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J Thorac Cardiovasc Surg 1994;107:152-161
© 1994 Mosby, Inc.

SURGERY FOR ACQUIRED HEART DISEASE

The use of unstented homograft valves for aortic valve reoperationsReview of a twenty-three–year experience

Middlesex and London, England

Address for reprints: Magdi Yacoub, FRCS, Academic Department of Surgery, Royal Brompton, National Heart and Lung Hospital, Sydney St., London SW3, England.

Abstract

Unstented homograft valves offer several theoretical advantages when used for patients who have had previous operations on the aortic valve. Between January 1970 and February 1993, 177 patients received unstented homograft valves after previous aortic valve operations. One hundred thirty-four patients had previous aortic valve replacement in the form of homografts (101 patients), mechanical prostheses (24 patients), and bioprostheses (9 patients), and 43 had previous valve repair. The indication for reoperation was deterioration of a noninfected valve (124 patients), infective endocarditis (40 patients), and failure of a noninfected mechanical valve (12 patients). Fresh homograft valves were implanted in 60 patients, homografts preserved in antibiotics were used in 111 patients, and 6 patients received cryopreserved valves. Aortic valve and root replacement was performed in 60 patients, and in 117 the homograft was inserted freehand in the subcoronary position. The early mortality was 5.1%. The actuarial survival at 10 years was 71%. Multivariate analysis demonstrated that patients with previous homograft replacement have a better long-term survival than patients who had previous mechanical valves (p = 0.017). The freedom from valve-related death and reoperation was 70% at 10 years. Fresh homografts faired better than antibiotic-sterilized homografts (p = 0.007). None of the patients had recurrence of endocarditis at 6 months, although 1 patient died of uncontrolled infection despite valve replacement. The freedom from recurrent endocarditis was 88% at 10 years. We conclude that unstented aortic homografts provide good early and long-term results for aortic valve reoperations, particularly in patients with previous homograft replacement. Recurrent endocarditis is uncommon even in patients operated on for prosthetic valve infections. (J THORAC CARDIOVASC SURG 1994;107:152-61)

Reoperation for aortic valve failure poses specific problems because of a variety of factors including advanced age, decreased myocardial function, and technical difficulties related to scarring and distortion of the aortic anulus and root. Unstented homografts offer many theoretical advantages, but their use is technically more demanding and their exact value and long-term performance for this specific group of patients remain to be defined.

This study reviews our experience with unstented homografts used for reoperations on the aortic valve. Their performance is evaluated and the determinants of early and long-term results are defined.

PATIENTS

From January 1970 to February 1993, 177 patients had aortic valve reoperations at two hospitals (Harefield Hospital, Middlesex: 124 patients; Royal Brompton, National Heart and Lung Hospital, London: 53 patients) by the same surgeon (M.Y.). These patients comprised 8% of our overall experience with aortic valve operations (Fig. 1). There were 131 male and 46 female patients with a mean age of 45 years (range 2 to 77 years). The indication for the first operation was aortic stenosis (transvalvular gradient >30 mm Hg) in 108 patients, aortic regurgitation in 42 patients, and infective endocarditis in 27 patients. One hundred thirty-four patients had previous aortic valve replacements in the form of homografts (101), mechanical prostheses (24), and bioprostheses (9). Forty-three patients had aortic valve repair (valvotomy in 22 and cusp augmentation in 21). The indications for reoperation were infection in 40 patients, noninfected tissue valve failure in 124, and mechanical valve failure in 12 patients. Tissue valve failures included mainly regurgitation for the homografts and calcification with stenosis for bioprostheses. Calcification and stenosis were also predominant in the patients with cusp augmentation and valvotomy. Mechanical valve failures included noninfective paravalvular leaks (8 patients), hemolysis (2 patients), repeated embolic episodes (1 patient), and structural deterioration (ball variance, 1 patient). Of the 40 patients with endocarditis requiring valve replacement, 25 had a previous homograft, 12 had mechanical valves, 2 had bioprosthetic valves, and 1 had a repair. Half of those with mechanical valves who required reoperation (12 of 24 patients) had endocarditis. The mean interval between the first and the second operation was 8.5 years (SD* ± 5.53) and for the various subgroups is shown in Table I. Nineteen patients had two or more previous aortic valve operations.

View larger version (25K): [in this window] [in a new window]   Fig. 1. Overall experience with aortic valve operations at Harefield Hospital and Royal Brompton and National Heart and Lung Hospital, 1970 to February 1993.

Table II

Aortic valve and root replacement was performed in 60 patients, and in 117 patients the homograft was inserted freehand in the subcoronary position. The aortic valve was exposed through a curved aortotomy starting in the midline anteriorly and extending into the middle of the noncoronary cusp. This allowed excellent exposure of the valve and possible enlargement of the root by extending the incision across the anulus into the subaortic curtain for 2 to 3 mm. The indication for root replacement was determined after exposure of the aortic valve and based on the size of the root, its distortion, and the need to exteriorize abscess cavities in the case of infective endocarditis. The techniques used for freehand and root replacement with homografts have been described earlier. ^{1, 2}

In cases of infective endocarditis the aim was to exteriorize the abscess cavities and to excise the infected tissue down to healthy noninfected tissue. When débridement extended below the level of the anulus, the homograft was then sutured to the noninfected myocardium of the left ventricular outflow tract, with care taken to avoid the area of the atrioventricular bundle.

Seventeen patients (9.6%) required emergency operations and 32 patients had concomitant procedures as presented in Table III.

Table IV

Statistical methods
The 2 test was used to compare frequencies of the different variables and a p value of less than 0.05 was considered significant. The probability of survival, freedom from valve-related death and reoperation, and freedom from endocarditis were calculated by the Kaplan-Meier method. ⁴

Logistic regression was used to analyze factors affecting early mortality. According to this model, P, the probability of early death, is related to the covariates, x₁, x₂, .... x_k as follows:

log (P/1 - P) = ß₀ + ß₁x₁ + ß₁x₂ + .... ß_kx_k

where ß₁, ß₂, .... , ß_k are the coefficients measuring the degree of influence of the covariates. Notice that if ß_i is positive then the increasing value of x_i leads to increasing probability of early death. Likelihood-based methods were used in fitting and testing the models. The statistical computer package BMDP (1990) was used to fit the models. ⁵

The Cox proportional hazard model ⁶ was used to analyze the time of follow-up after the reoperation and the time until death or second reoperation among those patients who did not die early after the operation. According to the Cox model, the hazard function at time t for a patient with covariates x₁, x₂, .... , x_k is given by the following equation:

h(t;x) = h₀(t) exp{ß₁x₁ + ß₂x₂ + .... + ß_kx_k}

where h₀(t) is the baseline hazard and the parameters ß₁, .... , ß_k are coefficients measuring the degree of influence of the covariates of the hazard function. The hazard is a measure of the rate at which death occurs, so that a positive value of ß_i indicates that the increasing value of the covariate x_i is associated with decreased survival time. Likelihood-based methods were used in fitting and testing models. ⁵ Qualitative covariates such as "cause offailure of the first valve" were fitted by means of indicator variables. In the case of quantitative covariates such as "time between operation" a linear relationship was initially assumed. In cases in which this was significant, the possibility of curvature was explored by adding a quadratic term. Investigations were also made into the possibility of interaction between covariates. The standard errors for the estimated survival probability were obtained by means of a boot-strapping method. ⁷

RESULTS

Early mortality and morbidity
The early (30-day) mortality for all the patients was 5.1% (9 of 177, 90% confidence limits [CL] 3.5% to 7.5%), the causes of which are listed in Table V. Logistic regression was used to analyze factors affecting early mortality. Possible covariates included age, gender, diagnosis at the first operation, multiple previous aortic operations, type of first operation, indication for the second operation, reoperation interval, date of the second operation, NYHA class, left ventricular function, urgency of operation (elective or emergency), type of operation (freehand or root), and concomitant procedures. NYHA class IV and poor left ventricular function were incremental risk factors for early deaths (p = 0.037 and 0.091, respectively). Early (30-day) mortality significantly decreased in the more recent era (p = 0.038, Fig. 2). Table VI shows the results of the logistic regression analysis. According to this model, the predicted early mortality for a patient with NYHA class II, good to moderate left ventricular function requiring replacement of a homograft valve in 1993 is 1.4% (90% CL 0.3% to 4.1%). In contrast, a matched patient but with more advanced symptoms (NYHA class IV) has a prediced early mortality of 11.4% (90% CL 2.5% to 39.3%).

View larger version (31K): [in this window] [in a new window]   Fig. 2. Aortic reoperations as percentage of total aortic operations performed over 23-year period.

Table VII

View larger version (18K): [in this window] [in a new window]   Fig. 3. Incidence of early death by year of operation as estimated by logistic regression model.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Kaplan-Meier estimate of probability of survival after reoperations with unstented homografts. Early mortality is excluded.

Late mortality
The actuarial survival curve is shown in Fig 4. The interval survivals at 5, 10, and 15 years were 92% (SE ± 2.6), 71% (SE ± 7.0), and 52% (SE ± 13.4), respectively. Table VIII outlines the causes of late death. Sudden death was not related to a preoperative diagnosis of aortic stenosis versus regurgitation (p = 0.6). Cardiac failure resulted from aortic regurgitation in one patient and mitral regurgitation with a normally functioning aortic homograft in one other patient. In the remaining four patients the cause of the cardiac failure, although not confirmed, was attributed to aortic homograft failure for the purpose of the analysis. The cause of death was confirmed at postmortem examination in 45% of cases and reported by the referring physician in the remainder. The Cox proportional hazard model was used to analyze the factors influencing long-term survival. The possible covariates included all those used in the analysis of early mortality, plus the type of homograft used, the presence of postoperative complications, functional status (NYHA), and evidence of aortic valve dysfunction by physical examination and echocardiography at the latest follow-up visit. The patients who had previous mechanical valve failure were at increased risk of late death (p = 0.017). The hazard rate decreases as the interval between the first and second operation increases (p = 0.005). The results of the multivariate analysis are shown in Table IX. According to this model a 60-year-old patient with NYHA class II symptoms who requires replacement of a homograft aortic valve implanted 12 years before has a 96% (SE ± 1.5%) probability of being alive at 5 years and a 90% (SE ± 3.9%) probability at 10 years. In contrast, a 60-year-old patient with NYHA class II symptoms who requires replacement of a mechanical aortic valve after 4 years has a 66% (SE ± 16.1%) probability of being alive at 5 years and a 33% (SE ± 18.5%) probability at 10 years (Fig. 5).

View larger version (23K): [in this window] [in a new window]   Fig. 5. Probability of survival for patients who had homografts implanted at first operation versus patients who had mechanical valve implanted at first operation. Predicted survival based on Cox proportional hazard model.

Table X

Infective endocarditis
Of the 40 patients with infective endocarditis requiring aortic valve replacement, 21 had active infection and 13 required emergency operation. The preoperative NYHA class was III or IV in 24 patients (60%). Aortic valve and root replacement was necessary in 15 patients (37.5%). The type of valve affected by endocarditis is shown in Table XI, and the organisms responsible for the infection are shown in Table XII. The early mortality was 10% (4 patients). One patient died at 45 days with uncontrolled infection complicated by brain abscesses despite valve replacement. Recurrence of infective endocarditis occurred in 1 patient 11 years after reoperation and required a second reoperation. Late infective endocarditis was seen in 2 patients at 12 months and 4 years, respectively, after reoperation. Neither had a history of infective endocarditis. The latter patient was successfully treated with antiobiotic therapy, but the former died of septic embolization. The actuarial freedom from infective endocarditis was 88% at 15 years (SE ± 8.6%) (Fig. 6). No difference in the prevalence of postoperative infective endocarditis was found between patients with or without infective endocarditis (p = 0.201).

View larger version (23K): [in this window] [in a new window]   Fig. 6. Kaplan-Meier estimate of probability of freedom from valve-related death and reoperations. Early mortality is excluded.

This study demonstrates that the use of unstented homograft valves in the setting of aortic valve reoperation offers good early and long-term results. The early mortality in the current era is low and similar to that of first-time operations. ⁸ Various authors reported a high early mortality of 15% to 40% ^9-11 in the late 1970s and early 1980s, although this poorer result may relate to differences in indications for reoperation in an earlier era and less advanced perioperative care. Our current findings show improved survival in patients who had their reoperation more recently.

The demand for aortic valve reoperation is higher in recent years because patients with bioprosthetic and homograft valves inserted 10 to 20 years ago are having symptoms stemming from valve degeneration. In addition, patients with mechanical prostheses are requiring treatment for thromboembolic phenomena, anticoagulant-related morbidity, and, rarely, mechanical failure. The benefits of early reoperation in symptom-free patients are evident in this study as well as in others. ^{12, 13}

View larger version (21K): [in this window] [in a new window]   Fig. 7. Kaplan-Meier estimate of probability of freedom from postoperative infective endocarditis. Early (30-day) mortality is excluded.

The freedom from homograft valve–related death and reoperation at 10 years has been reported to be between 30% and 90% for first-time aortic valve replacement. ^17-19 Our corresponding figure is 70% for aortic valve reoperation. Several factors have been implicated in homograft deterioration, including the preservation technique. ^17-19 Our experience lies predominantly with fresh and antibiotic-preserved homografts. Fresh homografts have only been available to us since 1980, which marked the beginning of our transplantation program, and our data show improved long-term results as compared with the antibiotic-preserved homografts. Viability of the homograft endothelium at the time of implantation may contribute to improved long-term valve performance. ²⁰ Previous concerns ²¹ over the immunogenicity of fresh homografts leading to early degeneration are not supported by our data.

The reoperation interval was found to influence the long-term performance of the homografts, particularly among patients with a previous homograft replacement. The significance of this effect is unclear and may relate to immunologic or other factors. Certainly, in patients who had previous homografts there were no early failures that might have then suggested accelerated degeneration owing to sensitization to previous allografts. ²²

Several studies report that paraprosthetic leak is more prevalent after aortic valve reoperation than after the initial procedure. ^{12, 13} We did not have any cases of paravalvular leak even in the setting of infective endocarditis. This favorable result may relate to conformational adaptation of the homograft and the preferential use of aortic root replacement to match the size and shape of the native root, compensate for its distortion, and facilitate exteriorization of root abscesses. Many authors advocate the use of homografts in patients with infective endocarditis, especially prosthetic valve endocarditis. ^23-25 Our results support such use even in cases of uncontrolled infection, wherein we found no increase in early mortality or recurrent infection. Moreover, postoperative (early and late) infective endocarditis was not prevalent and was not related to the presence of preoperative infective endocarditis.

Despite an active transplantation program and access to two homograft banks, we find limited homograft availability to be a persistent problem as reflected by a figure of approximately 60% homograft valve replacements for the first and second operations (Fig. 1). Our unit policy is to offer homograft valve replacement as a first choice unless one is not available or the patient refuses to accept one.

In conclusion, homograft valves offer good early and long-term results in aortic valve reoperations, and the early mortality in the present era is similar to that of first-time operations. Reoperations for "failed" homografts give better early and long-term results than for "failed" prosthetic valves. Fresh homograft valves give better results than antibiotic-sterilized valves. In patients with prosthetic valve endocarditis, unstented homograft valves are the valve substitutes of choice.

Appendix: DISCUSSION

Dr. Charles Yankah (Berlin, Germany)
I congratulate Dr. Albertucci and his colleagues for their study of the use of a second homograft in reoperations. Their results seem to support the evidence that a second homograft at a reoperation does not actually influence the long-term valve function; that is, early degeneration in not observed clinically, whereas in animal experiments a "second set reaction" is more often observed with accelerated rejection and consequently with early degeneration. At this juncture the question of viability should be raised. Dr. Albertucci, which of the standard techniques did you use in your reoperations? Did you use mostly aortic root replacement or subcoronary valve replacement? These two techniques might also influence the long-term performance of the valves. Second, on what basis do you use a homograft instead of other valve devices at reoperations?

Dr. Albertucci
We used a homograft as a freehand graft in about two thirds of the patients and as a root in about one third of the patients. Multivariate analysis did not demonstrate any difference in long-term outcome with regard to valve deterioration between freehand insertion or root insertion.

Because our unit is biased toward homografts, we will use a homograft whenever one is available unless the patient refuses to have another homograft. The homograft has been our valve of choice.

Acknowledgments

We thank Dr. Robinson, of the School of Mathematical and Physical Sciences at the University of Sussex, Brighton, Sussex, for the statistical analysis of the data and Eileen Boyland for her contribution in the collection and analysis of the data.

Footnotes

From the Academic Department of Surgery, Harefield Hospital and the Royal Brompton, National Heart and Lung Hospital,^a and the Department of Cardiology, Harefield Hospital, Middlesex,^b and the Royal Brompton, National Heart and Lung Hospital, London,^c England.

Read at the Seventy-third Annual Meeting of The American Association for Thoracic Surgery, Chicago, Ill., April 25-28, 1993.

*SD = Standard deviation.

*SE = Standard error.

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