When faced with a patient who needs a valve replacement, cardiologists and cardiac surgeons have a wide variety of valves from a number of manufacturers to select from. The decision as to which prosthesis should be implanted depends on a number of factors including the patient's age, gender (women of child-bearing age), life expectancy, comorbidities, location and size of the diseased valve, surgical expertise or preference, and considerations regarding anticoagulation (noncompliance or likely loss to follow-up).2 A detailed description of the process of valve selection is beyond the scope of this text and is discussed elsewhere.1
All valves can be classified as mechanical or biologic depending on the predominant material of composition. While mechanical valves are further classified according to the occlusion device, biologic valves are subclassified according to the presence of synthetic support structures (stented or stentless) and by the origin of the valve tissue (xenografts, homografts, or autografts).2–4
Mechanical valves are available in a variety of sizes and designs. All mechanical valves are comprised of a sewing ring (used to secure the valve in the patient) and an occluding device (designed to provide one-way flow through the prosthesis). The design of the occluding device is used to further subclassify this type of prosthesis into one of three major categories: (1) ball-in-cage, (2) tilting disk, and (3) bileaflet.
The first successful prosthetic heart valve implantation in the early 1960s was of the ball-in-cage design. The Starr-Edwards valve was the most widely used, with more than 200,000 prostheses placed since its introduction into clinical practice in 1965.3,5 Although it has undergone several modifications, the basic design remains unchanged, with a silastic ball constrained within a sewing ring comprised of Teflon or polypropylene cloth and two stellite alloy U-shaped arches that form a cage (Figure 11–1). The ball travels forward into the cage during antegrade flow, with the blood passing between the sewing ring and under the trailing edge of the ball between the struts. The occluder then moves back to seat snugly against the sewing ring when pressures equalize between the two chambers. There is no regurgitant flow during elevation of pressures in the receiving chamber.
Starr-Edwards ball-in-cage valve. (Reproduced with permission from Edwards LifeSciences.)
Their bulky design and flow characteristics (the smaller the valve, the more obstructive in nature the valve becomes) limit their use to the mitral position. They are also not typically implanted in individuals with small left ventricular cavities or with small aortic annular diameters. Although durable and therefore still seen in patients presenting for repeat surgery, these valves are no longer implanted, having been supplanted by valves with a better hemodynamic profile.
Representatives of this design include the Medtronic-Hall (Medtronic, Minneapolis, MN), OmniScience (MedicalCV Inc., Inver Grove Heights, MN), and the Bjork-Shiley valves (Figure 11–2). Although each of these valves has its own distinctive design features, a typical tilting disk valve is comprised of a circular sewing ring and an eccentrically hinged or pivoting disk occluder. The pyrolytic carbon disk opens to show two unequal (major and minor) orifices during antegrade blood flow. With reversal of pressures, retrograde flow against the larger leading portion of the disk results in closure by rotation on its hinge or pivot.
Single tilting disk designs of mechanical Medtronic-Hall prostheses. (Reproduced with permission from Medtronic, Inc.)
The Medtronic-Hall valve, first used clinically in 1977, is the most commonly implanted tilting disk valve and is second only to the St. Jude bileaflet valve as the most frequently implanted mechanical prosthesis. The pyrolytic carbon disk has a small central orifice through which the eccentrically placed hinge mechanism passes. The disk and struts are enclosed within a titanium housing that is attached to the native valve annulus with a Teflon sewing ring. The maximum opening angle of an aortic Medtronic-Hall valve is 75°, whereas a mitral prosthesis generally opens no more than 70°.3,6 The fact that the occluder disk opens less than 90° generates resistance to forward flow and small eddies of stagnant flow proximally that predispose to thrombus development. The small hole in the occluder mechanism results in a characteristic central regurgitant jet that is visible with CFD.
The OmniScience valve also consists of a pyrolytic carbon disk suspended within a titanium housing; however, the sewing ring is comprised of polyester knit rather than of Teflon. Unlike the Medtronic-Hall valve, the OmniScience prosthesis does not have a central hinge; rather, the motion of the disk is restricted by a series of struts. Because there is no central hinge and prerequisite hole, the OmniScience valve does not have a central regurgitant jet when in the closed position. The maximum opening angle of this design is approximately 80°.2
The Bjork-Shiley prosthesis, the first successful low-profile tilting disk design (1969), is no longer available in the United States because of problems with strut fracture. However, more than 360,000 valves of this type were implanted, so one occasionally may encounter a patient with this design in situ. The disk of this design has a convex-concave design, with a maximum opening angle of 70°.2
The major representatives of the bileaflet design are the St. Jude (St. Jude Medical, St. Paul, MN), the Carbomedics (Sulzer Carbomedics, Austin, TX) and the On-X (On-X LTI, Austin, TX) prostheses (Figure 11–3). The St. Jude bileaflet prosthesis is the most widely used mechanical valve in the world, with more than 600,000 implantations since its introduction in 1977. Each of these prostheses has two semicircular pyrolytic carbon occluders attached by small midline hinges to a support ring (pyrolytic carbon ring in St. Jude) and a Dacron sewing cuff. The manner in which the two occluder disks are hinged means they require no supporting struts, and therefore have an extremely favorable flow profile with lower transvalvular gradients as compared with the ball-in-cage and single tilting disk designs at similar annular diameters.3 With antegrade flow, both leaflets open to a maximum angle of 85°, resulting in two large, semicircular lateral orifices and a much smaller central rectangular orifice. With sufficient back pressure, the leaflets rotate on their hinges to close with an angle of approximately 25° to the plane of the supporting ring.2
Bileaflet tilting disk design prosthetic valves. A: St. Jude Medical. B: Sulzer Carbomedics. (Reproduced with permission from St. Jude Medical and Sulzer Carbomedics.)
The Carbomedics prosthesis is similar to the St. Jude in design; however, it has an adjustable titanium housing that can be rotated to position the leaflets in such a manner that they avoid contact with subvalvular tissue. The On-X valve is a newer valve made of pure pyrolitic carbon with a longer flow channel and an inlet that is flared outward to improve the flow dynamics through the valve. In addition, two leaflet guards are present to limit pannus encroachment onto the leaflets.7
Biologic tissue valves may be stented or stentless, depending on the presence of synthetic support structures. They may also be composed of either porcine or bovine leaflet tissue (xenografts), human cadaveric tissue (homografts), or native human tissue (autografts, as in the Ross procedure). Some biologic valves contain a mix of synthetic and natural biological materials (heterografts). Frequently, xenografts or homografts, especially for the aortic position, will include surrounding aortic tissue for improved natural structural support (composite grafts). The principal advantage of biologic over mechanical valves is related to the need for anticoagulation: mechanical valves require long-term anticoagulation, whereas biologic valves require only a short period (8 to 12 weeks) during endothelialization of the sewing ring.
Stented Biologic Prostheses
Stented biologic valves combine synthetic structural and supporting elements, with leaflets comprised of porcine valve leaflets (Hancock [Medtronic, Minneapolis, MN] and Carpentier-Edwards [Edwards Lifesciences, Irvine, CA] valves) or shaped pericardial tissue (Ionescu-Shiley [discontinued] and Carpentier-Edwards Perimount [Edwards Lifesciences, Irvine, CA] valves; Figure 11–4). The biologic elements of these valves are treated with glutaraldehyde to reduce antigenicity and increase tissue strength. However, this same process renders the tissue less pliable than native human valves and can promote calcification and degeneration in the long term.
Stented bioprostheses. A: Hancock valve (porcine; Medtronic). B: Carpentier-Edwards Perimount pericardial valve (bovine; Baxter Healthcare). C: St. Jude Epic (porcine; St. Jude Medical). (Reproduced with permission from Medtronic, Inc; Baxter Healthcare; and St. Jude Medical.)
The porcine valve leaflets in the Carpentier-Edwards prosthesis are mounted on a flexible Elgiloy frame, with stents manufactured from a single piece of wire attached to a Dacron sewing ring. The individual leaflets are mounted above the sewing ring, which allows for a larger orifice area and an improved hemodynamic profile as compared with valves with a similar ring size. However, the addition of the supporting struts (stents) does reduce the effective orifice area (EOA) as compared with a native human valve.8 The Hancock valve has many similarities to the Carpentier-Edwards valve on gross visual inspection. Nevertheless, each has a distinctive radiographic shape, with the Carpentier-Edwards valve appearing as a crown and the Hancock valve appearing as a circular ring because its stents are constructed from polypropylene, which is not radiopaque. During antegrade flow, the valve leaflets assume a somewhat irregular cone shape secondary to the slight restriction in opening caused by the valve stents. With pressure reversal, the leaflets coapt and commonly show a small central regurgitant jet, even in a normally functioning biologic valve.
Pericardial biologic valves (Ionescu-Shiley and Carpentier-Edwards Perimount) are not restricted by the physiologic size of the donor valve leaflets, thus allowing for the construction of larger valves. The Ionescu-Shiley valve was a low-profile prosthesis with three bovine pericardial cusps mounted to a Dacron-covered titanium frame using retention sutures. This prosthesis had difficulties with leaflet dehiscence and was discontinued after 10 years. The Perimount valve by Carpentier-Edwards has its pericardial leaflets mounted within the stent to maximize leaflet opening and to reduce abrasion between the leaflet and the stent.8
Stentless Biologic Prostheses
The development of stentless bioprostheses progressed with the desire to improve hemodynamics and long-term durability and preserve the benefits afforded by a tissue valve. The removal of the stent and sewing cuff permit the implantation of a relatively larger valve as compared with a stented biologic or mechanical valve of the same circumference. In addition, removal of the stent appears to significantly reduce stress at the base of the leaflets, which reduces calcification and slows valve degeneration. These valves are used only in the aortic position in which the patient's annulus and aortic root provide support and flexibility to the implanted materials. The typical stentless valve is comprised of an intact porcine aortic valve with an outer layer of polyester fabric to lend support and to facilitate implantation (Figure 11–5). Prior to the addition of the fabric, the valves are processed at very low fixation pressures to maintain collagen pliability. In addition, the Medtronic Freestyle valve is treated with amino-oleic acid to decrease calcium deposition. Several stentless valves are currently available, including the Toronto SPV (St. Jude Medical, St. Paul, MN), Medtronic Freestyle (Medtronic, Minneapolis, MN), and the CryoLife-O'Brien (CryoLife, Kennesaw, GA). These valves are technically more challenging to implant, and the exact surgical technique depends on how much of the recipient's aortic root and native sinus tissue are used in the process (full root technique with reimplantation of the coronary arteries, the root inclusion technique with preservation of the native coronary arteries, or a complete subcoronary or modified subcoronary technique).
Stentless bioprostheses. A: Toronto SPV valve (St. Jude Medical). B: Medtronic Freestyle valve (Medtronic). (Reproduced with permission from St. Jude Medical and Medtronic, Inc.)
The operative mortality rate for the stentless valves is slightly higher than that for other biologic valves (3% to 6%) partly due to the increased complexity of surgical technique. However, stentless valves have a very favorable 12-month complication rate (low rates of endocarditis, thrombosis, and hemorrhage) and survival rate of 91 ± 4% at 6 years. Currently, the engineered durability has held true, with no reported structural failures.9
Homografts and autografts
Homografts (human cadaveric tissue) are collected from the aortic and pulmonic positions within 24 hours of donor death and are treated with antibiotics before sterilization and preservation in liquid nitrogen at –196°C. These valves provide excellent hemodynamics, have low rates of thrombogenesis, are relatively resistant to infection, and are therefore ideal for use in the setting of acute native or prosthetic endocarditis.10,11 However, these cryopreserved valves are subject to accelerated degeneration as compared with native and mechanical valves. In addition, they require ongoing cryopreservation, have limited availability (most institutions carry a limited number of sizes, if at all), and require additional surgical time if the valve is not sized and thawed until the annulus is directly measured after arrest of the heart rather than relying on TEE valve sizing.12 The implantation of a homograft requires a highly skilled surgeon. Mitral homografts have been attempted in the past, but are no longer common.
Pulmonary autograft for aortic valve replacement (Ross procedure) uses the patient's native pulmonic valve and proximal main pulmonary artery to replace the diseased aortic valve and ascending aorta, with reimplantation of the native coronary arteries into the neo-ascending aorta. A stentless homograft (or other bioprosthesis) is usually placed in the pulmonic position so that anticoagulation is not required in the long term. Autografts are used most commonly in children, adolescents, adults with a longer than 20-year life expectancy, and those individuals in whom long-term anticoagulation is contraindicated or unwanted because of lifestyle factors.2,3 These valves have excellent hemodynamic profiles and are extremely durable with the “life expectancy” of a native valve. Limitations to its use include a technically more complex operation with the necessary replacement of two heart valves and the degeneration and/or obstruction of the new “pulmonic” valve and proximal pulmonary artery necessitating reoperation.
Transcatheter Aortic Valves
Endovascular transcatheter aortic valve implantation (TAVI) has recently become an option for those patients with calcific aortic stenosis who were previously deemed to be too high risk for a conventional valve replacement. The combination of advanced age with multiple comorbidities including renal, pulmonary, and cerebrovascular dysfunction increases the operative risk to over 25%.13 Following the first successful human implant in 2002, several different techniques have been developed (antegrade–transapical, and retrograde–transarterial) to more safely position and deploy the expandable bovine pericardial valves.
Currently there are two types of TAVI valves. The balloon expandable bovine pericardial “Sapien” valve (Figure 11–6) (Edwards Lifesciences, Irvine, California) is mounted onto a stainless steel stent and comes in two sizes (23 and 26 mm). The 23-mm valve is preferred for annular diameters of 18 to 21 mm, and the larger 26-mm valve for larger annular diameters up to 24 mm. The CoreValve (Medtronic, Minneapolis, MN) is also manufactured using bovine pericardium, but it is mounted on a self expanding Nitinol stent and is available in two sizes: a 26-mm valve for an annulus diameter of 20 to 23 mm, and a 29-mm valve for a 23- to 27-mm annulus. The valve chosen must be larger than the annulus diameter to reduce the risk of paravalvular leaks while allowing adequate anchoring of the valve system.
Sapien Transcatheter Aortic Valve Replacement bioprostheses (Edwards Lifesciences). (Reproduced with permission from Edwards Lifesciences Inc.)
Proper positioning of these valves is critical. Incorrect positioning can lead to embolization into the left ventricular cavity or anywhere along the aorta, paravalvular leaks, coronary ostial obstruction, or interference with the mitral valve (native or prosthetic). Both fluoroscopy and TEE are utilized in many centers, with TEE becoming the preferred imaging modality because of its versatility. TEE allows for a full assessment of ventricular function, a complete evaluation of all valves, sizing of the annulus and other structures, measurement of gradients, and quantification of trans- or paravalvular regurgitant jets. Echocardiography can also be utilized to direct the advancement of guidewires and delivery devices prior to and during the placement and delivery of the valve.14