Creating a bicuspid valve from the aortic wall: a new surgical approach on aortic valve disease (in vitro study).

We described a new technique (called 'bicuspidization'), which is performed by using autogenous material, without replacement of the aortic valves for the surgical treatment of aortic stenosis and/or insufficiency and tested it in in vitro sheep model. Different stress conditions were simulated by applying three different flow patterns (hemodynamic challenge tests) successively by using a centrifugal pump. It was demonstrated that the competency of the new bicuspid valves was excellent (zero insufficiency). There was a 10-11 mm Hg-increase on trans-valvular gradient comparing the normal hearts. The autogenous bicuspid valve has not blocked the way of the coronary flow in the closed position.     

The Angell-Shiley porcine xenograft

A 4-year clinical experience with fresh allograft tissue valves prompted a trial of 0.5% buffered glutaraldehyde as a valve fixative and sterilant. Tanned allograft and porcine xenograft valves were inserted into experimental animals, and, beginning in 1970, similar valves were implanted in a series of patients now totaling 312. The clinical results are excellent. The 5-year valve-related mortality is 6% for patients who had mitral valve replacement and 16% for those with aortic valve replacement. To date, the incidence of thromboembolism is 1.3% per patient-year, and valve-related morbidity and mortality for the combined groups is 27.4%. Valve stent design has evolved from symmetrically configured metal to anatomically molded plastic. The maintenance of natural valve configuration has optimized leaflet coaptation and support, decreased tissue stress, and eliminated valve-stent dehiscence and tissue rupture seen in valves deformed to fit symmetrical stents. Stent design, controlled glutaraldehyde solutions, and fixation techniques have improved leaflet flexibility and reduced valve orifice to annulus diameter ratios, thus producing transvalvular gradients comparable to both mechanical and modified orifice tissue valves. To date, tissue failure, observed in only 1.0% (3 of 287) of patients, is the result of calcification (2 patients) and cusp rupture due to incomplete fixation (1 patient).     

In situ venous valve construction

A technique of in situ deep venous valve construction based on vein wall intussusception is described. The technique has been used to construct a size-matched, autogenous valve in the femoral vein of the dog. Forty-one valves have been constructed. Immediately after construction, all 41 were patent at ascending phlebography and Harvey's test showed them to be competent. Descending phlebography demonstrated competence in 38 valves. Hydrostatic testing was performed on 27 valves in the vertical position at pressure gradients in the range 0.2-250 cmH2O. Of these, 24 valves were fully competent. Six valves were studied at intervals between 1 and 112 days after construction. All six were competent by Harvey's test and five were competent at descending phlebography. There was no evidence of thrombosis in any valve. This technique may be suitable for replacement of deep venous valves in patients with the calf pump failure syndrome caused by deep venous reflux.     

新型（XJ）异种猪瓣的研制与临床应用

目的介绍一种新型抗钙化异种猪瓣，并对其临床移植后的早期血流动力学进行评价。方法应用环氧氯丙烷（ＥＣ）化学改性戊二醛（ＧＡ）处理猪瓣。对这类改性后生物材料特性、弹性瓣架进行了实验研究，并应用Ｓｗａｎ－Ｇａｎｚ导管技术和二维超声心动图对换瓣病例进行了早期血流动力学观察。结果（１）ＥＣ改性后的猪瓣经移植于新西兰兔皮下２周、４周、６周后比对照组明显减轻钙化２．４～５．０倍；（２）ＥＣ改性后的猪瓣组织稳定性和机械抗张强度与对照组相比无明显差别；（３）游离羧基含量下降６．９倍，抗原性为阴性；（４）自行设计的三柱等距高弹性钢丝瓣架，对瓣叶承受的应力减小至９０％以下；（５）新型生物瓣体外流体力学测试平均跨瓣压差，有效开口面积，瓣口回流百分比以及流场均接近生理状态；（６）临床应用２４只新型生物瓣给２１例患者进行瓣膜移植，随访３月～５年未有瓣膜故障导致不良后果者。结论新型异种猪生物瓣既具有明显抗钙化效果，又保持了良好的组织结构和机械强度，组织相容性好，早期临床血流动力学满意。     

Repair of incompetent venous valves: a new technique

Current techniques for treating chronic venous disease caused by venous valve incompetence all have deficiencies. A need exists for a simple technique to restore competence to the diseased valves at multiple sites in the venous system. The Venocuff is an implantable device developed to restore venous valve competence by reducing the vein circumference. Two animal models are described that are useful in the study of venous valve incompetence; the device was tested in these two models in the jugular vein of sheep. The first model was of naturally incompetent valves. The device was implanted around 11 completely incompetent and seven partially incompetent valves in 18 veins of 11 sheep. After implantation competence was restored or improved in all valves. The second model made use of an arteriovenous fistula to produce incompetence in the jugular vein in four sheep. The implant increased the pressure difference across the valve from 16 to 68 mm Hg. The Venocuff implant was thus effective at restoring valvular competence in the two animal models studied. There is evidence that valvular incompetence in these models and the results observed may be applicable in some cases of chronic venous disease in humans.     

Orthotopic total aortic root replacement model in adult sheep.

Prosthetic heart valves undergo mandatory preclinical animal testing prior to human clinical trials. Historically, a non-site-specific placement of a valve prosthesis has been commonly performed; however, recently site-specific placement continues to attract interest. Various animal models have been used for preclinical evaluation of both aortic and mitral valve prostheses; however, a universally accepted animal model for orthotopic total aortic root replacement with acceptable early and late mortality for long-term evaluation has been lacking. This article reports a successful orthotopic model for placement of tissue valve conduit prosthesis for total aortic root replacement in adult sheep. This model utilized preoperative echocardiographic assessment, specific intraoperative surgical techniques, and both early and late postoperative management therapies. The combination of all of these components resulted in a successful model for orthotopic placement of a tissue valve prosthesis for total aortic root replacement in adult sheep for potential long-term assessment.     

Calcification resistance with aluminum-ethanol treated porcine aortic valve bioprostheses in juvenile sheep

BACKGROUND: Calcification of glutaraldehyde fixed bioprosthetic heart valve replacements frequently leads to the clinical failure of these devices. Previous research by our group has demonstrated that ethanol pretreatment prevents bioprosthetic cusp calcification, but not aortic wall calcification. We have also shown that aluminum chloride pretreatment prevents bioprosthetic aortic wall calcification. This study evaluated the combined use of aluminum and ethanol to prevent both bioprosthetic porcine aortic valve cusp and aortic wall calcification in rat subcutaneous implants, and the juvenile sheep mitral valve replacement model. METHODS: Glutaraldehyde fixed cusps and aortic wall samples were pretreated sequentially first with aluminum chloride (AlCl3) followed by ethanol pretreatment. These samples were then implanted subdermally in rats with explants at 21 and 63 days. Stent mounted bioprostheses were prepared either sequentially as previously described or differentially with AlCl3 exposure restricted to the aortic wall followed by ethanol pretreatment. Mitral valve replacements were carried out in juvenile sheep with elective retrievals at 90 days. RESULTS: Rat subdermal explants demonstrated that sequential exposure to AlCl3 and ethanol completely inhibited bioprosthetic cusp and aortic wall calcification compared with controls. However the sheep results were markedly different. The differential sheep explant group exhibited very low levels of cusp and wall calcium. The glutaraldehyde group exhibited little cusp calcification, but prominent aortic wall calcification. All sheep in the two groups previously described lived to term without evidence of valvular dysfunction. In contrast, animals in the sequential group exhibited increased levels of cusp calcification. None of the animals in this group survived to term. Pathologic analysis of the valves in the sequential group determined that valve failure was caused by calcification and stenosis of the aortic cusps. CONCLUSIONS: The results clearly demonstrate that a combination of aluminum and ethanol reduced aortic wall calcification and prevented cuspal calcification. Furthermore, this study demonstrates that exclusion of aluminum from the cusp eliminated the cuspal calcification seen when aluminum and ethanol treatments were administered in a sequential manner.     

Valve extension with glutaraldehyde-preserved autologous pericardium. Results in mitral valve repair

Preliminary experimental studies in our laboratory have shown that autologous pericardium treated with glutaraldehyde prevents late deterioration and calcification of the tissue. For this reason, glutaraldehyde-treated autologous pericardium has been used in a series of 64 patients who underwent operations for leaflet extension of the mitral valve between 1980 and 1989. Ages ranged from 2.5 to 60 years (mean 19 +/- 15). The causes of mitral valve insufficiency were rheumatic fever (69%), bacterial endocarditis (17%), congenital (8%), endomyocardial fibrosis (4.5%), and trauma (1.5%). The autologous tissue was fixed in a 0.62% glutaraldehyde solution for 15 minutes and rinsed in saline for an additional 15 minutes. Patching techniques varied depending on the site and the extent of the lesion. Associated mitral valve repair techniques (Carpentier's techniques) were mandatory in all patients. The period of follow-up extended from 6 months to 9 years (mean 3.1 +/- 2.5 years). There were no operative deaths in this series, and there was one late death (2%). In the six patients (12%) who underwent reoperation, there has been no case of calcification of the pericardial patch. Postoperative mitral valve function was assessed by bidimensional color Doppler echocardiographic techniques. Mitral valve insufficiency was trivial or absent in 80% of the patients. This experience permits us to conclude that leaflet extension is a simple and safe technique in valve reconstruction, allowing repair of mitral valves that otherwise would need to be replaced. It permits use of an adult-size prosthetic ring in children. Glutaraldehyde-treated autologous pericardium is the material of choice for this type of repair.     

Kangaroo vs. porcine aortic valves: calcification potential after glutaraldehyde fixation

The aim of this study was to evaluate and compare the calcification potential of kangaroo and porcine aortic valves after glutaraldehyde fixation at both low (0.6%) and high (2.0%) concentrations of glutaraldehyde in the rat subcutaneous model. To our knowledge this is the first report comparing the time-related, progressive calcification of these two species in the rat subcutaneous model. Twenty-two Sprague-Dawley rats were each implanted with two aortic valve leaflets (porcine and kangaroo) after fixation in 0.6% glutaraldehyde and two aortic valve leaflets (porcine and kangaroo) after fixation in 2% glutaraldehyde respectively. Animals were sacrificed after 24 h and thereafter weekly for up to 10 weeks after implantation. Calcium content was determined using inductively coupled plasma-mass spectrometry and confirmed histologically. Mean calcium content per milligram of tissue (dry weight) treated with 0.6 and 2% glutaraldehyde was 116.2 and 110.4 microg/mg tissue for kangaroo and 95.0 and 106.8 microg/mg tissue for porcine valves. Calcium content increased significantly over time (8.8 microg/mg tissue per week) and was not significantly different between groups. Regression analysis of calcification over time showed no significant difference in calcification of valves treated with 0.6 or 2% glutaraldehyde within and between the two species. Using the subcutaneous model, we did not detect a difference in calcification potential between kangaroo and porcine aortic valves treated with either high or low concentrations of glutaraldehyde.     

MONOCUSP AORTIC VALVE REPLACEMENT IN DOGS: AN EXPERIMENTAL MODEL

Aortic valve replacement using human allograft valves is a well established procedure, following which histological evidence of a host-graft interaction is seen .This varies in intensity, depending on the types of sterilizing and storage agents to which the allograft is exposed prior to insertion. A canine experimental model which enables study of these tissue ingrowth patterns in variously treated allografts has been devised. The technique is described in detail.     

Truly stentless molded autologous pericardial aortic valve prosthesis with single point attached commissures in a sheep model.

OBJECTIVE: Aortic valve cusp extension and free-hand aortic valve replacement with autologous pericardium has been described. The long-term results were shown to be comparable with commercially available aortic bioprostheses. Nevertheless the relatively demanding surgical technique could not find wide acceptance. We developed a new design of a molded aortic valve, fashioned from autologous pericardium, treated briefly with glutaraldehyde, and simplified the implantation technique using single point attached commissures (SPAC). METHODS: Molded autologous valve prostheses were implanted in the subcoronary aortic position in 10 sheep with the commissures connected to the aortic wall at three single commissural points (SPAC). The prosthesis mean size was 21.6+/-1.3 mm and the construction time (excluding 10 min glutaraldehyde treatment) was 6.2+/-1.2 min. Cardiopulmonary bypass and cross-clamp time was 111.1+/-12.4 min and 75.0+/-16.3 min, respectively. Six sheep were euthanized after 201.2+/-10.3 days (6 months) and four sheep were euthanized after 330.8+/-6.5 days (11 months) postoperatively. RESULTS: In all sheep, the valve was immediately competent. At sacrifice, SPAC has proven to be well anchored to the aortic wall and the pericardial valve to be pliable in all cases. The maximum transvalvular gradient after cardiopulmonary bypass and at sacrifice was 3.7+/-2.2 mmHg and 10.6+/-5.2 mmHg, respectively. CONCLUSIONS: This new truly stentless molded autologous aortic valve with simplified implantation technique (SPAC) makes a reliable implantation in a standard timeframe possible. The simplicity of construction, low cost and absent need for anticoagulation of this molded autologous aortic bioprosthesis offers an attractive alternative and not only for patients in the developing world.     

Durability of Prosthetic Heart Valves

Accelerated fatigue testing of clinical heart valves has been performed at cyclic rates of 33 to 35 cycles per second at 37° using water for non-biological valves and glutaraldehyde solutions for tissue valves. Flows were in the physiological range, and the pressure difference across each valve during closure was 100 ± 25 mm Hg. The results showed that major fatigue occurred for the Starr-Edwards 2320 at 150 million cycles, the Hufnagel trileaflet at 124 million cycles, the Björk-Shiley Delrin disc at 140, the Björk-Shiley Pyrolite disc at 973, the Beall 103 at 60, the Hancock porcine at 62, the Carpentier-Edwards porcine at 34, and the Ionescu-Shiley porcine pericardial prosthesis at 65 million cycles. The Lillehei-Kaster was removed after 762 million cycles without discernible wear.Three facts emerged from the testing data: (1) the component worn in vitro wears in vivo; (2) the sites of in vitro fatigue on the component are identical to clinical specimens; and (3) those valves that have high durability in vitro have given similar performance in patients. The in vitro and clinical data for tissue valves do not correlate. The possible reasons for the discrepancy are discussed, and a note of caution is made regarding realistic expectations of clinical durability of tissue valves.     

Evaluation of kangaroo aortic valved conduits in a juvenile sheep model: preliminary findings

BACKGROUND: Biological heart valve substitutes, manufactured from either porcine or bovine tissue, have been in use for more than 30 years. Despite low thrombogenicity and excellent performance, bioprosthetic heart valves tend to degenerate and calcify early in young patients because of patient and valve related factors. The aim of this study was to examine the calcification behavior of glutaraldehyde-preserved kangaroo heart valves in a juvenile sheep model. METHODS: Porcine (n = 10) and kangaroo (n = 10) valved conduits were implanted in the descending aortic position of juvenile sheep and retrieved after 6, 8, and 12 months. Retrieved valved conduits were examined for morphological changes and calcification of the valve tissue, using Von Kossa's stain technique and atomic absorption spectrophotometry. RESULTS: Structural valve deterioration, characterized by increased stiffness and severe calcification, occurred in 100% of the porcine conduits within 4 months. Kangaroo valve leaflets were significantly (p < 0.001) less calcified at 6 months (3.39+/-1.80 microg/mg), 8 months (5.86+/-4.57 microg/mg), and at 12 months (14.38+/-6.72 microg/mg), compared to porcine valves at 3 months (176.45+/-42.88 microg/mg ) and at 4 months (154.67+/-52.67 microg/mg ). Porcine aortic wall tissue was more calcified (118.24+/-42.86 microg/mg) than kangaroo aortic wall tissue (79.55+/-26.40 microg/mg). CONCLUSIONS: Kangaroo heart valves calcify less than porcine heart valves. These findings suggest that a different donor valve tissue has a lower calcification potential probably due to a difference in the morphological ultrastructure. This could result in improved long-term durability of kangaroo heart valves.     

Bioprosthetic heart valves of the future

Glutaraldehyde‐fixed bioprosthetic heart valves (GBHVs), derived from pigs or cows, undergo structural valve deterioration (SVD) over time, with calcification and eventual failure. It is generally accepted that SVD is due to chemical processes between glutaraldehyde and free calcium ions in the blood. Valve companies have made significant progress in decreasing SVD from calcification through various valve chemical treatments. However, there are still groups of patients (e.g., children and young adults) that have accelerated SVD of GBHV. Unfortunately, these patients are not ideal patients for valve replacement with mechanical heart valve prostheses as they are at high long‐term risk from complications of the mandatory anticoagulation that is required. Thus, there is no “ideal” heart valve replacement for children and young adults. GBHVs represent a form of xenotransplantation, and there is increasing evidence that SVD seen in these valves is at least in part associated with xenograft rejection. We review the evidence that suggests that xenograft rejection of GBHVs is occurring, and that calcification of the valve may be related to this rejection. Furthermore, we review recent research into the transplantation of live porcine organs in non‐human primates that may be applicable to GBHVs and consider the potential use of genetically modified pigs as sources of bioprosthetic heart valves.     

Micromechanics and Mathematical Modeling: An Inside Look at Bioprosthetic Valve Function

A major contributing factor in the degeneration of glutaraldehyde-treated porcine xenograft bioprostheses is tearing of the valve cusps near their commissural attachment to the supporting stent. We have been examining aortic valves at the micromechanical level, and have developed several sensitive techniques to evaluate the biomechanical changes produced by the glutaraldehyde fixation process. Additionally, we have developed a mathematical modeling technique that stimulates valve function during the entire cardiac cycle. Our micromechanical tests have shown that compressive buckling is common to all fixed tissues, occurs at physiological bending curvatures, and is likely to be the primary mode of mechanical failure of bioprosthetic valves. We have also shown that existing glutaraldehyde fixation techniques inhibit the natural internal shearing of the valve cusps, and disable the interaction of the fibrosa and the ventricularis. With our modeling technique, we have shown that flexural stresses are indeed concentrated near the valve commissures, and that appropriate modifications of the supporting stent can reduce flexural deformations. With these new, more revealing techniques at hand, prospective valve designs can be better evaluated prior to large scale animals and clinical testing.     

Computed tomography detects tissue formation in a stented engineered heart valve

Tissue-engineered heart valves (TEHV) are being explored as an alternative to conventional heart valve prostheses. Using the classic tissue engineering paradigm, a stented tri-leaflet valve is fabricated. Subsequently, the construct is implanted into the pulmonary position in a sheep. Follow-up by means of computed tomography, magnetic resonance imaging, and echocardiography was used to assess tissue formation. After 4 weeks, the scaffold of the TEHV has degraded and new tissue is formed. However, small areas without tissue formation were present at macroscopic inspection. This phenomenon was only visible on computed tomographic images. Therefore, computed tomography appears a promising technique for in vivo follow-up of tissue formation in tissue-engineered heart valves.     

Aortic valve allografts in sheep

Some of the mechnical and biological problems surrounding the use of fresh allograft inverted aortic valves as mitral valve substitutes are described. Certain aspects of the problem have been studied experimentally. In three sheep `fresh' aortic valve allografts were inserted, using cardiopulmonary bypass, into the main pulmonary artery, and were observed from 5 to 7 months after operation. The animals survived normally. Their normal pulmonary valves remained in situ. The technique is described. At subsequent necropsy, macroscopically the valves were found to be free from vegetation, and the cusps were pliable and apparently normal. Microscopically, the supporting allograft myocardium showed necrosis and early calcification. The valve cusp showed hyalinization of collagen, although beneath the endocardium this hyalinized collagen contained moderate numbers of fibroblasts with no evidence of proliferation. The endocardium and arterial intima of the allograft showed evidence of ingrowth from adjacent normal host endocardial tissues. The allograft itself was invested in a loose layer of fibro-fatty tissue, which, in view of the necrotic state of the graft myocardium, could well have been a reparative reaction rather than a homograft reaction. It is concluded that, although the cusps could function normally, the necrosis of the myocardium might in time lead to late failure of the graft. Further studies with the valve inserted at mitral level are indicated.     

Hemodynamic and pathologic evaluation of a unileaflet pericardial bioprosthetic valve

This study investigated the in vivo hemodynamics and pathologic changes of a unileaflet pericardial bioprosthetic valve 3 to 5 months after implantation in juvenile sheep. Group 1 had 10 sheep with tricuspid valve replacement. Group 2 had nine sheep with mitral valve replacement. Group 3 served as a control with 10 sheep that had tricuspid valve replacement with a trileaflet porcine bioprosthesis. Hemodynamic performance was satisfactory in all three groups despite prominent pathologic changes, particularly in unileaflet valves. Intrinsic cuspal calcification was present in 66% of the unileaflet tricuspid, 88% unileaflet mitral, and 25% porcine tricuspid valves. Neither cuspal tearing nor perforations were found. However, cuspal stretching and redundancy of the mobile cusp was present in six tricuspid, seven mitral unileaflet valves, and no porcine valves. Gross pericardial redundancy correlated with the microscopic appearance of distorted and separated collagen bundles. These findings suggest that multiple modes of primary tissue failure may limit the durability of this unileaflet pericardial valve.     

Kangaroo versus freestyle stentless bioprostheses in a juvenile sheep model: hemodynamic performance and calcification behavior

BACKGROUND: Glutaraldehyde-preserved bioprosthetic heart valve substitutes have limited performance and longevity due to tissue degeneration and calcification. The Freestyle valve (Medtronic Heart Valves, Inc, Minneapolis, MN) combines zero fixation pressure and proportional, variant-amino oleic acid (AOA) as antidegeneration and antimineralization measures. The aim of this study was to compare the calcification behavior of glutaraldehyde-preserved kangaroo aortic valves with Freestyle stentless bioprostheses in a juvenile sheep model. METHODS: Pulmonary artery and valve replacements were performed in juvenile sheep with Freestyle stentless aortic valves (n = 4) or glutaraldehyde-preserved kangaroo stentless aortic valves with no antimineralization measures (n = 6), and explanted at 200 postoperative days. RESULTS: Freestyle stentless valves and stentless kangaroo aortic valves showed normal valve function immediately postoperatively and up to 120 days. Valve leaflets of all valves were macroscopically free of visible calcification with normal histology. Valve leaflet calcification (microg Ca/mg tissue) was less in kangaroo than Freestyle (1.27 +/- 0.43 versus 2.38 +/- 1.37, p = 0.856). Aortic wall tissue calcification was severe in kangaroo and Freestyle (127.93 +/- 12.22 versus 122.19 +/- 11.99, p = 0.596). CONCLUSIONS: We conclude that glutaraldehyde-preserved kangaroo aortic valve leaflets are equal to AOA-treated Freestyle stentless valve leaflets with regard to calcification in juvenile sheep. Both bioprostheses are prone to aortic wall calcification. The low calcification features of the kangaroo aortic valve leaflets without antimineralization treatment may benefit the longevity of the valve.