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EDITORIAL |
| Department of Cardiothoracic and Vascular Surgery German Heart Institute Berlin, Germany |
This college must play its part in keeping alive the historic tradition of the medical profession, and must ever foster those high standards of professional behavior which distinguish a profession from a trade.– Winston S Churchill
It is a great privilege to be invited to write this editorial on the occasion of the 40th anniversary of clinical homograft valve surgery. Aware of my comparatively limited contributions to the homograft literature, I am engulfed by feelings of humility and gratitude to be able to express my views on an issue of great concern — clinical practice and the science of homograft surgery.
First, I would like to seize this great opportunity to honor 2 pioneer surgeons and teachers: Mr. Donald Ross in London and Sir Brian Barratt-Boyes in Auckland, New Zealand, for their achievements and contributions in homograft valve surgery. In June and July 1962, they independently pioneered the first successful replacement of a diseased aortic valve with a homograft aortic valve1,2 following experimental work by Duran and Gunning in Oxford. The procedure was revolutionary in the history of cardiac valve surgery in the last millennium because postoperative anticoagulation was not needed and there were no inherent thromboembolic complications, thus giving patients a better quality of life. I invite cardiac surgeons across the globe to join me in congratulating them on their achievements.
The homograft aortic or pulmonary valve is, by virtue of its natural properties, a valve of choice for reconstructing the aortic or pulmonary outflow tract in complex congenital or acquired valve lesions. It is easy to handle, is nonthrombogenic and thus requires no postoperative anticoagulation, and restores the anatomic units of the aortic or pulmonary outflow tract and thus offers an unimpeded blood flow and excellent hemodynamics. It has gained popularity in the management of acute endocarditis with root abscesses because of its resistance to infection and is in particular a valve of choice in regions with low economic and health standards, especially for women of childbearing age. Its superiority over prosthetic valves has been demonstrated and proven in the 25- and 29-year clinical follow-up studies of, respectively, Lund and colleagues3 and O'Brien and colleagues.4
It is tempting at a time like this to consider other problems surrounding the application of homograft valves and arteries: regulatory issues and clinical and laboratory research, including tissue engineering. Among many other distinguished homograft veterans and experts across the globe are Dr. Bodnar and Dr. Gonzalez-Lavin, Mr. Parker on the team of Mr. Ross in London, and Dr. Armiger, a co-worker of Sir Brian in Auckland, who have all contributed substantially to the clinical and laboratory research of homograft valve surgery.5,6
I spent the greater part of my resident years in pursuit of knowledge on a perfect homograft valve for clinical implantation. Much of this effort was directed towards learning to apply the technologically advanced, but con-ceptually simple, methods of evaluating the preservation techniques of homografts by defining the impact on viable cellular components and immune response, as well as evaluating the different implantation techniques in terms of the clinical performance and functional durability of the homografts. In 1981, 21 years ago, I had the opportunity to establish the first hospital-based homograft bank in Germany at the university surgical clinic in Kiel to start our homograft program.7 At the initial stages of our experimental work, I had an unexpected visit in summer 1984 by Mr. McNally of Cryolife in Atlanta, now the largest homograft bank in the world, whose mission and interest were to find the truth about immune response following homograft implantation. We have an ongoing exchange of ideas and knowledge, just as I have been doing with some national and international homograft bankers and implanting surgeons. The understanding on the use of homografts, from the bench to clinical practice, is probably universal for most homograft surgeons — especially of the young generation — homograft bankers, and scientists. What we have accepted as essentially critical and important to homograft recipients is the implanting surgeon's scientific and clinical knowledge as well as quality control. The surgeon has the medical and ethical responsibility to use a homograft valve professionally to treat a patient with a valvular disease.
Lerich put it simply and effectively: "The great problem of surgery is a problem of knowledge." Allan Callow in Boston complemented this by saying that "better science, which means better data, is making better medicine possible." One needs to recall that homograft surgery was introduced with enthusiasm in the 1960s,1,2 practiced for a short time, and then abandoned because of early structural deterioration related to preservation and decontamination techniques. The issue of homograft viability, raised by O'Brien and Angell,5 has persisted until today. The difficulties with decontamination and preservation have been largely overcome, resulting in the revival of homograft valve surgery with ensuing better clinical outcome. However, no evidence-based standard storage temperature and storage time have been proposed.
To provide perspective and to emphasize the "state of the art," a brief review of current clinical practice is warranted.
It is now recognized that the clinical results and durability of aortic and pulmonary homografts depend not only on tissue viability,5 recipient and donor age,8 and immune response,8 but also on valve sizing and the implantation technique.3,4,9 Quality assessment, sizing, and implantation techniques constitute the technical artistry of a homograft operation.
Comparison of geometrically matched and mismatched homografts with the recipient aortic outflow tract has been neglected in univariate and multivariate analyses for determining early nonstructural and late structural deterioration.3,4 In O'Brien's4 follow-up of patients with homograft aortic valves, the estimated freedom from reoperation for all causes was 76% and 50% at 15 and 20 years, respectively. Structural deterioration at 15 years was 47% in patients under 20 years of age and 81% in patients between 41 and 60 years. In Yankah's series,9 which compared undersized and oversized homografts, freedom from homograft explantation at 15 years was 48% for undersized valves and 92% for oversized valves. Undersizing of homografts is recognized as a determinant risk factor for early reoperation for various causes, such as paravalvular leak, pseudoaneurysm, central leak, and cusp rupture. Proper matching of a homograft to the recipient's root with either equal size or a 2-mm size difference without annular distortion will provide over 50% leaflet coaptation and excellent long-term clinical performance and durability. Since a cryopreserved homograft root has 15% to 20% distensibility, a homograft valve annulus with an internal diameter 3 mm less than that of the recipient annulus is defined as undersized.9
Freehand subcoronary aortic valve implantation with either scalloping of all 3 sinuses (Barratt-Boyes) or with retention of the noncoronary sinus (Ross) produces comparable short- and long-term clinical results to those of the root replacement technique if the homograft is properly sized and properly inserted. Freedom from reoperation at 15 years was 80% for the root replacement technique and 72% for subcoronary implantation.9
The choice of a full aortic root replacement or the miniroot technique with reimplantation of the coronary arteries for treating various types of aortic root pathology depends on the surgeon's experience with a particular technique.3–5,9 A full root replacement is associated with a low incidence of valve dysfunction and of early reoperation, whereas aortic root tailoring to accommodate a scalloped homograft in a subcoronary implantation appears to carry a high risk of valve failure and early reoperation.
In the learning phase of the homograft root replacement and freehand subcoronary implantation techniques, the rate of reoperation for technical, nonstructural valve failure will obviously be relatively high. The technical errors are associated with a geometric mismatch between the anatomic units of the aortic root,9 resulting in a para-valvular leak, central leak, cusp rupture, leaflet prolapse or distortion, commissural displacement, or aortic root dilatation from progressive dilatation (annuloectasia, Marfan's syndrome). Annulus reinforcement with Teflon strips or glutaraldehyde-fixed equine pericardial strips, as suggested by Ross5 to prevent progressive annular dilatation in patients with annuloectasia, has been practiced by many surgeons with good clinical results.
Undersized homografts suffer a progressive loss of leaflet extensibility owing to structural deterioration within a critical period of 5 years, which is peculiar to both implantation techniques.9 The learning phase therefore requires tutorial guidance on the selection and quality assessment of homografts and correct sizing for implan-tation to minimize early postoperative valve dysfunction and reoperation.
Aortic annulus patch enlargement using the Konno-Rastan aortoventriculoplasty technique with a synthetic material, an equine pericardium, or the Manougian technique, whereby the aortic incision is extended to the anterior mitral leaflet and the anterior mitral leaflet of the homograft is used to enlarge the aortic annulus in preparation for the replacement of the small aortic root with an oversized homograft (3 mm larger), poses no risk of postoperative valve incompetence following previous tutorial guidance. Clarke and Bishop in Denver and Daenen in Leuven reported excellent long-term clinical results in their series.6
Reduction annuloplasty, trigonum-commissuroplasty, or tailoring of the aortic root requires meticulous skills to avoid distortion of the homograft root during freehand subcoronary implantation. An alternative and more secure technique is the aortic root replacement operation, which was primarily developed by Ross in 1972 for the enlarge-ment of the small tunnel-type aortic root.5
The use of an antibiotic-treated cryopreserved homograft to replace the infected aortic valve or root embedded in pus fulfills surgical principles for infection management.8 Because it is a natural tissue, it excludes the periannular abscesses and necrotic tissues of the aortic root from the blood stream and exudes the antibiotic in its tissue to disinfect the surrounding tissue, thus allowing the infection to resolve.
Several investigators — Lund and Yacoub in Harefield and London,3 Miller in Stanford,10 Haydock's team in Auckland,11 and Yankah's group in Berlin8 — have analyzed their clinical experiences and proved that the homograft aortic valve is resistant to infection and is thus a superior valve substitute of choice for managing native and prosthetic aortic valve endocarditis with abscesses. The operative mortality of homograft patients was estimated to be 8% to 14%. The incidence of early reinfection was low (2.7%) and was associated with perivalvular leak and pseudoaneurysm. Analysis of the results of the freehand subcoronary implantation technique revealed an independent risk factor for early and late reoperation. Freedom from infection at 13 years was 91%.9
A homograft valve has no potential to grow with the patient and is compounded by immune response in young patients.8 Therefore, if the patient is expected to outgrow the donor homograft and late transvalvular gradients are to be avoided, one should consider implanting an oversized homograft or use the pulmonary autograft in the Ross operation. The Ross operation is a double-valve procedure consisting of subcoronary aortic valve or aortic root replacement with a pulmonary autograft, and replacement of the pulmonary outflow tract with a homograft. It has gained popularity and acceptance among today's pediatric surgeons.6
The development of procedures to reconstruct the right ventricular outflow tract has been a major advancement in the treatment of congenital heart disease. However, reconstruction with porcine valved Dacron conduits is not ideal. In 1966, Ross and Somerville12 repaired a pulmonary atresia and in 1967 McGoon and associates13 repaired a truncus arteriosus with aortic homografts, thereby paving the way for homograft reconstructive surgery of the right ventricular outflow tract. Fontan and Baudet14 in 1971 revolutionized the repair of tricuspid atresia using an aortic homograft to connect the right atrium with the pulmonary artery, thereby circumventing the obstructed, desaturated venous blood flow to the pulmonary circulation. In the 1980s, the pulmonary homograft became the valved conduit of choice over the aortic homograft because it is anatomically and structurally adapted to the pulmonary circulation and therefore has a lower rate of structural deterioration. The determinant factors of long-term durability and performance of cryopreserved aortic and pulmonary homografts in the pulmonary circulation in short- and long-term studies in children have been revealed to be conduit size and age at operation.
Homann and colleagues15 in Munich demonstrated in their 25-year clinical follow-up study that children with > 18 mm homograft conduits in the right ventricular outflow tract had 78% freedom from reoperation compared to 45% in children with < 18 mm conduits. These findings are congruent with those reported earlier by the teams of Somerville, Monro, and Clarke.5
In 1953 and 1954, Robicsek demonstrated experimentally that implantation of a tricuspid homograft with its sophisticated structures and functions is feasible. However, the procedure gained little interest or popularity because of unsatisfactory results.6 In 1967, on the basis of animal work by Braunwald and Welden, Angell reported the first successful use of premounted aortic homografts for mitral and tricuspid valve replacement.5 Senning pioneered the use of an unstented mitral homograft for mitral and tricuspid valve replacement in 1966, while in 1969 Ross used a pulmonary autograft and Yacoub the top-hat aortic homograft to replace the mitral valve. The clinical results were episodic.16 Subsequently, the procedure was abandoned with the exception of the top-hat technique until the surgical techniques were redeveloped in the 1980s and 1990s by Bernhard in Kiel, Revuelta in Santander, Mestres and Pomar in Barcelona, Acar in Paris, and Kumar and Duran in Riyadh. The techniques included valve sizing, determination of graft length, suturing of papillary muscle anastomosis, stabilization of annular anastomosis, and intraoperative echocardiographic assessment of chordae–papillary alignment. The extensive clinical experience in mitral valve reconstructive surgery of Carpentier and Acar as well as Mestres and Pomar, supported by intensive laboratory work on simplification of the procedure, enabled them to perform this operation more successfully with 89% freedom from reoperation at 3 and 5 years in Acar's series.6
In 1988, Barratt-Boyes, Yacoub, and Angell regarded the stented homograft valve as technically more feasible to implant and also durable in the mitral position even in children.5 They had patients with stented homograft valves that had lasted for 10 to 15 years.
Lastly, the improved technique of preserving arterial homografts has now allowed and encouraged revival of the surgical concept of homograft arterial reconstruction pioneered by Dubost17 in Paris in 1952, which was associated with aneurysmal dilatation, calcification, and reoperation at 5 years. Mestres and Pomar in Barcelona, Vogt in Zurich, and Hetzer and Yankah in Berlin have been pursuing this concept with cryopreserved arterial conduits for treating infected synthetic grafts of both abdominal and thoracic aortae as well as mycotic thoracic aorta aneurysm.6 The arterial homograft requires only a short-term postoperative antibiotic therapy as opposed to the synthetic graft with its inherent risk of reinfection.
Currently, interest is growing in stentless xenografts in aortic valve replacement.18 It is interesting that the clinical results of homografts and stentless xenografts are nearly equivalent in spite of their use in different patient populations. Xenografts, being readily available and their supply virtually unlimited, allow for wider application in older patients and in selected younger patients where a homograft may not be available.
In conclusion, a good-quality cryopreserved homograft, when properly implanted with an adequate size to match the aortic or the pulmonary outflow of the recipient, will lead to regression of ventricular hypertrophy and will provide excellent hemodynamic and clinical performance. Technical errors during homograft implantation will not only cause early failure in some patients, but may also be a cause for later structural deterioration. A small-caliber (< 18 mm) homograft conduit in the pulmonary position carries the risk of early structural deterioration, especially the aortic homograft.
Proof of the durability of viable homografts came from the longer follow-up series of the teams of Lund3 and O'Brien,4 valve failure rates, and the morphology of long-term valves.
The involvement of professional homograft bankers in the marketing of homograft valves has been extremely useful in terms of efficient procurement and preservation, quality control, and distribution of homograft valves and arteries to customers (surgeons). Their services are invaluable to implanting surgeons, who in turn are obliged to give feedback of their clinical experiences and results to enable homograft bankers to improve their services. Legislation to regulate procurement, storage, quality control, and distribution of homografts will be useful to protect both the patients and the users. The use of undersized homografts should be avoided since stentless xenografts have proven to be complementary to homo-grafts in clinical practice.
Finally, research in tissue engineering of trileaflet valves needs to be intensified and encouraged with adequate funding because its success will solve the major problems encountered in young patients, who are immunologically active and have growth potential.
REFERENCES
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