- Email: [email protected]
Structural alterations in tissue cardiac valves implanted in patients and in calves
Structural alterations in tissue cardiac valves implanted in patients and in calves John W. Yarbrough, M.D.,* William C. Roberts, M.D., and Robert L. Reis, M.D., ** Bethesda, Md.
Heterograft cardiac valves are supplied in a wide range of sizes and are prepared before implantation. Fresh, beta-propiolactone-treated, gamma-irradiated, freezedried, frozen, and formalin-fixed valves all have been utilized in patients.' Although initial hemodynamic results were good and thromboembolic complications were minimal, these valves have not been durable.":' Because it forms strong cross linkages of collagen while allowing flexibility of tissue and because its bonds are not biodegradable, glutaraldehyde is now being used as a preservative for heterograft valves.' Reports of heterograft valves previously fixed with glutaraldehyde have been encouraging." This paper describes anatomic findings in glutaraldehyde-fixed porcine aortic heterograft valves in 6 patients and in 1 calf. In addition, structural alterations in other types of tissue valves implanted in 4 patients and in 6 calves are described for comparison. From the Clinic of Surgery and Section of Pathology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014. Received for publication Sept. 6, 1972. Address for reprints: Dr. William C. Roberts, Section of Pathology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014. ·Present address: Department of Surgery, Duke University Medical Center, Durham, N. C. ··Present address: Department of Surgery, University of Kansas Medical Center, Kansas City, Kan.
364
Material Seventeen tissue valves, implanted for 7 to 240 days (mean 93), were examined at necropsy: Ten had been implanted in patients and seven in Holstein calves (Table I). Except for the three formalin-fixed valves, all had been placed on Reis-Hancock flexible stents.? Fourteen had been implanted at the National Heart and Lung Institute, whereas the three formalin-fixed porcine heterograft valves were implanted into human subjects elsewhere and later were submitted by Hancock Laboratories for histologic studies. The ten porcine heterografts were prepared before implantation by Hancock Laboratories, and the other seven heterotopic tissue valves were prepared at the National Heart and Lung Institute at the time of implantation by the method described previously.": 7 The tissue valves were implanted into the mitral position in all 7 calves. Of the 10 human subjects, 6 had mitral valve replacement, 3 had aortic replacement, and 1 had tricuspid replacement. At necropsy, the previously implanted valves were fixed in either 10 per cent buffered formalin or 3 per cent glutaraldehyde. After fixation, sections were taken from one or more leaflets of each valve, processed, cut, stained, and examined histologically. One section from each cusp
Volume 65 Number 3
Structural alterations in tissue cardiac valves
Morch, 1973
Fig. 1. Fresh autogenous pericardial valve implanted in the mitral position in a 50-year-old woman 39 days before death. There was no evidence of dysfunction of this tissue valve during the postoperative period. a, View of pericardial valve from above. The cusps are thickened and one is retracted. The valve orifice was competent when water was injected under pressure into the left ventricle at necropsy. b, Photomicrograph of section through one cusp. The surfaces of the cusp are lined by fibrin. The cusp is about four times thicker than normal. c, Close-up view of the area shown in brackets in b. This implanted parietal pericardium may be compared to its thickness at the time of implantation, a portion of which is shown in d. Both c and d were taken at the same magnification. The thickening of the implanted pericardium is due mainly to proliferation of fibrous tissue, which probably resulted from organization of the surface fibrin. e, Deposit of calcium in the implanted valve. t. Close-up of portion of implanted valve shows a multinucleated cell. Its significance is uncertain. (Hematoxylin and eosin stains. Original magnifications: b, xl l ; c and d, x70; e and t, x400.)
Fig. 2. Fresh autogenous pericardial valve implanted in the mitral position in a calf 7 days before death. The pericardial valve appeared to function normally during the entire postoperative period, and, at necropsy, injection of water into the left ventricle under pressure indicated that the valve was competent. a, Pericardial valve as viewed from left ventricle. The leaflets are mildly thickened. b, Low-power photomicrograph of section of one cusp. c, Blow-up of area in brackets shown in b. The cusp is nearly twice normal thickness. The thickening is due to fibrin-platelet thrombus on its surface. The margins of the original parietal pericardium are clearly visible. (Hematoxylin and eosin stains. Original magnifications: b, x6; c, x160.)
365
The Journal of
366
Yarbrough, Roberts, Reis
Thoracic and Cardiovascular Surgery
Fig. 3. Fresh autogenous parietal pericardium valve implanted in a calf 82 days before its death. The calf did poorly postoperatively, and signs of mitral regurgitation and infective endocarditis were evident at least 1 month before death. a, Tissue valve as viewed from left ventricle. The cusps are torn and perforated. There are no signs of infection at the site of attachment of the rigid frame to the natural mitral annulus. b, Section of portion of one cusp. The entire valve is necrotic and edematous. A few polymorphonuclear leukocytes are visible. No stainable colonies of organisms were observed, but the calf was being given antibiotics during the entire postoperative period. (Hematoxylin and eosin stain. Original magnification: b, xlOO.)
Fig. 4. Fresh autogenous pericardial valve implanted into the mitral position of a calf 116 days before its death. a, The cusps are diffusely thickened and retracted. b, One cusp tore and prolapsed into the left atrium. c, Photomicrograph of one of the nonprolapsed cusps shows the diffuse thickening. A close-up of the area in brackets is shown in d. There is considerable fibrous thickening of the implanted parietal pericardium. No colonies of organisms were seen on histologic study of the prolapsed cusp. (Hematoxylin and eosin stain. Original magnifications: c, x8; d, xlOO.)
Volume 65
Structural alterations in tissue cardiac valves
Number 3
367
Ma rch, 1973
Table I. Tissue valves evaluated Subjects and durati on of im planta tion PaTy pe of valve
tien ts
Fresh autogenous pericard ium Fresh autog enous fascia lata Glutaraldehydefixed human fascia lata Formalin -fixed porcine aortic valve Glutaraldehydefixed porcine aort ic valve Totals
39
3
7·116 80
0 0
48,56
2
3
80-240
6
14-180
10
14-240
0 110 7-116
7
Table II. Severity of morphologic changes in implanted tissue valves Changes af ter implan tation (0-4+)
Type of valve Fresh autogeno us pericard ium Fresh autogenous fascia lata Glutaraldehyde-fixed human fascia lata Formalin-fixed porcine aortic valve Glu taraldehyde-fixed por cine aortic valve Total
I
No. of Microsubjects Gross scopi c ++++
++++
+++
++++
2
++++
++++
3
++++
++++
7
+
+
4
17
was stained with hematoxylin and eosin, Verhoeff-van Gieson, methenamine silver, and periodic acid-Schiff.
Results The four fresh autogenous pericardial valves were thickened and retracted by 30 days after implantation (Figs. 1 to 4 ) (Table II). Two valves implanted in calves were perforated and detached from the commissures. Grossly, each appeared infected, but microscopically no stainable organisms were found in the leaflets. All
Fig. S. Fresh autogenous fascia lata valve impla nted into the mitral position of a calf 80 da ys before its death. In itiall y he did well posto peratively but later developed overt conge stive cardiac failure. A diastoli c rumble tha t suggested mitral stenosis became audible. At necropsy, the mitral orifice was part ially obstructed by a large fungal mass attached primarily to the rigid-frame por tion of the tissue valve . a, View of tissue valve from left atrium shows the large fun gal mass. b, View of valve from left ventricle again shows the fungal mass obs truc ting the orifice . c, Photomicrograph of a section of one cusp. T he fascia lata is necrotic, devoid of cells, and not thickened. d, Candida species in the fungal mass. ( Hema toxylin a nd eosin stain [c] and methenamine silver stai n [d] . Original magn ificat ions : c, x160; d, x400. )
valves were less pliable than they had been at implantation. Since no murmurs had been heard , valve funct ion was considered normal in both of the calves that lived 7 days and in the patient. The valves were competent when tested post mortem. Histologically, a few mononuclear cells were present on the surfaces of each valve cusp but not within the cusps. Minute calcific deposits were present in all valve cusps. Collagen fibers appeared intact in the valve implanted in the patient but were necrotic in each of the 3 calves. Fibrous tissue was present in each valve cusp, except in the valve implanted 7 days before death . Fibrin deposits were present on all valves. The thickening of the valve in the patient was primarily due to fibrous tissue, almost sure-
368
Yarbrough, Roberts, Reis
The Journol of Thorocic ond Cord iovosculor Surgery
Fig. 6. Glutaraldehyde-fixed human fascia lata valve implanted in the mitral position in a calf 56 days before its death. The calf did poorly from the time of implantation and died of severe congestive cardiac failure. The orifice of the valve became obstructed by a large fungal mass (a, b, and c). a, View of stenotic valve from left atrium. b, View from left ventricle. c. Photo micrograph shows fungal ball totally encapsulating the fascia lata cusp. d, Close-up view shows necrosis of the fascia lata with total absence of cells. e, Candida species identified in the obstructing mass. (Hematoxylin and eosin stains [c and d] and methenamine silver stain eel. Original magnifications : c, x5; d, x40; e, x400. )
ly the result of organization of fibrin. Vascular channels, located primarily at the base of the leaflet, were present in the valve of the human subject. The one fresh autogenous fascia lata valve (Fig. 5) implanted in a calf for 80 days was made stenotic by a large mycotic thrombus which occluded the orifice. This mass was attached to the sewing ring and to the atrial surface of the cusps. Candida species were found in the occluding mass. A few focal fibrin deposits were present on the leaflets, but generally the cusps were not thickened . However, the leaflets were necrotic. The two glutaraldehyde-fixed human fascia lata valves (Figs . 6 and 7) were examined 48 and 56 days, respectively, after implantation in calves. Both were stenotic, rigid,
focally but extensively calcified, and necrotic. Focal fibrin deposits were present on the implanted valves in both calves. The valvular stenosis in 1 calf was worsened by a large mycotic thrombus which was attached to two of the three cusps; it contained Candida species. No cellular infiltrates were present in the leaflets. Each of the three formalin-fixed porcine aortic valves (Figs. 8 to 10) was retrieved from a patient because of valve failure either due to detachment of a cusp from a commissure or to perforation or tearing of a cusp. Histologically, the leaflets in all 3 patients were necrotic and mildly thickened. The cusps were devoid of inflammatory cells except at their bases, where mononuclear cells were present. The adjacent aorta also was necrotic, and its intima was thickened
Volume 65 Number 3
Structural alterations in tissue cardiac valves
369
March, 1973
Fig. 7. Glutaraldehyde-fixed human fascia lata valve implanted in the mitral position of a calf 48 days before its death. The calf did poorly from the beginning of the postoperative period and died of progressive congestive cardiac failure. The valve cusps were not freely pliable at the time of insertion, and at necropsy the cusps were rigid. a, View of valve from left atrium. b, View from left ventricle. c, Photomicrograph of portion of one cusp. The fascia in this area is necrotic and free of cells. In other areas, fibrous tissue covered the ventricular surfaces. In most areas the cusps were extensively calcified. The rigidity of the cusps at necropsy could be attributed primarily to the presence of the calcific deposits. (Hematoxylin and eosin stain. Original magnification: c, x65.)
by a loose fibrous tissue that contained multinucleated cells. No calcific deposits were present. The seven porcine valves fixed in stabilized glutaraldehyde* (Figs. 11 to 16) appeared normal grossly. The leaflets were thin, translucent, and freely mobile. Valve function appeared normal in each. However, one of the seven valves contained a large thrombus in one of the three sinuses (Fig. 15). Microscopically, the cusps contained elongated cells, similar to those seen in a control valve (Fig. 17) . Small foci of necrosis were noted in three of the seven implanted valves (Figs. 12, 14, and 16). The sinus surfaces of the cusps of three valves were covered by. mesothelial-like cells arranged more or less perpendicular to 'Supplied by Hancock Laboratories, Inc., Orange, Calif.
Fig. 8. Formalin-fixed porcine aortic valve implanted in a patient 90 days before his death. a, Section' includes commissure, two aortic valve cusps, and adjacent aorta. The cusps at their bases contain mononuclear cells, and the adjacent intima of the aorta is thickened. b, Close-up of area in brackets shown in a. The media of the entire aorta is necrotic, the intima is thickened, and giant cells are present at the junction of the media and intima. (Hematoxylin and eosin stains. Original magnifications: a, x20; b, X 100.)
the cuspal surfaces (Fig. 12). These cells occasionally grouped together to form giant cells. Deposits of fibrin were superimposed on the lining cells. No lining cells, fibrin, or fibrous tissue were present on the upstream surfaces of the cusps except at their bases adjacent to the myocardium. No calcium was found in the cusps. Discussion
Although the numbers of patients (10) and calves (7) involved in the present study were small, the results of morphologic examination of the seventeen tissue valves are quite clear. Whether in human subjects or in calves, the parietal pericardial and fascia lata valves appear to be poor substitutes for natural valves. The fresh pericardial mitral valve in the patient was four times thicker than it had been when inserted only 39 days before death. Similar thickening and focal calcification of pericardial valves have been observed by others." 9 The thickening appeared to result from deposition of successive layers of fibrin which were transformed into fibrous tissue. In each of the 2 calves
370
Yarbrough, Roberts, Reis
Fig. 9. Formalin-fixed porcine aort ic valve implanted in a patient 150 days before his death . a, Low-power photomicrograph of one cusp. It is quite edematous. b, Close-up of area shown in brackets in a. Mononuclear cells infiltrate the core of the cusps, and that portion near the margins is totally necrotic. The core portion of the cusp is severely edematous. c, Close-up view of another area in brackets shown in a. The cusp in this area is totally necrotic, devoid of cells, and covered on one surface by a fibrin thrombus. (Hematoxylin and eosin stains . Original magnifications: a, x5; b, >-63; c, x250.)
that survived over 7 days with fresh pericardial valves, portions of the cusps became detached and both valves were necrotic. It is probable that infective endocarditis caused the major portion of the destruction in each of these animals, but that was not proved. All three fascia lata valves, whether implanted when fresh or after glutaraldehyde fixation, became rigid, their orifices stenotic, and the tissue necrotic. The rigidity was due
The Journo l of Thorac ic and Card iovascular Surgery
Fig. 10. Formalin-fixed porcine aortic valve irnplanted in a patient 240 days before his death. a, Section includes port ions of aorta and of two cusps. A close-up of one area in brackets is shown in b. Fibrin thrombus is present on the aortic surface of this cusp near its attachment to the aorta. Inflammatory cells also are present in the cusp at this point. c, Close-up of another area in brackets. Here the cusp is totally necrotic and devoid of cells. (Hematoxylin and eosin stains. Original magnifications : a, x5; b, x63; c, x31O.)
to deposition of fibrous tissue and extensive calcific deposits on the surfaces of the fascia lata cusps. The stenosis was produced by the rigidity of the cusps and by the presence of large mycotic thrombi obstructing the orifices (in 2 of the 3 calves). The cause of the latter in each was Candida species infection. Thus , the tissue valves were sites of infection in 4 of the 7 calves: The cusps were extensively damaged, and the valves were made purely incompetent in each of the 2 animals with pericardial valves; however, in the 2 with fascia lata valves, the
Volume 65 Number 3
Structural alterations in tissue cardiac valves
March. 1973
Fig. 11. Glutaraldehyde-fixed porcine aortic valve implanted in the mitral position of a 44year-old man 14 days before his death. Preoperatively he had severe mitral stenosis. His postoperative course was so smooth that he was discharged 12 days after operation. He collapsed suddenly at home 2 days later and died immediately. a, View of valve from left atrium. b, View from left ventricle. The cusps are freely movable and devoid of thrombus. c, Photomicrograph of section through one cusp. d, Close-up view shows many cells and no necrosis. These leaflets appear well preserved. (Hematoxylin and eosin stains. Original magnifications: c, x10; d, x290.)
Fig. 12. Glutaraldehyde-fixed porcine aortic valve implanted into tricuspid valve position in a 50-year-old woman 39 days before her death. There was no evidence of dysfunction of this valve in the postoperative period. a, View of valve as it appeared from right ventricle. The cusps are freely mobile. b, Photomicrograph of section of one cusp. The cusps are focally but minimally thickened. A small thrombus adheres to one surface. c, Close-up view through one area of cusp shows near normal appearance. d, Close-up view of another area of the same cusp. Here the surface of the cusp is covered by mesothelial-like cells. (Hematoxylin and eosin stains. Original magnifications: b, x20; c and d, x400.)
37 1
The Journol of
372
Yarbrough, Roberts, Reis
Thorocic and Card iova scular Surgery
Fig. 13. Glutaraldehyde-fixed porcine aortic valve implanted in the mitral position in a 60-year-old man for pure mitral regurgit ation. He died 50 days after the valve replacement, having been unconscious for 45 days. This implanted valve appeared to have functioned normally during the entire postoperative period. a. Implanted valve as viewed from left atrium. b, Same valve as viewed from left ventr icle. The orifice is competent and the leaflets are freely mobile. c, Some portions of the leaflets are thickened. A close-up view of a portion of one cusp is shown in d. The valve appears well preserved. (Hematoxylin and eosin stains. Original magnifications: c, x11; d, x160. )
infective process formed larger mycotic masses which caused the orifices to be stenotic. Although infection involving a Starr-Edwards caged-ball prosthesis occurs at the site of attachment of the prosthetic valve to the natural valve annulus/a' 11 the infection in the four tissue valves involved only the cusps . No infection was present beneath the prosthetic rings. The tissue valve may be particularly susceptible to infection by Candida species.P The most rigid of all the tissue valves utilized in this study were the glutaraldehydefixed fascia lata valves. These valves were stiff at the time of implantation. They were utilized in 7 calves, but 5 of the animals
died of acute pulmonary edema (probably from stenosis of the fascia lata valves) shortly after implantation. These 5 were not included in this study. The 2 included in this study lived 48 and 52 days , respectively, but each animal was ' in severe congestive cardiac failure during the entire postoperative period. The fresh fascia lata valve utilized in 1 calf in this study was quite mobile at the time of implantation, but it too was stiff and nonmobile when examined 80 days after implantation. McEnany and associates'> recently reviewed their results with autologous fascia lata mitral valves in 72 patients. In the 53 operative survivors, 60 per cent were dysfunctioning to varying degrees up to 2 years. These authors concluded that fascial valves were poor substitutes for the natural valves when used in the mitral position. This valve has become altered after insertion in the aortic position also .P - 14 The porcine aortic valve appears to ne a good substitute tissue valve if fixed in glutaraldehyde but a poor substitute if fixed in formalin. Each of the three formalin-fixed porcine valves evaluated in this study partially tore from its attachment between 90 and 150 days after implantation, causing severe valvular regurgitation in each. In contrast to the pericardial and fascia lata valves, the formalin-fixed porcine valves remained freely mobile. · However, they appeared necrotic on histologic examination. The glutaraldehyde-fixed porcine valves may well prove to be good substitute tissue valves. None of the 6 patients in this study who had these valves had evidence of valvular dysfunction postoperatively. Furthermore, these valves now have been implanted in 64 other patients by one of us (R. L. R.), and there has been no clinical evidence of valve dysfunction in any after a 20 month follow-up period. Examination of the glutaraldehyde-fixed porcine aortic valves utilized in the 6 patients and in 1 calf in this study showed that only minimal thickening occurred, and most areas of the cusps appeared well preserved by light microscopy. The cusps in all 7 subjects
Volume 65 Number 3
Structural alterations in tissue cardiac valves
March , 1973
Fig. 14. Glutaraldehyde-fixed porcine aortic valve implanted in the mitral position in a 55year-old woman for severe mitral stenosis 130 days before her death. This patient appeared to die from parenchymal pulmonary disease. There was no evidence that the implanted valve functioned abnormally at any time during the postoperative period . G, Implanted valve as viewed from left atrium. b, Valve as viewed from left ventricle . c, Photomicrograph of section of one cusp of the implanted valve. The cusps are not thickened and are freely mobile . d, Close-up of an area of the cusp. The cells are well preser ved, but the cusp is probably focally edematous. (Hematoxylin and eosin stains. Original magnifications: c, xl l ; d, x370.)
Fig. 15. Glutaraldehyde-fixed porcine aortic valve implanted in the aortic position for pure aortic regurgitation in a 51-year-old man 150 days before his death. This patient was asymptomatic postoperatively. He was found dead in bed at home. The cause of the sudden unexpected death is unclear. G, At necropsy, a thrombus filled one of the three sinuses of the implanted aortic valve. One week before he died, a 35 mm. Hg peak systolic pressure gradient was found between the left ventricle and brachial artery. The sinus thrombus may have accounted for the pressure gradient. b, Photomicrograph of section through implanted valve cusp and sinus thrombus. The distal portion of the cusp is mildly thickened. c, Close-up view of portion of cusp. The valve appears well preserved. (Hematoxylin and eosin stains. Original magnifications: a and b, xl l ; c, x400.)
373
The Journal of
374
Yarbrough, Roberts, Reis
Thorac ic and Cardiovascular Surgery
Fig. 16. Glutaraldehyde-fixed porcine aortic valve implanted into the mitral position of a calf 110 days before death. The valve functioned normally during the entire postoperative period. a, Implanted valve as viewed from left atrium. b, Valve as viewed from left ventricle. c. Photomicrograph of section through one cusp . d. Closeup view of portion of cusp. The valve cusps were freely mobile . The valve appears quite edematous and contains fewer cells than normally. (Hematoxylin and eosin stains. Original magnifications: c, x 10; d, x160. )
were freely mobile and competent at necropsy. Death in the 6 patients was due to factors unrelated to the tissue valve.
Summary Morphologic observations have been presented on tissue valves implanted in 10 patients and in 7 calves for periods ranging from 7 to 240 days. It appears that fresh autogenous pericardium, fresh autogenous or glutaraldehyde-fixed human fascia lata, and formalin-fixed porcine aortic valves are unacceptable substitutes for the natural cardiac valves when prepared by these methods . In contrast, the relatively minimal histologic changes in the glutaraldehyde-
Fig. 17. Section of a control porcine aortic valve fixed in stabilized glutaraldehyde but never implanted . a, Portion of a cusp straightened out. b, Higher-power view of base of another cusp. c, Close-up view of portion of one cusp. Compared to this and other control valves the glutaraldehyde fixed porcine valves showed the collagen fibers in the cusps to be aligned in a radial fashion. The collagen in the control valve, as shown here, ha s a more disorganized appearance. Although they did elongate, the nuclei of the glutaraldehyde-fixed valves were better maintained than the more spherical nuclei of the control valves as shown here . (Hematoxylin and eosin stains. Origin al magnifications : a, x8; b, x63; c. x400. )
fixed porcine aortic valves and the lack of signs of dysfunction for up to 20 months after implantation offer encouragement that this tissue valve will prove to be a good valve substitute.
Volume 65 Number 3
Structural alterations in tissue cardiac valves
375
Morch, 1973
REFERENCES
2
3
4
5
6
7
Gerbode, F., editor: Proceedings of the First International Workshop on Tissue Valves, October 4-5, 1969, Ann. Surg. 172: 1, 1970. O'Brien, M. F.: Heterograft Aortic Valves for Human Use: Valve Bank, Techniques of Measurement, and Implantation, J. THoRAe. CARDlOVASC. SURG. 53: 392, 1967. Ionescu, M. I., Mashhour, Y. A. S., and Uboler, G. H.: Reconstructed Heterograft Aortic Valves for Human Use, Thorax 23: 221, 1968. Buch, W. C., Kosek, J. C., and Angell, W. W.: Deterioration of Formalin-Treated Aortic Valve Heterografts, 1. THORAe. CARDIOVASC. SURG. 60: 673, 1970. Carpentier, A., Lemaigre, G., Robert, L., Carpentier, S., and Dubost, C.: Biological Factors Affecting Long-Term Results of Valvular Heterografts, J. THORAe. CARDIOVASC. SURG. 58: 467, 1969. Reis, R. L., Hancock, W. D., Yarbrough, J. W., Glancy, D. L., and Morrow, A. G.: The Flexible Stent: A New Concept in the Fabrication of Tissue Valve Prostheses, J. THORAe. CARDIOVASC. SURG. 62: 683, 1971. Kaiser, G. A., Hancock, W. D., Lukban, S. B., and Litwak, R. S.: Clinical Use of a New Design Stented Xenograft Heart Valve Prosthesis, Surg. Forum 20: 137, 1969.
8 Suzuki, A., Mackrell, J., and Kay, E. B.: Mitral Valve Replacement With Transplant Valves, J. THoRAc. CARDIOVASC. SURG. 60: 13, 1970. 9 Bjork, V. 0., and Hultquist, G.: Teflon and Pericardial Aortic Valve Prostheses, J. THORAC. CARDIOVASC. SURG. 47: 693, 1964. 10 Cohn, L. H., Roberts, W. c., Rockoff, S. D., and Morrow, A. G.: Bacterial Endocarditis Following Aortic Valve Replacement: Clinical and Pathologic Correlations, Circulation 33: 209, 1966. 11 Roberts, W. C., and Morrow, A. G.: Bacterial Endocarditis Involving Prosthetic Mitral Valves: Clinical and Pathologic Observations, Arch. Patho!. 82: 164, 1966. 12 Hudson, R. E. B.: Pathology of Heart Valve Grafting, ill Ionescu, M. I., Ross, D. N., and Wooler, G. H., editors: Biological Tissue in Heart Valve Replacement, London, 1972, Butterworth & Co., Ltd., pp. 835-886. 13 McEnany, M. T., Ross, D. N., and Yates, A. K.: Valve Failure in Seventy-Two Frame-Supported Autologous Fascia Lata Mitral Valves: Two-Year Follow-Up, J. THoRAc. CARDIOVASC. SURG. 63: 199, 1972. 14 Silver, M. D., and Trimble, A. S.: Structure of Autologous Fascia Lata Heart Valve Prostheses, Arch. Patho!' 93: 109, 1972.
Heterograft cardiac valves are supplied in a wide range of sizes and are prepared before implantation. Fresh, beta-propiolactone-treated, gamma-irradiated, freezedried, frozen, and formalin-fixed valves all have been utilized in patients.' Although initial hemodynamic results were good and thromboembolic complications were minimal, these valves have not been durable.":' Because it forms strong cross linkages of collagen while allowing flexibility of tissue and because its bonds are not biodegradable, glutaraldehyde is now being used as a preservative for heterograft valves.' Reports of heterograft valves previously fixed with glutaraldehyde have been encouraging." This paper describes anatomic findings in glutaraldehyde-fixed porcine aortic heterograft valves in 6 patients and in 1 calf. In addition, structural alterations in other types of tissue valves implanted in 4 patients and in 6 calves are described for comparison. From the Clinic of Surgery and Section of Pathology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014. Received for publication Sept. 6, 1972. Address for reprints: Dr. William C. Roberts, Section of Pathology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Md. 20014. ·Present address: Department of Surgery, Duke University Medical Center, Durham, N. C. ··Present address: Department of Surgery, University of Kansas Medical Center, Kansas City, Kan.
364
Material Seventeen tissue valves, implanted for 7 to 240 days (mean 93), were examined at necropsy: Ten had been implanted in patients and seven in Holstein calves (Table I). Except for the three formalin-fixed valves, all had been placed on Reis-Hancock flexible stents.? Fourteen had been implanted at the National Heart and Lung Institute, whereas the three formalin-fixed porcine heterograft valves were implanted into human subjects elsewhere and later were submitted by Hancock Laboratories for histologic studies. The ten porcine heterografts were prepared before implantation by Hancock Laboratories, and the other seven heterotopic tissue valves were prepared at the National Heart and Lung Institute at the time of implantation by the method described previously.": 7 The tissue valves were implanted into the mitral position in all 7 calves. Of the 10 human subjects, 6 had mitral valve replacement, 3 had aortic replacement, and 1 had tricuspid replacement. At necropsy, the previously implanted valves were fixed in either 10 per cent buffered formalin or 3 per cent glutaraldehyde. After fixation, sections were taken from one or more leaflets of each valve, processed, cut, stained, and examined histologically. One section from each cusp
Volume 65 Number 3
Structural alterations in tissue cardiac valves
Morch, 1973
Fig. 1. Fresh autogenous pericardial valve implanted in the mitral position in a 50-year-old woman 39 days before death. There was no evidence of dysfunction of this tissue valve during the postoperative period. a, View of pericardial valve from above. The cusps are thickened and one is retracted. The valve orifice was competent when water was injected under pressure into the left ventricle at necropsy. b, Photomicrograph of section through one cusp. The surfaces of the cusp are lined by fibrin. The cusp is about four times thicker than normal. c, Close-up view of the area shown in brackets in b. This implanted parietal pericardium may be compared to its thickness at the time of implantation, a portion of which is shown in d. Both c and d were taken at the same magnification. The thickening of the implanted pericardium is due mainly to proliferation of fibrous tissue, which probably resulted from organization of the surface fibrin. e, Deposit of calcium in the implanted valve. t. Close-up of portion of implanted valve shows a multinucleated cell. Its significance is uncertain. (Hematoxylin and eosin stains. Original magnifications: b, xl l ; c and d, x70; e and t, x400.)
Fig. 2. Fresh autogenous pericardial valve implanted in the mitral position in a calf 7 days before death. The pericardial valve appeared to function normally during the entire postoperative period, and, at necropsy, injection of water into the left ventricle under pressure indicated that the valve was competent. a, Pericardial valve as viewed from left ventricle. The leaflets are mildly thickened. b, Low-power photomicrograph of section of one cusp. c, Blow-up of area in brackets shown in b. The cusp is nearly twice normal thickness. The thickening is due to fibrin-platelet thrombus on its surface. The margins of the original parietal pericardium are clearly visible. (Hematoxylin and eosin stains. Original magnifications: b, x6; c, x160.)
365
The Journal of
366
Yarbrough, Roberts, Reis
Thoracic and Cardiovascular Surgery
Fig. 3. Fresh autogenous parietal pericardium valve implanted in a calf 82 days before its death. The calf did poorly postoperatively, and signs of mitral regurgitation and infective endocarditis were evident at least 1 month before death. a, Tissue valve as viewed from left ventricle. The cusps are torn and perforated. There are no signs of infection at the site of attachment of the rigid frame to the natural mitral annulus. b, Section of portion of one cusp. The entire valve is necrotic and edematous. A few polymorphonuclear leukocytes are visible. No stainable colonies of organisms were observed, but the calf was being given antibiotics during the entire postoperative period. (Hematoxylin and eosin stain. Original magnification: b, xlOO.)
Fig. 4. Fresh autogenous pericardial valve implanted into the mitral position of a calf 116 days before its death. a, The cusps are diffusely thickened and retracted. b, One cusp tore and prolapsed into the left atrium. c, Photomicrograph of one of the nonprolapsed cusps shows the diffuse thickening. A close-up of the area in brackets is shown in d. There is considerable fibrous thickening of the implanted parietal pericardium. No colonies of organisms were seen on histologic study of the prolapsed cusp. (Hematoxylin and eosin stain. Original magnifications: c, x8; d, xlOO.)
Volume 65
Structural alterations in tissue cardiac valves
Number 3
367
Ma rch, 1973
Table I. Tissue valves evaluated Subjects and durati on of im planta tion PaTy pe of valve
tien ts
Fresh autogenous pericard ium Fresh autog enous fascia lata Glutaraldehydefixed human fascia lata Formalin -fixed porcine aortic valve Glutaraldehydefixed porcine aort ic valve Totals
39
3
7·116 80
0 0
48,56
2
3
80-240
6
14-180
10
14-240
0 110 7-116
7
Table II. Severity of morphologic changes in implanted tissue valves Changes af ter implan tation (0-4+)
Type of valve Fresh autogeno us pericard ium Fresh autogenous fascia lata Glutaraldehyde-fixed human fascia lata Formalin-fixed porcine aortic valve Glu taraldehyde-fixed por cine aortic valve Total
I
No. of Microsubjects Gross scopi c ++++
++++
+++
++++
2
++++
++++
3
++++
++++
7
+
+
4
17
was stained with hematoxylin and eosin, Verhoeff-van Gieson, methenamine silver, and periodic acid-Schiff.
Results The four fresh autogenous pericardial valves were thickened and retracted by 30 days after implantation (Figs. 1 to 4 ) (Table II). Two valves implanted in calves were perforated and detached from the commissures. Grossly, each appeared infected, but microscopically no stainable organisms were found in the leaflets. All
Fig. S. Fresh autogenous fascia lata valve impla nted into the mitral position of a calf 80 da ys before its death. In itiall y he did well posto peratively but later developed overt conge stive cardiac failure. A diastoli c rumble tha t suggested mitral stenosis became audible. At necropsy, the mitral orifice was part ially obstructed by a large fungal mass attached primarily to the rigid-frame por tion of the tissue valve . a, View of tissue valve from left atrium shows the large fun gal mass. b, View of valve from left ventricle again shows the fungal mass obs truc ting the orifice . c, Photomicrograph of a section of one cusp. T he fascia lata is necrotic, devoid of cells, and not thickened. d, Candida species in the fungal mass. ( Hema toxylin a nd eosin stain [c] and methenamine silver stai n [d] . Original magn ificat ions : c, x160; d, x400. )
valves were less pliable than they had been at implantation. Since no murmurs had been heard , valve funct ion was considered normal in both of the calves that lived 7 days and in the patient. The valves were competent when tested post mortem. Histologically, a few mononuclear cells were present on the surfaces of each valve cusp but not within the cusps. Minute calcific deposits were present in all valve cusps. Collagen fibers appeared intact in the valve implanted in the patient but were necrotic in each of the 3 calves. Fibrous tissue was present in each valve cusp, except in the valve implanted 7 days before death . Fibrin deposits were present on all valves. The thickening of the valve in the patient was primarily due to fibrous tissue, almost sure-
368
Yarbrough, Roberts, Reis
The Journol of Thorocic ond Cord iovosculor Surgery
Fig. 6. Glutaraldehyde-fixed human fascia lata valve implanted in the mitral position in a calf 56 days before its death. The calf did poorly from the time of implantation and died of severe congestive cardiac failure. The orifice of the valve became obstructed by a large fungal mass (a, b, and c). a, View of stenotic valve from left atrium. b, View from left ventricle. c. Photo micrograph shows fungal ball totally encapsulating the fascia lata cusp. d, Close-up view shows necrosis of the fascia lata with total absence of cells. e, Candida species identified in the obstructing mass. (Hematoxylin and eosin stains [c and d] and methenamine silver stain eel. Original magnifications : c, x5; d, x40; e, x400. )
ly the result of organization of fibrin. Vascular channels, located primarily at the base of the leaflet, were present in the valve of the human subject. The one fresh autogenous fascia lata valve (Fig. 5) implanted in a calf for 80 days was made stenotic by a large mycotic thrombus which occluded the orifice. This mass was attached to the sewing ring and to the atrial surface of the cusps. Candida species were found in the occluding mass. A few focal fibrin deposits were present on the leaflets, but generally the cusps were not thickened . However, the leaflets were necrotic. The two glutaraldehyde-fixed human fascia lata valves (Figs . 6 and 7) were examined 48 and 56 days, respectively, after implantation in calves. Both were stenotic, rigid,
focally but extensively calcified, and necrotic. Focal fibrin deposits were present on the implanted valves in both calves. The valvular stenosis in 1 calf was worsened by a large mycotic thrombus which was attached to two of the three cusps; it contained Candida species. No cellular infiltrates were present in the leaflets. Each of the three formalin-fixed porcine aortic valves (Figs. 8 to 10) was retrieved from a patient because of valve failure either due to detachment of a cusp from a commissure or to perforation or tearing of a cusp. Histologically, the leaflets in all 3 patients were necrotic and mildly thickened. The cusps were devoid of inflammatory cells except at their bases, where mononuclear cells were present. The adjacent aorta also was necrotic, and its intima was thickened
Volume 65 Number 3
Structural alterations in tissue cardiac valves
369
March, 1973
Fig. 7. Glutaraldehyde-fixed human fascia lata valve implanted in the mitral position of a calf 48 days before its death. The calf did poorly from the beginning of the postoperative period and died of progressive congestive cardiac failure. The valve cusps were not freely pliable at the time of insertion, and at necropsy the cusps were rigid. a, View of valve from left atrium. b, View from left ventricle. c, Photomicrograph of portion of one cusp. The fascia in this area is necrotic and free of cells. In other areas, fibrous tissue covered the ventricular surfaces. In most areas the cusps were extensively calcified. The rigidity of the cusps at necropsy could be attributed primarily to the presence of the calcific deposits. (Hematoxylin and eosin stain. Original magnification: c, x65.)
by a loose fibrous tissue that contained multinucleated cells. No calcific deposits were present. The seven porcine valves fixed in stabilized glutaraldehyde* (Figs. 11 to 16) appeared normal grossly. The leaflets were thin, translucent, and freely mobile. Valve function appeared normal in each. However, one of the seven valves contained a large thrombus in one of the three sinuses (Fig. 15). Microscopically, the cusps contained elongated cells, similar to those seen in a control valve (Fig. 17) . Small foci of necrosis were noted in three of the seven implanted valves (Figs. 12, 14, and 16). The sinus surfaces of the cusps of three valves were covered by. mesothelial-like cells arranged more or less perpendicular to 'Supplied by Hancock Laboratories, Inc., Orange, Calif.
Fig. 8. Formalin-fixed porcine aortic valve implanted in a patient 90 days before his death. a, Section' includes commissure, two aortic valve cusps, and adjacent aorta. The cusps at their bases contain mononuclear cells, and the adjacent intima of the aorta is thickened. b, Close-up of area in brackets shown in a. The media of the entire aorta is necrotic, the intima is thickened, and giant cells are present at the junction of the media and intima. (Hematoxylin and eosin stains. Original magnifications: a, x20; b, X 100.)
the cuspal surfaces (Fig. 12). These cells occasionally grouped together to form giant cells. Deposits of fibrin were superimposed on the lining cells. No lining cells, fibrin, or fibrous tissue were present on the upstream surfaces of the cusps except at their bases adjacent to the myocardium. No calcium was found in the cusps. Discussion
Although the numbers of patients (10) and calves (7) involved in the present study were small, the results of morphologic examination of the seventeen tissue valves are quite clear. Whether in human subjects or in calves, the parietal pericardial and fascia lata valves appear to be poor substitutes for natural valves. The fresh pericardial mitral valve in the patient was four times thicker than it had been when inserted only 39 days before death. Similar thickening and focal calcification of pericardial valves have been observed by others." 9 The thickening appeared to result from deposition of successive layers of fibrin which were transformed into fibrous tissue. In each of the 2 calves
370
Yarbrough, Roberts, Reis
Fig. 9. Formalin-fixed porcine aort ic valve implanted in a patient 150 days before his death . a, Low-power photomicrograph of one cusp. It is quite edematous. b, Close-up of area shown in brackets in a. Mononuclear cells infiltrate the core of the cusps, and that portion near the margins is totally necrotic. The core portion of the cusp is severely edematous. c, Close-up view of another area in brackets shown in a. The cusp in this area is totally necrotic, devoid of cells, and covered on one surface by a fibrin thrombus. (Hematoxylin and eosin stains . Original magnifications: a, x5; b, >-63; c, x250.)
that survived over 7 days with fresh pericardial valves, portions of the cusps became detached and both valves were necrotic. It is probable that infective endocarditis caused the major portion of the destruction in each of these animals, but that was not proved. All three fascia lata valves, whether implanted when fresh or after glutaraldehyde fixation, became rigid, their orifices stenotic, and the tissue necrotic. The rigidity was due
The Journo l of Thorac ic and Card iovascular Surgery
Fig. 10. Formalin-fixed porcine aortic valve irnplanted in a patient 240 days before his death. a, Section includes port ions of aorta and of two cusps. A close-up of one area in brackets is shown in b. Fibrin thrombus is present on the aortic surface of this cusp near its attachment to the aorta. Inflammatory cells also are present in the cusp at this point. c, Close-up of another area in brackets. Here the cusp is totally necrotic and devoid of cells. (Hematoxylin and eosin stains. Original magnifications : a, x5; b, x63; c, x31O.)
to deposition of fibrous tissue and extensive calcific deposits on the surfaces of the fascia lata cusps. The stenosis was produced by the rigidity of the cusps and by the presence of large mycotic thrombi obstructing the orifices (in 2 of the 3 calves). The cause of the latter in each was Candida species infection. Thus , the tissue valves were sites of infection in 4 of the 7 calves: The cusps were extensively damaged, and the valves were made purely incompetent in each of the 2 animals with pericardial valves; however, in the 2 with fascia lata valves, the
Volume 65 Number 3
Structural alterations in tissue cardiac valves
March. 1973
Fig. 11. Glutaraldehyde-fixed porcine aortic valve implanted in the mitral position of a 44year-old man 14 days before his death. Preoperatively he had severe mitral stenosis. His postoperative course was so smooth that he was discharged 12 days after operation. He collapsed suddenly at home 2 days later and died immediately. a, View of valve from left atrium. b, View from left ventricle. The cusps are freely movable and devoid of thrombus. c, Photomicrograph of section through one cusp. d, Close-up view shows many cells and no necrosis. These leaflets appear well preserved. (Hematoxylin and eosin stains. Original magnifications: c, x10; d, x290.)
Fig. 12. Glutaraldehyde-fixed porcine aortic valve implanted into tricuspid valve position in a 50-year-old woman 39 days before her death. There was no evidence of dysfunction of this valve in the postoperative period. a, View of valve as it appeared from right ventricle. The cusps are freely mobile. b, Photomicrograph of section of one cusp. The cusps are focally but minimally thickened. A small thrombus adheres to one surface. c, Close-up view through one area of cusp shows near normal appearance. d, Close-up view of another area of the same cusp. Here the surface of the cusp is covered by mesothelial-like cells. (Hematoxylin and eosin stains. Original magnifications: b, x20; c and d, x400.)
37 1
The Journol of
372
Yarbrough, Roberts, Reis
Thorocic and Card iova scular Surgery
Fig. 13. Glutaraldehyde-fixed porcine aortic valve implanted in the mitral position in a 60-year-old man for pure mitral regurgit ation. He died 50 days after the valve replacement, having been unconscious for 45 days. This implanted valve appeared to have functioned normally during the entire postoperative period. a. Implanted valve as viewed from left atrium. b, Same valve as viewed from left ventr icle. The orifice is competent and the leaflets are freely mobile. c, Some portions of the leaflets are thickened. A close-up view of a portion of one cusp is shown in d. The valve appears well preserved. (Hematoxylin and eosin stains. Original magnifications: c, x11; d, x160. )
infective process formed larger mycotic masses which caused the orifices to be stenotic. Although infection involving a Starr-Edwards caged-ball prosthesis occurs at the site of attachment of the prosthetic valve to the natural valve annulus/a' 11 the infection in the four tissue valves involved only the cusps . No infection was present beneath the prosthetic rings. The tissue valve may be particularly susceptible to infection by Candida species.P The most rigid of all the tissue valves utilized in this study were the glutaraldehydefixed fascia lata valves. These valves were stiff at the time of implantation. They were utilized in 7 calves, but 5 of the animals
died of acute pulmonary edema (probably from stenosis of the fascia lata valves) shortly after implantation. These 5 were not included in this study. The 2 included in this study lived 48 and 52 days , respectively, but each animal was ' in severe congestive cardiac failure during the entire postoperative period. The fresh fascia lata valve utilized in 1 calf in this study was quite mobile at the time of implantation, but it too was stiff and nonmobile when examined 80 days after implantation. McEnany and associates'> recently reviewed their results with autologous fascia lata mitral valves in 72 patients. In the 53 operative survivors, 60 per cent were dysfunctioning to varying degrees up to 2 years. These authors concluded that fascial valves were poor substitutes for the natural valves when used in the mitral position. This valve has become altered after insertion in the aortic position also .P - 14 The porcine aortic valve appears to ne a good substitute tissue valve if fixed in glutaraldehyde but a poor substitute if fixed in formalin. Each of the three formalin-fixed porcine valves evaluated in this study partially tore from its attachment between 90 and 150 days after implantation, causing severe valvular regurgitation in each. In contrast to the pericardial and fascia lata valves, the formalin-fixed porcine valves remained freely mobile. · However, they appeared necrotic on histologic examination. The glutaraldehyde-fixed porcine valves may well prove to be good substitute tissue valves. None of the 6 patients in this study who had these valves had evidence of valvular dysfunction postoperatively. Furthermore, these valves now have been implanted in 64 other patients by one of us (R. L. R.), and there has been no clinical evidence of valve dysfunction in any after a 20 month follow-up period. Examination of the glutaraldehyde-fixed porcine aortic valves utilized in the 6 patients and in 1 calf in this study showed that only minimal thickening occurred, and most areas of the cusps appeared well preserved by light microscopy. The cusps in all 7 subjects
Volume 65 Number 3
Structural alterations in tissue cardiac valves
March , 1973
Fig. 14. Glutaraldehyde-fixed porcine aortic valve implanted in the mitral position in a 55year-old woman for severe mitral stenosis 130 days before her death. This patient appeared to die from parenchymal pulmonary disease. There was no evidence that the implanted valve functioned abnormally at any time during the postoperative period . G, Implanted valve as viewed from left atrium. b, Valve as viewed from left ventricle . c, Photomicrograph of section of one cusp of the implanted valve. The cusps are not thickened and are freely mobile . d, Close-up of an area of the cusp. The cells are well preser ved, but the cusp is probably focally edematous. (Hematoxylin and eosin stains. Original magnifications: c, xl l ; d, x370.)
Fig. 15. Glutaraldehyde-fixed porcine aortic valve implanted in the aortic position for pure aortic regurgitation in a 51-year-old man 150 days before his death. This patient was asymptomatic postoperatively. He was found dead in bed at home. The cause of the sudden unexpected death is unclear. G, At necropsy, a thrombus filled one of the three sinuses of the implanted aortic valve. One week before he died, a 35 mm. Hg peak systolic pressure gradient was found between the left ventricle and brachial artery. The sinus thrombus may have accounted for the pressure gradient. b, Photomicrograph of section through implanted valve cusp and sinus thrombus. The distal portion of the cusp is mildly thickened. c, Close-up view of portion of cusp. The valve appears well preserved. (Hematoxylin and eosin stains. Original magnifications: a and b, xl l ; c, x400.)
373
The Journal of
374
Yarbrough, Roberts, Reis
Thorac ic and Cardiovascular Surgery
Fig. 16. Glutaraldehyde-fixed porcine aortic valve implanted into the mitral position of a calf 110 days before death. The valve functioned normally during the entire postoperative period. a, Implanted valve as viewed from left atrium. b, Valve as viewed from left ventricle. c. Photomicrograph of section through one cusp . d. Closeup view of portion of cusp. The valve cusps were freely mobile . The valve appears quite edematous and contains fewer cells than normally. (Hematoxylin and eosin stains. Original magnifications: c, x 10; d, x160. )
were freely mobile and competent at necropsy. Death in the 6 patients was due to factors unrelated to the tissue valve.
Summary Morphologic observations have been presented on tissue valves implanted in 10 patients and in 7 calves for periods ranging from 7 to 240 days. It appears that fresh autogenous pericardium, fresh autogenous or glutaraldehyde-fixed human fascia lata, and formalin-fixed porcine aortic valves are unacceptable substitutes for the natural cardiac valves when prepared by these methods . In contrast, the relatively minimal histologic changes in the glutaraldehyde-
Fig. 17. Section of a control porcine aortic valve fixed in stabilized glutaraldehyde but never implanted . a, Portion of a cusp straightened out. b, Higher-power view of base of another cusp. c, Close-up view of portion of one cusp. Compared to this and other control valves the glutaraldehyde fixed porcine valves showed the collagen fibers in the cusps to be aligned in a radial fashion. The collagen in the control valve, as shown here, ha s a more disorganized appearance. Although they did elongate, the nuclei of the glutaraldehyde-fixed valves were better maintained than the more spherical nuclei of the control valves as shown here . (Hematoxylin and eosin stains. Origin al magnifications : a, x8; b, x63; c. x400. )
fixed porcine aortic valves and the lack of signs of dysfunction for up to 20 months after implantation offer encouragement that this tissue valve will prove to be a good valve substitute.
Volume 65 Number 3
Structural alterations in tissue cardiac valves
375
Morch, 1973
REFERENCES
2
3
4
5
6
7
Gerbode, F., editor: Proceedings of the First International Workshop on Tissue Valves, October 4-5, 1969, Ann. Surg. 172: 1, 1970. O'Brien, M. F.: Heterograft Aortic Valves for Human Use: Valve Bank, Techniques of Measurement, and Implantation, J. THoRAe. CARDlOVASC. SURG. 53: 392, 1967. Ionescu, M. I., Mashhour, Y. A. S., and Uboler, G. H.: Reconstructed Heterograft Aortic Valves for Human Use, Thorax 23: 221, 1968. Buch, W. C., Kosek, J. C., and Angell, W. W.: Deterioration of Formalin-Treated Aortic Valve Heterografts, 1. THORAe. CARDIOVASC. SURG. 60: 673, 1970. Carpentier, A., Lemaigre, G., Robert, L., Carpentier, S., and Dubost, C.: Biological Factors Affecting Long-Term Results of Valvular Heterografts, J. THORAe. CARDIOVASC. SURG. 58: 467, 1969. Reis, R. L., Hancock, W. D., Yarbrough, J. W., Glancy, D. L., and Morrow, A. G.: The Flexible Stent: A New Concept in the Fabrication of Tissue Valve Prostheses, J. THORAe. CARDIOVASC. SURG. 62: 683, 1971. Kaiser, G. A., Hancock, W. D., Lukban, S. B., and Litwak, R. S.: Clinical Use of a New Design Stented Xenograft Heart Valve Prosthesis, Surg. Forum 20: 137, 1969.
8 Suzuki, A., Mackrell, J., and Kay, E. B.: Mitral Valve Replacement With Transplant Valves, J. THoRAc. CARDIOVASC. SURG. 60: 13, 1970. 9 Bjork, V. 0., and Hultquist, G.: Teflon and Pericardial Aortic Valve Prostheses, J. THORAC. CARDIOVASC. SURG. 47: 693, 1964. 10 Cohn, L. H., Roberts, W. c., Rockoff, S. D., and Morrow, A. G.: Bacterial Endocarditis Following Aortic Valve Replacement: Clinical and Pathologic Correlations, Circulation 33: 209, 1966. 11 Roberts, W. C., and Morrow, A. G.: Bacterial Endocarditis Involving Prosthetic Mitral Valves: Clinical and Pathologic Observations, Arch. Patho!. 82: 164, 1966. 12 Hudson, R. E. B.: Pathology of Heart Valve Grafting, ill Ionescu, M. I., Ross, D. N., and Wooler, G. H., editors: Biological Tissue in Heart Valve Replacement, London, 1972, Butterworth & Co., Ltd., pp. 835-886. 13 McEnany, M. T., Ross, D. N., and Yates, A. K.: Valve Failure in Seventy-Two Frame-Supported Autologous Fascia Lata Mitral Valves: Two-Year Follow-Up, J. THoRAc. CARDIOVASC. SURG. 63: 199, 1972. 14 Silver, M. D., and Trimble, A. S.: Structure of Autologous Fascia Lata Heart Valve Prostheses, Arch. Patho!' 93: 109, 1972.