Expanded Polytetrafluoroethylene and Dacron Grafts for Atherosclerotic Lower Extremity Occlusive Disease



Expanded Polytetrafluoroethylene and Dacron Grafts for Atherosclerotic Lower Extremity Occlusive Disease



Steven M. Farley, Vikram S. Kashyap and Wesley S. Moore


Surgical revascularization for lower extremity arteriosclerotic occlusive disease, manifested by critical limb ischemia (CLI) and life-limiting claudication, has been enhanced by an expanding array of conduit choices. In the absence of a perfect arterial substitute, autologous vein possesses the most of these desired arterial characteristics and remains the preferred substitute. However, in up to 25% of patients with CLI, adequate autologous vein is unavailable because of prior use, intrinsic disease, or insufficient length or caliber for revascularization. In this setting an autologous vein is not necessarily the most appropriate conduit, and prosthetic conduit is preferred in some patients.


Expanded polytetrafluoroethylene (ePTFE), the most commonly used prosthetic graft for lower extremity bypass, was initially developed for industrial use as a chemically inert tubing and wire insulation. It was modified and first used as a vascular prosthesis in humans in 1976 to treat lower extremity atherosclerotic occlusive disease. Commercially available heparin-bonded ePTFE (HB-ePTFE) grafts have been developed in an attempt to improve graft patency. Since its initial application, the use of ePTFE has expanded to include dialysis grafts, grafts for abdominal aortic aneurysm repair, extra-anatomic grafts, and bypass grafts for aortoiliac and infrainguinal occlusive disease.


Dacron (polyethylene terephthalate) is a synthetic material that as a textile can be fashioned into a prosthetic graft by weaving, knitting, or braiding methods. Dacron grafts were used often in the early era of lower extremity arterial reconstruction, but initial mediocre results and the presumed increased thrombogenicity of this graft led to a decline in its use. However, Dacron grafts remain the graft of choice in aortic reconstruction for many surgeons, especially with coatings of collagen, albumin, or gelatin. There has been a reappraisal of and rekindled interest in Dacron grafts with heparin bonding for lower extremity arterial reconstruction.



Indications for Prosthetic Bypass


Lower extremity bypass grafting using prosthetic graft material has been supported in the literature when autologous vein is unavailable, as an equivalent conduit when compared to autologous saphenous vein (ASV) in the above-knee (AK) femoropopliteal position, as a competitive choice in the below-knee (BK) popliteal position when HB-ePTFE is used, and occasionally for femorocrural (FC) bypass using HB-ePTFE.


The natural history of arterial occlusive disease is unaltered by a bypass, and atherosclerosis can progress in the distal arterial tree, resulting in graft failure. Because every operation using a vascular graft has a finite patency and therefore a failure rate, patients with lower extremity ischemia have a great likelihood of requiring secondary or even tertiary interventions over their lifetimes. These factors mandate a long-term approach to these patients. Simple comparisons of primary patency fail to assess the impact of initial graft selection on overall long-term treatment.



Data


Evaluation of the relative effectiveness of synthetic grafts (mostly ePTFE) with ASV for infrainguinal bypass has been hampered by the omission of many variables that can influence graft patency other than graft material. To address these issues, a prospective multicenter randomized trial comparing ASV with ePTFE showed that 4-year patency rates for AK grafts were not statistically different from ASV. Limb-salvage rates were essentially identical for AK grafts. However, ASV had significantly better patency rates for distal BK popliteal and infrapopliteal bypasses when compared to ePTFE. Importantly though, ePTFE distal bypass produced acceptable results and was regarded as a better option than primary amputation in patients without ASV graft.


At the University of California at Los Angeles (UCLA), Quinones-Baldrich and colleagues reviewed the preferential use of ePTFE over saphenous vein in 258 patients undergoing 322 infrainguinal revascularizations: 59% for limb salvage and 41% for disabling intermittent claudication. Primary patency for all femoropopliteal bypasses performed with ePTFE was 59% at 5 years. Secondary patency achieved with additional procedures (e.g., thrombolysis, thrombectomy, anastomotic revisions) was 74% at 5 years. Factors associated with improved outcomes for ePTFE included intermittent claudication as the indication for bypass compared to limb salvage, and the AK target over BK popliteal locations. Femoral–infrapopliteal reconstructions using ePTFE had a dismal primary patency rate of 22% at 3 years.


The Bypass versus Angioplasty in Severe Ischaemia of the Leg (BASIL) trial is the most recent randomized trial to include data on prosthetic grafts and lower extremity bypass. Of the 228 patients randomized to the surgery-first arm, 25% underwent bypass with graft, two thirds of which were to the popliteal artery. The authors concluded that prosthetic graft was a conduit of last resort, citing its poor performance in amputation-free survival and overall survival when compared to autologous vein and primary balloon angioplasty. However, data regarding the use of heparin-bonded grafts were not included in the study results.


Data support improved outcomes using HB-ePTFE compared to traditional ePTFE. Also, HB-ePTFE results have challenged the belief that saphenous vein is a superior conduit compared to prosthetic graft for BK bypasses. A randomized Scandinavian study of 546 patients suggested improved primary and secondary 1-year patency with HB-ePTFE when compared to ePTFE but failed to reach statistical significance. Importantly, subgroup analysis of 232 patients who underwent femoropopliteal bypass (excluding the 307 patients who underwent femorofemoral bypasses), demonstrated a statistically significant difference between HB-ePTFE and ePTFE, reporting 1-year primary patencies of 80% and 70%, respectively. In 2009, a retrospective Belgian study of 350 patients with 1- and 2-year follow-up compared HB-PTFE to ASV and demonstrated no difference in primary patency of AK, BK, and FC bypasses between the two groups. The largest report to date is a multicenter, retrospective review of 425 patients, who underwent AK (23%), BK (56%), and FC (20%) bypasses for critical limb ischemia. With a reported 36-month follow-up, the cumulative primary patency, secondary patency, and limb-salvage rates were 61%, 70%, and 83%, respectively. These results compare favorably to historical results for ePTFE, considering 76% of patiens underwent BK bypasses.


Every operation, whether performed with ASV or graft, is subject to finite patency, and secondary interventions are often necessary. At UCLA, thrombectomy with or without patch angioplasty, extension with either vein or ePTFE, or extension and replacement with ePTFE yielded 30-month patency rates between 32% and 55%. In contrast, patients who underwent secondary revascularization with an entirely new bypass using ASV had a 30-month patency of 88%. These results have been mirrored by others. In contrast, Whittemore and coworkers found that revision of failed vein bypass grafts resulted in an unacceptable 3-year patency rate. Therefore, secondary interventions were often more successful when graft was used primarily as a result of the availability of saphenous vein and greater success with graft revisions.


Thus, a concept of staged revascularization for lower extremity ischemia has evolved with an initial prosthetic AK femoropopliteal bypass and then, if necessary, a vein distal bypass if the prosthetic graft fails. Recent randomized and retrospective studies of HB-ePTFE suggest improved patency, with results apparently comparable to ASV even in the BK position. This concept of staged infrainguinal reconstruction permits more conduit options and consequently maximizes the overall revascularization potential. This strategy allows adequate revascularization in most patients with ePTFE grafts and preserves the more optimal use of the ASV in the infrapopliteal location, if needed. Other groups have adopted a similar treatment strategy and have had acceptable results that compare favorably with institutions using an all-autologous policy. Variables that have been examined and found to lead to increased prosthetic graft failure include anastomoses to isolated popliteal segments, poor arteriographic runoff, continued cigarette smoking, and uncontrolled diabetes mellitus.


Some groups have advocated the use of Dacron prostheses with similarly good results. Abbott and colleagues reported on a multicenter randomized prospective trial comparing ePTFE and Dacron for AK femoropopliteal bypass grafting. Their study was predicated on the fact that prosthetic grafts in the AK popliteal position had acceptable results when compared with autogenous vein in many studies. Two hundred forty-four patients were enrolled and randomized to the two different graft choices. There was no statistically significant difference between the two prosthetic conduit types with respect to primary or secondary patencies. The primary patency at 3 years for Dacron was 62% ± 14.4% as compared with 57% ± 15.5% with ePTFE. Results were poorer for small-diameter grafts as opposed to larger grafts (6 mm vs. 8 mm) and for young (<65 years) smoking patients. The authors concluded that for preferential use of synthetic grafts, either conduit would function well, but should be restricted to elderly nonsmokers. Graft size should be carefully considered given the better experience with the larger conduit sizes. This study did not include a third arm of randomizing patients to saphenous vein grafts. Thus definitive comparative conclusions with autogenous conduit cannot be made.


More recently, heparin-bonded Dacron (HBD) has been evaluated in a randomized study with ePTFE. HBD demonstrated improved 3-year primary and cumulative patencies, as well as 5-year limb salvage, when compared to ePTFE. However, 5-year patencies failed to show statistical significance. Moreover, unlike HB-ePTFE, HBD results in the BK position were poor, with 1-, 3-, and 5-year patencies of 38%, 23%, and 23%, respectively.

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Aug 25, 2016 | Posted by in CARDIOLOGY | Comments Off on Expanded Polytetrafluoroethylene and Dacron Grafts for Atherosclerotic Lower Extremity Occlusive Disease

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