Ann Thorac Surg 1999;68:79-83
© 1999 The Society of Thoracic Surgeons
Original Articles
Reduction of intimal and medial thickening in sheathed vein grafts
Heinz Robert Zurbrügg, MDa,
Markus Wied,
Gianni D. Angelini, FRCSa,
Roland Hetzer, MD, PhDa
a Bristol Heart Institute, University of Bristol, Bristol, England, United Kingdom
Adress reprint requests to Dr Zurbrügg, Deutches Herzzentrum Berlin, Augustenburger Platz 1 13353 Berlin, Germany;
e-mail: zurbruegg{at}dhzb.de
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Abstract
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Background. Arterial pressures are described as an important factor in the development of graft degeneration and in reduced patency rate in vein bypass grafts. Sheathing of the graft with a pressure resistant mesh tubing might slow down this development.
Methods. Saphenous vein grafts were implanted into the carotid arteries of five pigs in order to evaluate the influence on myointimal hyperplasia of a compliant Phynox mesh tubing (a wrought Cobalt-Chromium-Nickel-Molybdenum-Iron Alloy), which surrounded autologous vein grafts that were exposed to arterial pressure. Each pig was operated on using a sheathed vein graft (biocompound-graft, a hybrid vascular prosthesis) on one side and an untreated saphenous vein on the other.
Results. After 4 weeks intimal hyperplastic changes were found in all histological sections. The wall thickness (medial and intimal layer) varied from 351 µm to 432 µm in the biocompound-graft and from 391 µm to 1196 µm in the native vein grafts (p < 0.05, n = 5). Severe myocytial and fibroblast proliferation was only found in the control grafts. Cellularity of the medial layer differed at sites of maximal cellular density and ranged from 11 to 12 cells in the biocompound-graft and from 17 to 18 cells per counting field in the native vein grafts (p < 0.05, n = 5).
Conclusions. External support of vein grafts reduces intimal and medial layer proliferation. The findings of this study are in accordance with the results reported by other research groups.
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Introduction
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Structural and functional changes in vein bypass graft walls might be seen as a symptom or a pathogenic factor for graft degeneration and reduced patency rate. Brody and associates described arterial pressure as an important factor in the development of these changes [1]. Angelini and associates revealed reduced adenosine triphosphate-diphosphate concentration ratios in graft veins that was caused by distention before implantation [2].
Various attempts have been made to cope with arterial pressures as a wall stress factor for venous graft walls [3–5]. The biocompound-graft (Alpha Research) (sheathed graft), a hybrid prosthesis intraoperatively assembled out of the patients vein and a highly flexible Phynox mesh (Alpha Research, Luterbach, Switzerland) mesh, also seeks to overcome this problem. During the process of assembly a balloon catheter is inserted into the graft vein. Then a fine mesh tubing is pushed onto the vein and they are glued together with fibrin sealant [6, 7]. In this study intimal and medial layer changes, together with aspects of biocompatibility where close contact of mesh material to the adventitial tissue in a pulsating environment occurs, were examined. Therefore the use of human fibrin sealant in pigs has been omitted.
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Material and methods
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Five white race pigs, weighing 20 to 25 kg, were operated on (Bristol Heart Institute, University of Bristol, Bristol, England). All animals were handled under humane conditions in accordance with the Home Office Animals (Scientific Procedures) Act 1998 (HMSO 1990). They were subjected to the common carotid artery surgery medication and anesthesia. A detailed description of the animal model has been previously described [8]. An unmanipulated saphenous vein graft was interposed into one carotid artery, and for the other carotid artery the graft vein was sheathed with 7.8-mm mesh tubing. A compliant, braided Phynox mesh tubing, with fiber diameters of 32 µm, was slipped over the vein (sheathed graft). Vein grafts of at least 25 mm in length were used. The principle behind the mesh tubing is that of a Chinese finger, which decreases in diameter when longitudinal tension is applied. The mesh tubing was smoothed out over the vein in order to make it fit evenly over the vein wall. The steps of construction are described elsewhere [9]. The original bonding procedure with human fibrin sealant was not performed so as to avoid impairment of the biocompatibility examinations.
Pressure fixation
Four weeks after implantation histological sections of the native veins were made using elastic van Gieson’s stain. Toluidine blue stain was used for the sheathed veins because of the special treatment applied (see below). An equal number of control sections (unsheathed grafts) were made and stained with toluidine blue in order to achieve comparability for the cell counts and for the wall thickness comparisons.
All grafts were divided into proximal, medial, and distal pieces and a randomly selected section was made for each piece, which was at least 5 mm away from the anastomosis and the next section. Six sections per pig (for each 3 biocompound-graft and each 3 saphenous vein control sections) were evaluated. To avoid the destruction of the sheathed samples during the cutting process, caused by the fibers of the mesh tubing, the tissue was embedded into Epon Araldit and cut with a diamond-coated microtome saw into 70 µm slices. Transverse sections were selected at the midportion of each piece. Toluidine blue was used for staining as described by Hehrlein and associates [10, 11]. Because toluidine blue only stains the core, the distinction of cell types is impaired. Therefore, the total number of cells was counted in both intimal and medial layers in five microscopic fields (32 µm x 32 µm x 0.65 µm depth1 ) per section at x400 magnification. The narrowest parts of the grafts are decisive for the fate of the graft. Therefore the maximal and minimal values of wall thickness (medial and intimal together) and cellularity found in each sheathed graft were compared with their corresponding unsheathed control grafts. Medians were used for presentation of the data and the Wilcoxon’s matched pairs test for statistical comparisons was used. Means and standard deviations are shown in the tables for completeness reasons only.
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Results
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Graft vessel diameters varied from approximately 6 mm for the sheathed grafts to approximately 8 mm for the control grafts. Whereas areas of intimal hyperplastic changes were found in all histological sections (Figs 1, 2) the cell counts of the medial layer as an indicator of inflammatory response differed significantly (Figs 3, 4).
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Fig 1. Histological section (toluidine blue stain; x 10 before % reduction) of a sheathed vein graft (pig) 4 weeks after implantation into the arterial circuit. The Phynox fibers are visible in the adventitial layer.
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Fig 2. Histological section (elastic van Gieson’s stain; x 10 before % reduction) of a vein graft (pig) 4 weeks after implantation into the arterial circuit.
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Fig 3. Wall segment of a sheathed graft (toluidine blue stain; x 100 before % reduction) with areas of intimal and medial layer thickening 4 weeks after implantation into a pig carotid artery. The section shows an irregular distribution of cells with a concentration around the Phynox fibers.
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Fig 4. Histological section (elastic van Gieson’s stain; x 100 before % reduction) of a vein segment with low intimal and significant medial layer thickening 4 weeks after implantation into a pig carotid artery. A loss of myocytes with a concentration of inflammatory cells in the medial layer is visible.
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The maximal wall thickness at each section site for the sheathed grafts (intimal and medial layer) varied from 351 µm to 432 µm (median 379 µm), whereas the unsheathed grafts ranged from 391 µm to 1196 µm with a median value of 939 µm (p < 0.05, n = 5 ). Maximal intimal layer thickness varied from 191 µm to 222 µm (median 199 µm) for sheathed grafts, and from 224 µm to 748 µm (median 528 µm) for vein grafts (p < 0.05, n = 5). Maximal medial layer thickness varied not significantly from 167 µm to 210 µm, (median 180 µm) for sheathed grafts and from 180 µm to 448 µm (median 403 µm) for vein grafts. The measurements are given in Table 1.
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Table 1. Results of the Measurements of Wall Thickness Which Give the Maximal Values for Each Section Site - in the Intimal and Medial Wall Layer of Biocompound-Grafts (bcg) and Unsheathed Veins (nv) 4 Weeks After Implantation in Pigs That Underwent Surgery on Both Carotid Arteries. The bcg Significantly Reduced Overall Wall Thickness (Medial and Intimal Layer) as Well as Intimal Layer Thickness. Wilcoxon’s Matched Pairs Test was Applied for p-Values
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Minimal wall thickness per section site went from 134 µm to 170 µm with a median of 160 µm in the sheathed vessels and from 160 µm to 261 µm with a median 173 µm in unsheathed veins. The minimal intimal thickness varied from 68 µm to 83 µm with a median of 76 µm in the sheathed grafts and from 76 µm to 177 µm with a median of 86 µm for the unsheathed veins. This difference was found to be significant (p < 0.05, n = 5). In the medial wall layer minimal wall thickness per section site varied from 66 µm to 97 µm (median 82 µm) in the sheathed grafts and 69 µm to 108 µm (median 82 µm) in the unsheathed veins. The measurement results are given in Table 2.
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Table 2. Results of the Measurements of Wall Thickness Which Give the Minimal Values for each Section Site - in the Intimal and Medial Wall Layer of Biocompound-Grafts (bcg) and Unsheathed Veins (nv) 4 Weeks After Implantation in Pigs That Underwent Surgery on Both Carotid Arteries. The bcg Significantly Reduced Overall Wall Thickness (Medial and Intimal Layer). Wilcoxon’s Matched Pairs Test Was Applied for p-Values.
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The maximal intimal layer cellularity for each section site ranged from 6 cells to 8 cells (median 7 cells) in the sheathed grafts and from 7 to 8 cells (median 8 cells) in the unsheathed veins. The maximal cellularity for the medial layer varied from 11 to 12 cells (median 12 cells) in the sheathed grafts and 17 to 18 cells (median 18 cells) for the native vein grafts (p < 0.05, n = 5, Table 3 ).
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Table 3. Results of the Cellularity Count Which Give the Maximal Values for Each Section Site - in the Intimal and Medial Wall Layer of Biocompound-Grafts (bcg) and Unsheathed Veins (nv) 4 Weeks After Implantation in Pigs That Underwent Surgery on Both Carotid Arteries. The bcg Significantly Reduced Medial Wall Layer Cellularity. Wilcoxon’s Matched Pairs Test was Used for p-Values.
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Minimal cellularity values per section site in the intimal wall layer ranged from 4 to 5 cells (median 4 cells) in sheathed graft samples and from 4 to 5 cells (median 5 cells) in the unsheathed veins. The minimal values in the medial layer went from 5 to 6 cells (median 5 cells) and 12 to 15 cells (median 14 cells). These differences were found to be significant (p < 0.05 and n = 5). These results are given in Table 4.
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Table 4. Results of the Cellularity Count Which Give the Minimal Values for Each Section Site - in the Intimal and Medial Wall Layer of Biocompound-Grafts (bcg) and Unsheathed Veins (nv) 4 Weeks After Implantation in Pigs That Underwent Surgery on Both Carotid Arteries. The bcg Significantly Reduced Medial Wall Layer Cellularity. Wilcoxon’s Matched Pairs Test was Used for p-Values.
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One to 2 giant cells were found in half of the sheathed sections. Low rates of foreign body giant cells, together with moderate inflammatory cell infiltration adjacent to the fibers of the mesh, indicate that a mild foreign body response took place. Inflammatory cells were limited to an area of about 30 µm around the fibers, whereas generalized proliferative activity was to be seen in the unsheathed graft sections (Figs 3, 4).
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Comment
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A significant reduction of intimal and overall wall thickness from the maximal values at each section site was found. As the maximal wall thickness is crucial to the outcome of the bypass graft, the introduction of a sheath for a graft may reduce the risk of occlusion.
Whereas there was an obvious and significant difference in the intimal and overall wall thickness, the values for the medial tissue did not differ significantly. Equalizing processes might be the reason for this effect. The mesh tubing prevented the veins from becoming extensively distended by the arterial pressure and thus prevented the medial layer from becoming thinner. As wall distention occurs in the unsheathed grafts, the wall injury may induce a proliferation of the thin medial layer and therefore equalize the differences when compared with sheathed grafts.
This coincides with the increased medial layer cellularity, which was not observed in the sheathed veins. Significantly less inflammation occurred in the sheathed vein grafts, which supports the concept that less wall injury and reparative activity took place. Decreased intimal proliferation, observed in the sheathed vein grafts, might consequently be the result of the optimization of tangential stress on the endothelial cell layer due to an almost regular inner diameter of the sheathed vein.
The adventitial foreign body reaction around the mesh fibers did not induce proliferation in the medial layer. The inflammatory cells are in close distance to the fibers, which suggests a high degree of histocompatibilty with the mesh graft employed.
Overall external support of vein grafts reduces intimal and medial layer proliferation. The findings of this study are in accordance with the results of other groups [4, 5, 12–14].
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Footnotes
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1 A Leica DMRBE Microscope (Leica Mikrosystem GmbH, Bensheim, Germany) was used and only focused within the cell cores were counted. The depth can be calculated as follows: 
where D = depth, 
= wavelength (550nm) and a = aperture of the objective (for x400 magnification : 0.65). 
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References
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Accepted for publication January 22, 1999.
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Abstract
Introduction
