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Vascular Grafts: Characteristics and Rational Selection
Despite advances in endovascular interventions, the use of surgical bypass is still both relevant and fundamental to the treatment of a wide variety of vascular surgery conditions. The technical details of the bypass procedure and its subsequent outcomes are dependent on the conduit used. The ideal bypass conduit should be as similar as possible to the native vessel it is replacing. It also needs to be readily available, durable, and easy to handle; it needs to hold a low likelihood of infection and thrombosis and be inexpensive. Although such conduit does not exist, autologous blood vessels are closest to the ideal ( Box 16.1 ).
Box 16.1
Conduit Characteristics
Vein grafts, specifically the great saphenous vein (GSV), are the most commonly used autologous conduit. Autologous arterial grafts have been used in selective cases, including coronary revascularization. However, because autologous conduits are not always available or appropriate, artificial grafts such as expanded polytetrafluoroethylene (ePTFE, W.L. Gore, Newark, Delaware) and polyethylene terephthalate (Dacron, DuPont, Wilmington, Delaware) have been used in clinical practice. In general, such grafts are harder to handle, best not used in infected fields, and, compared with autologous conduits, are at increased risk for structural deterioration, occlusion, and infection. Biological conduits, such as cryopreserved vein, have been used for reconstruction when autologous vein is not available and use of prosthetic graft is judged to be suboptimal. Clinical research involving the use of tissue engineered grafts as conduits is ongoing and holds promise ( Table 16.1 ).
TABLE 16.1
Conduit Types
| Conduit Type | Advantages | Disadvantages |
|---|---|---|
| Autologous superficial vein |
|
|
| Autologous deep vein |
|
|
| Autologous arteries |
|
|
| Prosthetic grafts |
|
|
| Cryopreserved allografts and xenografts |
|
|
Autologous Vein
Autologous vein has traditionally been the preferred conduit for vascular bypass in multiple anatomic locations and for a variety of indications, the most common of which remain infrainguinal bypass and creation of arteriovenous access. Autologous vein grafts can also be used for vascular reconstructions in other locations, including other peripheral arteries and more central vessels. For example, the carotid artery can be reconstructed using interposition bypass with GSV in the setting of occlusive disease, aneurysmal disease, and traumatic injury. Alternatively, autologous vein can be used in the setting of an arm bypass as part of a distal revascularization and interval ligation (DRIL) procedure to treat ischemic steal syndrome after hemodialysis access creation.
Histologically, vein is composed of three layers. The tunica intima consists of the endothelium, plays an important role in vasomotor regulation, and is a barrier and interface for the vein from circulating mediators. The tunica media is composed of smooth muscle cells and elastic fibers, whereas the tunica adventitia is composed of connective tissue which provides structural support for the vessel. Veins have a thinner and less well-defined medial layer compared with arteries, as the venous system is a lower pressure system. The adventitia in the vein comprises the majority of the vessel wall volume, whereas it is only a small part of an arterial wall. Arterial endothelial cells are generally long and narrow, whereas vein endothelial cells are short and wide.
Similar to arteries, arterialized veins can develop intimal hyperplasia, which can lead to graft stenosis and ultimately failure.
Multiple pieces of autologous vein, including saphenous and arm vein, can be anastomosed together to form what are called composite venous grafts. Such grafts are often used in infrainguinal lower extremity bypass. Patency of such grafts is lower than single segment autologous grafts; however, they remain a viable option when the former is not available.
Saphenous Vein
In clinical practice the most commonly used autologous vein conduit is GSV because it is easily accessible, long lengths are available for harvest, and removal is inconsequential. The GSV can be harvested using traditional open techniques, using either continuous or skip incisions. Wound infection rates are similar between continuous and skip incisions. Alternatively, GSV can be harvested endoscopically. Comparison of these techniques reveals that, although results are heterogeneous, the overall trend is that endoscopic vein harvest has fewer wound complications but is associated with lower patency rates.
GSV conduits have been used in many revascularization scenarios including peripheral artery reconstruction in the lower and upper extremities as well as renal and visceral revascularization. GSV is often used for infrainguinal bypass, using a variety of conduit configurations such as in situ, reversed, and nonreversed reconstruction. Infrainguinal bypass has been applied in both occlusive and aneurysmal disease. The largest series of saphenous vein bypasses analyzed include 2058 in situ bypasses, the majority of which were for critical limb ischemia (CLI). Primary patency rates of 85%, 72%, and 55% were reported at 1, 5, and 10 years, respectively. Taylor reported a series of 516 reversed vein bypass grafts, showing comparable 5-year primary and secondary patency rates of 75% and 81%.
When used for infrainguinal bypass, GSV conduits have been shown to have superior patency to other conduits. Subgroup analysis of the Bypass versus Angioplasty in Severe Ischaemia of the Leg (BASIL) trial, the only published randomized trial comparing bypass to endovascular interventions in CLI, showed that vein bypass, of which 90% was performed using GSV, was associated with improved amputation free survival compared with prosthetic conduit.
Data from the Project of Ex-vivo Vein Graft Engineering via Transfection (PREVENT) III trial, a large prospective cohort of infrainguinal bypass performed for CLI, revealed that single segment GSV and vein diameter were associated with improved outcomes. Of 1404 total bypass procedures, 604 were performed using single segment GSV greater than 3.5 mm, and 1-year primary patency, primary-assisted patency, secondary patency, and limb salvage rates were 71.5%, 84.4%, 86.9%, and 90.6%, respectively. Conduit characteristics were the main determinant of both early and late graft failure.
The small saphenous vein (SSV) is also an additional source of conduit. It can be used as a single conduit or be spliced with other veins. SSV use can be limited by available length and often requires the patient to be positioned prone for successful harvest.
GSV can be used for vascular reconstructions in other locations. This includes other peripheral arteries and more central vessels. For example, the carotid artery can be reconstructed with GSV in the setting of occlusive disease, aneurysmal disease, or traumatic injury. Reconstruction can be either an interposition bypass or an external to internal carotid bypass. GSV can also be used for upper extremity reconstruction for subclavian or axillary occlusive disease or traumatic injury. GSV is also often commonly used for DRIL procedures in the setting of steal syndrome after hemodialysis access creation. Finally, GSV can also be used as conduit for renal and visceral bypass for both occlusive and aneurysmal disease.
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