Heart valve implants

Premarket Approval applies to life-supporting or life-sustaining devices. Class III devices are those that support or sustain human life, are of substantial importance in preventing impairment of human health, or which present a potential, unreasonable risk of illness or injury. Those devices that if used improperly may cause unreasonable risk or illness or injury to a person. These devices must meet the standards set forth in Class I and Class II in addition to premarket approval of Class III. Examples include heart pacemakers, replacement heart valves, implanted spinal cord stimulators, silicone gel-filled breast implants. 

In Vitro and In Vivo Evaluation of Vascular and Heart Valve Implants... 

Bauer F,Eltchaninoff H andTron C (2004), Acute improvement in global and regional left ventricular systolic function after percutaneous heart valve implantation in patients with symptomatic aortic stenosis . Circulation, 110,1473-1476. 

Artificial heart valve (Charles Hufnagel) Hufnagel develops an artificial heart valve and performs the first heart-valve implantation surgery in a human patient the following year. 

It often happens tiiat only a part of the heart, such as the aortic valve, fails and needs replacement. Repair could be made by using foreign tissue (for example, a pig heart valve) or a mechanical heart valve implant to replace a diseased one. About 250,000 valve replacement procedures are performed annually worldwide. In the United States about 45% of the procedures involve a mechanical valve. The most widely used valve is shown in Figure 12.19 . It has two semicircular discs tiiat move to allow blood to flow m the desired direction as the heart pumps, then they fall back together to form a seal against backflow. 

Biomaterials have many cardiovascular applications (that is, pertaining to the heart, blood, and blood vessels). Heart valve implants are often mechanical devices. The presentation of a smooth surface is important, to reduce blood clotting and the loss of red blood cells. Vascular grafts are commonly constructed of Dacron a polyester material that integrates with surrounding tissues. Artifi-... 

Several years ago a biomedical company produced and marketed a new, efficient heart valve implant. It was later withdrawn from the market, however, because patients using it suffered from severe loss of red blood cells. Describe what properties of the valve could have been responsible for this result. 

Silicones have a wide variety of applications (Figure 22.27). Due to the strong Si—O and Si—C bonds, silicones are generally imreactive and can be synthesized to perform in temperature ranges of 75°F to 400°F. Silicone oils can be found in cosmetics, car wax, and hydrauhc fluids. The silicone elastomers are used for medical tubing, heart valve implants, electrical tape, caulk, and gaskets. Silicone resins are used to insulate electrical equipment and in the molding process of electronic circuit boards. These examples represent only a small portion of the uses for silicone compounds. Because of this extensive use, several hundred thousand tons of silicones are manufactured each year. 

Biomedical Applications. In the area of biomedical polymers and materials, two types of appHcations have been envisioned and explored. The first is the use of polyphosphazenes as bioinert materials for implantation in the body either as housing for medical devices or as stmctural materials for heart valves, artificial blood vessels, and catheters. A number of fluoroalkoxy-, aryloxy-, and arylamino-substituted polyphosphazenes have been tested by actual implantation ia rats and found to generate Httle tissue response (18). 

One successful total artificial heart is ABIOMED s electric TAH. This artificial heart consists of two seamless blood pumps which assume the roles of the natural heart s two ventricles (Fig. 7). The pumps and valves are fabricated from a polyurethane, Angioflex. Small enough to fit the majority of the adult population, the heart s principal components are implanted in the cavity left by the removal of the diseased natural heart. A modest sized battery pack carried by the patient suppHes power to the drive system. Miniaturized electronics control the artificial heart which mns as smoothly and quietly as the natural heart. Once implanted, the total artificial heart performs the critical function of pumping blood to the entire body (6). 

Hea.rt Va.Ives. Since the early 1960s nearly 50 different heart valves have been developed. The most commonly used valves as of the mid-1990s include mechanical prostheses and tissue valves. Nearly 75,000 of these prosthetic valves are implanted aimually worldwide, and about 30,000 in the United States alone. Caged-baH, caged disk, and til ting-disk heart valves are the types most widely used. 

Biomedical. Heart-valve parts are fabricated from pyrolytic carbon, which is compatible with living tissue. Such parts are produced by high temperature pyrolysis of gases such as methane. Other potential biomedical apphcations are dental implants and other prostheses where a seal between the implant and the living biological surface is essential. Plasma and arc-wire sprayed coatings are used on prosthetic devices, eg, hip implants, to achieve better bone/tissue attachments (see Prosthetic and BiOLffiDiCALdevices). 

The major biological application of isotropic carbon is in heart valves. The material is performing well and several hundred thousand units have been produced so far. Other applications include dental implants, ear prostheses, and as a coating for in-dwelling catheters. 

Subcutaneous in vivo testing of these polymers (13,14) has shown minimal tissue response—similar, in fact, to the response to poly-(tetrafluoroethylene). These materials are candidates for use in heart valves, heart pumps, blood vessel prostheses, or as coating materials for pacemakers or other implantable devices. 

Chemoprophylaxis has been extended to other surgieal proeedures where the risk of infection m be low but its oecurrence has serious eonsequences. This is especially frue for the implantation of prosthetic joint or heart valves. These are major surgical procedures and although infeetion be infiequent its eonsequences are serious and on balance the use of chemoprophylaxis is cost-effeetive. 

S. epidermidis is implicated in many medical implant infections. Mechanical heart valves, shunts, catheters and orthopedic devices are examples of implanted devices... 

Prosthetic-valve endocarditis Endocarditis that occurs in patients with bioprosthetic or synthetic implanted heart valves. 

Siloxane-containing devices have also been used as contact lenses, tracheostomy vents, tracheal stents, antireflux cuffs, extracorporeal dialysis, ureteral stents, tibial cups, synovial fluids, toe joints, testes penile prosthesis, gluteal pads, hip implants, pacemakers, intra-aortic balloon pumps, heart valves, eustachian tubes, wrist joints, ear frames, finger joints, and in the construction of brain membranes. Almost all the siloxane polymers are based on various polydimethylsiloxanes. 

There is a great need for strong materials such as alloys that can snap back into shape. Medical applications include prostheses— artificial limbs—and implanted devices such as heart valves. Most biological substances are smart, and the ability to replace lost or injured tissues and organs with smart materials would be a tremendous medical advance. 

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