Conduits for Hemodialysis Access

Conduits for Hemodialysis Access

Conduits for Hemodialysis Access Eric C. Scott, MD, and Marc H. Glickman, MD The role of prosthetic arteriovenous (AV) access is still important in th...

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Conduits for Hemodialysis Access Eric C. Scott, MD, and Marc H. Glickman, MD The role of prosthetic arteriovenous (AV) access is still important in the management and care of the renal dialysis patients. Multiple new modalities are available to the surgeon today and it is imperative that their role be understood so that optimum care can be delivered to this complex group of patients. This article describes significant changes in prosthetic management and newer configurations available to the surgeon. Semin Vasc Surg 20:158-163 © 2007 Elsevier Inc. All rights reserved.

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ATIENTS WITH END-STAGE renal disease rely on durable, high-flow vascular conduits to obtain effective hemodialysis. To this end, a wide variety of arteriovenous (AV) fistulas and vascular grafts have been designed to provide patients with suitable means of hemodialysis. Beginning in the 1990s, evidence began to accumulate that AV fistulas had improved patency, required fewer secondary procedures to maintain patency, and had reduced incidence of infection compared to AV grafts and catheters.1-4 In response, the National Kidney Foundation published guidelines in 1997 that emphasized the benefits of AV fistulas and established a nationwide goal of creating AV fistulas in 50% of new patients who require hemodialysis access placement each year.5 Despite preferential placement of AV fistulas, many patients lack suitable veins for autogenous fistula creation. In some instances, native superficial veins of the forearm are too small to use effectively and, in others, all suitable veins have been utilized in prior access procedures, peripheral vascular operations, or for coronary artery bypass grafting. Thus, despite our understanding of the advantages of AV fistulas, there is a clear and ever-present portion of patients with end-stage renal disease that depend on prosthetic or biological AV grafts for hemodialysis. In 2004, these patients accounted for 38% of the more than 309,000 patients receiving hemodialysis in the United States.6 Following decades of research, there are now numerous prosthetic and biological conduits available for use in hemodialysis access.

Division of Vascular Surgery, Eastern Virginia Medical School, Norfolk, VA. Address reprint requests to Marc H. Glickman, MD, Division of Vascular Surgery, Eastern Virginia Medical School, 600 Gresham Drive, Suite 8620, HH 6th Floor, Norfolk, VA 23507. E-mail: mglickman@ vascularandtransplant.com

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0895-7967/07/$-see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1053/j.semvascsurg.2007.07.007

Biological Grafts Bovine Carotid Artery The bovine carotid artery conduit was first described in 1966, and is formed by enzymatic digestion of the musculoelastic portion of the arterial wall, leaving behind a strong, nonantigenic collagen matrix7 (Fig 1). Its first use as an AV graft for hemodialysis was described by Chinitz and colleagues in 19728 and, within a decade, numerous other centers had begun using bovine carotid artery for patients without suitable vein. The graft is soft, naturally compliant, pulsatile, and relatively easy to use. Cumulative patency data is variable, ranging from 21% to 86% at 1 year and 45% to 76%% at 2 years.9-16 In 1975, three years after introduction of bovine carotid artery grafts, expanded polytetrafluoroethylene (ePTFE) was first used as a conduit for AV access.17 Soon after, numerous case series and one prospective, randomized trial were published comparing outcomes in patients receiving bovine carotid artery and ePTFE grafts (Table 1).10,12-16,18,19 Five of these studies, including the only prospective, randomized trial, failed to demonstrate a significant difference in cumulative patency between the two conduits.14-16,18,19 Each of the remaining three studies, however, found superior patency in ePTFE grafts compared to bovine carotid artery.10,12,13 While superior cumulative patency of bovine carotid artery or ePTFE remains debatable, several important differences in graft complications have emerged. First, ePTFE grafts are more often salvaged in the event of infection. Bovine carotid arteries become infected in 9% to 20% of cases, with similar rates found in ePTFE grafts.10,12,14,15 However, as an infected biologic conduit is at risk for degeneration and rupture, these grafts are consistently removed when infection is confirmed. In contrast, many ePTFE graft infections that do not involve an anastomosis are successfully treated conservatively, preserving the graft. Second, bovine carotid artery grafts are equally prone to pseudoaneurysm formation (1% to

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Figure 1 Bovine carotid artery graft (Artegraft; Artegraft, Inc, North Brunswick, NJ).

Figure 2 Bovine mesenteric vein graft (ProCol Vascular Bioprosthesis; Hancock Jaffe Laboratories, Irvine, CA).

8%),13-15,18 but closure of the defect is more difficult than for ePTFE, and often requires graft revision or resection. Third, true aneurysms are predominantly seen in bovine carotid artery grafts, and at a rate of 1% to 7%.12-14,18 This is speculated to result from gradual degeneration of the graft, in part due to repeated needle punctures in focal areas. Aneurysmal degeneration is an additional indication for bovine graft removal. Finally, bovine carotid artery conduits are typically several hundred dollars more expensive than ePTFE grafts. In light of these considerations, use of bovine carotid artery conduits has diminished. Nevertheless, bovine carotid artery remains a useful alternative to prosthetic grafts at some centers. It is presently available in 6-, 7-, and 8-mm diameters with lengths of 15 to 45 cm (Artegraft; Artegraft, Inc, North Brunswick, NJ).

Bovine Mesenteric Vein Bovine mesenteric vein, a newer bioprosthetic conduit with higher elastin content than bovine carotid artery, was developed in the 1990s and became commercially available in 2003 (ProCol Vascular Bioprosthesis; Hancock Jaffe Laboratories, Irvine, CA). Mesenteric vein is harvested, treated with a patented process of glutaraldehyde crosslinking, and sterilized with gamma radiation. The process maintains the collagen-derived strength and elastin-generated compliance of the vein while rendering it nonantigenic. Once prepared for implantation, its compliance is very similar to that of human saphenous vein, and only slightly less than that of human femoral artery.20 It is presently available in a 6-mm diameter with lengths of 10 to 40 cm (Fig 2). Bovine mesenteric vein was used in several European trials

Table 1 Published Reports Comparing Cumulative Patency of Bovine Carotid Artery and Expanded Polytetrafluoroethylene Arteriovenous Grafts Cumulative Patency (%) First Author

Year

Grafts (n)

6 Months

12 Months

24 Months

Kaplan19

1976

Butler15

1977

Tellis12

1979

Lilly14

1980

Anderson10

1980

Sabanayagam13

1980

Hurt18

1983

Anderson16

2005

BCA, 16 ePTFE, 15 BCA, 103 ePTFE, 184 BCA, 71 ePTFE, 66 BCA, 113 ePTFE, 83 BCA, 113 ePTFE, 83 BCA, 402 ePTFE, 225 BCA, 62 ePTFE, 78 BCA, 245 ePTFE, 446

NR NR 94 85 NR NR NR NR NR NR 57* 94* NR NR NR NR

NR NR 83 75 33* 62* 73 84 70* 87* 21* 91* 84 65 86 82

NR NR 76 74 NR NR NR NR 45* 73* NR NR 72 63 NR NR

Abbreviations: BCA, bovine carotid artery; ePTFE, expanded polytetrafluoroethylene; NR, not reported. *Significant difference between conduits.

E.C. Scott and M.H. Glickman

160 of infrainguinal reconstruction prior to its US Food and Drug Administration approval for use as a hemodialysis graft in the United States. In these two small trials, a total of 38 patients with critical limb ischemia and no suitable vein for bypass underwent revascularization with bovine mesenteric vein.21,22 Results were disappointing in each trial, as the conduit was prone to early postoperative thrombosis in this location. Primary patency ranged from 0% to 16% at 3 months, and these results led to the abandonment of bovine mesenteric vein for infrainguinal reconstruction. Use of bovine mesenteric vein as a conduit in complicated hemodialysis patients has been explored by several authors, with more promising results. In 1999, a case series was published of 49 patients who received 50 bovine mesenteric vein grafts for vascular access. Patients had previously undergone placement of an average of 3.6 access fistulas or grafts and were followed for 30 months to assess patency and frequency of complications.23 In the early postoperative period, three graft thromboses and two graft infections were noted. Primary and secondary patency was 62% and 71% at 30 months, respectively. Aneurysmal degeneration of the conduit was not observed in any patient during the 30-month period. In a US multicenter trial of bovine mesenteric vein AV grafts in 183 patients who had previously failed at least one prosthetic graft, results were similarly encouraging.24 Primary patency at 12 months was 36% compared to 28% in similar control patients receiving a new prosthetic graft (P ⫽ .52). Secondary patency at 24 months was 60% in mesenteric vein patients and 43% in patients with prosthetic grafts (P ⫽ .036). Thrombotic and infectious complications were likewise significantly lower in patients receiving bovine mesenteric vein grafts. Pseudoaneurysm formation occurred at a rate of 0.1 events per year and was similar to the rate observed in patients with prosthetic grafts. Bovine mesenteric vein is currently approved by the US Food and Drug Administration for use in the United States as a hemodialysis access graft, and is an appealing alternative to prosthetic graft, particularly for patients who have previously experienced prosthetic graft failure. Although there is limited data, primary and secondary patency of bovine mesenteric vein grafts appear to be equivalent or superior to synthetic grafts, and postoperative complications appear to be reduced with its use.

1970s, ePTFE has become the most widely used type of synthetic graft. Numerous studies, consisting largely of case series and retrospective reviews, have been published detailing patency data and complications incurred during its use. Primary patency of ePTFE grafts at 1 and 2 years is reported to be 17% to 52% and 37% to 38%, respectively.25,26 Secondary patency, more widely reported in these studies, ranges from 50% to 96% at 1 year, 61% to 87% at 2 years, and 50% to 56% at 3 years.10,18,25-27 A meta-analysis was published in 2004 that assessed patency in autogenous and ePTFE AV grafts placed in the upper extremities.2 In this review of 34 studies, a total of 1,245 ePTFE grafts were identified with accompanying primary patency data, and 703 ePTFE grafts were identified with secondary patency data. The aggregate primary patency of these AV grafts was 58% and 33% at 6 months and 18 months, respectively. Secondary patency was 76% and 55% at these time intervals. In comparison, the primary patency rate for autogenous fistulas was 72% and 51% at 6 and 12 months, respectively, and secondary patency rates were 86% and 77% at these intervals. The authors concluded that the best available evidence demonstrated superior patency in autogenous fistulas compared to ePTFE grafts, but cautioned that the vast majority of this evidence came from case series. Complications associated with use of ePTFE grafts include thrombosis, infection, pseudoaneurysm, steal syndrome, venous hypertension, and seroma formation. Of these, the propensity for ePTFE grafts to thrombose is its greatest detriment, as demonstrated by the low primary patency rates above. In a study of 190 ePTFE grafts, 1.32 to 1.38 thrombectomies per graft were required to maintain patency over the course of 2 years.25 The number of surgical revisions required to maintain patency ranged from 0.26 to 0.29 per graft. Graft infections occurred in 6% to 12% of all grafts, 55% to 66% of which were preserved with local treatment and antibiotics.18,25,27,28 Pseudoaneurysm formation is reported in 0% to 6% of patients,18,27 and steal syndrome is reported in 2% to 3%.25,27 Venous hypertension and seroma formation are less commonly reported in the literature, and occur in ⬍10% of patients.25,27 Two ePTFE grafts are marketed in the United States (GoreTex; W.L. Gore, Flagstaff, AZ; Impra; Bard Inc, Tempe, AZ) and although they are manufactured differently, no significant differences in patency, complications, or cost have been shown.25,29

Synthetic Grafts ePTFE

Cuffed ePTFE Grafts

Expanded polytetrafluoroethylene was the first synthetic material to be used in the construction of a prosthetic vascular graft. The ePTFE polymer has numerous qualities that make it an appealing graft material. It is an inert polymer and thus avoids activation of the host’s immune system; it has the lowest coefficient of friction of any known solid material, minimizing resistance to flow; and in an “expanded” form, allows for tissue incorporation and improved pliability. Since its first use in hemodialysis access surgery during the

Numerous modifications of the standard ePTFE graft have been made, most of which have failed to improve patency.30 However, the introduction of a cuffed ePTFE graft (Venaflo II; Bard Inc, Tempe, AZ) that creates a large hood at the venous anastomosis has proven beneficial. Designed to reduce turbulence and shear stress at the venous anastomosis, the graft is intended to reduce neointimal hyperplasia at this vulnerable location and, thereby, improve graft patency. In addition, the larger anastomosis that results from the hooded

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Figure 3 Expanded polytetrafluoroethylene graft with hooded distal cuff for creation of enlarged venous anastomosis (Venaflo II; Bard Inc, Tempe, AZ).

design may delay the formation of significant stenoses at this location (Fig 3). In the only prospective, randomized trial of cuffed versus standard ePTFE grafts for hemodialysis access, significant improvements in secondary patency and flow rates were observed.31 Patients who received cuffed ePTFE grafts were found to have twice the secondary patency of standard ePTFE at 1 year (64% v 32%), a difference that was maintained at 2 years (58% v 21%). In addition, the average time to first intervention on the cuffed grafts was 138 days in comparison to 68 days for standard grafts (P ⬍ .05). Primary patency was superior in the cuffed ePTFE patients at 1 year, but not overall. The graft is produced in 6- to 8-mm diameters, lengths of 10 to 50 cm, and is also available in a variety of tapered configurations.

Helical ePTFE Grafts A helical-shaped ePTFE graft (SwirlGraft; Veryan Medical Ltd., London, UK) has recently been introduced in Europe and is currently being evaluated in a prospective, randomized trial in the United States. The graft’s shape induces a

Figure 4 Helical expanded polytetrafluoroethylene graft designed to establish helical blood flow in the access, reducing regions of low flow or stasis and subsequent formation of intimal hyperplasia. Presently available in Europe and under evaluation in the United States (SwirlGraft; Veryan Medical Ltd., London, United Kingdom).

swirling pattern of blood flow within, eliminating or substantially reducing areas of low shear stress and flow stagnation. This alteration of design is intended to reduce intimal hyperplasia at the venous anastomosis and prolong graft function. Reductions in neointimal hyperplasia and improved patency have been observed in porcine models32 (Fig 4).

Early-Access Grafts Placement of a hemodialysis access is typically accompanied by a significant delay after the procedure before the conduit can be used. Following AV fistula placement, a maturation period of 6 weeks to 6 months can be expected, while tissue incorporation of AV grafts requires 2 to 4 weeks. Patients require temporary hemodialysis catheters during this period, which results in added expense and the potential for catheter-related complications.

Vectra A new, “early-access” conduit constructed of self-sealing polyurethane has been introduced (Vectra; Bard Inc, Tempe, AZ; Fig 5), that effectively eliminates this waiting period and the need for temporary catheters. Constructed of a tri-layer of

Figure 5 Early-access graft containing a self-sealing polyurethane layer that allows for safe access 24 hours following implantation (Vectra; Bard Inc, Tempe, AZ).

E.C. Scott and M.H. Glickman

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Figure 6 Hybrid vascular access graft that combines expanded polytetrafluoroethylene limb for arterial anastomosis with a silicone catheter that is tunneled into the superior vena cava or right atrium (GRAFTcath, Inc. Eden Prairie, MN). Both portions of the device can be cut to an appropriate length and are joined by a titanium connector.

Thoralon, a proprietary material containing polyetherurethaneurea, this graft can safely be cannulated 24 hours after placement. In a prospective, randomized trial of polyurethane versus ePTFE grafts, 142 patients (71 patients in each group) were followed for 12 months and no differences in primary or secondary patency were observed.33 Within 2 weeks of placement, over two-thirds of the polyurethane grafts had been accessed, yet there were no differences in adverse events between the groups, including bleeding. An additional case series of 133 patients receiving polyurethane grafts reported similar results, in which 81% of the grafts were accessed within 4 days of placement.34 In this series, no patient required placement of a temporary dialysis catheter. In these two studies, primary patency was 51% and 33% to 44% at 6 and 12 months, respectively, while secondary patency was 78% to 85% and 61% to 74% for the same time periods. Together, these studies demonstrate that polyurethane grafts significantly reduce catheter dependence and can be accessed within 24 hours of placement without compromising safety or patency.

Fusion A multicenter, prospective trial is presently underway in the United States to assess the safety and efficacy of a new earlyaccess graft (Fusion, Boston Scientific, Natick, MA). The graft is composed of an inner layer of ePTFE and an outer layer of woven polyester. The layers are fused together with polycarbonate-urethane (Corethane), a polyurethane-like material that renders the graft rapidly hemostatic and ready for hemodialysis use within 72 hours of placement. Superior hemostasis and similar handling characteristics compared to conventional ePTFE have been observed in unpublished animal model data. No additional clinical data is available at present.

Hemodialysis Access Hybrids The Achilles’ heal of hemodialysis grafts is most often the venous anastomosis. When a biologic or prosthetic hemodialysis graft fails, it is most commonly secondary to intimal hyperplasia at the venous anastomosis and resultant thrombosis.3,28 To avoid this anastomosis entirely, a novel conduit

(GRAFTcath; GRAFTcath, Inc. Eden Prairie, MN) has been introduced that transitions an ePTFE graft with traditional arterial anastomosis into a single-lumen silicone catheter that is inserted via the internal jugular vein into the superior vena cava or right atrium. The conduit is fully implanted and accessed in the typical percutaneous fashion. Unlike traditional catheters that obtain inflow from the central venous circulation and are often disabled by the formation of fibrin sheaths at the catheter tip, the “catheter” portion of the GRAFTcath secures venous outflow only and thereby avoids fibrin sheath-mediated inflow restriction. A randomized, multicenter trial is underway comparing this conduit to standard ePTFE for technical success and outcomes at 1 year (Fig 6).

Conclusion The provision of functional, lasting hemodialysis access for patients with end-stage renal disease remains a daunting task. The large number of biologic and prosthetic conduits presently available represents the widest range of options surgeons have ever had, yet this breadth reflects our inability to identify the “ideal” access conduit. In fact, there are few procedures in vascular surgery that result in primary patency rates as low as those obtained for AV grafts. Data from prospective, randomized trials suggest cuffed ePTFE grafts provide superior patency in patients who have experienced prior graft failure, and that “early access” grafts can provide a safe, accessible conduit, obviating the need for temporary dialysis catheter use. In all other instances, the optimal graft remains a function of the individual patient’s clinical situation, the surgeon’s experience, and the local availability of various conduits.

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JL: Longitudinal comparison of dialysis access methods: risk factors for failure. J Vasc Surg 26:1009-1019, 1997 Gibson KD, Gillen DL, Caps MT, Kohler TR, Sherrard DJ, StehmanBreen CO: Vascular access survival and incidence of revisions: a comparison of prosthetic grafts, simple autogenous fistulas, and venous transposition fistulas from the United States Renal Data System Dialysis Morbidity and Mortality Study. J Vasc Surg 34:694-700, 2001 National Kidney Foundation: NKF-DOQI clinical practice guidelines for vascular access. National Kidney Foundation-Dialysis Outcomes Quality Initiative. 1997. Available at: http://www.ncbi.nlm.nih.gov/ entrez/query.fcgi?cmd⫽Retrieve&db⫽PubMed&dopt⫽Citation&list_ uids⫽9339150. Accessed November 18, 2006 US Renal Data System: USRDS 2006 Annual Data Report: Atlas of End-Stage Renal Disease in the United States, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD, 2006 Rosenberg N, Martinez A, Sawyer PN, Wesolowski SA, Postlethwait RW, Dillon ML Jr: Tanned collagen arterial prosthesis of bovine carotid origin in man. Preliminary studies of enzyme-treated heterografts. Ann Surg 164:247-256, 1996 Chinitz JL, Tokoyama T, Bower R, Swartz C: Self-sealing prosthesis for arteriovenous fistula in man. Trans Am Soc Artif Intern Organs 18:452457, 1972 Hutchin P, Jacobs JR, Devin JB, Shaughnessy S, Roland AS: Bovine graft arteriovenous fistulas for maintenance hemodialysis. Surg Gynecol Obstet 141:255-258, 1975 Anderson CB, Sicard GA, Etheredge EE: Bovine carotid artery and expanded polytetrafluroethylene grafts for hemodialysis vascular access. J Surg Res 29:184-188, 1980 Hertzer NR, Beven EG: Venous access using the bovine carotid heterograft: techniques, results, and complications in 75 patients. Arch Surg 113:696-700, 1978 Tellis VA, Kohlberg WI, Bhat DJ, Driscoll B, Veith FJ: Expanded polytetrafluoroethylene graft fistula for chronic hemodialysis. Ann Surg 189:101-105, 1979 Sabanayagam P, Schwartz AB, Soricelli RR, Lyons P, Chinitz J: A comparative study of 402 bovine heterografts and 225 reinforced expanded PTFE grafts as AVF in the ESRD patient. Trans Am Soc Artif Intern Organs 26:88-92, 1980 Lilly L, Nighiem D, Mendez-Picon G, Lee HM: Comparison between bovine heterograft and expanded PTFE grafts for dialysis access. Am Surg 46:694-696, 1980 Butler HG 3rd, Baker LD Jr, Johnson JM: Vascular access for chronic hemodialysis: polytetrafluoroethylene (PTFE) versus bovine heterograft. Am J Surg 134:791-793, 1977 Anderson C, Richardson CJ, Ney AL, et al: Renewed interest in bovine heterograft for vascular access; a comparison between polytetrafluoroethylene and bovine, in Henry ML (ed): Vascular Access for Hemodialysis IX. Los Angeles, CA, Bonus Books, Inc., 2005, pp 185-193 Baker LD Jr, Johnson JM, Goldfarb D: Expanded polytetrafluoroethylene (PTFE) subcutaneous arteriovenous conduit: an improved vascular access for chronic hemodialysis. Trans Am Soc Artif Intern Organs 22:382-387, 1976 Hurt AV, Batello-Cruz M, Skipper BJ, Teaf SR, Sterling WA Jr: Bovine carotid artery heterografts versus polytetrafluoroethylene grafts. A prospective, randomized study. Am J Surg 146:844-847, 1983

163 19. Kaplan MS, Mirahmadi KS, Winer RL, Gorman JT, Dabirvaziri N, Rosen SM: Comparison of “PTFE” and bovine grafts for blood access in dialysis patients. Trans Am Soc Artif Intern Organs 22:388-393, 1976 20. Katzman H, Glickman MH, Schild AF, Fujitani RM, Lawson JH: Prospective, non-randomized, multicenter evaluation of the mesenteric vein bioprosthetic graft for hemodialysis access in patients with prior failed prosthetic grafts, in Henry ML (ed): Vascular Access for Hemodialysis IX; Los Angeles, CA, Bonus Books, Inc., 2005, pp 169-183 21. Kovalic AJ, Beattie DK, Davies AH: Outcome of ProCol, a bovine mesenteric vein graft, in infrainguinal reconstruction. Eur J Vasc Endovasc Surg 24:533-534, 2002 22. Schmidli J, Savolainen H, Heller G, et al: Bovine mesenteric vein graft (ProCol) in critical limb ischaemia with tissue loss and infection. Eur J Vasc Endovasc Surg 27:251-253, 2004 23. Bourquelot P: Procol bioprosthetic vascular grafts for dialysis access in, Henry ML (ed): Vascular Access for Hemodialysis VI. Chicago, IL, Precept Press, 1999, pp 223-229 24. Katzman HE, Glickman MH, Schild AF, Fujitani RM, Lawson JH: Multicenter evaluation of the bovine mesenteric vein bioprostheses for hemodialysis access in patients with an earlier failed prosthetic graft. J Am Coll Surg 201:223-230, 2005 25. Hurlbert SN, Mattos MA, Henretta JP, et al: Long-term patency rates, complications and cost-effectiveness of polytetrafluoroethylene (PTFE) grafts for hemodialysis access: a prospective study that compares Impra versus Gore-tex grafts. Cardiovasc Surg 6:652-656, 1998 26. Bacchini G, Del Vecchio L, Andrulli S, Pontoriero G, Locatelli F: Survival of prosthetic grafts of different materials after impairment of a native arteriovenous fistula in hemodialysis patients. Asaio J 47:30-33, 2001 27. Kherlakian GM, Roedersheimer LR, Arbaugh JJ, Newmark KJ, King LR: Comparison of autogenous fistula versus expanded polytetrafluoroethylene graft fistula for angioaccess in hemodialysis. Am J Surg 152:238243, 1986 28. Palder SB, Kirkman RL, Whittemore AD, Hakim RM, Lazarus JM, Tilney NL: Vascular access for hemodialysis. Patency rates and results of revision. Ann Surg 202:235-239, 1985 29. Kaufman JL, Garb JL, Berman JA, Rhee SW, Norris MA, Friedmann P: A prospective comparison of two expanded polytetrafluoroethylene grafts for linear forearm hemodialysis access: does the manufacturer matter? J Am Coll Surg 185:74-79, 1997 30. Berardinelli L: Grafts and graft materials as vascular substitutes for haemodialysis access construction. Eur J Vasc Endovasc Surg 32:203211, 2006 31. Sorom AJ, Hughes CB, McCarthy JT, et al: Prospective, randomized evaluation of a cuffed expanded polytetrafluoroethylene graft for hemodialysis vascular access. Surgery 132:135-140, 2002 32. Caro CG, Cheshire NJ, Watkins N: Preliminary comparative study of small amplitude helical and conventional ePTFE arteriovenous shunts in pigs. J R Soc Interface 2:261-266, 2005 33. Glickman MH, Stokes GK, Ross JR, et al: Multicenter evaluation of a polyturethaneurea vascular access graft as compared with the expanded polytetrafluoroethylene vascular access graft in hemodialysis applications. J Vasc Surg 34:465-472; discussion 72-73, 2001 34. Jefic D, Reddy PP, Flynn LM, Provenzano R: A single center experience in the use of polyurethaneurea arteriovenous grafts. Nephrol News Issues 19:44-47, 2005