Asian Cardiovasc Thorac Ann 2003;11:314-318
© 2003 Asia Publishing EXchange Ltd
Polyurethane Grafts: a Viable Alternative for Dialysis Arteriovenous Access?
Peng Chan Wearn, MMed,
Tan Seck Guan, FRCS
Department of Surgery, Singapore General Hospital, Singapore, Republic of Singapore
For reprint information contact: Tan Seck Guan, FRCS Tel: 65 6321 4051 Fax: 65 6220 9323 email: angelasg{at}singnet.com.sg Department of Surgery, Singapore General Hospital, Outram Road, Singapore 169608, Republic of Singapore.
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ABSTRACT
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Polyurethane grafts, promoted for their self-sealing properties and low complication rates, were recently introduced for hemodialysis access. We review our experience with this graft material to determine its complication and patency rates. Between January 1995 and December 1999, 118 patients, 75 female and 43 male, with a mean age of 51 years, had a total of 163 polyurethane arteriovenous grafts inserted. The grafts were mostly 6 mm in diameter (69.3%), placed in a loop configuration (91.4%) in the forearm (49.7%). The median follow-up period was 12 months. The grafts were first cannulated for hemodialysis at a median time of 19 days after implantation, with 12% used within 3 days. Thrombosis and infection were the most common complications at rates of 32.7% and 30.0%, respectively. Infection was the most common cause of graft loss (61.5%). These complications were amenable to salvage intervention. The 1-year primary and secondary patency rates were 73% and 86%, respectively; the 3-year secondary patency rate, 72%. The 1-year serviceability rate was 64%. With satisfactory patency rates and the advantage of its self-sealing properties permitting early cannulation, polyurethane grafts provide a viable alternative for hemodialysis access. Early recognition and appropriate management of complications can prolong graft survival.
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INTRODUCTION
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Since the introduction of extracorporeal dialysis of blood in 1943,1 the survival of patients with endstage renal disease has been prolonged. However, the maintenance of continuous vascular access has been a constant challenge. The forearm arteriovenous (AV) fistula described by Brescia and colleagues2 in 1966 remains the preferred form of vascular access for long-term hemodialysis as it causes the fewest complications and offers the best long-term patency rate of all available forms of vascular access. However, for patients whose superficial veins are too small or who have undergone repeated vascular access surgery, the use of prosthetic grafts becomes necessary.
Expanded polytetrafluoroethylene (ePTFE) has stood the test of time as an alternative to autogenous vein for AV access in hemodialysis and is the most widely used material. However, it has several drawbacks. It requires 3 to 4 weeks to mature before it can be cannulated. It does not have self-sealing properties, and perigraft fibrous tissue needs to firm up to provide hemostasis after puncture. Postoperative swelling takes time to subside before puncturing can be done. "Weeping" of serous fluid through the graft can cause swelling and obscure the graft.
More recently, polyurethane vascular access grafts have been introduced. The graft was demonstrated in animal studies to cause minimal anastomotic neointimal hyperplasia and have self-sealing properties (Figure 1
), which resulted in minimal postoperative swelling and allowed early cannulation after implantation. Most of the patients with endstage renal failure are started on dialysis through necklines, so early cannulation will enable early removal of these lines and thus avoid the attendant complications.
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Figure 1. Self-sealing properties of a polyurethane graft (right) after puncture compared with a conventional graft (left).
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Since 1995, our department has used polyurethane grafts as an alternative for vascular access. The purpose of this study was to review our experience with the use of this graft, especially in terms of its rates of complication and patency, and hence to determine if it is a viable option for AV access in hemodialysis.
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PATIENTS AND METHODS
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From January 1995 to December 1999, a total of 118 patients with 163 polyurethane AV grafts (Thoratec; Thoratec Corp., Pleasanton, CA, USA) placed for chronic hemodialysis at our hospital were reviewed. There are 75 female and 43 male patients with a female/male ratio of 1.7:1. At the time of implantation, their mean age was 51 years (range, 21 to 88 years). Of the 163 grafts, 50 (30.7%) were placed in diabetic patients and 8 (4.9%) in patients on regular steroid therapy.
The graft characteristics are summarized in Table 1. The majority of the grafts were placed in a loop configuration (91.4%) in the forearm (49.7%) and were 6 mm in diameter (69.3%).
The median follow-up period was 12 months (range, 0 to 60 months). All the graft and salvage procedures were performed by a single senior surgeon. Graft status was evaluated by clinical examination during follow-up visits and through telephone interviews.
The SPSS version 10.0 statistical package (SPSS, Inc., Chicago, IL, USA) was used for life table analysis and construction of Kaplan-Meier patency curves. Categorical data were analyzed by the chi-squared test.
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RESULTS
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The grafts were first cannulated for hemodialysis at a median period of 19 days (range, 1 to 40 days) after implantation, with 20 grafts (12.3%) being used without complications within 3 days of implantation while the patients were still in the hospital.
The graft complications encountered after implantation are listed in Table 2. Altogether there were 78 grafts lost: 26 (33.3%) from thrombosis, 48 (61.5%) from infection, and 4 (5.1%) from pseudoaneurysm.
Thrombosis was the most common complication, occurring in 84 cases (32.7%) at a median time of 9 months after implantation. Patient age, sex, diabetes, as well as the location, size, and configuration of the graft were not shown by the chi-squared test to be independent risk factors for thrombosis. Where information was available, venous causes, mainly venous intimal hyperplasia, were cited as the reason for blockage. Early thrombosis within 6 weeks following implantation occurred in 14 grafts. Thrombectomy was attempted in 12 of these patients, and all were successful.
Of the 84 cases of late thrombosis, no remedial measures were taken for 16 cases because they were deemed unsuitable for salvage. Salvage procedures were performed for 68 cases (81%), of which 10 failed and 58 were successful (a success rate of 85%). Salvage procedures included thrombectomy with a Fogarty balloon catheter (Cobe Cardiovascular Inc., Arvada, CO, USA) with or without thrombolysis; and where there was associated stenosis of the venous anastomosis, balloon angioplasty, vein patching, or interposition grafting was done concomitantly. The operative techniques for the 58 successful salvage cases included thrombectomy alone (33 cases) and thrombectomy with angioplasty (7 cases) or anastomotic revision (7 cases) or interposition graft (11 cases). Of the 7 cases treated by thrombectomy with angioplasty, 5 had angioplasty when angiography showed stenosis at the completion of thrombectomy. Another case had only thrombectomy done initially but was found to have poor flow postoperatively; angioplasty was performed the next day when the angiogram showed stenosis. The last case had thrombolysis and angioplasty performed in the radiology suite, without the need for thrombectomy. The salvaged grafts functioned for a median period of 3 months (range, 1 day to 35 months) before blockage recurred, and repeated interventions were performed in 22 grafts to maintain patency.
Infection accounted for 77 cases (30.0%) of complications, which occurred at a median time of 5.7 months after implantation. It is the most common cause of graft loss at 61.5%. Of these cases, 3 were associated with pseudoaneurysm and 7 with blockage. Where information was available, the most common organism found was Staphylococcus aureus. Diabetes and steroid therapy were not shown to be significant risk factors for infection.
Grafts with extensive infection were removed. Locally infected grafts where the infection was confined to one part of the graft were treated by pus drainage, removal of the infected segment, and graft interposition. Of the 67 cases with infection alone, 40 required complete removal of the graft, 9 were treated with intravenous antibiotics, and 18 were successfully revised with segment interposition. Of the 7 blocked grafts with infection, 5 were removed and 2 had thrombectomy with, in 1 of them, interposition grafting. One of these patients had a subclavian vein occlusion and cellulitis of the arm. The cellulitis subsided on antibiotic therapy, but the graft could not be used anyway. All the 3 grafts with infection and pseudoaneurysm were removed. Thus, overall, 20 of the 77 cases (26%) were successfully salvaged.
Pseudoaneurysm without concomitant infection occurred in 6 grafts at a median interval of 1.3 months after implantation. The pseudoaneurysm was located at the arterial anastomosis in 4 cases and at the "A" limb of the graft in 2 cases. Four of the affected grafts were removed, 1 was revised with an interposition graft, and 1 was left alone.
There were 9 cases of steal syndrome. One was treated by constriction of the graft with a silk tie until the radial pulse was just palpable. Doppler studies showed good flow in the distal vessels in the remaining 8 cases, 2 of whom were only mildly symptomatic and thus were not treated while the other 6 cases improved with physiotherapy. Of 8 cases with venous stenosis, 1 was left alone, 5 had angioplasty, and 2 had bypass grafts. The 2 bypass grafts consisted of a subclavian external jugular Thoratec bypass graft in 1 case and a subclavian axillary PTFE bypass graft in the other, to treat subclavian vein occlusion in both cases.
The primary and secondary patency rates of the grafts were 73% and 86%, respectively, at 1 year and 55% and 72% at 3 years (Figures 2 and 3). Primary patency refers to the graft remaining patent and unobstructed following the initial operation. Secondary patency refers to a graft requiring intervention to maintain patency but otherwise remaining intact. As a result of complications such as graft infection and pseudoaneurysm, the graft serviceability rate of 64% at 1 year (Figure 4) was lower than the patency rate. Serviceability refers to the ability to use the graft for hemodialysis.
During the course of the study, 12 patients died. The causes of death were myocardial infarction in 4 patients, sepsis in 3, endstage renal disease in 2, and carcinoma of the cervix, stroke, and acute pulmonary embolism in 1 patient each. None of the patients died within 30 days following surgery.
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DISCUSSION
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Vascular grafts used for blood access should have features that include ease of puncture on repeated occasions without aneurysm formation; minimal bleeding from the puncture site; potential for early cannulation; and resistance to thrombosis, infection, and edema or seroma formation. Vascular grafts such as ePTFE and biologic grafts, including saphenous veins,3 bovine veins, and umbilical cord veins,4 have been used for this purpose. However, each has unique problems related to its structural characteristics.
The polyurethane graft is a recently introduced alternative. It consists of a compliant 3-layered polyurethane cast. The innermost textured layer is designed to heal in at the anastomosis while preventing thrombus formation. The middle layer is a solid tube of BPS-215M, a material that provides strength and accounts for the graft’s self-sealing properties. The outer textured layer optimizes longer-term tissue ingrowth. Although both the inner and outer surfaces of the graft are porous, the solid middle layer prevents any direct liquid passage across the graft wall and so preclotting is not required. The graft has a total wall thickness of 0.5 mm and is available as a 40-cm graduated reinforced graft of either 5- or 6-mm diameter. It is reinforced by a high-modulus polymer fiber embedded within the graft wall that provides kink resistance and allows it to be used as a looped conduit.
Owing to its self-sealing properties, dialysis needle puncture can normally be started 48 hours after implantation. However, prior to first use, the site should be examined to ensure there is no inflammation, swelling, or edema, the presence of any of which would call for more healing time. Because of pressure for hospital beds, most of our patients are discharged by the 1st or 2nd postoperative day when thrill is felt in the graft. They are then followed up in the clinic 2 weeks later. Once there is no swelling or inflammation, the patient is referred for cannulation. Although the median time to cannulation was 19 days in this study, 12% of the grafts were cannulated within 3 days, allowing early use of the graft for hemodialysis. Another study5 reported cannulation of 58% of the grafts within 3 days. The potential for early cannulation may allow the graft to be used in acute cases needing hemodialysis, thus obviating the need for a concomitant internal jugular line. It will also allow shorter hospitalization and follow-up, which would mean cost savings and convenience for patients. Further investigation with a randomized controlled trial to compare early and late cannulation results should be conducted.
The main advantages of autogenous AV fistulas over available synthetic grafts are their long-term patency of up to 50% at 5 years and low risk of infection.6,7 Based on the results of this study in terms of patency and complication rates, polyurethane grafts are unlikely to replace radiocephalic fistulas as the access of choice for hemodialysis.
Thrombosis was the most common complication found in this study. Theoretically, factors such as gender (women) and diabetes should increase the risk of thrombosis on account of the small vessel caliber in women and the presence of distal vascular disease in diabetics. Moreover, upper arm grafts should pose a lower risk of thrombosis than do forearm grafts as the vessels there are larger with higher flow rates. However, this study as well as others5,8 did not show any differences in the thrombosis rates in these patient groups.
Early thrombosis occurring within 6 weeks following implantation is usually due to technical or mechanical error. Thus, it is generally amenable to operative revision. In fact, 12 of the 14 cases of early thrombosis in this study were successfully revised to restore patency. Late thrombosis during follow-up is usually secondary to venous intimal hyperplasia, venous stenosis proximal to the graft, hypotension after dialysis, or excessive pressure on the puncture site after dialysis. Late thrombosis accounted for the majority of thrombosis in this study.
Overall, salvage was attempted in 81% of the cases with thrombosis, and 85% of these salvage attempts were successful. In addition, 22 of these grafts had repeated salvage interventions, thus prolonging graft life and preserving other sites for future access. These results are not dissimilar to those of ePTFE grafts.7 Hence, salvage intervention should be attempted wherever possible.
Graft infection is a major concern with an incidence of 30.0% and a graft loss rate of 61.5% in our study. Staphylococcus aureus was the most common organism. Diabetes and steroid therapy were not shown to increase the risk of infection. The median time when infection occurred was 5.7 months after implantation, indicating that secondary infection of grafts already in use for dialysis was the main cause. This is largely attributed to repeated cannulation of the grafts for dialysis in immunocompromised uremic renal patients,9 who are mostly nasal carriers of Staphylococcus aureus.10 Thus, aseptic cannulation with rotation of puncture sites is important to prevent secondary cannulation infection. This is a preventable complication and can significantly increase the graft serviceability rate.
Salvage was successful in 26% of the infected grafts. More grafts could have been salvaged as revision was attempted only at a later period of the study. Studies of ePTFE grafts have reported successful salvage of up to 60% of infected grafts.11,12 Early recognition of infection followed by prompt treatment with antibiotics and revision is important to improve the chances of success.
Pseudoaneurysm with infection accounted for 1.2% of the cases of complications, and all the affected grafts were removed. Other complications included pseudoaneurysm alone, steal syndrome, and venous anastomotic stenosis with high venous pressure in the returning cannulas during dialysis. These were amenable to salvage intervention.
The cumulative primary patency rate of 73% and secondary patency rate of 86% at 1 year are similar to those reported by another study.5 These rates are comparable to those of ePTFE grafts at 1 year of 77% (primary) and 64% (secondary).7,8,13–15 The 3-year primary and secondary patency rates in our study of 55% and 72%, respectively, fall within the standards set by the National Kidney Foundation,16 which recommends that the cumulative patency rates of all dialysis AV grafts should be at least 70% at 1 year, 60% at 2 years, and 50% at 3 years.
Polyurethane grafts offer satisfactory patency rates for hemodialysis access. Moreover, their self-sealing properties and potential for early cannulation benefit patients in urgent need of reliable medium- to long-term hemodialysis. Thrombosis and infection are major complications, but both are amenable to salvage. In conclusion, this retrospective review shows that polyurethane grafts are a viable alternative for patients not suitable for autogenous AV fistulas, and early recognition with appropriate management of complications can extend overall graft life. Further studies in the form of randomized controlled trials will help elucidate how polyurethane grafts compare with other types of vascular access.
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REFERENCES
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ABSTRACT
INTRODUCTION
