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Vascular and Interventional Radiology |
1 From the Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia. Received September 10, 1998; revision requested November 3; revision received March 24, 1999; accepted June 8. Address reprint requests to the author, Department of Radiology, New York Presbyterian Hospital (Columbia Presbyterian), MHB 4-100, 177 Fort Washington Ave, New York, NY 10032.
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Abstract |
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MATERIALS AND METHODS: Fourteen PTFE-covered Wallstents were placed in 13 patients with TIPS: seven at shunt creation and seven during revision of TIPS with one to five prior thromboses at 1 day to 1 year after initial TIPS formation. In six cases, prior to stent-graft placement persistent biliary-TIPS fistulas were demonstrated despite repeated shunt revisions with additional metallic stents.
RESULTS: All but one graft-lined TIPS were widely patent at a mean duration of venographic follow-up of 19 months (median, 17 months; range, 5–32 months). The limiting percentage of stenosis within the grafted shunts was 0%–10%. One patient developed stent-graft thrombosis; the prior biliary-TIPS fistula was seen despite the graft. A second, parallel PTFE-lined transcaval shunt was created in this patient; it was widely patent at 11-month follow-up. In two asymptomatic patients, stenoses developed in the short, nongrafted portions of the outflow hepatic veins.
CONCLUSION: PTFE stent-grafts can markedly prolong TIPS patency, potentially reducing the need for shunt follow-up and revision and the risk of recurrent symptoms associated with shunt stenosis or occlusion.
Index terms: Grafts, 95.1268 • Hypertension, portal, 95.711 • Liver, interventional procedures, 761.1269, 95.1268 • Shunts, portosystemic, 95.453 • Stents and prostheses, 95.1268 • Veins, grafts and prostheses, 95.1268
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Whereas routine shunt surveillance and revisions form a necessary part of the care of every patient with TIPS, they are both costly and invasive and do not protect patients against shunt malfunctions and associated symptoms that may occur between imaging intervals. Furthermore, current noninvasive follow-up techniques may lack previously reported sensitivity and specificity (6–9). These drawbacks may limit the perceived long-term value of TIPS, which is still considered by many as largely a "bridge to transplantation" (10) or a procedure to be performed in desperate straits. Durable TIPS may be looked to more readily in patients with relatively milder forms of hepatic disease in whom long-term survivals are anticipated because of mitigation of concerns about lifelong shunt surveillance and repeated revisions.
The ability to prevent primary and recurrent shunt stenoses by mating a porous metallic stent with graft material has been validated in both animal studies and early human applications. In 1995, Nishimine et al (11) reported the first use of polytetrafluoroethylene (PTFE) stent-grafts they made themselves to successfully prolong de novo TIPS patency in pigs. In 1997, my colleagues and I reported similar results with use of a PTFE-encapsulated TIPS stent-graft that extended shunt patency to 5 months compared with 2–4 weeks in the control group (12). In 1997, Saxon et al (13) reported prolongation of shunt patency in a pilot study in which PTFE grafts were used to revise TIPS in six patients with recurrent stenoses. Herein I expand on these data by reporting results with PTFE stent-grafts used for both creation and revision of TIPS.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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-year-old shunt that had been revised with a stent-graft 3 weeks earlier. The causes of hepatic disease included alcoholism in seven patients, viral hepatitis in four, diffuse hepatic metastases from breast carcinoma causing cirrhosis in one, and polycythemia vera causing Budd-Chiari syndrome in one. None of the patients were febrile or had any laboratory evidence of systemic infection. In the two patients with marked ascites, the ascitic fluid was tapped and revealed no evidence of spontaneous bacterial peritonitis or infection.
In the seven patients undergoing shunt revision, one to five prior TIPS thromboses had occurred from 1 day to 1 year after initial TIPS formation. Biliary-TIPS fistulae were documented in six cases by means of gentle injection of iodinated contrast material into the occluded shunts. This was performed either during retrograde transjugular catheterization or, in one case, by means of direct transhepatic percutaneous puncture of the TIPS. In each of these cases, the identical persistent biliary fistula was demonstrated at each shunt thrombosis. The fistulae persisted despite shunt thrombectomies and repaving of the lumina with additional Wallstents (Boston Scientific, Natick, Mass).
We administered 1 g of cefazolin sodium intravenously before each graft implantation. The stent-grafts were constructed on a sterile tabletop at the time of TIPS revision or initial shunt creation. The length of the segment to be lined with PTFE was measured by using a kinked guide wire technique. In all cases, we measured from the portal venous entry site to the caval ostium, with the intention of lining this length, including the outflow hepatic vein, with PTFE. Metallic clips were placed on the patient's abdomen to mark the desired sites of the hepatic and portal venous ends of the graft.
A 3- or 4-mm-diameter standard Thin Wall PTFE graft (Impra, Tempe, Ariz) was dilated briefly by using 10- or 12-mm dry high-pressure balloons (Blue-Max; Boston Scientific) (Fig 1). The internodal distance of the graft was 30 µm. This measurement characterizes the permeability and microstructure of the graft; PTFE with a 30-µm internodal distance currently accounts for the majority of commercially available PTFE grafts. The radially expanded graft material was then cut to length by using a scalpel or sharp scissors. The graft material was sutured solely at the leading end of the Wallstents because the constrained stents were 32–54 mm longer than when deployed. This change in length far exceeded the longitudinal elasticity of the PTFE.
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This graft-sheath assembly was then introduced into the patient through a 30-cm-long 16-F sheath (Cook). The device was advanced into position over a Super Stiff (Boston Scientific) or Extra Stiff (Cook) Amplatz guide wire and deployed by sequentially withdrawing the sheaths and then finishing deployment of the partially expanded Wallstent. The stent delivery catheter was removed, and the stent-graft was immediately dilated and tacked down with a 10-mm angioplasty balloon (Boston Scientific).
The second technique requires use of the newer reconstrainable Wallstent (Unistep; Boston Scientific) and takes advantage of the larger catheter profile and slight additional space within the stent compartment of the delivery catheter. It has become my preferred approach for creating and delivering the stent-grafts and is now used exclusively. In this technique, the graft is back loaded onto the stent delivery catheter and the stent is partially deployed, exposing a longitudinal length of approximately four to six stent interstices. The PTFE suture is knotted to a stent interstice within three interstices of its leading end (Fig 2). It is then passed through the graft wall and looped back in toward the metallic stent, and a surgeon's knot is loosely tied. As the Wallstent is gently recaptured within the delivery catheter, the knot is slowly tightened. Once the stent is nearly fully recaptured (1–2 mm exposed), the knot is tied firmly, and several knots are added. This process is repeated at one or two additional points approximately 180° or 120° apart.
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The graft is then furled around the stent delivery catheter and tightly bound to it with a single 4-0 braided polyester suture. The suture is laced back and forth across the stent, starting from the trailing end of the device, akin to a pair of Roman sandals (Fig 3). The bound stent-graft is slowly advanced into a shortened 13-F sheath (Peel-Away; Cook), while the suture lacing is slowly unwrapped in the reverse direction (Fig 4). If desired, this graft can be loaded into a lower profile delivery system than the one described. In this TIPS application, there was little reason to do this, as the devices were introduced through a venous approach; a slight reduction in the system's delivery profile would have little effect on the potential for neck hematoma or the duration of manual compression at the jugular puncture site.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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All but one graft-lined TIPS in both de novo and revision groups were widely patent at follow-up venography (Figs 6, 7). The limiting stenosis within the grafted segments was 0%–10% in all cases. In other words, the degree of encroachment on the endoluminal surface was 0–1 mm. In one asymptomatic patient at 18-month follow-up, a minimal narrowing at the hepatic venous end of the shunt was identified (Fig 8); the portosystemic gradient was 10 mm Hg. The finding was present within the bare, uncovered trailing end of the stent-graft. At the site of graft coverage, this narrowing disappeared. In another asymptomatic patient at 21-month follow-up, a portion of the outflow hepatic vein similarly uncovered by graft material had developed a flow-limiting stenosis with resultant excessive portal venous pressures. The stenosis was treated with hepatic venous angioplasty and placement of a single, overlapping, bare Wallstent.
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In the case of stent-graft occlusion in our study, the TIPS was abandoned. A new stent-graft was not placed within its lumen because the residual shunt diameter, even when widely patent, was deemed insufficient to adequately decompress the patient's portal venous system, which had a portosystemic gradient of 18 mm Hg; this was in part due to the five Wallstents that had been used to revise the shunt during the preceding 1 years of repeated shunt revisions. As no suitable second hepatic veins were present, a parallel transcaval shunt was created between the cephalic portion of the retrohepatic vena cava and the left portal vein and was immediately lined with a new stent-graft. At 11-month follow-up, this second graft-lined shunt remained free of stenosis or tissue encroachment.
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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In 1995, Nishimine et al (11) reported a systematic study of the potential benefits of lining porcine TIPS with graft material. Shunts created in pigs typically become highly stenotic or occluded within 2–4 weeks of creation. Thirteen porcine TIPS were lined with handmade PTFE stent-grafts sewn to a combination of Gianturco Z stents (Cook) and Wallstents and were compared with TIPS lined with conventional Wallstents. At 1-month follow-up, nine of 13 grafted shunts revealed a stenosis of less than 50%, whereas only one control TIPS was patent.
We expanded on these results by evaluating an encapsulated expanded PTFE, or ePTFE, stent-graft designed specifically for TIPS (12). Venographic follow-up was performed at monthly intervals with necropsy and specimen retrieval at 1, 3, 4, and 5 months. Histologic and venographic study of the animals with grafts revealed near absence of any tissue within the shunt lumina. The good overall patency of the graft group was in marked contrast to that in control animals, which developed occlusions or marked stenoses (40%–72%) within 4 weeks of TIPS. Our longer porcine stent-graft patencies may have been related to the thicker graft material in our devices or the use of a different breed of miniswine.
Unlike the results of Nishimine et al (11), our results showed no bile staining was present in any of the described porcine TIPS or in 50 subsequent porcine TIPS. The abluminal surfaces of the porcine grafts in our study were encompassed by a thick rind of myofibroblasts and extracellular collagen matrix identical to that seen narrowing the lumina of human TIPS. Interestingly, though, this tissue was prevented from reaching the shunt lumen or growing around the metallic stent struts by the presence of the graft.
These findings lend support to the theory that shunt tract stenosis may also represent a fibrotic healing response to the trauma of shunt creation, one that can develop in the absence of bile leaks and can be controlled with an exclusionary, biocompatible shunt liner. In placing stent-grafts in both newly created shunts and TIPS with recurrent thromboses due to biliary fistulae, one is ideally treating or preventing all causes of shunt stenosis, including thrombus formation and smooth muscle cell inhibition caused by the presence of bile (25), as well as the fibrotic tissue healing response mediated by other proliferative factors.
In contrast, porcine TIPS created with polyethylene teraphthalate–covered stents failed to exclude the stenotic tissue from the porcine TIPS lumina; in most cases, a diffuse, thick layer of fibroblast tissue lined the site of the fabric (26,27). The porosity of polyester grafts is much lower than that of PTFE, which potentially allows transgraft tissue growth, a phenomenon that may limit the success of such grafts in humans. The bile-resistance of polyethylene terephthalate was not assessed in the first series, as none of the stenotic porcine shunts demonstrated bile staining. On one hand, the inflammatory response of the polyester graft may elicit a rapid fibroblast proliferative response within the host, sealing any bile leaks. Alternatively, the fabric may fail as a barrier to bile or serve as a wick when in contact with bile and exaggerate the effect of a bile-to-shunt fistula.
One argument against using a porous fabric–covered stent-graft for TIPS is the case of stent-graft failure during TIPS revision in our study, which occurred because of a previously unreported event—inadvertent gas denucleation of the PTFE during radial expansion. In that case, the increased graft permeability corresponded to the site of the biliary fistula so that the graft failed to provide a barrier to continued bile leakage. Using a structure of greater porosity in this setting, such as polyethylene terephthalate, might prove that it is equally unable to prevent bile permeation into the TIPS.
These questions will remain unanswered until large-scale human trials of polyethylene terephthalate stent-grafts are conducted, although it is arguable that such trials are unwarranted given the comparatively successful results with PTFE. An effective TIPS graft, or other shunt-directed therapy, ideally would address all three modes of shunt stenosis: bile-related and bile-unrelated tract stenoses and hepatic venous stenosis. Arguably, the graft material used would be biocompatible, microporous, nonthrombogenic, and relatively impermeable to bile and tissue and would provide a substrate for endothelial lining. The microstructure of PTFE can, for example, be altered to suit many of these needs. For example, it can be made less permeable on its abluminal surface while retaining a higher porosity on the endoluminal surface to encourage endothelialization and healing.
Results of TIPS grafts in humans are still limited (13,28–31). Saxon et al (13) published the results of an important pilot study evaluating stent-grafts for revisions of TIPS stenoses and occlusions. Six patients with an initial mean primary TIPS patency of 50 days (range, 9–100 days) had their TIPS lined with a modified Gianturco Z stent endoskeleton supporting a 4-mm PTFE graft that had been dilated to 14 mm in diameter. Once delivered, the PTFE was sandwiched against the shunt lumen with a standard Wallstent. Three patients had initially demonstrable biliary fistulae. Of five surviving patients, three had TIPS that remained patent at a mean venographic follow-up of 315 days. One shunt occluded and one became stenotic owing to graft misplacement. The authors concluded that PTFE-covered stent-grafts were effective for revision of TIPS in patients with tract stenosis and occlusion.
More recently, Ferral et al (30) reported use of a modified stent-graft (Cragg Endopro System I; Mintec, Bahamas), a polyester fabric–coated nitinol stent, for creation of TIPS in 13 patients. By using unspecified US criteria, one shunt was deemed stenotic at an unspecified follow-up interval, whereas two shunts were found occluded at 2- and 3-month follow-up. Whereas performance of shunt venography, the examination standard of reference for evaluating the degree of shunt stenosis, was described as having been performed, the results of these portograms were not; this technical note focused largely on the acute and short-term results of the devices in a TIPS application.
The results of the series reported herein, confirm the promising results of Saxon et al (13) with use of relatively similar PTFE to line repeatedly stenosed or occluded TIPS. In addition, the data suggest that use of PTFE stent-grafts in patients with TIPS can provide prolonged shunt patency directly from the time of shunt creation. We did not create a stock of preassembled sterilized stent-grafts in different lengths, attach them to custom-built Gianturco Z stent skeletons, or find it necessary to dilate the graft material for hours prior to use (32). Instead we created our stent-grafts at the time of the procedure and measured to fit the patient. With practice, it was possible to create and deliver the stent-graft, dilate it with a balloon, and perform final shunt venography within 40 minutes of the decision to proceed. The ability to rapidly create such flexible stent-grafts may be useful in other emergent or urgent trauma or vascular applications.
We chose to build TIPS grafts on the Wallstent platform because of the longitudinal flexibility of the stent, the long stents available that allow one stent-graft to pave an entire tortuous shunt tract, and the long-standing suitability of the Wallstent as a TIPS stent. A number of balloon-expandable flexible stents recently have become available, and we also have experimented successfully with graft delivery on these stents. They may prove equally suited to this and other stent-graft applications. These technical issues ultimately will become moot as commercially available stent-grafts suited to these applications become available in the United States.
Our second described technique for rigging the graft to the stent proved expedient and provided very reliable deployments. We were, however, initially cautious about extending the graft material to the very trailing end of the Wallstent for fear that that portion of the stent would not expand, making it difficult to recatheterize the stent and dilate it and the graft after deployment. This explains both the stenoses seen in two patients—one within the portion of the hepatic vein lined only with bare metallic stent and the other in an area of entirely bare hepatic vein uncovered by metallic stent. It serves as an object lesson, emphasizing the need for extending a TIPS stent-graft right up to the hepatic venous ostium, if not a very short distance into the vena cava, both for revision and for new shunt formation. Not grafting this segment leaves a second mode of shunt failure intact: hepatic venous stenosis.
In summary, it is likely that in a few years interventional radiologists will both create and revise TIPS with off-the-shelf commercially available TIPS stent-grafts. Our results indicate that PTFE TIPS stent-grafts have the potential to provide both newly created and revised TIPS the durable, uninterrupted patency they presently lack.
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Footnotes |
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Author contributions: Guarantor of integrity of entire study, Z.J.H.
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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