Degeneration and Cacfi cation of Bioprosthetic Cardiac Valves

Transcription

1 ANIMAL MODEL OF HUMAN DISEASE Degeneration and Cacfi cation of Bioprosthetic Cardiac Valves Bioprosthetic Tricuspid Valve Implantation in Sheep GLENN R. BARNHART, MD, MICHAEL JONES, MD, TOKUHIRO ISHIHARA, MD, PhD, DANIEL M. ROSE, MD, ALTAGRACIA M. CHAVEZ, MD, and VICTOR J. FERRANS, MD, PhD From the Surgery Branch and the Pathology Branch, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland Human Disease The development of an ideal cardiac valve substitute remains a major problem in cardiac surgery. Mechanical prosthetic valves are usually durable and hemodynamically satisfactory; however, their thrombogenicity necessitates chronic anticoagulant therapy. Even with anticoagulent therapy, the linearized embolic complication rate for mechanical valves may be as high as 5(07 per patient year'; the bleeding complication rate from taking anticoagulants is similar.2 Because of these problems, nonthromboembolic alternatives to mechanical valves are desirable. Bioprosthetic valves are constructed partially from biologic tissues (the leaflets) and partially from prosthetic materials (the leaflet-supporting structures). At the present time, the most widely used bioprosthetic valves are the glutaraldehyde-preserved porcine aortic valves, which were first implanted in human beings in Glycerol-treated human dura mater and glutaraldehyde-treated bovine pericardium are also being employed as materials for the construction of the leaflets of bioprosthetic valves. The hemodynamic performance of currently used bioprostheses is generally comparable to that of mechanical valves; chronic anticoagulant therapy is usually not necessary. These advantages have made bioprosthetic valves the valves of choice for implantation at many institutions /82/ $00.70 American Association of Pathologists 136 Nevertheless, clinically important degenerative changes, including calcific deposits and collagen breakdown, develop in a high percentage of bioprostheses within a few years after implantation.34 These alterations result in significant degrees of bioprosthetic stenosis and regurgitation. Compared with those in older individuals, such calcific deposits develop more rapidly in children and young adults, in whom the need for long-term durability of the prosthetic valve and for avoidance of anticoagulant therapy assumes critical importance.56 For these reasons, development of an animal model of bioprosthetic valve degeneration and calcification is highly desirable for studies of interventions designed to improve the durability of bioprostheses. Animal models, including dogs, baboons, and calves, have been used extensively for the study of Publication sponsored by the Registry of Comparative Pathology of the Armed Forces Institute of Pathology and supported by Public Health Service Grant RR from the Division of Research Resources, National Institutes of Health, under the auspices of Universities Associated for Research and Education in Pathology, Inc. Address reprint requests to Michael Jones, MD, Surgery Branch, National Heart, Lung and Blood Institute, Building 10, Room 2N242, National Institutes of Health, Bethesda, Maryland University of Michigan Exhibit 2009 St. Jude Medical v. University of Michigan IPR

2 Exh Page 1 ANIMAL MODEL OF HUMAN DISEASE Degeneration and Cacfi cation of Bioprosthetic Cardiac Valves Bioprosthetic Tricuspid Valve Implantation in Sheep GLENN R. BARNHART, MD, MICHAEL JONES, MD, TOKUHIRO ISHIHARA, MD, PhD, DANIEL M. ROSE, MD, ALTAGRACIA M. CHAVEZ, MD, and VICTOR J. FERRANS, MD, PhD From the Surgery Branch and the Pathology Branch, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland Human Disease The development of an ideal cardiac valve substitute remains a major problem in cardiac surgery. Mechanical prosthetic valves are usually durable and hemodynamically satisfactory; however, their thrombogenicity necessitates chronic anticoagulant therapy. Even with anticoagulent therapy, the linearized embolic complication rate for mechanical valves may be as high as 5(07 per patient year'; the bleeding complication rate from taking anticoagulants is similar.2 Because of these problems, nonthromboembolic alternatives to mechanical valves are desirable. Bioprosthetic valves are constructed partially from biologic tissues (the leaflets) and partially from prosthetic materials (the leaflet-supporting structures). At the present time, the most widely used bioprosthetic valves are the glutaraldehyde-preserved porcine aortic valves, which were first implanted in human beings in Glycerol-treated human dura mater and glutaraldehyde-treated bovine pericardium are also being employed as materials for the construction of the leaflets of bioprosthetic valves. The hemodynamic performance of currently used bioprostheses is generally comparable to that of mechanical valves; chronic anticoagulant therapy is usually not necessary. These advantages have made bioprosthetic valves the valves of choice for implantation at many institutions. Nevertheless, clinically important degenerative changes, including calcific deposits and collagen breakdown, develop in a high percentage of bioprostheses within a few years after implantation.34 These alterations result in significant degrees of bioprosthetic stenosis and regurgitation. Compared with those in older individuals, such calcific deposits develop more rapidly in children and young adults, in whom the need for long-term durability of the prosthetic valve and for avoidance of anticoagulant therapy assumes critical importance.56 For these reasons, development of an animal model of bioprosthetic valve degeneration and calcification is highly desirable for studies of interventions designed to improve the durability of bioprostheses. Animal models, including dogs, baboons, and calves, have been used extensively for the study of Publication sponsored by the Registry of Comparative Pathology of the Armed Forces Institute of Pathology and supported by Public Health Service Grant RR from the Division of Research Resources, National Institutes of Health, under the auspices of Universities Associated for Research and Education in Pathology, Inc. Address reprint requests to Michael Jones, MD, Surgery Branch, National Heart, Lung and Blood Institute, Building 10, Room 2N242, National Institutes of Health, Bethesda, Maryland /82/ $00.70 American Association of Pathologists 136

3 Exh Page 2 Vol. 106 * No. 1 BIOPROSTHETIC TRICUSPID VALVES 137 Figure 1-View of the outflow surface of an unimplanted pericardial valve bioprosthesis, showing three leaflets made of bovine parietal pericardium and mounted on a Dacron sewing ring. Figure 2-Outflow surface of a pericardial bioprosthesis removed 6 months after implantation in the tricuspid position in a sheep. The leaflets are thickened and retracted. Figure 3-Roentgenogram of the bioprosthesis shown in Figure 2. Calcific deposits are present in each leaflet and are most prominent at commissures. mechanical valve prostheses.' Investigations of bioprosthetic valves implanted in animals, however, have been limited in number,7-10 and no study has been published analyzing the clinical and pathologic features of bioprosthetic valve failure in experimental animals. Animal Model We have found juvenile sheep, aged 4 to 5 months, to be suitable as a model for the study of bioprosthetic valve failure for several reasons. They grow rapidly, increasing in average weight from 29 kg at the time of valve implantation to 45 kg at the time of study, 4 to 7 months later. Their blood pressure, heart rate, cardiac output, and intracardiac pressure are essentially the same as those of young human beings. Their valve annular sizes are suitable for implantation of commerically available bioprosthetic valves, and their vessels and body sizes are adequate for standard cardiopulmonary bypass techniques. They tolerate anesthesia well and present no major difficulty in postoperative management. Most important, however, the bioprostheses implanted for a few months in these animals show changes comparable to those that take several years to develop with bioprostheses implanted in humans. For tricuspid valve replacement, anesthesia is induced with sodium thiamylal and is maintained with halothane following endotracheal intubation and mechanical ventilation. A thoracotomy incision is made in the fourth right intercostal space. Cardiopulmonary bypass is instituted, a right atriotomy is performed, and the tricuspid valve is excised and replaced by the bioprosthetic valve. After operation the rams are placed in cages with warm humidified oxygen for 24 hours. They usually stand and feed on the first postoperative day. Thereafter, they are maintained with conventional husbandry techniques for 4-7 months, when late postoperative hemodynamic and morphologic studies are performed. Biologic and Pathologic Features We performed tricuspid valve replacement with glutaraldehyde-preserved bioprosthetic valves ( 1 bovine pericardial valves and 9 porcine aortic valves) in 20 juvenile rams. Sixteen rams were studied electively 4-7 (average 6) months postoperatively. One of these rams had a clinically unsuspected valve infection. Three other rams died from bioprosthetic infection 1-6 months after the operation, and an additional animal died of prosthetic valve failure 3 months after operation. All 20 valves had morphologic and/or hemodynamic evidence of bioprosthetic valve failure. In the 16 rams undergoing elective hemodynamic studies, there was no difference in immediate or late postoperative mean diastolic transvalvular gradients; however, end-diastolic gradients averaged 1.4 ± 0.7 mm Hg immediately postoperatively, but increased to 4.8 ± 0.7 mm Hg later (P< 0.001, n = 10). Each of these 16 rams had necropsy evidence of hepatic congestion, and 5 of these also had ascites. Morphologic abnormalities were found in each of the 16 noninfected bioprostheses (Figures 1-7). They consisted of thrombi (8 valves), calcific deposits (16 valves), and fibrous sheathing (16 valves). Calcific deposits were documented by roentgenograms of the explanted valves (Figure 3), by histologic study (Figure 6), and by quantitative analysis of leaflet tissue. Average values obtained (n = 13) were 401 ± 94 mg of calcium/g dry weight of tissue, as compared

4 138 BARNHART ET AL AJP * January S -.& --- -,. Figure 4-Histologic section of the leaflet of an unimplanted pericardial bioprosthesis. The outflow surface is shown at the top; the inflow surface, which is rougher, at the bottom. The leaflet is composed of bundles of collagen, small elastic fibers (not shown), connective tissue cells, and small vessels. (One-micron-thick plastic section, alkaline toluidine blue, x 140) Figure 5-Histologic section of the leaflet from the same bioprosthesis as in Figures 2 and 3. Microthrombus on the outflow surface is shown at the top; the inflow surface is covered by a thick fibrous sheath. (One-micron-thick plastic section, alkaline toluidine blue, x 70) Figure 6-Histologic section of another area of the same bioprosthesis as in Figures 2, 3, and 5, showing a large, darkly stained calcific deposit, which is located near the leaflet tip and forms part of the outflow surface. The inflow surface (bottom) is covered by a fibrous sheath. (Hematoxylin and eosin, x 60) Figure 7-Scanning electron micrograph showing endothelial cells of host origin covering a fibrous sheath that has grown over the surface of the leaflet of a pericardial for 6 months. bioprosthesis implanted (x 2600) with 0.24 mg/g in unimplanted tissue. Histologic and ultrastructural features of calcific deposits, thrombi, platelet aggregates, degenerated collagen, and fibrous sheaths (Figures 5-7) were similar to those in bioprostheses recovered after implantation in man.3 48 Comparison With Human Disease Juvenile rams also present the accelerated bioprosthetic valve calcification that occurs in children and young adults. The growth of juvenile rams to maturity in 4-7 months after implantation temporally telescopes those metabolic processes that are associated with 10 or more years of growth in man. The serum calcium levels in young rams, like those of young human beings, are in the high normal range ( mg/dl); their serum phosphorus (8.2 ± 0.4 mg/dl) and alkaline phosphatase (290 ± 25 IU/A) levels are elevated, as compared with those of adults. These alterations (which appear related to bone growth) Exh Page 3

5 Exh Page 4 Vol. 106 * No. I BIOPROSTHETIC TRICUSPID VALVES 139 are thought to be of importance in the pathogenesis of calcific deposits in bioprostheses.6 The clinical, hemodynamic, and morphologic features (including collagen degeneration, calcific deposits and fibrous sheathing) of bioprosthetic valve failure are similar in young sheep and in humans. However, fibrous sheathing is more prominent in valves removed from sheep. This fibrous sheath may play an important role in producing thickening, retraction, and reduced mobility of the leaflets."' Usefulness of the Model If indeed 50, ,000 bioprosthetic heart valves have been implanted in persons in this country alone,3 the clinical importance of anticipated bioprosthetic valve failure is enormous. Alterations of bioprosthetic valves in juvenile rams appear to be clinically and pathologically very similar to those occurring in human beings; however, they develop much more rapidly, making the sheep a valuable animal model for study of the pathogenesis of bioprosthetic valve failure. Studies of interventions for retarding or eliminating the degeneration of bioprosthetic valves are in progress. References 1. Gott VL, Brawley RK, Jones M: Mitral valve prostheses: The problem of thromboembolism. Second Henry Ford Hospital International Symposium on Cardiac Surgery. Edited by JE Davila. New York, Appleton-Century-Crofts, 1978, pp Kloster FE: Complications of artificial heart valves. JAMA 1979, 241 : Angell WW, Angell JD: Porcine valves. Prog Cardiovasc Dis 1980, 23: Ferrans VJ, Boyce SW, Billingham ME, Jones M, Ishihara T, Roberts WC: Calcific deposits in porcine bioprostheses: Structure and pathogenesis. Am J Cardiol 1980, 46: Magilligan DJ Jr, Lewis JW Jr, Jara FM, Lee MW, Alam M, Riddle JM, Stein PD: Spontaneous degeneration of porcine bioprosthetic valves. Ann Thorac Surg 1980, 30: Sanders SP, Levy RJ, Freed MD, Norwood WI, Castaneda AR: Use of Hancock porcine xenografts in children and adolescents. Am J Cardiol 1980, 46: Lefrak EA, Starr A: Cardiac Valve Prostheses. New York, Appleton-Century-Crofts, Ferrans VJ, Spray TL, Billingham ME, Roberts WC: Structural changes in glutaraldehyde-treated heterografts used as substitute cardiac valves. Am J Cardiol 1978, 41: Yarbrough JW, Roberts WC, Reis RL: Structural alterations in tissue cardiac valves implanted in patients and in calves. J Thorac Cardiovasc Surg 1973, 65: Geroulanos S, Gossler W, Walpoth B, Turina M, Senning A: Fruhe rasterelektronenoptische Oberflachenveranderungen nach orthotoper Pulmonalklappen- Xenotransplantation durch glutaraldehyd-konditionierte Schweineklappen. Helv Chir Acta 1979, 46: Ishihara T, Ferrans VJ, Jones M, Boyce SW, Roberts WC: Occurrence and significance of endothelial cells in implanted porcine bioprosthetic valves. Am J Cardiol 1981, 48: Acknowledgments We wish to acknowledge the assistance of the staff of the Section of Laboratory Animal Medicine and Surgery, National Heart, Lung, and Blood Institute, and especially the veterinary expertise of Drs. Joseph E. Pierce, Delwin K. Buckhold, and James F. Harwell.