Effects of fixation back pressure and antimineralization treatment on the morphology of porcine aortic bioprosthetic valves

Current indications for stentless aortic bioprostheses

The best aortic prostheses have been debated for decades. The introduction of stentless aortic bioprostheses was aimed at improving hemodynamics and potentially the durability of aortic bioprostheses. Despite the good short- and long-term outcomes after implantation of stentless aortic bioprostheses, their use remains limited owing to the technically demanding implantation techniques. Nevertheless, stentless aortic bioprostheses might be of special benefit in certain indications, where they could be a valuable addition to the surgical armamentarium.

Current Progress in Anticalcif ication for Bioprosthetic and Polymeric Heart Valves

The use of bioprosthetic valves fabricated from fixed heterograft tissue (porcine aortic valves or bovine pericardium) in heart valve replacement surgery is limited because of calcification-related failures. The mechanism of calcification of bioprosthetic valves is quite complex and has a variety of determinants, including host factors, tissue fixation conditions, and mechanical effects. Currently, there is no effective therapy to prevent calcification in clinical settings. This article reviews a variety of anticalcification strategies that are under investigation either in advanced animal models or in clinical trials. Bisphosphonates, such as ethan hydroxybisphosphonate (EHBP), inhibit calcium phosphate crystal formation. However, because of their systemic toxicity, they are used as either tissue treatments or polymeric site-specific delivery systems. Detergent treatment, such as sodium dodecyl sulfate (SDS), extracts almost all phospholipids from bioprosthetic heart valve cuspal tissue. Procedures, such as amino oleic acid pretreatment, inhibit calcium uptake. Polyurethane trileaflet valves, investigated as alternatives to bioprosthetic or mechanical valve prostheses, undergo intrinsic and thrombus-related calcification and degradation. Calcification- and thrombus-resistant polyurethanes synthesized in our laboratory by covalent linking of EHBP or heparin (either in bulk or on surface) by unique polyepoxidation chemistry are attractive candidates for further research. Tissue-engineered heart valves may have an important place in the future.

Approach to the analysis of cardiac valve prostheses as surgical pathology or autopsy specimens

Pathologists are likely to encounter substitute heart valves with increasing frequency. Informed evaluation of such valves provides valuable information that contributes to both patient care and our understanding of the pathobiology of host interactions with mechanical devices. This article summarizes the most important considerations underlying such analyses-including valve identification, common morphologic features and modes of failure, technical details of evaluation, and potential pitfalls.

Effect of glutaraldehyde fixation on the frictional response of immature bovine articular cartilage explants

This study examined functional properties and biocompatibility of glutaraldehyde-fixed bovine articular cartilage over several weeks of incubation at body temperature to investigate its potential use as a resurfacing material in joint arthroplasty. In the first experiment, treated cartilage disks were fixed using 0.02, 0.20 and 0.60% glutaraldehyde for 24h then incubated, along with an untreated control group, in saline for up to 28d at 37°C. Both the equilibrium compressive and tensile moduli increased nearly twofold in treated samples compared to day 0 control, and remained at that level from day 1 to 28; the equilibrium friction coefficient against glass rose nearly twofold immediately after fixation (day 1) but returned to control values after day 7. Live explants co-cultured with fixed explants showed no quantitative difference in cell viability over 28d. In general, no significant differences were observed between 0.20 and 0.60% groups, so 0.20% was deemed sufficient for complete fixation. In the second experiment, cartilage-on-cartilage frictional measurements were performed under a migrating contact configuration. In the treated group, one explant was fixed using 0.20% glutaraldehyde while the apposing explant was left untreated; in the control group both explants were left untreated. From day 1 to 28, the treated group exhibited either no significant difference or slightly lower friction coefficient than the untreated group. These results suggest that a properly titrated glutaraldehyde treatment can reproduce the desired functional properties of native articular cartilage and maintain these properties for at least 28d at body temperature.

Evolving concepts of cardiac valve dynamics: the continuum of development, functional structure, pathobiology, and tissue engineering

Considerable progress has been made in recent years toward elucidating a conceptual framework that integrates the dynamic functional structure, mechanical properties, and pathobiological behavior of the cardiac valves. This communication reviews the evolving paradigm of a continuum of heart valve structure, function, and pathobiology and explores its implications. Specifically, we discuss (1) the interactions of valve biology and biomechanics (eg, correlations of function with structure at the cell, tissue, and organ levels and mechanical considerations, development, endothelial cell and interstitial cell biology, extracellular matrix biology, homeostasis, and adaptation to environmental change); (2) mechanisms of disease (eg, valve cell and matrix pathobiology in congenital anomalies, aortic valve calcification, and mitral valve prolapse); (3) considerations in replacement and repair (eg, cell/matrix biology of tissue valve substitutes and their degeneration and durability of repairs); and (4) the potential for tissue engineering approaches to therapeutic regeneration of the cardiac valves. Opportunities for research and clinical translation are highlighted.

Tissue-engineered heart valve prostheses: ‘state of the heart’

In this article, we will review the current state of the art in heart valve tissue engineering. We provide an overview of mechanical and biological replacement options, outlining advantages and limitations of each option. Tissue engineering, as a field, is introduced, and specific aspects of valve tissue engineering are discussed (e.g., biomaterials, cells and bioreactors). Technological hurdles, which need to be overcome for advancement of the field, are also discussed.

Limited long-term durability of the Cryolife O'Brien stentless porcine xenograft valve

BACKGROUND

Despite the fact that early and midterm hemodynamic and clinical results of the Cryolife O'Brien (CLOB) stentless valve have been reported to be favorable, the long-term durability and clinical results of this valve are largely unknown. Accordingly, we analyzed 10-year outcomes after aortic valve replacement with this valve.

METHODS AND RESULTS

From January 1994 to September 2004, 185 patients (67, 73, and 75 years, 25th, 50th, and 75th quartiles, respectively; 38% older than 75 years; 56% females) underwent aortic valve replacement with the CLOB valve. Sixty-eight percent of patients were in NYHA class 3 to 4. Standard EuroSCORE was 7.1+/-2.7. Pure aortic stenosis accounted for 42% (n=79), and pure insufficiency for 12% of cases (n=22). Concomitant surgery: 28% coronary artery bypass (n=51), 11% mitral valve replacement/annuloplasty (n=21), and 2% ascending aorta replacement (n=3). Sixty-one percent of patients received a 23-mm valve or smaller size. Follow-up was 100% completed, and cumulative follow-up was 65 months/patient. The 30-day mortality was 5.4% (none were valve related). Actuarial survival at 5 and 10 years were 68% and 40%, respectively. Actuarial freedom from structural valve deterioration was 91% at 5 years and dropped to 44% at 10 years. Actuarial freedom from reoperation was 94% at 5 years and declined to 57% at 10 years.

CONCLUSIONS

In a population with a high prevalence of elderly females with small aortic root, the CLOB valve demonstrated satisfactory clinical results till 8-years. Afterward, a significant increase in hazard for structural valve deterioration and reoperation occurred in late follow-up.

Factors influencing the mean postoperative gradients across stentless porcine valves

BACKGROUND

To study the preoperative and intraoperative variables influencing the mean post-operative transvalvular gradient across stentless porcine valves.

METHODS

From 1995 to 2002, 84 patients underwent stentless valve insertion. The mean age was 73 years, and 63% were male. The valve pathology was aortic stenosis (AS) in 79%, aortic regurgitation (AR) in 12%, and mixed in 9%. Valve sizes ranged from 21 to 29 with size 27 being most frequent. 54% of patients had concomitant procedures. Patients had at least yearly clinical and echocardiographic follow-up.

RESULTS

There was no operative mortality. 9.5% of the patients had significant postoperative complications. The average echo interval was 18.6 months (range 1-88). The overall mean transvalvular gradient was 9.88+/-5.67 (SD) mmHg. Variables associated with significantly reduced gradients were: larger valve sizes (p=0.002), younger age (p=0.05), pre-op AR (p=0.008), and increasing post-operative interval (p=0.05). The mean gradients decreased by 0.28 mmHg for each post-operative year. The method of implantation did not significantly affect gradients (p=0.26).

CONCLUSIONS

Excellent mean transvalvular gradients were achieved with stentless valves studied, with a low operative risk. The gradients did not appear to be related to intra-operative factors, suggesting that insertion techniques can be tailored to suit patient conditions and surgeon preferences.

Prevention of calcification in bioprosthetic heart valves: challenges and perspectives

Surgical replacement with artificial devices has revolutionised the care of patients with severe valvular diseases. Mechanical valves are very durable, but require long-term anticoagulation. Bioprosthetic heart valves (BHVs), devices manufactured from glutaraldehyde-fixed animal tissues, do not need long-term anticoagulation, but their long-term durability is limited to 15 - 20 years, mainly because of mechanical failure and tissue calcification. Although mechanisms of BHV calcification are not fully understood, major determinants are glutaraldehyde fixation, presence of devitalised cells and alteration of specific extracellular matrix components. Treatments targeted at the prevention of calcification include those that target neutralisation of the effects of glutaraldehyde, removal of cells, and modifications of matrix components. Several existing calcification-prevention treatments are in clinical use at present, and there are excellent mid-term clinical follow-up reports available. The purpose of this review is to appraise basic knowledge acquired in the field of prevention of BHV calcification, and to provide directions for future research and development.

Long-Term Results of Aortic Valve Replacement With the St. Jude Toronto Stentless Porcine Valve

BACKGROUND

Long-term survival and freedom from valve-related events of the St. Jude Toronto stentless porcine valve (SPV) are unknown. The aim of this study was to investigate late clinical outcomes after aortic valve replacement with the Toronto SPV.

METHODS

Between 1992 and 2000, 200 patients (131 males, 69 females) underwent aortic valve replacement with the Toronto SPV. Mean patient age at implantation was 64.6 +/- 10.9 years (range 33 to 82 years). At the time of operation, 32%, 51%, and 17% of patients were in New York Heart Association class I/II, III, and IV, respectively. Aortic stenosis, aortic insufficiency, and combined lesions were present in 64%, 13.5%, and 22.5% of patients preoperatively. Concomitant coronary artery bypass grafting was performed in 34.5% of patients.

RESULTS

Perioperative mortality occurred in 2.5% (5/200) of patients. There were 31 late deaths. Actuarial survival at 5 and 10 years was 89.2% and 68.0%, respectively. There was no significant difference in overall actuarial survival between isolated valve patients and valve plus coronary artery bypass grafting patients, 71% versus 62% respectively, p = 0.85. Actuarial freedom from valve reoperation at 5 and 10 years was 97.6% and 79.9%, respectively. Actuarial freedom from structural valve deterioration was 98.8% at 5 years and declined to 77.9% at 10 years. Freedom from structural valve deterioration was poorer in patients with preoperative aortic insufficiency or bicuspid disease. Actuarial freedom from embolic events and endocarditis at 10 years were 94.6% and 95.9%, respectively.

CONCLUSIONS

Although early clinical results were excellent, a significant increase in hazard for structural valve deterioration occurred in late follow-up.

The Medtronic Intact Bioprosthesis: Clinical and Hemodynamic Performance Over 13 Years

We evaluated our results over 13 years with the aortic-position Medtronic Intact bioprosthesis. Our study involved 91 consecutive patients with isolated aortic valve replacement with the Medtronic Intact bioprosthesis. The follow-up was complete for 95%. Mean follow-up was 6.61 years (range 16 days-13 years), 590 patient years. Early mortality rate was 3.3%. Late mortality was 23 patients. Survival at 13 years was 53.52% (SD = 7.63%). The linearized rate of major thromboembolism was 0.34% per patient year; rate of major bleeding events was 0.33% per patient year. The rate of nonstructural dysfunction was 0.16% per patient year. Rate of reoperation was 0.53% and rate of structural valve deterioration was 0.16% per patient year. New York Heart Association (NYHA) postoperative classes were I to II in 92.21%. Gradients were as follows: 21 to 23.87 mm Hg, 23 to 18 mm Hg, 25 to 15.5 mm Hg, and 27 to 16.50 mm Hg. Structural valve deterioration was low during the 13 years of follow-up. Valve gradients and areas remained the same over the follow-up period. The Medtronic Intact bioprosthesis shows excellent clinical and hemodynamic performance at 13 years of follow up.

Tissue reactions to epoxy-crosslinked porcine heart valves post-treated with detergents or a dicarboxylic acid

Calcification limits the long-term durability of xenograft glutaraldehyde (GA)-crosslinked heart valves. Previously, a study in rats showed that epoxy-crosslinked heart valves reduced lymphocyte reactions to the same extent as the GA-crosslinked control and induced a similar foreign-body response and calcification reaction. The present study was aimed at reducing the occurrence of calcification of epoxy-crosslinked tissue. Two modifications were carried out and their influence on cellular reactions and the extent of calcification after 8 weeks' implantation in weanling rats was evaluated. First, epoxy-crosslinked valves were post-treated with two detergents to remove cellular elements, phospholipids and small soluble proteins, known to act as nucleation sites for calcification. The second approach was to study the effect of the impaired balance between negatively and positively charged amino acids by an additional crosslinking step with a dicarboxylic acid. The detergent treatment resulted in a washed-out appearance of especially the cusp tissue. With the dicarboxylic acid, both the cusps and the walls had a limited washed-out appearance. The wall also demonstrated some detachment of the subendothelium. After implantation, both detergent and dicarboxylic acid post-treatment histologically resulted in reduced calcification at the edges of cusps and walls. However, total amounts of calcification, measured by atomic emission spectroscopy, were not significantly reduced. Data concerning the presence of lymphocytes varied slightly, but were in the same range as the GA-crosslinked control, i.e., clearly reduced compared with a noncrosslinked control. It is concluded that both the double detergent and the dicarboxylic acid post-treatment of epoxy-crosslinked heart valve tissue do not represent a sound alternative in the fabrication of heart valve bioprostheses.

Retrospective clinical analysis of stented vs. stentless porcine aortic bioprostheses

OBJECTIVE

The study was designed to compare hemodynamic performance, structural failure and survival of patients undergoing aortic valve replacement (AVR) with a composite aortic stented or stentless porcine bioprosthesis.

METHODS

From January 1990 to June 1999, the clinical data of 725 patients undergoing AVR with stented porcine aortic bioprosthesis were reviewed. We defined two groups of patients with similar clinical characteristics: 202 patients receiving aortic stented and 205 patients stentless valves. The two patients groups were similar in age, sex, valve lesion, valve size, preoperative New York Heart Association (NYHA) class status and follow-up.

RESULTS

The number of patients available for follow-up, excluding hospital and late mortality, reoperations and patients lost to follow-up, was 157 for the stented and 175 for the stentless group. There was a higher incidence of rheumatic heart disease in the stented (59%) vs. stentless group (44%), (P=0.003). Fewer patients had prior aortic bioprosthetic dysfunction in the stented (7.6%) compared to the stentless group (25%) (P<0.001). The mean intensive care unit stay, hospital mortality and late mortality were similar (P, NS). The total complication rate was higher in the stented (12%) than the stentless (3.4%)(P=0.005). Valve related death was higher in the stented (2.5%) than the stentless (0%) (P=0. 049). Postoperatively, the aortic effective orifice area (AEOA) was larger (P<0.001) and the transvalvular peak and mean gradients were lower in the stentless group (P<0.001). The leaflet tissue degeneration analysis was 8.0% in patients at risk for stented and 0. 6% for stentless (P=0.001). Actuarial analysis disclosed no statistical difference in patient survival between groups (P=0.18). Reoperations were less frequent in the stentless group (P=0.010).

CONCLUSIONS

Hemodynamic benefits in the stentless group were evident and expressed by larger AEOA, lower gradients, better left ventricular remodeling with significant decrease of the left ventricular mass. Lower complication rates, lower reoperation rates, less leaflet tissue degeneration, and lower valve related mortality rates were seen in the stentless group. A controlled clinical comparison trial with longer follow-up will be required to confirm these clinical and hemodynamic benefits.

Founder's Award, 25th Annual Meeting of the Society for Biomaterials, perspectives. Providence, RI, April 28-May 2, 1999. Tissue heart valves: current challenges and future research perspectives

Substitute heart valves composed of human or animal tissues have been used since the early 1960s, when aortic valves obtained fresh from human cadavers were transplanted to other individuals as allografts. Today, tissue valves are used in 40% or more of valve replacements worldwide, predominantly as stented porcine aortic valves (PAV) and bovine pericardial valves (BPV) preserved by glutaraldehyde (GLUT) (collectively termed bioprostheses). The principal disadvantage of tissue valves is progressive calcific and noncalcific deterioration, limiting durability. Native heart valves (typified by the aortic valve) are cellular and layered, with regional specializations of the extracellular matrix (ECM). These elements facilitate marked repetitive changes in shape and dimension throughout the cardiac cycle, effective stress transfer to the adjacent aortic wall, and ongoing repair of injury incurred during normal function. Although GLUT bioprostheses mimic natural aortic valve structure (a) their cells are nonviable and thereby incapable of normal turnover or remodeling ECM proteins; (b) their cuspal microstructure is locked into a configuration which is at best characteristic of one phase of the cardiac cycle (usually diastole); and (c) their mechanical properties are markedly different from those of natural aortic valve cusps. Consequently, tissue valves suffer a high rate of progressive and age-dependent structural valve deterioration resulting in stenosis or regurgitation (>50% of PAV overall fail within 10-15 years; the failure rate is nearly 100% in 5 years in those <35 years old but only 10% in 10 years in those >65). Two distinct processes-intrinsic calcification and noncalcific degradation of the ECM-account for structural valve deterioration. Calcification is a direct consequence of the inability of the nonviable cells of the GLUT-preserved tissue to maintain normally low intracellular calcium. Consequently, nucleation of calcium-phosphate crystals occurs at the phospholipid-rich membranes and their remnants. Collagen and elastin also calcify. Tissue valve mineralization has complex host, implant, and mechanical determinants. Noncalcific degradation in the absence of physiological repair mechanisms of the valvular structural matrix is increasingly being appreciated as a critical yet independent mechanism of valve deterioration. These degradation mechanisms are largely rationalized on the basis of the changes to natural valves when they are fabricated into a tissue valve (mentioned above), and the subsequent interactions with the physiologic environment that are induced following implantation. The "Holy Grail" is a nonobstructive, nonthrombogenic tissue valve which will last the lifetime of the patient (and potentially grow in maturing recipients). There is considerable activity in basic research, industrial development, and clinical investigation to improve tissue valves. Particularly exciting in concept, yet early in practice is tissue engineering, a technique in which an anatomically appropriate construct containing cells seeded on a resorbable scaffold is fabricated in vitro, then implanted. Remodeling in vivo, stimulated and guided by appropriate biological signals incorporated into the construct, is intended to recapitulate normal functional architecture.

Assessment of pericardium in cardiac bioprostheses. A review

Cardiac valve bioprostheses are assessed in terms of their present and future clinical utility. The problems concerning durability basically involve early failure due to tears in the valve leaflets and late failure mainly associated with calcification of the biological tissue. New strategies for selection and chemical treatment of the biomaterials employed are analyzed, and the available knowledge regarding their mechanical behavior is reviewed. It is concluded that the durability of these devices, and thus their successful use in the future, depends on the knowledge of the interactions among the different biomaterials of which they are composed, the development of new materials, and the engineering design applied in their construction.

Carpentier-Edwards standard and supraannular porcine bioprostheses: comparison of technology

BACKGROUND

Performance with regard to structural valve deterioration (SVD) with the Carpentier-Edwards standard (CE-S) and supraannular (CE-SAV) (Baxter Healthcare Corp, Irvine, CA) porcine bioprostheses was evaluated to determine whether progress in reduction of structural failure has been achieved with technological changes.

METHODS

The CE-S was implanted during 567 aortic valve replacement (AVR) and 486 mitral valve replacement (MVR) procedures, and the CE-SAV was implanted during 1,670 AVR and 1,096 MVR procedures. The failure mode of early stent dehiscence with the CE-SAV prosthesis, thought to be controlled by manufacturing changes in 1986 and 1987, supported comparison of the CE-SAV with censored cases of stent dehiscence. Stent dehiscence accounted for only 1.2% (1 of 81) and 14.1% (29 of 205) of AVR and MVR CE-SAV failures, respectively.

RESULTS

The only difference for AVR for freedom from SVD occurred in the 21- to 40-year age group at 15 years and was 68% for the CE-SAV and 31% for the CE-S (p<0.05). In the 61- to 70-year age group, freedom from SVD at 15 years was 76% for the CE-S and 84% for the CE-SAV; for the 71-year or higher age group, freedom from SVD was 89% and 95%, respectively (p = NS). For MVR freedom from SVD was different only in the 71-year or higher age group and was 90% for the CE-S and 59% for the CE-SAV (p<0.05). Freedom from SVD was reduced but was similar (p = NS) for the other age groups. For AVR the actual freedom from SVD at 15 years for the CE-S and CE-SAV was, respectively, 79% and 72% for the 51- to 60-year age group, 86% and 91% for the 61- to 70-year age group, and 98% and 98% for the 71-year or higher age group. For MVR, these rates were, respectively, 69% and 75% for the 61- to 70-year age group and 96% and 89% for the 71-year and higher age group.

CONCLUSIONS

The technologic advancements made in the second-generation CE-SAV bioprosthesis to reduce the incidence of structural failure have not uniformly been successful. The actual freedom from SVD provides evidence for implantation of porcine bioprostheses for AVR in age groups 61 to 70 years and 71 years or higher and for MVR in the age group 71 years or higher.

The mosaic bioprosthesis in the aortic position: hemodynamic performance after 2 years

BACKGROUND

The Mosaic bioprosthesis is a porcine valve combining several new features to improve hemodynamics and durability: a low profile stent for reduced flow obstruction, zero pressure fixation to maintain the natural collagen crimp, and the amino oleic acid antimineralization treatment to enhance durability.

METHODS

Fifty-five Mosaic valves were implanted in the aortic position since February 1994. Data from these patients (group 1) were compared with data from 52 patients who had received a Hancock Modified Orifice II aortic valve (group 2). The mean patient age was 72.0 (+/-5.9) years for group 1 and 76.8 (+/-4.7) years for group 2. Clinical examinations including transthoracic echocardiography were performed 6 and 24 months postoperatively.

RESULTS

Mild aortic insufficiencies were found in 2 patients in group 1 and in 3 patients in group 2. There were no embolic or bleeding complications. One Mosaic patient required reoperation due to mitral insufficiency. During the reoperation, a small (approximately 3 mm) thrombus was noted on the outflow side of a Mosaic cusp. The valve was removed and replaced prophylactically. In the 2-year follow-up, hemodynamic measurements showed mean pressure gradients of 12.4 mm Hg for the 21 mm, 11.3 mm Hg for the 23 mm, and 15.4 mm Hg for the 25 mm prostheses in the Hancock group. In the Mosaic group, mean pressure gradients were 14.8 mm Hg for the 21 mm, 10.9 mm Hg for the 23 mm, and 11.5 mm Hg for the 25 mm valves. Differences between pressure gradients and effective orifice areas of the Hancock and the Mosaic valves were not statistically significant. Early mortality in group 1 was 3.6% and in group 2 3.8%. Overall mortality was 12.7% and 13.5%, respectively.

CONCLUSIONS

The Mosaic valve has low pressure gradients for all sewing ring diameters. Compared with the Hancock Modified Orifice valve, there was no statistically significant gradient difference but a tendency toward better hemodynamics was noted in the Mosaic group after 2 years.

The aortic valve microstructure: effects of transvalvular pressure

We undertook this study to establish a more quantitative understanding of the microstructural response of the aortic valve cusp to pressure loading. Fresh porcine aortic valves were fixed at transvalvular pressures ranging from 0 mmHg to 90 mmHg, and small-angle light scattering (SALS) was used to quantify the gross fiber structure of the valve cusps. At all pressures the fiber-preferred directions coursed along the circumferential direction. Increasing transvalvular pressure induced the greatest changes in fiber alignment between 0 and 1 mmHg, with no detectable change past 4 mmHg. When the fibrosa and ventricularis layers of the cusps were re-scanned separately, the fibrosa layer revealed a higher degree of orientation while the ventricularis was more randomly oriented. The degree of fiber orientation for both layers became more similar once the transvalvular pressure exceeded 4 mmHg, and the layers were almost indistinguishable by 60 mmHg. It is possible that, in addition to retracting the aortic cusp during systole, the ventricularis mechanically may contribute to the diastolic cuspal stiffness at high transvalvular pressures, which may help to prevent over distention of the cusp. Our results suggest a complex, highly heterogeneous structural response to transvalvular pressure on a fiber level that will have to be duplicated in future bioprosthetic heart valve designs.

Prevention of bioprosthetic heart valve calcification by ethanol preincubation. Efficacy and mechanisms

BACKGROUND

Calcification of the cusps of bioprosthetic heart valves fabricated from either glutaraldehyde cross-linked porcine aortic valves or bovine pericardium frequently causes the clinical failure of these devices. Our investigations studied ethanol pretreatment of glutaraldehyde cross-linked porcine aortic valves as a new approach to prevent cuspal calcification. The hypothesis governing this approach holds that ethanol pretreatment inhibits calcification resulting from protein structural alterations and lipid extraction.

METHODS AND RESULTS

Results demonstrated complete inhibition of calcification of glutaraldehyde-pretreated porcine bioprosthetic aortic valve cusps by 80.0% ethanol in rat subdermal implants (60-day ethanol-pretreated calcium level, 1.87 +/- 0.29 micrograms/mg tissue compared with control calcium level, 236.00 +/- 6.10 micrograms/mg tissue) and in sheep mitral valve replacements (ethanol-pretreated calcium level, 5.22 +/- 2.94 micrograms/mg tissue; control calcium level, 32.50 +/- 11.50 micrograms/mg tissue). The mechanism of ethanol inhibition may be explained by several observations: ethanol pretreatment resulted in an irreversible alteration in the amide I band noted in the infrared spectra for both purified type I collagen and glutaraldehyde cross-linked porcine aortic leaflets. Ethanol pretreatment also resulted in nearly complete extraction of leaflet cholesterol and phospholipid.

CONCLUSIONS

Ethanol pretreatment of glutaraldehyde cross-linked porcine aortic valve bioprostheses represents a highly efficacious and mechanistically based approach and may prevent calcific bioprosthetic heart valve failure.

Efficacy of the chitosan posttreatment in calcification prevention of the glutaraldehyde-treated porcine aortic noncoronary cusp implanted in the right ventricular outflow tract in dogs

Calcific tissue failure results in poor performance of the bioprosthetic heart valve. Chitosan post-treatment has been shown to be effective in calcification prevention of the glutaraldehyde-treated bovine pericardium when implanted subdermally in rats for 12 weeks. The present study investigated the effectiveness of the chitosan posttreatment in prevention of calcification of the glutaraldehyde-treated porcine aortic noncoronary cusp 5 months after implantation in the right ventricular outflow tract (RVOT) in mongrel dogs. Either 0.625% glutaraldehyde-treated (Group 1, n = 6) or glutaraldehyde-chitosan-treated (Group 2, n = 6) porcine aortic noncoronary cusp with the aortic wall was sewn to the RVOT. Gross histological observations showed moderate calcification of the glutaraldehyde-treated cusps, but no calcification was noticed in the glutaraldehyde-chitosan-treated grafts at 5 months. This was confirmed by results of quantitative analyses for calcium in half of each explanted cusp with aortic wall. The calcium content of the 0.625% glutaraldehyde-treated cusps (Ca, 40.6 +/- 24.9 mg/g dry wt) was significantly (p < 0.01) higher than that of glutaraldehyde-chitosan-treated cusps (Ca, 1.3 +/- 0.29 mg/g dry wt). These findings suggest that chitosan post-treatment is effective in complete calcium mitigation of the glutaraldehyde-treated porcine aortic noncoronary cusps implanted in the RVOT in dogs.

Pathology of substitute heart valves: new concepts and developments

All types of contemporary cardiac valve substitutes suffer deficiencies and complications that limit their success. Mechanical and bioprosthetic valves are intrinsically obstructive, especially in small sizes. Mechanical valves are associated with thromboembolic problems; the chronic anticoagulation used in virtually all mechanical valve recipients causes hemorrhage in some. Calcification limits the success of porcine and pericardial bioprostheses, allograft valves, and the yet experimental trileaflet polymeric prostheses. The predominant mechanism of calcification in porcine, pericardial, and allograft valves is cell mediated, being nucleated at the membranes and in organelles of the transplanted cells. In polymeric leaflet valves, calcification is both extrinsic (in adherent thrombus) and intrinsic (subsurface and acellular in the solid elastomer). Nevertheless, except for a few notable exceptions, contemporary mechanical valves are durable. Other important potential complications of prosthetic and bioprosthetic valves include paravalvular leak, endocarditis, or extrinsic interference with function. Moreover, aortic valvular allografts undergo progressive noncalcific degeneration, tearing, sagging, and/or retraction. Studies of retrieved long-term cryopreserved allograft explants demonstrate severe degeneration, with distortion of normal architectural detail, loss of endothelial and deep connective tissue cells, and variable inflammatory cellularity. Thus, they are morphologically nonviable valves, whose structural basis for function seems primarily related to the largely preserved collagen, and they are unlikely to have the capacity to grow, remodel, or exhibit active metabolic functions. Since calcification intrinsic to the cusps is the major pathologic process necessitating bioprosthetic valve reoperations, efforts to prevent formation of mineral deposits are active.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of calcification of glutaraldehyde pretreated porcine aortic valve cusps with sodium dodecyl sulfate: preincubation and controlled release studies

Calcification of bioprosthetic heart valves fabricated from glutaraldehyde pretreated bovine pericardium or porcine aortic valves (PAV) is a frequent cause of the failure of these devices. Of all strategies considered thus far, only detergent preincubations using compounds such as sodium dodecyl sulfate (SDS) inhibited PAV bioprosthetic mineralization in circulatory sheep bioprosthetic valve replacements. The present study sought to characterize the mechanism of action of SDS preinicubation. Results of transport and material characterization studies showed that SDS had a relatively high affinity for PAV, with a maximum uptake of 167.1 +/- 6.8 micrograms SDS/mg tissue over 24 h at 37 degrees C with a partition coefficient of 19.3. The PAV diffusion of SDS was 1.95 +/- 0.35 10(-6) cm2/sec. The principal effect of SDS on PAV was phospholipid extraction. The residual organic phosphate in the SDS pretreated tissue was 2.22 +/- 0.72 nmol/mg tissue compared to the control untreated group with 18.52 +/- 2.1 nmol/mg tissue. Incubations of PAV specimens in a 1% SDS solution for 24 h significantly inhibited calcification after 21 days in subdermal implants in 3-week-old male rats (PAV Ca2+ = 18.0 +/- 11.8 micrograms/mg) compared to control (177.8 +/- 6.0 micrograms/mg). In contrast, coimplants of 30% SDS silicone rubber polymers, for regional sustained SDS administration, did not impede PAV calcification in 21 day implants Ca2+ = 166.0 +/- 14.0 micrograms/mg compared to the nondrug silicone matrix controls, Ca2+ = 173.0 +/- 6.6 micrograms/mg). Thus, we conclude that the mechanisms of SDS inhibition of PAV calcification is due to material effects which occur during preincubation, and is not facilitated by sustained SDS administration.