Biological replacement heart valves: Identification and evaluation
Introduction
Valvular heart disease is a worldwide problem. While most valvular disease is of inflammatory etiology in developing countries, in the developed world, most valvular heart disease today is degenerative. Regardless of the cause, valvular heart disease ultimately ends up as stenosis or incompetence and leads to progressive cardiac changes and secondary involvement of other organs in the body.
Most patients with significant valvular heart disease need valve replacement. Annually, approximately 60,000 patients in the United States undergo heart valve replacement, while, worldwide, the number is over 250,000. Patients who undergo valve replacement lead a better lifestyle than those managed medically. In spite of significant progress in the development of prosthetic heart valves, prosthesis-related problems continue. It is therefore critical for pathologists and other medical staff to be able to recognize prostheses and their associated problems for further progress to be made in the development and improvement of prosthetic heart valves. Worldwide, over 55% of implanted prosthetic heart valves are mechanical valves and about 45% are biological. In many countries, the proportion of biological prostheses implanted is increasing fairly rapidly. At the same time, the repair of native heart valves is also increasing. In a previous article in this journal (Butany et al. Cardiovasc Pathol 12(1)), we have discussed the identification and evaluation of mechanical heart valve prostheses. In this article, we discuss the identification of replacement heart valves (having biological tissue components) and some of their salient features. Biological replacement heart valves may be divided into bioprosthetic valves, which usually have aldehyde-treated porcine aortic valve tissue or bovine pericardial tissue mounted on a fabric-covered stent and tissue heart valves, which are homograft aortic valves or autografts (the hosts own, e.g., pulmonary valve autografts).
In spite of the progress made, contemporary heart valve substitutes still do not meet all the criteria for an optimal valve, as enunciated over 50 years ago by Dwight Harken et al. [1].
All prosthetic heart valves (including mechanical valves) have a fabric-covered sewing cuff that surrounds the base of the prosthesis, which provides attachment site to suture the valve (mechanical or biological) into the native valvular annulus, from which the native valve has usually been removed. Bioprosthetic heart valves or biological tissue valves generally imitate the flow and materials properties, that is, the design, of the native counterpart. This is a much closer match than mechanical valves. The most essential component of the bioprosthesis is made of biological tissues, usually porcine aortic valves or a three-cusp valve made of bovine pericardium. A popular though somewhat limited (availability) type of valve is the tissue or homograft valve, which is usually a cadaveric aortic or pulmonary valve harvested and prepared for use as a replacement valve. Today, homografts are often cryopreserved after previous preparation and are then made available for use as replacement valves.
The majority of bioprosthetic valves continue to be made out of xenografts and treated with glutaraldehyde. Valves made from these chemically treated tissues are mounted on a frame or stent made of metal or plastic, covered with synthetic fabric. Like native heart valves, bioprosthetic heart valves have central flow and good hemodynamics. The biological surfaces usually have good thrombo-resistance in comparison to mechanical heart valves, and, hence, patients having a single bioprosthetic heart valve need not be maintained on anticoagulant treatment, with all its attendant problems.
Today, the most commonly used porcine bioprostheses are those that have been treated with glutaraldehyde of varying strengths (0.2% for the Hancock and 0.6% for the Carpentier–Edwards (CE)). The porcine right coronary cusp (RCC) has a muscle shelf at its base, which tends to promote early mineralization and tissue degeneration. For this reason, a few contemporary prostheses have this shelf excluded from the orifice or the right porcine coronary cusp is completely replaced with a left or noncoronary cusp from another porcine valve. These valves are commonly referred to as “composite valves”. The last 25 years or so gave rise to the development of a bioprosthesis made of glutaraldehyde-treated bovine pericardium. These bioprostheses have better hemodynamics over stented porcine valves, and several designs were brought into the market. The first generation of these bioprostheses (the Ionescu–Shiley valve) unfortunately suffered from design-related problems and failed fairly rapidly due to cusp tears. Many of these were withdrawn from the market in the mid-1980s. The second generation of pericardial bioprostheses (the CE PERIMOUNT pericardial bioprosthesis) has demonstrated equivalent hemodynamics to stentless valves and has fared considerably better with good long-term durability for over 17 years. Unfortunately though, it too tends to fail, even if at later intervals. The modes of failure are somewhat different and are predominantly related to mineralization of cusp tissue.
Due to their design, stented porcine bioprostheses (like mechanical heart valve prostheses) have a sewing ring that is associated with the formation of a residual gradient across the prostheses, so that the cure suffers from the same problem that the illness had. To try and overcome this, a new generation of porcine bioprostheses was developed. These are the stentless porcine bioprostheses. One of the first amongst these was the Toronto-Stentless Porcine Valve (T-SPV) developed by Dr. Tirone David at the Toronto General Hospital [2]. These stentless bioprostheses have minimal cloth covering or no fabric cover and a very minimal, if any, residual gradient. While there is no long-term follow-up available yet, some of the stentless valves have now been in place for over 10 years, and the results so far are very encouraging. These valves have very low to zero residual gradients, good to excellent hemodynamics and are associated with regression of left ventricular hypertrophy [3].
Amongst the newer and somewhat more complex surgical procedures used today is the use of the autograft pulmonary valves to replace diseased aortic valves, especially in young individuals. The pulmonary valve, in turn, is usually replaced with a homograft. The overall results of this procedure are so far excellent.
This paper reviews biological replacement heart valves, discusses their characteristic features and features that help in their identification and goes into a brief discussion of the potential complications associated with them.
We have discussed as many of the currently available bioprostheses as we could get details for. The prostheses are presented alphabetically (by manufacturer) and by type of prosthesis. A few of the bioprostheses discussed are not currently in use, but they were implanted in significant numbers so that they continue to be explanted and continue to be seen on the surgical pathology bench.
Section snippets
The Angell–Shiley (AS) xenograft valve
Model: AS Xenograft Valve.
Type: Stented porcine tissue valve.
Technical information: In 1970, Dr. William Angell and associates began experimenting with glutaraldehyde-treated xenografts. Five years later, Shiley Laboratories in conjunction with Dr. Angell developed a new xenograft: the AS stented porcine bioprosthesis [4].
Range and available dimensions: Available with annular diameters of 23–34 mm [5].
Physical characteristics: The AS xenograft is made from glutaraldehyde-treated porcine tissue.
The bovine pericardial CE PERIMOUNT valve
Models: CE PERIMOUNT Models 2700 and 2800 (aortic) and PERIMOUNT Plns Model 6900P (mitral) valves (Fig. 4A–D).
Type: Stented bovine pericardial valve.
Technical information: Manufactured and sold by Edwards Life Sciences LLC. These pericardial valves are designed for both mitral and aortic implantation.
Size range and available dimensions: Aortic: 19–29 mm; mitral: 25–33 mm.
Physical characteristics: Central flow. Neutralogic stress-free fixation with glutaraldehyde [46]. XenoLogiX is used to
The CryoLife stentless porcine bioprosthesis
Models: (A) CryoLife–O'Brien (formerly the Bravo Cardiovascular Valve Model 300 and the O'Brien–Angell) (Fig. 8). (B) CryoLife–Ross.
Types: (A) Supraannular aortic. (B) Pulmonary.
Technical information: Manufactured by CryoLife International (Kenesaw, GA). The CryoLife–O'Brien stentless porcine aortic valve was introduced to the European heart valve market in 1991 and the CryoLife–Ross pulmonary porcine valve was introduced in 1998. CryoLife acquired the rights to the O'Brien and the Ross valves
The CryoLife homograft/allograft valve
Models: Cryopreserved human heart valves are distributed in the USA under the names (A) CryoValve and (B) CryoValve SG.
Type: The CryoValve (Fig. 13A) and CryoValve SG (Fig. 13B) aortic and pulmonary allografts are cryopreserved and come with or without conduit.
Technical information: The CryoValve SG undergoes a gentle enzymatic treatment to reduce the cellularity of the tissue while leaving the extracellular matrix intact. Clinical studies indicate a reduced immune response and the potential to
Conclusion
In this paper, we have taken data from a large number of sources, synthesized it and provided the practicing pathologists with a practical guide to prosthetic heart valves. For each valve, we have given some detail regarding its appearance, structure, components and a few of the major morphological lesions seen in failed/failing cases. We have also offered the pathologist a diagnostic tree Table 1, Table 2. We hope that this will be of help to all in identifying prosthetic heart valves.
The
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