Precongress Symposium: Vascular Prostheses

Abstract: The U.S. Food and Drug Administration (FDA) believes that a performance standard for vascular graft prostheses ≥6 mm in diameter is necessary to control the risks associated with such devices. These include infection, occlusion, noncompliance, porosity issues, structural integrity, and nonbiocompatibility. In 1979 the FDA published its proposed rule to classify these grafts into Class II devices. The data supporting the need for a performance standard have come from FDA Advisory Committee Proceedings, contract safety and performance studies (1979), and the recent Vascular Graft Safety and Update Symposium (1984). Voluntary standards have been submitted by the AAMI (Association for the Advancement of Medical Instrumentation) VP-D/84 and the ISO/DP 7198. The amendment to the Federal Food, Drug, and Cosmetic Act, known as the Medical Device Amendment of 1976, under Section 514 presents the procedure for the development of performance standards. This article describes the participation by those regulatory, manufacturing, tester, and user groups most affected by the standards and most directly involved in standards development procedures.     

Vascular Prostheses Performance Assessment of Vascular Prosthesis Failure

During 30 years of active participation in clinical vascular surgery and the screening and development of vascular prostheses, greater than 450 different fabrications have been evaluated. Vascular prosthesis retrieval and analysis during this period of time have been systematized, and greater than 40 different causes of failure of vascular prostheses have been classified into six classes: failure of diagnosis, error of implantation, patient contribution, complications of healing, fabrication failure, and infection. Such analyses allow for scientific evaluation, testing development, and the elaboration of performance standards so badly needed at present. Such a task is being organized by American Society for Testing and Materials (ASTM) Subcommittee F04.04 on Cardiovascular Materials and Devices and will require the active participation of all knowledgeable individuals.     

Myristoyl Gelatin as a Sealant for Dacron Vascular Prostheses

Abstract: Myristoyl gelatin (MG) retains its gel structure at temperatures above body temperature without any crosslinking. As a coating material, MG adheres well to polyester fibers. and the outermost layers of the sealant that are in contact with blood or surrounding tissue become hydrophilic. We produced MG-impregnated knitted Dacron vascular prostheses (MG graft [MGG]) and investigated the usefulness of MG as a sealant by replacing the thoracic aorta of dogs. MGGs (5 cm long with an inner diameter of 10 mm) were implanted in 5 mongrel dogs (10–20 kg), and the grafts were retrieved at intervals of 4 h and 2. 4. 8, and 15 weeks after grafting. There was no thrombus formation on the flow surface of the MGGs, indicating adequate antithrombogenic properties. No re-sorption of MG occurred until after 2 weeks, and neither immune reaction nor excessive foreign body reaction was noted. Fragmentation of the sealant induced by cell infiltration began to occur at 4 weeks, yet the sealing effect persisted. The organization of MGG was almost complete at 8 weeks. Because of its pliability and effective adhesion to polyester fibers, its antithrombogenicity, and the persistent sealing effect due to delayed biodegradation and resorption, we conclude that MG is an extremely useful sealant for polyester vascular prostheses.     

The Vicious Cycle of Nonhealing Neointima in Fabric Vascular Prostheses

Abstract: Delayed neointimal healing of a fabric vascular prosthesis was investigated in an animal study focusing on the relationship between red thrombus, fibrinolysis, and endothelialization on the luminal surface. Fabric vascular prostheses were implanted into the descending aortas of 72 dogs. Fifty-nine grafts were explanted from 1 h to 1,705 days after implantation. One hour after implantation, the graft wall was red in color due to fresh thrombus; however, at 1 day the luminal surface became white. Red thrombus reappeared at 1 week and remained present in the long-term. Microscopically the initial red thrombus contained numerous erythrocytes. The white thrombus at 1 day was composed of a dense fibrin network without erythrocytes. At 2 days numerous lacunae appeared in the fibrin layer, and at 3–5 days cavernae and low density fibrin areas were present secondary to fibrinolysis. These areas allowed the blood components to infiltrate into the fibrin layer, and as a result red thrombus reformed within it. The thrombi on the luminal surface in the long-term was always red in color and composed of complicated, multiple stages of thrombus formation, i.e., fresh thrombus with erythrocytes, dense fibrin without erythrocytes, low fibrin density areas, lacunae and cavernae in the fibrin layer, and blood component infiltration into these spaces. Thrombus was always newly formed and present, and involuted in parallel due to fibrinolysis, suggesting that these phenomena perpetuated in a vicious cycle. However, at the anastomoses fibrinolysis was present, but blood component infiltration was prevented by the endothelial cell lining. These results suggest that endothelialization may arrest the vicious cycle of non-healing neointima in fabric vascular prostheses.     

A Comparative Evaluation of two Types of Dacron Vascular Prostheses

The function of two types of Dacron vascular prostheses was evaluated in dogs using the abdominal aorta, inferior vena cava and iliac artery as host vessels. The transplants used were woven and knitted DeBakey prostheses and knitted Cooley prostheses. It is concluded that the function of a prosthesis with a diameter less than 5 mm is unsatisfactory. The same is true of a prosthesis implanted on the venous side, even in a large vein, such as the inferior caval vein, with a high flow. There is no significant difference between the function of DeBakey and Denton Cooley prostheses.     

Succinylated Collagen Crosslinked by Thermal Treatment for Coating Vascular Prostheses

Vascular prostheses coated with collagen carefully prepared to avoid contamination were tested to see if it could induce endothelial cell lining throughout the graft surface in a natural way. The collagen fibers were succinylated. Hydrogel produced with the succinylated collagen was used for the sealant to reduce the amount of solid substance. To avoid contamination and the side effects of chemical reagents, the collagen thermally crosslinked under sterile conditions. A suspension of the collagen fibers was enmeshed in the interstices of Dacron fibers of fabric prostheses, which were then thermally crosslinked at 130°C for 20 h. The prostheses were porous when the collagen fiber network was dry. Under wet conditions, however, the water permeability of the grafts was reduced to 0.1 ml/min from the 1,250 ml/min of the original prostheses. Three weeks after implantation in the abdominal aortas of dogs, 81.2 ± 11% of the luminal surface was macroscopically thrombus free, and 56 ± 14% was endothelialized. More than 95% of the coated collagen had been absorbed. Numerous fibroblasts had migrated into the graft walls, and capillary blood vessels had infiltrated the inside of the graft walls without foreign body reaction. In the controls, thrombus free areas averaged 9.0 ± 5%, and endothelialized areas averaged 5.2 ± 4%. Many giant cells, plasma cells, and lymphocytes had migrated into the graft walls, but no fibroblasts. These results suggest that rapid endothelialization is possible when clean collagen is used.     

New Concept of Microporous Structure in Small Diameter Vascular Prostheses

Abstract: Six types of skinned and microporous vascular prostheses (1.5 mm ID) with different hydraulic permeabilities (HP) were fabricated with either a polyurethane polydimethylsiloxane (PU-PDMS) or a polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) by the spray phase inversion technique: skinned PU-PDMS grafts with an HP of 0 ml/min/cm² (PU-PDMS-S-0), microporous surfaced PU-PDMS grafts with an HP of 2.7 ml/min/cm² (PU-PDMS-2.7), microporous surfaced PU-PDMS grafts with an HP of 39 ml/min/cm² (PU-PDMS-39), poled skinned PVDF-TrFE grafts with an HP of 0 ml/min/cm²(poled PVDF-TrFE-S-0), poled microporous surfaced PVDF-TrFE grafts with an HP of 41 ml/min/cm² (poled PVDF-TrFE-41), and unpoled microporous surfaced PVDF-TrFE grafts with an HP of 41 ml/min/cm² (unpoled PVDF-TrFE-41). Straight segments of these grafts (1.5–2.2 cm in length) and one loop PU-PDMS-39 (10 cm in length) were implanted in the rat infrarenal aorta. Graft surface morphology and wall porosity as reflected by hydraulic permeability were the main determinants of early patency and completeness of healing including endothe-lialization.     

Materials and Their Specifications for Vascular Prostheses. Specifications and Performance Standards

Specification and performance standards represent distinctly different criteria for safe and effective cardiovascular materials and devices, in distinct areas of materials and device conversions, with consideration of implant as well as physiological assault. Cardiovascular device failures need more definitive, realistic, and creative approaches to devising standards of performance with relevance to the physiological environment.     

Plasminogen Activators, Matrix Metalloproteinases and Their Inhibitors in Implanted Vascular Prostheses

OBJECTIVES: To examine the role of plasminogen activators (PAs) and matrix metalloproteinases (MMPs) in the healing of prosthetic grafts. METHODS: Thirty explanted grafts (16 Dacron and 14 PTFE) were studied immunohistochemically using antibodies to PAs, MMPs, and their inhibitors. The percentages of immunostain-positive multinucleated giant cells (MGC) were related to duration of implantation (early vs late), type of lesion (stenosis vs false aneurysm), graft material (Dacron vs PTFE), and graft status (occluded vs patent). RESULTS: All specimens were positive for PAs and MMPs. There were no significant differences in the percentages of MGCs positive for PAs, MMPs, or tissue inhibitor type 2 of MMP (TIMP-2) between the groups. The percentage of TIMP-1 in the aneurysm group (mean, 26%) was significantly lower than that of the stenosis group (mean, 46%) (p<0.05). CONCLUSION: After the implantation of a vascular prosthesis, PAs and MMPs are expressed in cell migration, proliferation and matrix construction. Under-expression of TIMP-1 may be related to the formation of an anastomotic aneurysm.     

Endoluminal surgery

Background: Progress in laparoscopic surgery and the employment of minimally invasive techniques have led to the emergence of a new branch in this field, laparoscopic endoluminal surgery (LES). Methods: LES encompasses all surgical procedures that involve intentional trocar or instrument penetration into the lumen of the gut in order to perform a surgical procedure. The integrity of the organ is preserved except for the small enterotomy sites and the operated area. The procedure is technically demanding and requires sound endoscopic skills. Results: The experience in LES has primarily involved the stomach and right colon for two reasons. First, these organs have a larger volume/surface ratio in the gut; therefore, they are accessible for the endoluminal instruments. Second, even limited resection of the stomach or colon can potentially carry a significant morbidity. LES minimizes the extent of this resection. Conclusions: In this paper we review recent developments in LES, discuss the technical aspects of the procedure, and recommend its applicability.