Volume 25
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Number 8
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August
’
2014
Re: “Evaluating Patency Rates of an Ultralow-Porosity Expanded Polytetrafluoroethylene–Covered Stent in the Treatment of Venous Stenosis in Arteriovenous Dialysis Circuits” From: Bart Dolmatch, MD Department of Interventional Radiology Palo Alto Medical Foundation 701 E. El Camino Real Mountain View, CA 94040
1315
characterized AV access stent-grafts (though, admittedly, we still await a stent-graft specifically cleared for AV fistulae). Still, it is far easier for me to justify the use of an AV graft device in an AV fistula. Why would anyone consider using a biliary device this way? Without substantial prospective comparative human clinical data, any interventionalist who uses the VIABIL device in an AV circuit simply cannot defend that decision when the access becomes infected, or when the VIABIL device migrates, erodes, fractures, dislodges, develops stenosis or thrombosis, or has any other harmful outcome. Even though none of these events may ever occur, until the VIABIL device meets the same rigorous in vivo and human clinical standards that other AV access devices have achieved, it should be reserved for malignant biliary strictures.
Editor: The recent report by Chan et al (1) describes the first use of nonporous VIABIL stent-grafts (W.L. Gore and Associates, Flagstaff, Arizona) in hemodialysis arteriovenous (AV) access circuits. Although novel, their results should be taken with great caution. As early as 1975, Campbell et al (2) showed that surface porosity is required for patency of vascular polytetrafluoroethylene (PTFE) conduits. Porosity permits tissue ingrowth, and this limits PTFE thrombosis. We showed that nonporous PTFE stent-grafts developed as much in-stent tissue as control bare metal stents (3). However, unlike bare metal stents, nonporous PTFE stent-grafts formed tissue that was poorly attached and contained seromas and hematomas. Although Chan et al (1) write that “the nonporous nature of the VIABIL stent-graft might have biologic and clinical effects that are as yet unclear,” nearly 40 years of research have shown that nonporous PTFE vascular conduits are inferior to porous ones. Regarding any AV access intervention, only when the entire circuit remains functional will there be clinical benefit. Yet, the primary circuit patency rate for the VIABIL device was only 45% at 6 months. In addition, one access became infected, there were no 6-month angiographic images, and, despite a 2-month angiographic image showing patency during elbow flexion, circuit patency was lost in four of six circuits in which the VIABIL device crossed a joint. Without a comparative study versus angioplasty, stent placement, or use of another stent-graft, we cannot tell if use of the VIABIL device offers any benefit compared with other interventions. Once upon a time, when we had only biliary stents, interventionalists used these Food and Drug Administration–approved devices for unindicated (or “off-label”) vascular situations. The 2005–2007 study by Chan et al (1) overlapped a time when no AV access stents or stentgrafts were available. Today there are two very well
We appreciate the points raised by Dolmatch (1) concerning the use of the VIABIL biliary stent-graft (W.L. Gore and Associates, Flagstaff, Arizona) for venous outflow stenoses in arteriovenous grafts and fistulae. Although it is certainly not our contention that stent-grafts should be used routinely in an off-label fashion—a point we make clear in our article (2)—our goal was merely to evaluate a timely and pressing unmet medical need confounding a specific patient population when the study was performed. Indeed, the data were obtained more than 5 years ago, primarily at a time when no stent-graft options for dialysis access circuits existed, and any stent-grafts used in dialysis access circuits would, by necessity, be used on an off-label basis. Nonetheless, we would note several points worth mentioning in regard to this letter. Although the author provides data regarding the benefits of medium porosity in stent-grafts, the data reported by Campbell et al (3) and Dolmatch et al (4) are based on the use of various stentgrafts solely used in the arterial system. Although dialysis access circuits are subject to relatively high flow rates and
B.D. has a royalty agreement with CR Bard (Summit, New Jersey) and is a paid consultant for CR Bard and Merit Medical Systems (South Jordan, Utah).
None of the authors have identified a conflict of interest.
http://dx.doi.org/10.1016/j.jvir.2014.03.013
http://dx.doi.org/10.1016/j.jvir.2014.04.006
REFERENCES 1. Chan MG, Miller FJ, Valji K, Bansal A, Kuo MD. Evaluating patency rates of an ultralow-porosity expanded polytetrafluoroethylene-covered stent in the treatment of venous stenosis in arteriovenous dialysis circuits. J Vasc Interv Radiol 2014; 25:183–189. 2. Campbell CD, Goldfarb D, Roe R. A small arterial substitute: Expanded microporous polytetrafluoroethylene: patency versus porosity. Ann Surg 1975; 182:138–143. 3. Dolmatch B, Dong YH, Heeter Z. Evaluation of three polytetrafluoroethylene stent-grafts in a model of neointimal hyperplasia. J Vasc Interv Radiol 2007; 18:527–534.
Drs. Chan et al respond:
1316
’
Letters to the Editor
pressure, data repeatedly show that these circuits and their vascular biology are distinct from the arterial system. In line with this, the stents are placed in veins, and therefore, to our knowledge, as hypothesized at the time, it was unclear whether the same histologic predictors of patency in the arterial system would hold true in the high-flow venous system. Our group, along with others, previously reported on the use of the VIABAHN stent-graft (2), a medium-porosity stent-graft approved for the arterial system, in a small series of arteriovenous access cases, and reported results similar to those of our recent study (2). With hopes of reducing restenosis caused by neointimal hyperplasia, the VIABIL device was used based on the theory that the nonporous nature may limit tissue ingrowth of any kind and therefore provide some degree of prolonged patency. Albeit not in the same vascular environment, Jin et al (5) recently evaluated a nonporous graft material for arteriovenous graft creation in an animal model in comparison with porous polytetrafluoroethylene and found that the nonporous graft showed reduced neointimal hyperplasia. Therefore, contrary to the author’s contention, the data do not clearly support the issue of graft porosity in all vascular circuits as a closed avenue of further investigation, as, clearly, knowledge of vascular biology and materials science has and continues to advance since the original research referenced almost 40 years ago. Previous notions of “fact” must continually be revisited in the light of new knowledge, as history is replete with examples in which previously long held and dear assumptions are later found to be incorrect. The limitations of our study (2) were identified, and, clearly, one cannot draw strong conclusions regarding the statistical data. We believe the data did not indicate that the VIABIL device is clearly inferior to currently approved devices; for example, the 6-month circuit patency in our study (2) was 45% compared with 38% in the trial of Haskal et al (6). However, our study (2) was an uncontrolled, retrospective study and included a small number of patients, and we therefore agree and state as such that we cannot clearly suggest that the VIABIL device is equal to or better than any other stentgraft device, or any other intervention.
Chan et al
’
JVIR
Now that approved devices for use in arteriovenous grafts exist, the approved devices are certainly recommended for use in grafts and are probably the best current option for use in fistulae if needed. We do not suggest that covered biliary stents be used routinely in the vascular system. However, our goal in presenting our data was to suggest that, with the rapid ongoing advances occurring in vascular biology and materials sciences that have resulted in new device capabilities far exceeding those of each previous generation, the porosity issue be reconsidered, particularly in the venous side of arteriovenous circuits. Michael G. Chan, MD Franklin J. Miller, MD Karim Valji, MD Michael D. Kuo, MD Department of Radiology, University of California, Los Angeles, Los Angeles, California (M.G.C., M.D.K.) Department of Radiology, University of California, San Diego, San Diego, California (M.G.C., F.J.M.) Department of Radiology, University of Washington, Seattle, Washington (K.V.)
REFERENCES 1. Dolmatch B. Re: Evaluating patency rates of an ultralow-porosity expanded polytetrafluoroethylene–covered stent in the treatment of venous stenosis in arteriovenous dialysis circuits. J Vasc Interv Radiol 2014; 25:314. 2. Chan MG, Miller FJ, Valji K, Kuo MD. Evaluation of expanded polytetrafluoroethylene-covered stents for the treatment of venous outflow stenosis in hemodialysis access grafts. J Vasc Interv Radiol 2011; 22: 647–653. 3. Campbell CD, Goldfarb D, Roe R. A small arterial substitute: expanded microporous polytetrafluoroethylene: patency versus porosity. Ann Surg 1975; 182:138–143. 4. Dolmatch B, Dong YH, Heeter Z. Evaluation of three polytetrafluoroethylene stent-grafts in a model of neointimal hyperplasia. J Vasc Interv Radiol 2007; 18:527–534. 5. Jin D, Takai S, Li Z, et al. Outside fibroblasts play a key role in the development of inner neointima after the implantation of polytetrafluoroethylene grafts. J Pharmacol Sci 2012; 119:139–149. 6. Haskal ZJ, Trerotola S, Dolmatch B, et al. Stent graft versus balloon angioplasty for failing dialysis-access grafts. N Engl J Med 2010; 362: 494–503.
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Number 8
’
August
’
2014
Re: “Evaluating Patency Rates of an Ultralow-Porosity Expanded Polytetrafluoroethylene–Covered Stent in the Treatment of Venous Stenosis in Arteriovenous Dialysis Circuits” From: Bart Dolmatch, MD Department of Interventional Radiology Palo Alto Medical Foundation 701 E. El Camino Real Mountain View, CA 94040
1315
characterized AV access stent-grafts (though, admittedly, we still await a stent-graft specifically cleared for AV fistulae). Still, it is far easier for me to justify the use of an AV graft device in an AV fistula. Why would anyone consider using a biliary device this way? Without substantial prospective comparative human clinical data, any interventionalist who uses the VIABIL device in an AV circuit simply cannot defend that decision when the access becomes infected, or when the VIABIL device migrates, erodes, fractures, dislodges, develops stenosis or thrombosis, or has any other harmful outcome. Even though none of these events may ever occur, until the VIABIL device meets the same rigorous in vivo and human clinical standards that other AV access devices have achieved, it should be reserved for malignant biliary strictures.
Editor: The recent report by Chan et al (1) describes the first use of nonporous VIABIL stent-grafts (W.L. Gore and Associates, Flagstaff, Arizona) in hemodialysis arteriovenous (AV) access circuits. Although novel, their results should be taken with great caution. As early as 1975, Campbell et al (2) showed that surface porosity is required for patency of vascular polytetrafluoroethylene (PTFE) conduits. Porosity permits tissue ingrowth, and this limits PTFE thrombosis. We showed that nonporous PTFE stent-grafts developed as much in-stent tissue as control bare metal stents (3). However, unlike bare metal stents, nonporous PTFE stent-grafts formed tissue that was poorly attached and contained seromas and hematomas. Although Chan et al (1) write that “the nonporous nature of the VIABIL stent-graft might have biologic and clinical effects that are as yet unclear,” nearly 40 years of research have shown that nonporous PTFE vascular conduits are inferior to porous ones. Regarding any AV access intervention, only when the entire circuit remains functional will there be clinical benefit. Yet, the primary circuit patency rate for the VIABIL device was only 45% at 6 months. In addition, one access became infected, there were no 6-month angiographic images, and, despite a 2-month angiographic image showing patency during elbow flexion, circuit patency was lost in four of six circuits in which the VIABIL device crossed a joint. Without a comparative study versus angioplasty, stent placement, or use of another stent-graft, we cannot tell if use of the VIABIL device offers any benefit compared with other interventions. Once upon a time, when we had only biliary stents, interventionalists used these Food and Drug Administration–approved devices for unindicated (or “off-label”) vascular situations. The 2005–2007 study by Chan et al (1) overlapped a time when no AV access stents or stentgrafts were available. Today there are two very well
We appreciate the points raised by Dolmatch (1) concerning the use of the VIABIL biliary stent-graft (W.L. Gore and Associates, Flagstaff, Arizona) for venous outflow stenoses in arteriovenous grafts and fistulae. Although it is certainly not our contention that stent-grafts should be used routinely in an off-label fashion—a point we make clear in our article (2)—our goal was merely to evaluate a timely and pressing unmet medical need confounding a specific patient population when the study was performed. Indeed, the data were obtained more than 5 years ago, primarily at a time when no stent-graft options for dialysis access circuits existed, and any stent-grafts used in dialysis access circuits would, by necessity, be used on an off-label basis. Nonetheless, we would note several points worth mentioning in regard to this letter. Although the author provides data regarding the benefits of medium porosity in stent-grafts, the data reported by Campbell et al (3) and Dolmatch et al (4) are based on the use of various stentgrafts solely used in the arterial system. Although dialysis access circuits are subject to relatively high flow rates and
B.D. has a royalty agreement with CR Bard (Summit, New Jersey) and is a paid consultant for CR Bard and Merit Medical Systems (South Jordan, Utah).
None of the authors have identified a conflict of interest.
http://dx.doi.org/10.1016/j.jvir.2014.03.013
http://dx.doi.org/10.1016/j.jvir.2014.04.006
REFERENCES 1. Chan MG, Miller FJ, Valji K, Bansal A, Kuo MD. Evaluating patency rates of an ultralow-porosity expanded polytetrafluoroethylene-covered stent in the treatment of venous stenosis in arteriovenous dialysis circuits. J Vasc Interv Radiol 2014; 25:183–189. 2. Campbell CD, Goldfarb D, Roe R. A small arterial substitute: Expanded microporous polytetrafluoroethylene: patency versus porosity. Ann Surg 1975; 182:138–143. 3. Dolmatch B, Dong YH, Heeter Z. Evaluation of three polytetrafluoroethylene stent-grafts in a model of neointimal hyperplasia. J Vasc Interv Radiol 2007; 18:527–534.
Drs. Chan et al respond:
1316
’
Letters to the Editor
pressure, data repeatedly show that these circuits and their vascular biology are distinct from the arterial system. In line with this, the stents are placed in veins, and therefore, to our knowledge, as hypothesized at the time, it was unclear whether the same histologic predictors of patency in the arterial system would hold true in the high-flow venous system. Our group, along with others, previously reported on the use of the VIABAHN stent-graft (2), a medium-porosity stent-graft approved for the arterial system, in a small series of arteriovenous access cases, and reported results similar to those of our recent study (2). With hopes of reducing restenosis caused by neointimal hyperplasia, the VIABIL device was used based on the theory that the nonporous nature may limit tissue ingrowth of any kind and therefore provide some degree of prolonged patency. Albeit not in the same vascular environment, Jin et al (5) recently evaluated a nonporous graft material for arteriovenous graft creation in an animal model in comparison with porous polytetrafluoroethylene and found that the nonporous graft showed reduced neointimal hyperplasia. Therefore, contrary to the author’s contention, the data do not clearly support the issue of graft porosity in all vascular circuits as a closed avenue of further investigation, as, clearly, knowledge of vascular biology and materials science has and continues to advance since the original research referenced almost 40 years ago. Previous notions of “fact” must continually be revisited in the light of new knowledge, as history is replete with examples in which previously long held and dear assumptions are later found to be incorrect. The limitations of our study (2) were identified, and, clearly, one cannot draw strong conclusions regarding the statistical data. We believe the data did not indicate that the VIABIL device is clearly inferior to currently approved devices; for example, the 6-month circuit patency in our study (2) was 45% compared with 38% in the trial of Haskal et al (6). However, our study (2) was an uncontrolled, retrospective study and included a small number of patients, and we therefore agree and state as such that we cannot clearly suggest that the VIABIL device is equal to or better than any other stentgraft device, or any other intervention.
Chan et al
’
JVIR
Now that approved devices for use in arteriovenous grafts exist, the approved devices are certainly recommended for use in grafts and are probably the best current option for use in fistulae if needed. We do not suggest that covered biliary stents be used routinely in the vascular system. However, our goal in presenting our data was to suggest that, with the rapid ongoing advances occurring in vascular biology and materials sciences that have resulted in new device capabilities far exceeding those of each previous generation, the porosity issue be reconsidered, particularly in the venous side of arteriovenous circuits. Michael G. Chan, MD Franklin J. Miller, MD Karim Valji, MD Michael D. Kuo, MD Department of Radiology, University of California, Los Angeles, Los Angeles, California (M.G.C., M.D.K.) Department of Radiology, University of California, San Diego, San Diego, California (M.G.C., F.J.M.) Department of Radiology, University of Washington, Seattle, Washington (K.V.)
REFERENCES 1. Dolmatch B. Re: Evaluating patency rates of an ultralow-porosity expanded polytetrafluoroethylene–covered stent in the treatment of venous stenosis in arteriovenous dialysis circuits. J Vasc Interv Radiol 2014; 25:314. 2. Chan MG, Miller FJ, Valji K, Kuo MD. Evaluation of expanded polytetrafluoroethylene-covered stents for the treatment of venous outflow stenosis in hemodialysis access grafts. J Vasc Interv Radiol 2011; 22: 647–653. 3. Campbell CD, Goldfarb D, Roe R. A small arterial substitute: expanded microporous polytetrafluoroethylene: patency versus porosity. Ann Surg 1975; 182:138–143. 4. Dolmatch B, Dong YH, Heeter Z. Evaluation of three polytetrafluoroethylene stent-grafts in a model of neointimal hyperplasia. J Vasc Interv Radiol 2007; 18:527–534. 5. Jin D, Takai S, Li Z, et al. Outside fibroblasts play a key role in the development of inner neointima after the implantation of polytetrafluoroethylene grafts. J Pharmacol Sci 2012; 119:139–149. 6. Haskal ZJ, Trerotola S, Dolmatch B, et al. Stent graft versus balloon angioplasty for failing dialysis-access grafts. N Engl J Med 2010; 362: 494–503.
