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Replacement Heart Valves

By comparison with the revolutionary changes in many other fields of medicine, the development of artificial heart valves has progressed rather gradually since its beginning in 1960. The main problems recognized with the first designs-thromboembolism in mechanical valves and structural deterioration in tissue valve were reduced to an acceptable level by 1965-1970, and have not been substantially reduced further in the ensuing 25 years. However, the past few years has seen a resurgence in the development cycle, and there are currently many new devices are under investigation


Heart valves can be classified into two broad categories according to the origin of their occluding mechanism: mechanical and biological. After the heart-lung machine made valve replacement a possibility in 1955, many replacement devices were attempted. Mechanical and biologic valves of all kinds were tried, including designs with one, two, three and four leaflets. The first valve replacements that led to long term survivors were mechanical cage ball valves used by the Harken in the aortic position and Starr in the mitral position both in 1960.
Once the early problems of valve fixation and durability were solved, the major force that drove mechanical valve development was the reduction of thromboembolic complications. One favorite design was the tilting disc valve of which several varieties exist. The original versions of the bileaflet valve did not endure, but this design was successfully implemented in 1977 on the basis of the transference of pyrolytic carbon technology from spacecraft to heart valves. Thus the mechanical valve designs that have prevailed until today are the ball valve, tilting disc valve, and the bileaflet valve (figure107). Current development of mechanical valves is concentrated in attempting to enhance the bileaflet vave design.

The first biological valves used successfully were transplants from human cadavers, called homografts or allographs pioneered by Ross and Barratt-Boyes in 1962. Successful use of autologous grafts was begun with the pulmonary autograph in 1967. The goal of these biologic valves was to reduce the complications associated with thromboembolism and the need for anticoagulation. Several homologous and heterologous materials were used to fabricate tissue valves but eventually abandoned. During the 1960s, a major advance was the use of glutaraldehyde preservation of porcine valves pioneered by Carpentier and coworkers. Glutaraldehyde-fixed valves currently in use are aortic porcine valves and, in resurgence, valves fabricated of bovine pericardium. Current developments in tissue valve technology include improved methods of fixation, calcification mitigation treatments, and stentless designs.

Valve Descriptions

A heart valve functions as a check valve, opening to permit forward blood flow and closing to prevent retrograde flow, about 40 million times per year. Heart valve prostheses consist of an orifice, through which blood flows, and an occluding mechanism that closes and opens the orifice. There are two fundamental approaches to valve design: mechanical prostheses with rigid manufactured occluders, and biological prostheses also called tissue valves with flexible leaflet occluders of animal origin. The latter category includes replacement valves of human origin .

Mechanical Valves

The type of a mechanical valve is designated by its occluder: a ball, a circular disk or two semi- circular leaflets (bileaflet). For ball (Fig.107a-upper left) and disk valves (fig.107b-upper right), the occluder is guided and retained by structural members call struts attached to the orifice. The combination of orifice and struts is referred to as the valve housing. For bileaflet valves (fig.107c-mid left) the leaflets are retained and guided by a hinge or pivot mechanism: projections of the leaflets fit into indentations or sockets in the housing, which serve to retain the leaflets and define their limits of travel.

Figure 107 click to enlarge

Caged Ball Valve

The first clinically successful heart valve was the Starr-Edwards caged ball valve introduced in 1960. For 5 years the valve underwent several slight design modifications resulting in the model currently used. The ball is a silicone rubber polymer impregnated with barium sulfate for radiopacity. The cobalt- chromium alloy struts are joined at the apex to form a cage (fig.107a-upper left). When the ball opens by moving to the end of its cage it creates a circular primary orifice and a ring shaped secondary orifice between the ball and the housing. In the aortic position there is a tertiary orifice between the equator of the ball and the aorta.

Figure 107f below is copied from the New England Journal of Medicine, May 22, 2008 edition, Image of the Week,

Starr–Edwards Heart Valves after 38 Years

This 67-year-old woman presented with dyspnea and severe tricuspid regurgitation. The mitral and aortic valves from 38 years earlier were functioning normally. Click here for video

Tilting Disk Valve

Tilting disk valves have separate projections into the orifice, either single arms or closed loops to retain and guide the disk-shaped occluder. Among the metals use for the housing are stainless steel and titanium. The disks are graphite with a coating of pyrolitic carbon.When the disk pivots to the open position, the primary orifice is separated into twoareas, called the major and minor orifices. The Bjork-Shiley valve was the first successful tilting valve. It became available in 1971 with a carbon-coated disk and both struts (inflow and outflow) welded to the chromium alloy orifice. The Convexo - Concave model introduced in 1979, had an integral inflow strut to eliminate the few inflow strut fractures that had occurred with previous models. Sorin manufactured a tilting disk valve patterned after the Shiley valve disk, but with both struts integral to the housing to avoid the possibility of strut fracture. It is currently available with a pyrolitic carbon-coated sewing ring and and entire housing.

The Medtronic Hall valve has a titanium housing machined from a solid cylinder and a thin carbon coated disk with flat parallel sides (fig.107b-upper right) . The disk opens to 75° in the aortic model and 70 degrees in in the mitral. The disc is retained and guided by an S-shaped guide strut that protrudes through a central hole in the disk. Four structural elements project perpendicularly from the annulus into to the orifice: a guide strut and three pivot struts (one inflow two outflow). The two inflow pivot struts from near the top (inflow) edge of the orifice toward each other; the disk seats on the on the flat triangular bottom surfaces of these struts. The Omniscience valve is a streamlined elegant looking valve. It has a curved pyrolitic Carbon disk with no indentations, a one- piece titanium cage, and a seamless polyester knit sewin ring. The disk opens to 80 degrees and closes at an angle of 12 degrees to the plane of the orifice. It has been in use since 1978 but underwent a design change in 1981 to 82 involving a significant modification of the sewing ring.

Bileaflet Valves

The currently available bileaflet valves vary with regard to several design features Although the features of the valves manufactured by St. Jude, Baxter, Carbomedics, Soren, ATS, and Medtronic are described here , clinical performance information is included only for the St. Jude valve, for which a large amount of long-term information is available. The two leaflets of a bileaflet valve swing apart during opening, resulting in three separate flow areas. the bileaflet valve housings are either solid pyrolytic carbon over a graphite ( titanium in the Sorin valve) substrate. All except the St. Jude and Sorin valves have stiffener rings to strengthen the housing, shield it from needles during implantation, and improve radiographic visualization (Sorin valve has a titanium substrate, rather than graphite, which may confer some benefit). All except St. Jude are rotatable after the plantation. Since its first implant in 1977, the St. Jude bileaflet valve has been used half a million times (fig.107c-mid left). Previous bileaflet valves were unsuccessful; but the results with this design, with pyrolitic carbon coated housing and leaflets, introduced a new generation of mechanical prostheses. The housing of the valve included two rounded tabs, called pivot guards, that project out from the inflow side. The inside surfaces of these tabs contain the butterfly shaped indentations that serve to retain the leaflets. The tabs containing the hinge sockets extend above the housing, whereas with the other bileaflet valves, the cavities in the housing containing the pivot recesses are located within the main body of the housing, approximately at the plane of the annulus. The leaflets open to 85° and swing through an arc of 55 to 60°, depending on valve size, from fully closed to fully open (travel arc).

The TEKNA valve (Baxter Edwards) originally called Duromedics introduced several modifications of the bileaflet configuration. The leaflets are curved and translate slightly in the direction of flow as they open. Unlike other by leaflet valves, the leaflets seat on a lip or shelf molded into the housing. This seating design may reduce regurgitation and the possibility for suture entrapment, but at the expense of an increase gradient and higher closing impact. In the aortic model, the leaflets open to 77°, with a travel arc of 62 degrees; these dimensions are in the mitral, 73° and 58° ,respectively. The original Duromedicson valve experienced some fractures of leaflets and housing; it was withdrawn from the market in 1988, reintroduced in 1990, Carbomedics, the company that made pyrolitic carbon components, in 1993 received the third marketing approval for a bileaflet valve in the United States. The carbomedics bileaflet valve has flat leaflets that open 78 to 80°, with the resultant travel arc of 53 to 55°; it has a carbon coated blood contacting surface on the sewing ring.

The Medtronic Parallel bileaflet valve is unique in that the leaflets open to the maximum possible angle, 90°, to the plane, of the housing with the travel arc of 50°. Unique design features include an active dual mode pivotl washing and a housing profile optimized for flow.

Biological Valves

Biological valves include as wide a variety as do mechanical valves.

1. An autograft is a valve that has been translocated within the same individual (e.g., the pulmonary valve in the aortic position).

2. An autologous tissue valve is a valve that has been fabricated from the patient's own nonvalvular tissue (e.g. pericardium).

3. A homograft valvet is one that has been transplanted from a donor of the same species (a donor's aortic or pulmonary valve into a recipient's aortic or pulmonary position).

4. A heterograft is one that has been transplanted from another species; it may be either an intact valve ( e.g., a porcine aortic valve, as in fig. 107d-mid right) or a valve fashioned from heterologous tissue (e.g., bovine pericardium as in fig. 107e-lower left). The first successful biological valves were homografts. The homograft valve is not a homogenous type of valve, but it has appeared in a range of subtypes according to many variable factors. Sterilization methods used include chemical (ethylene oxide, beta propiolactone), rradiation, and antibiotics, with antibiotics being favored today. Preservation for a short time (months) is accomplished with nutrient storage of 4°C but cryopreservation, which allows for indefinite storage, greatly increases the availability of homografts. Because of supply limitations with homografts, the most widely used valves are the partially manufactured heterograft valves. Bioprosthesis is a term Carpentier and Dubost introduced for a biological tissue that has been treated to render it nonviable. Glitaraldehyde is used for fixing and preserving prosthetic heart valves because of three important biological actions: it sterilizes the tissue, renders it bio acceptable by destroying antigenicity, and stabilizes the molecular cross- links between the collagen fibers to enhance durability.

Homograft, Autograft

The homograft valve is considered to be the preferred substitute for aortic valve replacement, especially for younger patient. It has excellent hemodynamics, no anticoagulant requirements, and low (or in some cases zero) thrombogenicity. The drawback is low availability and a more technical the demanding operation. The pulmonary autograph procedure consists of an autotransplant of the pulmonary valve to the aortic position.The pulmonary valve is then replaced by an aortic or pulmonary homograft. The pulmonary autograft is perhaps the best aortic valve substitute for younger patients, as there is potential for growth of the pulmonary valve in the aortic position ; but this operation involves a double valve replacement with the attendant early and late risks.

Autologous Percardial Valve

An innovative Valve concept has recently has been deloped and is being investigated. This is a new category of valve: an attempt to combine the reproducibility and the ease of insertion of the commercial stented heterograft valve and the benefits of autologous tissue. It is a frame-mounted autologous pericardial valve, which is assembled from a kit in the surgical theater. The kit consist of the tools to create the valve in a matter of minutes: a cookie- cutter- type tool for obtaining the correctly shaped piece of pericardium, a frame that snaps together around the tissue, and holder to precisely align two pieces for assembly.

Porcine Heterograft Valves (Stented)

Most heterograft valves are mounted on rigid or flexible stents, to which are attached the leaflets and the sewing ring. Implantation involves fixing the sewing ring into place in (or above) the patient's annulus. The Hancock standard porcine valve (Medtronic, Inc.) was the first commercially available porcine valve. The stent is made up of flexible polypropylene cylinder with the radiopaque ring of cobalt-chromium alloy added for rigidity (fig.107d-mid right). The Hancock modified orifice was designed to overcome the undesirable hemodynamics caused by the muscular shelf of the porcine right coronary cusp by replacing that leaflet with one of the other two leaflets from another valve. The Hancock II valve incorporates second generation features such as low- pressure fixation, calcification retardant treatment and a thinner stent. The MO II valve is a modified Orifice valve with a modified scalloped sewing ring. The Medtronic's intact valve is distinctive in that the calcification retardant treatment colors it blue. It is fixed in zero pressure leading the leaflets thinner and more flexible. Medtronic has recently introduced the Mosaicm valve, which incorporates features from both the intact valve and the Handcock II valve.

The Carpentier-Edwards standard porcine valve became available shortly after the Hancock valve and has been widely used. the frame of the valve is a flexible wire stent intended to reduce stresses on the leaflets and orifice yet retain its original contour over time. A one piece, cylindrical, flexible Mylar support sur rounds a flexible wire frame. The annulus is asymmetrical rather than circular to incorporate the muscular septal ridge of the porcine right coronary cusp. In the Carpentier-Edwards Supra Annular Valve, The mounting structure of the aortic valve has been redesigned for the positioning above rather than within the annulus.The fixation treatment and the stent have been modified in an attempt to improve leaflet durability. The sewing ring was reconfigured to increase the effective orifice of the valve. The St. Jude BioImplant porcine valve is available internationally. Clinical investigation has begun on the X-Cell porcine valve, developed in conjunction with St. Jude Medical and Hancock -Jaffe laboratories. The innovative design features of this valve include an extraction process to selectively remove calcification sites from the porcine tissue, sterilization bygamma irradiation treatmento reduce leaflet stiffness and a clothless stent. It also features zero-pressure fixation and, in smaller (2.5mm) sizes, a composite leaflet arrangement.

Porcine Heterograft Valves (Unstented)

The homograft is considered to have properties superior to those of the heterograft with regard to hemodynamics and thromboembolism. In an attempt to incorporate some of the advantages of a homograft into an easily available commercial product several manufacturers have recently begun clinical testing of stentless porcine valves. This potential benefit is achieved at the expense of a more difficult implant technique. As with homografts there are potentially three ways of implanting a stentless porcine valve :
(1) as a replacement for the aortic root with reimplantation of the coronary arteries;
(2) as a miniroot replacement ,where the leaflets remain attached to the donor aortic wall, which is inserted within the host aorta; and
(3) as a valve- only replacement,where the sides of the donor aorta are scalloped and the valve is sewn freehand so free into the the subcoronary position in the host's aorta.

Bovine Pericardial Heterograft Valves

Pericardial valves are assembled using biological tissues as a fabric, rather than being harvested directly, as are porcine valves. The theoretical advantage include more symmetrical and complete opening for optimal hemodynamics, the opportunity to allow extra tissue for eventual shrinkage, any higher intrinsic percentage of collagen than in porcine valves. Since it is the collagen that is cross- linked during fixation with glutaraldehyde, a stronger and more durable tissue should result. The Ionescu-Shiley was was the first commercially available pericardial valve, but it had an unacceptable rate of structural failure and was taken off the market. The Carpentier- Edward Pericardial Bioprosthesis received FDA approval in 1991 and has become quite well accepted. It uses a sophisticated method of mounting the leaflets to the stent which does not depend on stitches passing through the the leaflets (fig.107e-lower left). The leaflets are secured behind the stent pillar by a plastic plug, which serves as an anchor to prevent them from being pulled through the opening in the wire frame. An international model has a modified sewing ring, which is reinforced and more cone -shaped.


(1) Structural deterioration refers to any change in valve function resulting from an intrinsic abnormality causing stenosis or regurgitation.

(2) Nonstructural dysfunction: any abnormality resulting in stenosis or regurgitation that is not intrinsic to the valve itself. This includes inappropriate sizing, also called prosthesis- patient mismatch.

(3) Thromboembolism includes any valve thrombosis or embolus except those secondary to infection or hemorrhage. This includes any neurologic deficit and any peripheral arterial emboli unless proved to have resulted from another cause. Patients who do not awaken postoperatively or who awaken with a stroke or myocardial infarction are excluded. Valve thrombosis is listed as a subcategory of thromboembolism.

(4) Anticoagulant-related hemorrhage includes any episode of internal or external bleeding ( in patients taking anticoagulants or antiplatelets) that is fatal, causes a stroke, or serious enough to require hospitalization or transfusion.

(5) The diagnosis of prosthetic vave endocarditis is based on clinical criteria, including an appropriate combination of positive blood cultures and clinical signs or histologic confirmation at reoperation or autopsy. Morbidity associated with active infection, such as thromboembolism or paravalvular leak is included in this category only.

The Comparative Clinical Performance

The choice between valve types involves a tradeoff between an increased risk of thromboembolism-thrombosis-bleeding complex for mechanical valves versus the structural deterioration of tissue valves.

Structural Deterioration of Biological Valves

Structural valve deterioration with tissue valves is not a constant risk event but increases with time.Thus linearized rates are not appropriate and actuarial methods must be used to describe and compare them.

Porcine Valves

Stented porcine prostheses represent by far the most commonly used biological valves. Freedom from structural deterioration at ten years ranges from about 60 to 90 percent for aortic position and from about 60 to 80 percent from the mitral position.

Pericardial Valves

After an initial unsatisfactory experience to with the Ionescu-Shiley valve, which did not terribly but was not equal to the standards of other contemporary bioprostheses, the Carpentier-Edwards valve seems to have rescued the concept of bovine pericardium as an acceptable alternative for valve fabrication. There has now been over twelve years experience with it, it has received FDA approval for marketing in the aortic position, and the results to date have been encouraging.


Thromboembolism rates for homograft valves are considered quite low or even zero by some investigators. For structural deterioration, homografts should be evaluated according to the various methods of sterilization and preservation. It appears that the series that used a chemical or irradiation process for sterilization have the highest rates of failure, about 40 percent structural valve deterioration free at ten years. Those sterilize by antibiotic whether stored in nutrient solution or cryopreserved,are more durable with about 75 percent structural valve deterioration free at ten years.

Pulmonary Autograph

The pulmonary autograft used as an the aortic valve replacement is considered a "permanent" valve and is especially appropriate for young patients. Excellent results have also been reported in treating endocarditisin. Freedom from replacement for all reasons including endocarditis has been reported to be as high as 85 percent at twenty years but has also been found to be 48.5% at 19 years.

Choice of Valve

The embolic-thrombosis-bleeding complex with mechanical valves and structural failure with biological valves serve to distinguish between the two valve types. But, judging from the wide variation in reported results with each model of valve, patient-specific factors must influence the results more than valve-specific ones, and it is impossible to rank valves, within valve types, on the basis of complication rates. However, some general recommendations can be made with regard to valve selection. Though not covered in this review, valve repair, when practical, should be considered preferable to replacement, especially in the mitral position, but also the aorctic positions. When replacement is necessary an argument can be made for a particular class of valve under certain circumstances. A biological valve should be used when the patient cannot or will not take anticoagulants, desires pregnancy, or has a short life expectancy. A mechanical valve should be used if the patient will be on anticoagulants anyway (because of a trial fibrillation or mechanical valve in another position), is in renal failure or on dialysis, or has a long life expectancy. Mechanical valves should also be considered first for double valve replacement, because the thromboembolic risk is not an additive with two valves but the risk of structural deterioration is additive.

Future Developments

The current trend in mechanical valves is toward further development of bileaflet valve principle enhancing the very successful St. Jude valve design. New directions include the search for better hemodynamics, for example parallel leaflets and lower thrombogenicity Better anticoagulant management has the potential to reduce bleeding complications and also to reduce thromboembolic events. If markers of thrombogenicity can be identified and measured preoperatively, tissue valves can be preferentially used in high risk patients and mechanical valves and low risk patients, even an older ages. The primary advantage of biological valves is a reduced need for anticoagulation, but the offsetting disadvantage is poorer durability. The search for im prve durability will define the newer generation of biological valves. A major factoris low or zero pressure fixation, which allows the leaflets to be fixed in the neutral position and to retain more of the natural flexibility. Anticalcification treatments may improve durability and the elimination of the stent should provide improved with hemodynamics.

Reference:Grunkemeier,G.L. and others,Hurst's The Heart Update I,1996,Pp.98-123.