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Clinical Observations |
1 Department of Radiology, The Churchill Hospital, Oxford, United Kingdom.
2 Department of Radiology, The John Radcliffe Hospital, Headley Way, Oxford OX3
9DU, United Kingdom.
3 Department of Nephrology, The Churchill Hospital, Oxford, United
Kingdom.
4 Department of Vascular Surgery, The Churchill Hospital, Oxford, United
Kingdom.
Received May 6, 2005;
accepted after revision August 17, 2005.
Address correspondence to D. R. Warakaulle
(dinuke2{at}hotmail.com).
Abstract
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CONCLUSION. The SG 100 shows initial promise as a conduit for hemodialysis fistulas in patients with difficult vascular access. The SG 100 has characteristic appearances on diagnostic imaging and is prone to similar pathologic processes that affect autogenous venous and synthetic grafts. These grafts are readily amenable to imaging-guided percutaneous intervention, which plays a major role in prolonging graft function.
Keywords: dialysis • fistulas • hemodialysis • interventional radiology • renal disease • venography
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Xenografts modified by tissue engineering technology may prove to be alternatives to PTFE conduits in the absence of suitable autogenous veins. The bovine ureter has several characteristics that render it suitable for this purpose: adequate length, internal diameters similar to those of prosthetic grafts, and a strong tissue matrix. The SynerGraft 100 (SG 100 [CryoLife Inc.]) bovine ureter graft is novel in that the collagen matrix is not cross-linked by aldehydes. A loss of antigenicity is achieved as the ureter is decellularized using hypotonic cell lysis, DNA and ribonuclease digestion, and isotonic washing. Both historical and current xenografts have had antigenicity removed by chemical fixation, but this changes their biocompatibility, compliance, and behavior in vivo.
When used as arteriovenous grafts in canine models, the SG 100 has been shown to appropriately remodel to the host environment through a process of recellularization and neovascularization [5]. The behavior of this graft in humans has been reported (Darby C, Cornall AJ, presented at the Advances in Tissue Engineering and Biology of Heart Valves 2004 meeting). We expected that the graft would have novel, previously unobserved radiologic features and responses to radiologic intervention.
A secondary patency rate of 76% at 6 months has been reported for PTFE conduits [6] and 88-89% for autogenous hemodialysis fistulas [7]. A study of 62 bioprosthetic hemodialysis fistulas manufactured from denatured bovine mesenteric veins followed up over a mean period of 453 days showed a secondary patency rate of 73% at 6 months [8]. The authors of that study concluded that these denatured bovine xenografts could be used as conduits for hemodialysis fistulas with reasonable results in high-risk patients without autogenous veins.
The aim of our study was to report the appearances on diagnostic imaging and the use of percutaneous imaging-guided intervention in the treatment and preservation of function of SG 100s used as hemodialysis fistulas and to evaluate the clinical performance and longevity of this novel prosthetic conduit.
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A total of 19 SG 100s were implanted in the 17 patients in the study group during the review period, with two patients receiving a second graft after failure of the first. The mean follow-up time was 76.9 weeks (range, 11-140). Three patients died of unrelated causes during the study period. The follow-up times for these patients were 86, 80, and 43 weeks. Two of these patients had functioning SG 100s at the time of death. Eight other patients also had functioning SG 100s at the end of the study period. The types of hemodialysis fistulas were brachiobasilic, n =3; brachiocephalic, n = 6; brachioaxillary (all straight interposition grafts sited in the arm), n = 8; axilloaxillary loop (sited in the arm), n = 1; and femoral loop (sited in the thigh), n = 1. None of the patients was lost to follow-up or received a kidney transplant.
Surgical thrombectomy was performed in four thrombosed grafts, surgical thrombectomy and revision were performed in two grafts, and two failed grafts required replacement. Neither percutaneous thrombolysis nor radiologic thrombectomy was performed on any patient during the study period. All four explanted specimens underwent histologic analysis.
The routine follow-up protocol for these patients included six monthly vascular duplex studies performed by one of the authors, a radiologist experienced in vascular sonography and vascular intervention. In addition, these patients also underwent six monthly access blood flow studies using an ultrasound dilution test performed with a Flow-QC unit (Transonic Systems). These studies were performed by one of the authors, an experienced hemodialysis access nurse specialist.
Technique
Patients were referred for digital subtraction fistulography if they met
one or more of the following criteria: a hemodynamically significant stenosis
had been identified on a routine duplex study; the patient had a total access
blood flow rate of less than 500 mL/min or a decrease in blood flow of more
than 25% on an ultrasound dilution test; or there was clinical evidence of
elevated venous pressure during hemodialysis (i.e., high venous pressure,
prolonged bleeding, extremity swelling, or dilated collateral veins).
Digital subtraction fistulography was performed to the level of the right atrium. The method of access depended on operator preference and was either puncture of the fistula—with images obtained after initial fistulography after the inflation of a blood pressure cuff to reflux contrast medium across the arterial anastomosis—or direct arterial puncture. More than one puncture was performed if deemed necessary.
Patients underwent percutaneous transluminal fistuloplasty if a significant lesion amenable to percutaneous intervention was seen on digital subtraction fistulography. Fistuloplasty was performed at the time of digital subtraction fistulography when possible. However, some patients had to be recalled for percutaneous transluminal fistuloplasty later for logistic reasons. Written informed consent was obtained from all patients before the procedure.
The puncture site was selected to allow intervention on stenoses seen during the previous duplex or digital subtraction fistulography study (or both). Direct puncture of the fistula was performed for lesions in the graft. Central venous intervention was performed via either a direct graft puncture (n =5) or a common femoral vein approach (n = 3), depending on operator preference. An appropriately sized (6- to 9-French, depending on the size of the balloon catheter) sheath was placed at the access site. The lesion was crossed with an angled hydrophilic guidewire with the aid of a suitable catheter, a 4-French multi-purpose catheter if access was via the fistula or a 5-French headhunter catheter if access was via the common femoral vein. The catheter was passed across the lesion, and the hydrophilic guidewire was exchanged for a stiff guidewire. High-pressure balloons (Blue Max, Boston Scientific) and cutting balloons were used if deemed necessary.
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Technical success of the procedure was defined as restoration of flow in the fistula and a residual stenosis of less than 30%. The interventional radiologist determined whether this was achieved at the time of the procedure. Endovascular stents were deployed at the discretion of the interventional radiologist for stenoses that responded poorly to percutaneous transluminal fistuloplasty. Stent-grafts were deployed in cases of percutaneous transluminal fistuloplasty-related venous rupture that did not respond to prolonged low-pressure balloon inflation. All digital subtraction fistulography studies and percutaneous transluminal fistuloplasty studies were performed either by or under the supervision of two other authors, both experienced vascular interventional radiologists.
Statistical Analysis
For the purposes of statistical analysis, primary patency was defined as
patency during the interval between the first percutaneous intervention and
repeated intervention or fistula thrombosis. Secondary patency was defined as
patency during the interval between the first percutaneous intervention and
the time when the fistula was surgically declotted, revised, or abandoned.
These definitions are in accordance with Society of Interventional Radiology
reporting standards and quality improvement guidelines
[9,
10].
Primary and secondary patency rates were estimated using the Kaplan-Meier technique. The two survival curves were compared using the log-rank test. The data were censored if a patient died of unrelated causes with a functional graft or if the graft had worked until the end point of the study.
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The mean time to the first percutaneous intervention was 35 weeks (range, 5-89 weeks). The mean time interval between subsequent percutaneous interventions (percutaneous transluminal fistuloplasty or stenting) per patient during the study period was 7.8 months. The calculated primary patency rates plus the SEs of the estimates at 3, 6, and 12 months were 95% ± 11%, 63% ± 9%, and 27% ± 15%, respectively. The secondary patency rates for the same time intervals were 95% ± 11%, 83% ± 10%, and 83% ± 10%, respectively. The Kaplan-Meier survival curves for the primary and secondary patency rates are shown in Figure 6. The median primary patency was 7.7 months, and the median secondary patency was 21 months.
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The complications of percutaneous transluminal fistuloplasty and stent deployment were as follows: puncture site hematoma, n = 1; perforation that sealed after prolonged low-pressure balloon inflation, n = 5; and perforation requiring covered stent deployment, n = 1. Therefore, the major complication rate per episode of percutaneous intervention was 3%. The mean time interval between percutaneous interventions (percutaneous transluminal fistuloplasty or stenting) per patient during the study period was 7.8 months.
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Chemically treated bovine ureters used as peripheral vascular bypass grafts may be a viable alternative to PTFE grafts in patients with no suitable autogenous veins. These chemically treated bioprostheses do not undergo tissue ingrowth and intimal hyperplasia as PTFE grafts do [11]. This was thought to be due to the low porosity of their wall structure. However, bovine xenografts treated with glutaraldehyde have been shown to be prone to aneurysm formation, neointimal hyperplasia, stenosis, and occlusion [12, 13].
The process of decellularization involved in manufacturing the SG 100 allows repopulation of the graft by host cells, thereby potentially preventing the long-term complications of chemically treated xenografts. Interestingly, aneurysmal degeneration has been reported in an SG 100 used as a femoral-posterior tibial bypass graft [14]. However, this phenomenon was not seen in our series, although a degree of fusiform dilatation was seen in the body of the graft at the site of needling in two patients. Neither graft proceeded to aneurysmal dilatation or loss of graft function. In one patient, a false aneurysm developed early at an arterial anastomosis. The mechanism for the development of a false aneurysm was unknown and may have been related to graft trauma at the time of insertion.
The untreated bovine graft has improved compliance due to the absence of collagen cross-linking. It was hoped that this feature would lead to a reduced incidence of stenosis at the distal end of the graft. However, we observed a significant incidence of outflow stenosis. The most common site of stenosis seen in our study was at or just before the venous anastomosis (48%), followed by the body of the SG 100. These findings are similar to those seen with PTFE hemodialysis fistulas. A prospective randomized controlled trial of surgical versus endovascular management of 115 patients with thrombosed PTFE grafts found that 55% of stenoses occurred less than 4 cm from the venous anastomosis [15]. Thrombosis, stenosis, or both are the most common causes of PTFE graft impairment or loss and are primarily attributable to venous outflow stenosis due to intimal hyperplasia [16].
Despite the theoretic advantage of the SG 100 over the PTFE graft in that it allows repopulation of the graft matrix by host cells, our experience has shown that neointimal hyperplasia, stenosis, and thrombosis can still occur, probably because of the similar hemodynamics at the venous anastomosis. In autogenous venous fistulas, the most common sites of stenosis are within 3 cm of the anastomosis and at the cephalic arch [7]. In addition, because all the patients in our study population had long-term hemodialysis before SG 100 implantation, the central venous stenoses seen were most likely a consequence of prior central venous catheters.
To our knowledge, only one other recent study has been published about the medium-term outcome of bioprostheses used as hemodialysis fistulas [8]. None of the patients in that study underwent reported interventional radiologic procedures to salvage dysfunctional grafts. The grafts were a completely different technology and tissue source; hence, the results are not directly comparable to our findings.
The role of interventional radiology is well established in maintaining the patency of autogenous and prosthetic hemodialysis fistulas. The primary and secondary patency rates from our study are comparable to the patency rates achieved for autogenous hemodialysis fistulas, which are recognized as having the best long-term survival rates [7]. The patency rates at 6 months in our study population compare favorably with the results of a meta-analysis of studies of PTFE hemodialysis fistula patency by Huber et al. [6]. However, both surgical and percutaneous interventions were included together in the definition of patency in the Huber et al. population, making direct comparison with our findings difficult.
We performed this retrospective review of our experience of SG 100s used as hemodialysis fistula conduits at our institution with a small study population. This has resulted in a relatively large SE of the estimates of patency rates. Because this was a nonrandomized study, selection bias is possible. However, the difference between the primary and secondary patency rates indicates that repeated percutaneous intervention has a potentially important role in prolonging the survival of these grafts and delaying the need for surgical intervention.
In summary, the SG 100 is a novel prosthetic material that shows initial promise as a hemodialysis fistula conduit in patients with no available autogenous veins. The grafts have distinctive appearances on duplex sonography. Stenoses due to intimal hyperplasia have been shown to occur. The SG 100 is amenable to percutaneous intervention (balloon dilatation and stent placement), and this feature makes a major contribution to its long-term patency.
Acknowledgments
We thank S.-J. Holt for compiling the radiologic images.
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