| BackgroundNowadays, coronary atherosclerosis has become one of the main factors which can cause serious diseases or death in our human being. In the last several years , the attack rate of coronary atherosclerosis has took a great increase following with the improvement of living standard in the world. Apart from the PTCA (percutaneous transcoronary angioplasty), CABG (Coronary Artery Bypass Grafting)has become the most effective method for clinical therapy. With the widely use and development of coronary artery bypass surgery, the operative mortality has become much lower than before ,and the living quality of the ill has remarkably improved. In CABG two kinds of blood vessel were commonly used. One is artery , including arteria mammaria interna and arteria radialis, with the merit of high long-term patency rates (about 90% after 10 years) ;The other is vein ,most is the saphenous vein, which is more widely used for its macro-aperture ,easy harvesting and fitting for multivessel disease.However, the effectiveness of saphenous vein grafts is limited by patency rates approaching 50% 10 years after surgery, and 60% of them had to receive another operation. The major occlude is due to the progressive medial thickening , neointimal hyperplasia , and subsequent superimposed atherosclerosis. Many different therapeutic modalities have been investigated in an attempt to prevent from saphenous vein graft failure, including drug treatment, radiation therapy, gene therapy and support of the external synthetic stents and sheaths. Among them ,the gene therapy and support of the external stents and sheaths are mostly been researched .But apart from lipid lowering therapy, no intervention has hitherto proved clinically effective in preventing late vein graft failure which clearly constitutes a major clinical and economic problem that needs to be urgently resolved.After years research, it was found that perivenous support could remarkably reduce neointimal hyperplasia and promote the formation of a neo-vasa vasorum. According to the relative diameter, The external stent is classify into two type. One is non-restrictive external sheath,the other, restrictive external sheath.,It also can be distinguished as degradable and non-degradable sheath for its character. Alough a great deal of progress has been made,no external sheath was clinically used nowadays. ObjectiveIn our experience,two kinds of PGA external sheath which were different with each other in thickness and a group of Dacron sheath were applied. we also divided them into different groups for their relative diameter. through these work,we wish to get a first sight about this new material which was to be made into external sheath,and to do more research on the mechanisam under the external stents.Methods and results1.Adult mongrel dogs, male or females, will be used in this experiment. We divide them into 5 teams. Dogs in team A will be grafted with autogenous veins as the control group. Team B will be grafted with external 0.4mm restrict PGA stent around autogenous veins. Team C will be grafted with external terylene stent. Team D will be grafted with external 0.4mm norestrict PGA stent. Team E will be grafted with external 0.15mm norestrict PGA stent. After 1/2/4/6 weeks the grafts were re-exposed, checked forpatency and then removed, Grafts were pressure fixed ex vivo at 100 mmHg using Carson's fixative and post-fixed in the same solution for approximately 24 h before being processed for wax embedding. After that,the veins were stained and examined by light and electron microscopy for Histology,Morphometric and Proliferating cell nuclear antigen (PCNA) analyses.2.A11 the grafts with either stent material were patent 4 and 6 weeks after implantation. The cross-sectional thickness of neointima and media in external 0.15mm and 0.4mm no-restrict PGA stent was significantly smaller than the same part of the control team. But in the external 0.4mm restrict PGA stent team and the terylene stent group,they also get a significant grow in neointima and media. As previously reported, placement of external macro-porous stents leads to a highly vascularised neoadventitia in the space between the stent and the graft. We show here that the placement of 0.15mm no-restrict PGA stents and the terylene stent elicits strikingly different adventitial responses. Firstly, with 0.15mm PGA stenting, adventitial microvessels are larger and more fully developed and extend into the media/adventitial interface. This is well illustrated with specific endothelial cell staining for lectin. On the contrary, with Dacron and 0.4mm restrict PGA stenting, the neovasculature was diffuse and comprised many small microvessels. Secondly, large microvessels appeared to penetrate the macroporous stent forming a continuous connection between the external compartment and the vein-graft itself. Percentage of cells expressing PCNA proteina .There were large numbers of giant cells within the cavernous regions of the PGA stent , which were absent in the Dacron stented grafts and the native grafts. A significantly smaller percentage of PCNA positive (proliferating) cells were detected in the intimal and inner medial regions of the 0.15mm and 0.4mm no-restrict PGA stented vessels compared with the darcon stented and the control vessels. In contrast, significantly more proliferating cells were detected in the adventitial layer of PGA stented vessels than terylene stented vesselsConclusionsThis work clearly demonstrates that this new type of material with the character like felt can be a good choice to be made into external stents,which can reduce neointimal and medial thickening after graft. This effect may be due, in part, to the development of a complex network of adventitial microvessels (a neovasa vasorum), which could improve the supply of nutrients and oxygen to the graft and obviate hypoxia. The stent also promotes the infiltration of large numbers of inflammatory cells, which may redirect the migration of VSMC to the adventitia rather than to the intima and produce cytokines that inhibit VSMC proliferation. It remains to be established whether this type of external stent will prevent late vein graft failure in patients undergoing CABG However, since the concensus is that the inhibition of neointima formation and medial thickening precedes and predisposes to subsequent atherogenesis ,it is reasonable to suggest that the external stent will prove effective in treating late vein graft failure. Clearly, when clinical trials are undertaken, the new material are to be well considered.
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