Allograft aortic valve replacement: long-term comparative clinical analysis of the viable cryopreserved and antibiotic 4 degrees C stored valves

Preservation of human heart valves for replacement in children with heart valve disease: past, present and future

Valvular heart disease affects 30% of the new-borns with congenital heart disease. Valve replacement of semilunar valves by mechanical, bioprosthetic or donor allograft valves is the main treatment approach. However, none of the replacements provides a viable valve that can grow and/or adapt with the growth of the child leading to re-operation throughout life. In this study, we review the impact of donor valve preservation on moving towards a more viable valve alternative for valve replacements in children or young adults.

Impact of Three Different Processing Techniques on the Strength and Structure of Juvenile Ovine Pulmonary Homografts

Homografts are routinely stored by cryopreservation; however, donor cells and remnants contribute to immunogenicity. Although decellularization strategies can address immunogenicity, additional fixation might be required to maintain strength. This study investigated the effect of cryopreservation, decellularization, and decellularization with additional glutaraldhyde fixation on the strength and structure of ovine pulmonary homografts harvested 48 h post-mortem. Cells and cellular remnants were present for the cryopreserved group, while the decellularized groups were acellular. The decellularized group had large interfibrillar spaces in the extracellular matrix with uniform collagen distribution, while the additional fixation led to the collagen network becoming dense and compacted. The collagen of the cryopreserved group was collapsed and appeared disrupted and fractured. There were no significant differences in strength and elasticity between the groups. Compared to cryopreservation, decellularization without fixation can be considered an alternative processing technique to maintain a well-organized collagen matrix and tissue strength of homografts.

Vitrification of Heart Valve Tissues

Application of the original vitrification protocol used for pieces of heart valves to intact heart valves has evolved over time. Ice-free cryopreservation by Protocol 1 using VS55 is limited to small samples (1-3 mL total volume) where relatively rapid cooling and warming rates are possible. VS55 cryopreservation typically provides extracellular matrix preservation with approximately 80% cell viability and tissue function compared with fresh untreated tissues. In contrast, ice-free cryopreservation using VS83, Protocols 2 and 3, permits preservation of large samples (80-100 mL total volume) with several advantages over conventional cryopreservation methods and VS55 preservation, including long-term preservation capability at -80 °C; better matrix preservation than freezing with retention of material properties; very low cell viability, reducing the risks of an immune reaction in vivo; reduced risks of microbial contamination associated with use of liquid nitrogen; improved in vivo functions; no significant recipient allogeneic immune response; simplified manufacturing process; increased operator safety because liquid nitrogen is not used; and reduced manufacturing costs. More recently, we have developed Protocol 4 in which VS55 is supplemented with sugars resulting in reduced concerns regarding nucleation during cooling and warming. This method can be used for large samples resulting in retention of cell viability and permits short-term exposure to -80 °C with long-term storage preferred at or below -135 °C.

Audit of homograft valve bank

Introduction

Even today, the search for the ideal cardiac valve continues. With advantages of having superior flow dynamics, avoidance of anticoagulation, and resistance to infection, homograft has been shown to have an edge over conventional prosthetic and bioprosthetic valves. But they suffer from disadvantages of limited availability and durability. Our center operates one of the oldest functioning valve banks in the country. We present our experience with homograft valve banking with antibiotic and cryopreserved homografts spread over a quarter century.

Methods

For donor selection, procurement, sterilization, and preservation, the recommendations of the American Association of Tissue Banks are being followed in accordance with statutory provisions of the Transplantation of Human Organs Act, 1994.

Results

During 25-year period (1993-2017), 777 hearts were procured. Age of the donors ranged from 2 to 60 years and hearts were procured within 24 h of death. A total of 1646 homografts (774 pulmonary, 774 aortic, 60 mitral valves, 20 descending thoracic aortae, and 18 monocusps) were harvested. A total of 546 (32%) homografts were rejected for various reasons. Nine hundred sixty-seven (56.7%) homografts were used in different procedures. Of these, 478 were pulmonary homografts, 425 were aortic homografts, 39 mitral homografts, 18 monocusps, and 7 descending thoracic aorta homografts. One hundred fifty-four (16%) homografts were antibiotic preserved and the rest 813 (84%) were cryopreserved.

Conclusions

It is possible to run a homograft valve bank with minimum costs. Though, cryopreservation is more expensive, it provides an opportunity to store the valves for an indefinite period and maintain an uninterrupted supply of homografts.

The cryoprotectant trehalose could inhibit ERS-induced apoptosis by activating autophagy in cryoprotected rat valves

Valvular diseases are common health problems that are strongly related to high morbidity and mortality; aortic valve allograft transplantation may be a promising way to improve survival and relieve symptoms. However, ideal tissue viability has not been observed with common valve cryopreservation methods, which could lead to apoptosis and necrosis in cryopreserved tissue. It has been observed that trehalose plays a positive role by acting to maintain cell structures and protect cells from stress responses. In this study, we studied the effects of trehalose in protecting rat valve tissue from the cooling process. We found improved higher cell function in rat valves treated with trehalose during cryopreservation than in those treated with dimethyl sulphoxide (DMSO). To further explore the mechanisms, we found that trehalose could down-regulate the expression of cleaved caspase-3, an important molecule involved in cell apoptosis. In addition, treatment with trehalose also decreased Glucose-regulated protein 78 (GRP78) and CCAAT/enhancer-binding protein homologous protein (CHOP), the key proteins in the endoplasmic reticulum stress (ERS) process. Intriguingly, we observed that trehalose promotes cryoprotected rat valve cell autophagy via an mTOR-independent but p38 MAPK-dependent signaling pathway. Additionally, miR-221 and miR-32 have been implicated in such cell activities. In summary, our study offers a new and meaningful cryopreservation approach for valve allograft storage.

Surgery for Diseases of the Aortic Root

The aortic root is the junction between the heart and aorta, containing the aortic valve and the coronary artery ostia. Various pathologic conditions arise in this region requiring complex surgical correction. These include aneurysmal dilatation with and without aortic regurgitation, acute aortic dissection extending below the sinotubular junction, and infective endocarditis with valve and periannular destruction. Multiple strategies for correction of these complex surgical issues exist, with excellent early results and long-term survival.

Decellularized Cryopreserved Allografts as Off-the-Shelf Allogeneic Alternative for Heart Valve Replacement: In Vitro Assessment Before Clinical Translation

Cryopreserved allogeneic conduits are the elective biocompatible choice among currently available substitutes for surgical replacement in end-stage valvulopathy. However, degeneration occurs in 15 years in adults or faster in children, due to recipient's immunological reactions to donor's antigens. Here, human aortic valves were decellularized by TRICOL, based on Triton X-100 and sodium cholate, and submitted to standard cryopreservation (TRICOL-human aortic valves (hAVs)). Tissue samples were analyzed to study the effects of the combined procedure on original valve architecture and donor's cell removal. Residual amounts of nucleic acids, pathological microorganisms, and detergents were also investigated. TRICOL-hAVs proved to be efficaciously decellularized with removal of donor's cell components and preservation of valve scaffolding. Trivial traces of detergents, no cytotoxicity, and abrogated bioburden were documented. TRICOL-hAVs may represent off-the-shelf alternatives for both aortic and pulmonary valve replacements in pediatric and grown-up with congenital heart disease patients.

Early Experience with Homograft Valve Banking

Homograft cardiac valves have been shown to have several advantages over conventional prosthetic valves. From October 1993 through November 1996, 273 homografts (262 valved and 11 non-valved) were used in various procedures at the All India Institute of Medical Sciences, New Delhi, India. The recommendations of the American Association of Tissue Banks were followed for procurement, harvesting, and storage of the valves. One hundred and ninety-six hearts were procured yielding a total of 439 homograft valves; 192 were pulmonary homografts, 187 were aortic homografts, and 60 were mitral homografts. Eighty-five homografts were used in the Ross procedure, 64 were used in homograft replacement of the aortic valve, 28 were used in replacement of the mitral valve, 85 were used in various operations for heart disease as valved conduits, and 11 homografts were used as either non-valved conduits or for patch repair. One hundred and thirty-five homografts (31%) were discarded for various reasons. Our early experience of valve banking is discussed.

Heart valve tissue engineering: how far is the bedside from the bench?

Heart disease, including valve pathologies, is the leading cause of death worldwide. Despite the progress made thanks to improving transplantation techniques, a perfect valve substitute has not yet been developed: once a diseased valve is replaced with current technologies, the newly implanted valve still needs to be changed some time in the future. This situation is particularly dramatic in the case of children and young adults, because of the necessity of valve growth during the patient's life. Our review focuses on the current status of heart valve (HV) therapy and the challenges that must be solved in the development of new approaches based on tissue engineering. Scientists and physicians have proposed tissue-engineered heart valves (TEHVs) as the most promising solution for HV replacement, especially given that they can help to avoid thrombosis, structural deterioration and xenoinfections. Lastly, TEHVs might also serve as a model for studying human valve development and pathologies.

Vitrification of heart valve tissues

Application of the original vitrification protocol used for pieces of heart valves to intact heart valves has evolved over time. Ice-free cryopreservation by Protocol 1 using VS55 is limited to small samples where relatively rapid cooling and warming rates are possible. VS55 cryopreservation typically provides extracellular matrix preservation with approximately 80 % cell viability and tissue function compared with fresh untreated tissues. In contrast, ice-free cryopreservation using VS83, Protocols 2 and 3, has several advantages over conventional cryopreservation methods and VS55 preservation, including long-term preservation capability at -80 °C; better matrix preservation than freezing with retention of material properties; very low cell viability, reducing the risks of an immune reaction in vivo; reduced risks of microbial contamination associated with use of liquid nitrogen; improved in vivo functions; no significant recipient allogeneic immune response; simplified manufacturing process; increased operator safety because liquid nitrogen is not used; and reduced manufacturing costs.

The Ross operation: utilization of the patient’s own pulmonary valve as a replacement device for the diseased aortic valve

Prosthetic heart valves have been outpaced by progress in cardiac surgery. Early biologic valve prostheses consisted of tissues mounted on a rigid stent, and did not require anticoagulation, but rarely survived two decades. Subsequently, durable mechanical valve prostheses dominated despite, the requisite anticoagulation. The mechanical design remains imperfect, with obstruction to flow, turbulence, hematological changes and also, occasionally audible clicks. Reports documenting superior function for cryopreserved human aortic heart valves (homografts) without these problems, albeit with limited durability, followed. The marketing of 'stentless biologic valves', mimicking these attributes was a reaction to the shortage of homografts. These imperfections explain the rediscovery of the Ross operation, in which the patient's pulmonary valve (autograft) is excised to replace the aortic valve. The autograft is living tissue, complete with attributes of a healthy heart valve, including growth and durability. The pulmonary valve, where lower pressure and oxygen saturation retards degeneration, is substituted with a pulmonary homograft. The Ross operation is exacting and leaves the patient with two potentially malfunctioning valves.

International heart valve bank survey: a review of processing practices and activity outcomes

A survey of 24 international heart valve banks was conducted to acquire information on heart valve processing techniques used and outcomes achieved. The objective was to provide an overview of heart valve banking activities for tissue bankers, tissue banking associations, and regulatory bodies worldwide. Despite similarities found for basic manufacturing processes, distinct differences in procedural details were also identified. The similarities included (1) use of sterile culture media for procedures, (2) antibiotic decontamination, (3) use of dimethyl sulfoxide (DMSO) as a cryoprotectant, (4) controlled rate freezing for cryopreservation, and (5) storage at ultralow temperatures of below −135°C. Differences in procedures included (1) type of sterile media used, (2) antibiotics combination, (3) temperature and duration used for bioburden reduction, (4) concentration of DMSO used for cryopreservation, and (5) storage duration for released allografts. For most banks, the primary reasons why allografts failed to meet release criteria were positive microbiological culture and abnormal morphology. On average, 85% of allografts meeting release criteria were implanted, with valve size and type being the main reasons why released allografts were not used clinically. The wide variation in percentage of allografts meeting release requirements, despite undergoing validated manufacturing procedures, justifies the need for regular review of important outcomes as cited in this paper, in order to encourage comparison and improvements in the HVBs' processes.

Xenograft Failure of Pulmonary Valved Conduit Cross-linked with Glutaraldehyde or Not Cross-linked in a Pig to Goat Implantation Model

Background

Biologic valved grafts are important in cardiac surgery, and although several types of graft are currently available, most commercial xenografts tend to cause early disfiguration due to intimal proliferation and calcification. We studied the graft failure patterns on non-fixed and glutaraldehyde-fixed pulmonary xenograft in vivo animal experiment.

Materials and Methods

Pulmonary valved conduits were obtained from the right ventricular outflow tract of eleven miniature pigs. The grafts were subjected to 2 different preservation methods; with or without glutaraldehyde fixation: glutaraldehyde fixation (n=7) and non-glutaraldehyde fixation (n=4). The processed explanted pulmonary valved grafts of miniature pig were then transplanted into eleven goats. Calcium quantization was achieved in all of the explanted xenograft, hemodynamic, histopathologic and radiologic evaluations were performed in the graft which the transplantation period was over 300 days (n=7).

Results

Grafts treated with glutaraldehyde fixation had more calcification and conduit obstruction in mid-term period. Calcium deposition also appeared much higher in the glutaraldehyde treated graft compared to the non-glutaraldehyde treated graft (p<0.05).

Conclusion

The present study suggests that xenografts prepared using glutaraldehyde fixation alone appeared to have severe calcification compared to the findings of non-glutaraldehyde treated xenografts and to be managed with proper anticalcification treatment and novel preservation methods. This experiment gives the useful basic chemical, histologic data of xenograft failure model with calcification for further animal study.

Acute regeneration and chronic acellular transformation of rabbit cryopreserved aortic allografts

An analysis of rabbit cryopreserved aortic allografts excised on postoperative days (POD) 2, 5, 11, 60, 210, 360, and 720, as well as controls that were untransplanted native aortas and cryopreserved aortas, was performed. On POD2, the number of medial smooth muscle cells in the allografts was reduced to approximately 50%. Ki-67 analysis revealed that medial smooth muscle cells in the allografts proliferated from the 2nd day. By the 11th day, their proliferation ceased and the number of medial smooth muscle cells was restored to almost at the same level as in the controls. Polymorphic microsatellite DNA marker analysis disclosed that the restored medial smooth muscle cells were of donor origin. From 7 months through 2 years, the media of cryopreserved aortic allografts were transformed into acellular structures, in which the elastic fibers were preserved. On the other hand, newly accumulated smooth muscle cells were observed in the adventitia just outside of acellular media after 7 months. In some cases, scattered lamellar calcium deposition was observed in the same regions. This study presents a comprehensive documentation of regeneration and acellular transformation in cryopreserved aortic allografts based on short and long-term analysis.

Aortic reoperation after freestanding homograft and pulmonary autograft root replacement

BACKGROUND

Human allografts and pulmonary autografts offer many advantages as an aortic valve and root substitute. The progressive degeneration of the aortic allograft and the pulmonary autograft has been seen as an important disadvantage, and the need for a reoperation has been perceived as challenging and risky for the patients.

METHODS

Between March 1992 and October 2009, 53 consecutive patients (mean age 50 ± 13 years; 38 male), who had a previous aortic root replacement, underwent redo surgery for failure of the aortic homograft (n = 42) or the pulmonary autograft (n = 11). The median follow-up (available for 47 of 51 patients) was 44 months.

RESULTS

Structural valve deterioration was the main indication for reoperation on the homograft (86%), with an earlier presentation in patients who received homografts from donors more than 55 years old. Failure of the pulmonary autograft occurred primarily because of severe aortic regurgitation predominantly due to dilation of the autograft (n = 5) and autograft valve prolapse (n = 5). The total in-hospital mortality was 3.8% (n = 2). No deaths occurred among patients who previously underwent a Ross procedure. The course was complicated in 25 cases (48%). The cumulative 1-year, 5-year, and 8-year survival rates were 92%, 90%, and 77%, respectively. No late deaths were encountered after reoperation on the pulmonary autograft (maximum follow-up 218 months). Freedom from reoperation (excluding early in-hospital operation) for recurrent aortic valve or root pathology was 97% at 8 years.

CONCLUSIONS

Reoperation after freestanding homograft and pulmonary autograft root replacement can be accomplished safely. The total postoperative morbidity rate is still high.

What is the proper place of the ross procedure in our modern armamentarium?

Despite nearly four decades of experience, the role of pulmonary valve autotransplantation (Ross procedure) in the treatment of aortic valve disease in adults and children continues to evolve and remains controversial. As the picture of late results has unfolded, alternating waves of enthusiasm and caution have characterized its use and have led to ongoing refinements in indications and operative technique. At present, it is seen as indispensable in the treatment of aortic valve disease in infants and small children (for whom no satisfactory replacement alternative exists and for whom growth is essential), attractive for adolescents and young adults who wish to avoid anticoagulants because of childbirth and lifestyle considerations, a reasonable option for selected adults who desire biologic solutions with potentially better durability than conventional bioprostheses, and contraindicated for the elderly and those with connective tissue disorders. Young patients with bicuspid aortic valve are the most common potential recipients, but also the most controversial, because of the risk of autograft dilatation. Optimal matching of prosthesis to patient is a clinical challenge for all caretakers involved in the treatment of valvular heart disease; this review provides guidelines to identify those patients who will benefit most from the Ross procedure, and those for whom it is inadvisable.

Allograft tissue for use in valve replacement

Homograft or allograft tissue has been available for use as replacement for diseased valves or reconstruction of major vessels for decades. However, with respect to replacement of diseased valvular tissue the search for the ideal valve still continues. In this review we will discuss the clinical indications, surgical techniques, and outcome of aortic homografts.

Fresh homografts obtained through a national organ-sharing programme for repair of congenital heart disease

Homografts have been used in congenital cardiac surgery for over 30 years. We utilized the resources of a national organ-sharing programme to obtain fresh homografts and report their use in correcting cardiac pathologies in 20 children between March 2001 and May 2003. In 16 patients, a valved conduit was used to form a connection between the pulmonary ventricle and the pulmonary artery. In three patients, a non-valved aortic conduit was used to form an extra-cardiac Fontan circulation and in one patient, non-valved pulmonary and aortic conduits were used to repair an infected aortic aneurysm. Three patients died following surgery. Survivors were followed up using echocardiography between 2 and 24 months post-surgery. Results demonstrate that, with the help of a national organ-sharing programme, the use of fresh homograft conduits is feasible in a paediatric patient population with reasonable waiting times.

Six-year monitoring of the donor-specific immune response to cryopreserved aortic allograft valves: Implications with valve dysfunction

BACKGROUND

The immune rejection has been anticipated as one of the major causes of allograft aortic valve (AAV) degeneration. The purpose of this study was to prospectively serially measure the magnitude and evolution of the recipient anti-HLA class I antibody response up to 6 years from AAV implant and to correlate serologic data with valve performance by means of a concurrent echocardiographic survey.

METHODS

Cryopreserved AAVs were obtained from multiorgan HLA-typed donors. Nineteen patients younger than 50 years (mean age, 43.3 +/- 8 years) were prospectively studied. After successful surgery, all AAV recipient underwent at 3 and 6 months and each year postoperatively (mean follow-up, 71.9 months) concomitant serum sample collection and two-dimensional transthoracic echocardiography. The presence of anti-HLA antibodies was tested against a panel of lymphocytes obtained from 30 blood donors.

RESULTS

Progressive structural valve deterioration was seen in 6 patients (31.5%) of whom 4 (21%) were reoperated. All pretransplant recipients sera were panel-reactive antibody negative. Seventeen patients (89.4%) demonstrated significant panel-reactive antibody levels, which peaked at 6 months postoperatively, declined from 6 to 24 months, and slowly decreased afterward. In 14 of 19 cases (73.6%) donor-specific HLA antibodies were identified. A strong immunization (6-year persistence of panel-reactive antibody > 70% and peak panel-reactive antibody > 80%) was detected in 31.5% and 36.8% of recipients, respectively. Strong immunization was found to be significantly associated with progressive structural deterioration.

CONCLUSIONS

The immune reaction after cryopreserved AAV implantation is a peculiar long-lasting response occurring in the majority of recipients younger than 50 years of age. An association between a sustained and pronounced immunization and an aggressive AAV degeneration was observed.

Dynamics of cryoprotectant permeation in porcine heart valve leaflets

Valve replacement is a common cardiovascular procedure for the treatment of a variety of congenital and acquired defects. Many surgical programs rely on cryopreserved heart valves from regional tissue bank programs to meet clinical demands. Current cryopreservation strategies for heart valves are empirically derived. The aim of this study was to use proton nuclear magnetic resonance spectroscopy (NMR) to monitor changes in cryoprotectant concentration in isolated heart valve leaflets. Porcine aortic valves were locally obtained, freshly isolated, and allowed to equilibrate at various experimental temperatures (22 degrees C, 10 degrees C, 4 degrees C) for 1 h prior to immersion in 1 M Me2SO solution. At defined intervals (0, 0.25, 0.5, 1, 2, 6, and 24 h) the valves were removed from the Me2SO and the leaflets were rapidly dissected and equilibrated in deuterium oxide (D2O). Using previously described techniques the Me2SO concentration in the heart valve leaflets was determined by NMR and the diffusion coefficient was calculated as a function of time and temperature. Heart valve leaflets were fully equilibrated with Me2SO after approximately 2 h of exposure at 22 degrees C while equilibrium was not reached >6 h or more at 10 degrees C and 4 degrees C. These results indicate that that permeation of Me2SO in heart valves is strongly temperature dependent Furthermore, this study provides a quantitative measure of Me2SO permeation and cryoprotectant at equilibration in heart valve leaflets. The clinical applications of these findings may help to optimize the balance between the protective and toxic effects of cryoprotectants and lead to improved methods of preservation of heart valves.

Allograft Aortic Valve Replacement in the Adult: A Review

Aortic valve replacement using an allograft has been used continuously for over 40 years. Its advantages are excellent haemodynamic function, low thrombogenicity, resistance to infection and avoidance of the complications of anticoagulation. The main concern is its long-term durability, with the high hazard phase for failure between 10 and 20 years. We have only recently been able to judge the true long-term behaviour of the contemporary allograft with two recently published series of patients having reached follow-up beyond 20 years in significant numbers. This review of allograft aortic valve replacement in the adult covers the areas of history, benefits, techniques of sterilisation and preservation, operative methods and outcomes.

Viability and histologic structure of porcine valves after cryopreservation

BACKGROUND

Increased awareness of the limitations of current cardiac valve substitutes has generated a renewed interest in the use of allograft valves. The effects of currently used preservation techniques on the viability of the valve leaflets and the longevity of the implantation however remain controversial. The objective of this study is to analyze the influence of ischemic time, sterilization methods with or without fungicides, and storage procedures on the viability of the valve leaflets and on the histologic structure of the arterial wall, valve leaflet, and myocardium.

METHODS

The tissue sources were hearts from 40 pigs with 1 hour of warm ischemic time. The aortic and pulmonary valves were dissected after 2 or 24 hours of cold ischemic time. They were stored in antibiotic solution for 20 hours at 4 degrees C with or without an antifungal agent. The samples were cryopreserved using a programmed temperature decrease method. After 1 week of storage in a liquid nitrogen tank, either in a gas or a liquid phase, the cardiac valves were slowly thawed and examined.

RESULTS

Pulmonary valves showed greater viability than aortic valves. Decreased cellular viability was observed independent of cold ischemic time, treatment with amphotericin B, or the storage method used. Treatment with or without amphotericin B had no influence on cellular viability. Conversely it was observed that there was greater cellular viability among those valves stored in a liquid phase. As far as the histologic structure of the valve is concerned we did not observe any influence either in the treatment with amphotericin B or the storage method used although it was observed that reduction of the cold ischemic time minimized histologic injury.

CONCLUSIONS

Optimization of preservation methods may decrease the negative effects of cryopreservation on cell viability and histologic structure of the valve.

Role of inflammation and ischemia after implantation of xenogeneic pulmonary valve conduits: histological evaluation after 6 to 12 months in sheep

OBJECTIVE

Commercially available biological heart valve prostheses undergo degenerative changes, which finally lead to complete destruction. Here we evaluate the role of inflammation and ischemia after implantation of xenogeneic heart valve conduits (XPC) generated by novel concepts of tissue engineering.

METHODS

Acellularized (a-)XPC and autologus re-seeded (s-)XPC were implanted into sheep. Samples were taken as follows: after acellularization (n = 2), after re-seeding (n = 2), 6 months (seeded/non-seeded: n = 3/5), 9 months (n = 2/5), and 12 months (n = 3/2) post implantation. Five native porcine conduits served as control. Using histological methods, samples were evaluated for pathological changes and existence/density of microvessels.

RESULTS

Prior to implantation a-XPC were completely free of cells. Six months after implantation, leaflets and pulmonary arteries of s-XPC and a-XPC showed good endothelial surface coverage. Microvessel density within the myocardial cuffs and pulmonary vessel walls were comparable to control in all grafts. However, 6, 9 and 12 months after implantation pathological severe microvessel ingrowth, calcification and cellular infiltrations were observed on a-XPC and s-XPC valves, whereas myocardial cuffs and XPC-artery walls showed only mild degenerative alterations.

CONCLUSIONS

Inflammatory reactions play a pivotal role in the degeneration of a-XPC and s-XPC. Thus, since ischemia seems to have little or no influence on this process, inflammation inductive factors should be the center of interest.

Mechanisms of bioprosthetic heart valve calcification

BACKGROUND

Cryopreserved human heart valves are used in approximately 20% of the tissue heart valve procedures performed annually. The pathophysiology of allograft failure is not fully understood. The authors proposed the hypothesis that the rapid deterioration observed in some allograft heart valve recipients is caused by disruptive interstitial ice damage that occurs during cryopreservation and subsequently leads to accelerated valve degeneration on implantation.

METHODS

This hypothesis was tested by comparison of the standard commercial heart valve freezing method of cryopreservation and an ice-free, vitrification method of cryopreservation with fresh controls in a subcutaneous, juvenile rat implant model of calcification. Calcium concentration in explants was determined by atomic absorption spectroscopy.

RESULTS

Statistically significant calcification (P<0.05) was observed in both syngeneic and allogeneic cryopreserved valves relative to fresh valves. The ice-free cryopreservation method demonstrated significant reduction of allogeneic heart valve calcification (P<0.01). Comparison of fresh syngeneic and allogeneic grafts at the 3-week time point demonstrated significantly higher calcium content in allograft valve explants (P<0.005).

CONCLUSIONS

These findings demonstrate that allogeneic valve calcification is influenced by two factors, the cryopreservation method used and immunogenicity. Alternative cryopreservation methods that avoid ice formation may improve the in vivo performance of cryopreserved allogeneic heart valves.

Choice of prosthetic heart valve for adult patients

This review summarizes the major long-term (> or =10 to 15 years) patient outcomes after insertion of many Food and Drug Administration approved prosthetic heart valves (PHV). Mechanical PHV was associated with a better survival (p < 0.02) at 15 years after aortic valve replacement (AVR) than with a bioprosthesis in the Department of Veterans Affairs (DVA) trial. In both the DVA and the Edinburgh Heart Valve trials, bioprosthesis were associated with structural valve deterioration (SVD) (mitral valve replacement [MVR] > AVR) and, therefore, for replacement of the PHV. Thromboembolism and bleeding rate were higher with mechanical PHV. Mortality after AVR and MVR is high at 10 to 15 years because of the associated comorbid conditions and older age of patients. Outcomes with "new" good valves are similar to that with "older" good valves. Complication rates of thromboembolism, bleeding, endocarditis, and leak vary widely; the rates of these complications are not different among different mechanical PHV and among different bioprosthetic PHV. Structural valve deterioration is rare with mechanical PHV. Structural valve deterioration of bioprosthesis after MVR is higher than after AVR; after AVR, homografts and bioprosthesis have similar rates of SVD. The exact rate of SVD of the pulmonary autograft is uncertain. Valve prosthesis-patient mismatch is clinically important when it is severe and in selected patients when it is moderate. Bioprosthesis have a low rate of SVD in the older patient and, thus, are the PHV of choice for AVR in patients > or =60 to 65 years of age and for MVR in patients > or =65 to 70 years of age; in younger patients mechanical valves are the PHV of choice. In individual patients there may be exceptions to these general rules.

Primary aortic valve replacement with cryopreserved aortic allograft: an echocardiographic follow-up study of 570 patients

BACKGROUND

Despite the many advantages of an aortic allograft valve (AAV) over a prosthetic aortic valve, its durability is suboptimal. The aims of the present study were to document characteristic features of AAV dysfunction and to investigate factors influencing the development of such dysfunction.

METHODS AND RESULTS

A group of 570 patients (mean age, 48+/-16 years) with a cryopreserved AAV underwent a follow-up echocardiographic study (mean time after surgery, 6.8 years; range, 1.0 to 22.9 years). Significant AAV regurgitation was present in 14.7% of patients, and AAV stenosis was present in 3.2%. The root replacement subgroup had the smallest number of patients with significant AAV regurgitation (5.0%) compared with the subcoronary (23.0%) or the inclusion cylinder technique subgroup (14.7%). After 10 to 15 years after AAV replacement, grade > or =2 AAV dysfunction was present in 40% of patients in the subcoronary subgroup, but no significant dysfunction was observed in patients in the root replacement subgroup (P<0.001). Smaller host aortic annulus size in both subcoronary (coefficient, -0.145; P=0.013) and root replacement subgroups (coefficient, -0.249; P=0.011) was associated with more frequent AAV dysfunction (grade > or =2). In addition, significant AAV dysfunction was more frequent when patients were younger (coefficient, -0.020; P=0.015) in the subcoronary subgroup and the donor was older (coefficient, 0.054; P=0.019) in the root replacement subgroup.

CONCLUSIONS

The present study indicates that the root replacement technique is associated with less frequent AAV degeneration. Our findings should help in establishing more strict selection criteria for surgical replacement procedure type and patient/donor factors for AAV replacement and, therefore, could lead to improve AAV longevity.

Aortic allografts for left ventricular outflow tract replacement in children

Aortic allografts provide many advantages in children requiring left ventricular outflow tract (LVOT) reconstruction. The low risk of thromboembolic events and freedom from the requirement for anticoagulation are primary benefits. Additionally, excellent hemodynamic results are possible even in the presence of multilevel obstruction. The pulmonary autograft has become the favored approach in most pediatric centers, as the limited longevity of the aortic allograft has now become apparent. However, some children are not candidates for the pulmonary autograft. Thus, the aortic allograft remains a useful aortic valve substitute in children. Using standard aortic root replacement (ARR) or extended aortic root replacement (EARR) techniques, aortic allografts can be used in any circumstance. Young age and small size are predictive of shortened valve longevity and higher operative mortality compared with older children. Reoperation to replace a degenerated aortic allograft can be accomplished safely. Copyright 2000 by W.B. Saunders Company

Risk factors for late pulmonary homograft stenosis after the Ross procedure

BACKGROUND

We reviewed our experience with the Ross procedure to identify the prevalence and predictors of late pulmonary homograft stenosis.

METHODS

Between June 1992 and December 1997, 109 consecutive patients (age 34.5 +/- 8.6 years) underwent the Ross procedure, with reconstruction of the right ventricular outflow tract with a cryopreserved pulmonary homograft (22 to 30 mm diameter). There was one early and one late death. Echocardiographic follow-up was available in 105 of 108 patients (97%), with a follow-up of 39 +/- 20 months. Homograft donor and preservation measurements and patient variables were subjected to multivariable analyses to identify independent predictors of late homograft performance.

RESULTS

The major physiopathologic finding was homograft stenosis. Peak systolic gradients across the homograft were 20 mm Hg or more in 30 of 105 patients (28.5%) and 40 mm Hg or more in 4 of 105 patients (3.8%). One patient required two re-replacements of her homograft for severe stenosis. Moderate or severe homograft insufficiency was noted in 10 of 105 patients (9.5%). The independent predictors of late pulmonary homograft stenosis were younger donor age (p = 0.03), shorter duration of cryopreservation (p = 0.01), and smaller homograft size (p = 0.06). Beating heart donor status, short homograft ischemic time, and other factors that have been shown to be associated with increased graft viability were associated with graft stenosis but did not reach statistical significance. However, mean gradients across the homograft were significantly related (p = 0.002) to the number of these risk factors in each patient.

CONCLUSIONS

Stenosis of the pulmonary homograft can be a significant problem following the Ross procedure, and was predicted by younger donor age and shorter duration of cryopreservation. These factors may be related to increased cellular viability, which might actually predispose to late homograft stenosis in a subgroup of patients.

Hemodynamic and clinical impact of prosthesis-patient mismatch in the aortic valve position and its prevention

Prosthesis-patient mismatch is present when the effective orifice area of the inserted prosthetic valve is less than that of a normal human valve. This is a frequent problem in patients undergoing aortic valve replacement, and its main hemodynamic consequence is the generation of high transvalvular gradients through normally functioning prosthetic valves. The purposes of this report are to present an update on the concept of aortic prosthesis-patient mismatch and to review the present knowledge with regard to its impact on hemodynamic status, functional capacity, morbidity and mortality. Also, we propose a simple approach for the prevention and clinical management of this phenomenon because it can be largely avoided if certain simple factors are taken into consideration before the operation.

Revascularization of canine cryopreserved tracheal allografts

BACKGROUND

We examined the blood supply of a cryopreserved tracheal allograft and its morphohistologic changes after transplantation.

METHODS

In each of 22 dogs, a five-ring tracheal segment was replaced by one of the following tracheal grafts: fresh autografts (n = 8), cryopreserved tracheal allografts (n = 8), or fresh allografts (n = 6). The cryopreserved tracheal allografts were preserved at -196 degrees C for 60 days. No immunosuppressant was given to any of the animals. All grafts were retrieved at 1 and 12 weeks and assessed by microangiography and histology.

RESULTS

The epithelial denudation and the revascularization of the transverse intercartilaginous arteries were recognized within 7 days as common to each of the three types of grafts. In the cryopreserved tracheal allografts, neither cartilage degradation nor graft shrinkage occurred at 7 days. However, the recanalized transverse intercartilaginous arteries completely disappeared at 12 weeks, and marked shrinkage occurred; the cartilage cells were accompanied by karyolysis and were significantly decreased in number (p < 0.05). Recanalization of the transverse intercartilaginous arteries was also demonstrated in the fresh allografts; however, necrosis abruptly occurred as a result of acute rejection responses.

CONCLUSIONS

Cryopreservation of a tracheal allograft provided sufficient reduction of the acute rejection responses, and blood supply to the cryopreserved tracheal allograft was established through the recanalized transverse intercartilaginous arteries within 7 days; however, subsequent chronic rejection responses resulted in occlusion of the transverse intercartilaginous arteries and atrophy.

A size-matching heterotopic aortic valve implantation model in the rat

BACKGROUND

Structural failure of cardiac valve allografts may be related to technical factors such as size mismatch, resulting in early intimal proliferation and fibrosis or immunological reactions against the transplanted valves, featuring lymphocytic infiltration.

OBJECTIVE

To develop a heterotopic aortic valve implantation model in the rat to study the immunological factors leading to graft failure in the setting of a technical adaptation for size mismatch.

METHODS

Syngeneic (WAG-WAG or DA-DA) and allogeneic (WAG-BN or WAG-DA) rat strain combinations were used to study the effect of the allogeneic response on valve properties. An end-to-side anastomosis was made between the U-shaped aortic root graft and the recipient's abdominal aorta to resolve the problems of size matching.

RESULTS

No animals suffered from ischemic or neurological complications during the study period. One hundred percent survival and patency of the aortic grafts were achieved at the end of a 21-day observation period. In the syngeneic group 9 of 10 valves were still competent when assessed during retrograde injection. In contrast, 2 of 10 allogeneic valve grafts were competent on postoperative Day 21. Microscopic evaluation revealed no fibrosis or intimal thickening in the syngeneic valve grafts while the allogeneic valve grafts demonstrated rejection-like morphology.

CONCLUSION

The absence of fibrosis and intimal thickening in the syngeneic transplanted valve grafts indicates that this implantation model is not influenced by nonimmunological-based structural changes. Therefore, this new model enables us to study the association between donor-directed immune responses and allograft degeneration in a technically unbiased manner.

Correlations in uncertainty analysis for medical decision making: an application to heart-valve replacement

A Monte Carlo uncertainty analysis with correlations between parameters is applied to a Markov-chain model that is used to support the choice of a replacement heart-valve. The objective is to quantify the effects of uncertainty in and of correlations between probabilities of valve-related events on the life expectancies of four valve types. The uncertainty in the logit- and log-transformed parameters-mostly representing probabilities and durations-is modeled as a multivariate normal distribution. The univariate distributions are obtained through values for the median and the 0.975 quantile of each parameter. Correlations between parameters are difficult to quantify. A sensitivity analysis is suggested to study their influences on the uncertainty in valve preference prior to further elicitation efforts. The results of the uncertainty analysis strengthen the conclusions from a preceding study, which did not include uncertainty in the model parameters, where the homograft turned out to be the best choice. It is concluded that the influence of correlations is limited in most cases. Preference statements become more certain when the correlation between valve types increases.

Effect of storage temperature on cell viability in cryopreserved canine aortic, pulmonic, mitral, and tricuspid valve homografts

We determined how long cryopreserved aortic, pulmonic, mitral, and tricuspid valve homografts could be stored in a deep freezer (-80 degrees C) without compromising fibroblast viability. Valves harvested from 20 anesthetized mongrel dogs were grouped into nonfrozen control, frozen and stored in liquid nitrogen (-196 degrees C), and frozen and stored in a deep freezer (-80 degrees C). Frozen groups were divided into subgroups and stored for 2, 4, 8, or 12 weeks. A leaflet of each valve was divided into three fragments, and fibroblast viability was analyzed by flow cytometry. Cell viability was defined as staining by fluorescent diacetate but not by propidium iodide. The viability of untreated control valves from all four sites was about 70%, decreasing to about 50% when treated with low doses of antibiotics. The viability of frozen valves stored in liquid nitrogen was about 45% without a significant difference among storage periods. The viability of valves frozen and stored in a deep freezer was significantly lower than for the liquid nitrogen group at 2 weeks for the mitral valve and at 4 weeks for other valves. These results suggest that homografts can be stored in a deep freezer for up to 2 weeks without deterioration.

Allograft heart valves: the role of apoptosis-mediated cell loss

OBJECTIVE

The purpose of this study was to determine whether apoptosis of endothelial and connective tissue cells is responsible for the loss of cellularity observed in implanted aortic allograft valves.

METHODS

Fresh (n = 6) and cryopreserved (n = 4) aortic allograft valves were retrieved at 2 days to 20 weeks after implantation in an ovine model. Sections of these valves were studied with the use of histologic and electron microscopic methods, nick end-labeling and dual immunostaining for factor VIII-related antigen and proliferating cell nuclear antigen, followed by counterstaining for DNA and laser scanning confocal fluorescence microscopic observation.

RESULTS

The endothelial cells and cusp connective tissue cells of implanted valvular allografts showed loss of proliferating cell nuclear antigen (indicative of cessation of mitotic activity) and evidence of apoptosis (nick end labeling). The latter was manifested by nuclear condensation and pyknosis, positive nick end labeling, and formation of intra- and extracellular apoptotic bodies derived from the fragmentation of apoptotic cells. These changes began to develop at 2 days after implantation, peaking at 10 to 14 days, and became complete by 20 weeks, at which time the valves had the typical acellular morphologic features of allografts implanted for long periods of time.

CONCLUSIONS

Apoptosis occurs in endothelial cells and cuspal connective tissue cells of implanted allografts and appears to be a cause of their loss of cellularity. This apoptosis may be related to various factors, including immunologic and chemical injury, and hypoxia during valve processing and reperfusion injury at the time of implantation.

Primary aortic valve replacement with allografts over twenty-five years: valve-related and procedure-related determinants of outcome

OBJECTIVES

Allografts offer many advantages over prosthetic valves, but allograft durability varies considerably.

METHODS

From 1969 through 1993, 618 patients aged 15 to 84 years underwent their first aortic valve replacement with an aortic allograft. Concomitant surgery included aortic root tailoring (n = 58), replacement or tailoring of the ascending aorta (n = 56), and coronary artery bypass grafting (n = 87). Allograft implantation was done by means of a "freehand" subcoronary technique (n = 551) or total root replacement (n = 67). The allografts were antibiotic sterilized (n = 479), cryopreserved (n = 12), or viable (unprocessed, harvested from brain-dead multiorgan donors or heart transplant recipients, n = 127). Maximum follow-up was 27.1 years.

RESULTS

Thirty-day mortality was 5.0%, and crude survival was 67% and 35% at 10 and 20 years. Ten- and 20-year rates of freedom from complications were as follows: endocarditis, 93% and 89%; primary tissue failure, 62% and 18%; and redo aortic valve replacement, 81% and 35%. Multivariable Cox analyses identified several valve- and procedure-related determinants: rising allograft donor age and antibiotic-sterilized allograft for mortality; donor more than 10 years older than patient for endocarditis; rising donor age minus patient age, rising implantation time (from harvest to aortic valve replacement), and donor age more than 65 years for tissue failure; and rising donor age minus patient age, young patient age, rising implantation time, and subcoronary implantation preceded by aortic root tailoring for redo aortic valve replacement. Estimated 10- and 20-year rates of freedom from tissue failure for a 70-year-old patient with a viable valve from a 30-year-old donor and no other risk factors were 91% and 64%; the figures were 71% and 20% if the donor age was 65 years. The rates of freedom from tissue failure for a 30-year-old patient with a 30-year-old donor were 82% and 39%; the figures were 49% and 3% with a 65-year-old donor. Beneficial influences of a viable valve were largely covered by short harvest time (no delay for allografts from brain dead organ donors or heart transplant recipients) and short implantation time.

CONCLUSIONS

Primary allograft aortic valve replacement can give acceptable results for up to 25 years. The late results can be improved by the use of a viable allograft, by matching patient and donor age, and by more liberal use of free root replacement with re-implantation of the coronary arteries rather than tailoring the root to accommodate a subcoronary implantation.

Effect of HLA mismatching and antibody status on "homovital" aortic valve homograft performance

BACKGROUND

Recipients of "homovital" aortic valve homografts are known to produce specific antibodies to human leukocyte antigen (HLA) determinants present on the cellular compartment of the valve tissue; however, the clinical significance of these antibodies is unknown. Data from 182 patients receiving homovital aortic valve homografts has been analyzed to determine the impact of HLA disparity and HLA antibody production on survival and function of the homograft.

METHODS

Human leukocyte antigen mismatch data were available for 127 patients (mean follow-up, 6.02+/-0.26 years). Two patients were considered well matched for HLA A+B antigens (zero or one mismatch) compared with 125 poorly matched (two to four mismatches). Nine patients had a zero HLA-DR mismatch compared with 52 with one mismatch and 59 patients completely mismatched for DR antigens.

RESULTS

There was no significant association between the degree of HLA mismatch for either class I or class II antigens whether the loci were considered alone or in combination (ie, A, B, DR, AB, or ABDR mismatching) with markers of long-term valve function including patient mortality, reoperation, valve degeneration, valve stenosis, presence of regurgitation, and postoperative New York Heart Association class. One hundred thirty-six of 167 (82%) were found to have produced antibodies after operation (mean time after operation, 6.42+/-0.58 years). In 61 cases both antibody specificity and donor HLA typing was available. In 92% of these, the antibodies were of the IgG subclass and were specific for the HLA class I molecules of the donor. The presence of HLA antibodies was associated with an increase in the frequency of mild valve stenosis (not significant) compared with those patients who did not develop HLA antibodies (antibody negative = 9.7%; panel reactive antibodies <50% = 29.1%; and panel reactive antibodies >50% = 22.2%; not significant). There was also an increased prevalence of valve degeneration associated with HLA antibodies. The actuarial freedom from valve degeneration for the 35 HLA antibody-negative patients was 100% at 1, 5, and 10 years compared with 100% at 1 year, 97% at 5 years, and 92% at 10 years for 55 patients with panel reactivity less than 50%, and 98% at 1 year, 94% at 5 years, and 88% at 10 years for the 77 patients who were highly sensitized (not significant). There was no correlation with other markers of long-term valve function.

CONCLUSIONS

The influence of the immune response on valve function requires further studies involving large numbers of patients followed for a longer period of time. We believe prospective matching for HLA antigens is warranted to produce a well-matched cohort of patients for analysis and to reduce antibody sensitization, which would help to clarify this issue.