sheep. The valves are made by growing tissue from the artery of a lamb on a matrix of the correct dimensions in an artificial culture medium.

 

Conclusion

 

   When one of the valves of the heart becomes infected with a disease, it can be replaced with one of several different types of prosthetic valves.  These prosthetic valves must create a non-return flow system and must meet certain standards with regard to inertia, strength, elasticity, and electrochemical properties.  The replacement valve must also be durable, as the human body is a harsh place for foreign objects, which can corrode or break down.

 

    Of the different prosthetic valve types, the ball and cage mechanical valve was first used.  Other mechanical valve types include a tilting disc model and a bileaflet valve.  Mechanical heart valves make satisfactory valves replacements, but they all encounter similar problems.  Flow pattern disruptions can lead to thrombus formation, a fact that requires valve recipients to take anticoagulants on a long term basis.  Also, mechanical stresses can damage blood cells and bacterial infections can lead to further damage.

 

    Bioprosthetic tissue valves do not encounter as many problems as mechanical valves. Tissue valves can be made from human or animal tissue. Valves of human tissue are classified as homografts, which are valves transplanted from another human being or autografts, which are valves transplanted from one position to another within a patient.  This is most often done using the Ross Procedure. Prosthetic valves made of animal tissue are often referred to as heterografts or xenografts and can be made of porcine (pig) tissue or bovine peacardial (cow) tissue.