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EDITORIAL |
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Department of Cardiac Surgery Guy's Hospital & National Heart Hospitals London, UK |
Among other considerations, the initial sizing of the homograft is of importance and it should be remembered that the given size of a homograft means the internal diameter. If to this we add the thickness of the wall, then the effective external diameter of the implant will be 2 to 3 mm larger. This fact should be kept in mind. Also, a double-layer suture insertion is commonly used together with interrupted sutures at the lower margin and a continuous suture along the upper margin, ensuring that the commissures are elevated under some tension to prevent subsequent cusp prolapse. Finally, the noncoronary sinus is kept intact to maintain the spatial relationships between the adjacent commissures, which is an important feature in achieving competence. With these guidelines, it is certainly possible to achieve a competent subcoronary valve placement, whether it is a homograft, autograft, or xenograft but, because of the initial learning curve, many surgeons wishing to use a biological valve opted for the less satisfactory and bulky stented variety.2
The first free-standing aortic root was used in 1972 in a case of aortic root abscess, in which there was aorto-left ventricular discontinuity.3 The pus was evacuated, the ventricle and aorta were re-anastomosed to a homograft root and the coronaries were reimplanted. While there were still early learning problems and difficulties in inserting a subcoronary replacement, the concept of using a free-standing root replacement was increasingly appealing, since it almost guarantees a competent valve.4 This applies even in relatively inexperienced hands and, not surprisingly, the technique is used almost exclusively in the autograft operation.
Scanning the surgical journals over the years, it has been fascinating to read what is supposed to constitute a long-term valve follow-up: sometimes it is 3 years, sometimes 5, and others talk of long-term follow-up with two or three patients available for assessment at about 7 to 8 years in the case of many of the so-called stentless valves. The thing we have learned from our considerable experience with over 2,000 homograft valves, is that 7 years is the watershed which determines whether the valve will continue to function satisfactorily ("7-year itch"). Up to that point, all valves are excellent, so to make a reasonable assessment one needs a satisfactory cohort of patients at least 10 years and preferably 20 years post-implantation.
Homograft valves are collected from cadavers under 45 years of age, (preferably, nearer 20 years) and processed within 24 hours of death when the body has been kept in a morgue refrigerator. They are then either stored for up to 3 weeks in antibiotics at 4°C or frozen rapidly with a cryopreservative and stored in liquid nitrogen. From our clinical experience, we have learnt many things; for example, unstented homografts do not give rise to emboli, turbulence, or hemolysis, nor do they impair ventricular function from splinting of the valve ring, as occurs with stented and mechanical prostheses. In other words, the homograft functions exactly like the normal valve and without the need for anticoagulant or anti-platelet therapy.
We have also learnt that homograft valves rapidly become acellular by a process of rejection and subsequently undergo degeneration, wear and tear, or calcification as a result of the absence of living cells to service the matrix. Late degeneration is usually heralded by the onset of a diastolic murmur or a change in the systolic murmur and is an indication that the valve will need to be changed within 3 to 5 years, giving an average homograft functional life of 15 to 18 years. However, we have noted that the degenerative process in homografts where the collagen has not been denatured by glutaraldehyde, is only slowly progressive and never catastrophic. This is an important advantage and consequently, there is plenty of warning to both the clinician and patient of impending valve failure. Consequently, a planned operation can be carried out, making valve-related deaths rare.
Also, sudden unexplained deaths that are not infrequent with mechanical valve replacement are very rare indeed, with 3 out of 555 patients followed up over a 25-year period. Because of the probability of being able to achieve a safe second operation, patient survival which after all is what we should be interested in, is regularly better than valve survival. Also, the quality of life in a homograft patient, removed from the dangers of embolism and anticoagulant hemorrhage, medical checks, restrictions on pregnancy, and the danger of sudden death, is undoubtedly considerably better than the quality of life in a mechanical or bioprosthetic valve patient; and it is very close to normal. Usually there is no need for any form of medication.
Although we have inserted approximately 2,000 aortic valve homografts, in order to bring the results up to date and to analyze them statistically, we have hard data on 555 operative survivors with aortic valve disease, operated on by one surgeon in the same hospital and followed up to 25 years. Operative mortality has stabilized at 2%, making the operation comparable with any form of aortic valve replacement. Late deaths occurred in 81 patients in the period under review, that is 15% of the 555 but only 3 of these were sudden deaths. By far, the most common cause of late death was slowly progressive primary tissue failure. Patient survival is well over 70% at 10 years and 53% at 20 years, and these are much better than most reported valve series during that time interval. I make no apologies for giving patient survival rather than valve survival, since the patient's primary interest is in himself rather than the valve; and we should not forget that it is the patient's interest and not the surgeon's nor those of the commercial company which is of paramount importance.
We have not been able to identify any other definite determinants of long-term performance with regard to the patient's age or the age of the donor valve. However, storage time in antibiotics has a bearing, with an early onset of diastolic murmurs in valves stored up to 3 weeks before insertion. Since durability is a chief determinant of long-term performance, we need to direct our efforts to improve the functional life of homografts. One way is to look upon them as a form of organ transplantation. This is a realistic concept since viable valves can be removed from transplant recipient hearts. Like stored homografts, unfortunately, they rapidly become acellular and to retain the viability and function of the cells implies the need for some form of immunosuppression. This negates some of the unique advantages of the homograft. We cannot, however, talk about preserving living cells, especially the vulnerable endothelial cells, while we ignore their immunological susceptibility. Hopefully, with improved methods of tissue preservation and other techniques for maintaining the structural integrity of the valve collagen, it may be induced to act as a favorable matrix on which the body will deposit its own living fibroblasts and endothelium. Only when we have achieved these goals by tissue engineering will the homograft achieve its true potential, although even in its present form, it already outperforms most other biological valves.
Meanwhile, may I remind you that autotransplantation of the patient's own living pulmonary valve to the aortic site offers the only permanent biological valve replacement we have.5 It represents the gold standard against which we can compare homografts and the vast selection of currently available bioprosthetic and mechanical valves.
Finally, the homograft has its major application in the aortic position but it has, of course, been used in the right ventricular outflow and the tricuspid valve area. As a true mitral homograft, excellent pioneering work in this direction has been carried out by Professor Acar of Paris, and we await his further reports with interest.6 At the same time, we are experimenting with the use of inverted autografts (top hats) as mitral replacements and, again, this work is in its preliminary stages and will be reported upon in due course.7
In summary, the aortic homograft has certainly established itself as judged by a recent publication by Yacoub's group8 reporting on a randomized trial to include the pulmonary autograft for comparison.
References
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