| Abstract|| |
Vascular access surgery is a permanent challenge for the vascular surgeon and is a very dynamic, inter-disciplinary field in modern medicine. High technical skill, extreme care and continuity to gain as much experience as possible are required. Priority should be given to the patients' native vessels. Nevertheless, grafts do have a place provided special attention is paid to the restricted indications. Close co-operation of the vascular surgeon, the nephrologist and the radiologist is essential. The nephrologist is responsible for access surveillance and for early detection of fistula dysfunction. Dialysis related parameters should be observed routinely; clinical examination and if necessary ultrasonographic and angiographic technique will give further information. Recently introduced methods such as permanent central venous catheters and an increasing variety of interventional procedures must be critically evaluated in the future.
Keywords: Hemodialysis, Vascular Access, AV fistula, ePTFE grafts, Angioplasty.
|How to cite this article:|
Konner K. Recent Developments in Vascular Access for Hemodialysis. Saudi J Kidney Dis Transpl 1997;8:113-8
| Introduction|| |
Angioaccess is the lifeline for the hemodialysis patient. Since Brescia and Cimino  introduced the surgically created arteriovenous (AV) fistula thirty years ago, two basic aspects have changed: first, more and more elderly patients and diabetics are being accepted for long-term dialysis treatment and secondly, a rising proportion of patients with a history of dialysis therapy for more than twenty years is emerging, thus presenting special vascular problems. Additionally, graft materials, permanent central venous catheters, diagnostic and interventional radiologic techniques and ultrasonographic investigations have established their place in vascular surgery.
| Arteriovenous Fistula|| |
It seems to be the image of operations to create AV fistula to be classified as simple procedures, performed by the relatively younger vascular surgeons. This is a basic misunderstanding because these operations require high technical skill and experience, A distinct feel is needed to handle the tiny vessels, especially the veins, and to choose the type and location of the AV anastomosis. The aim of the procedure should be to achieve a vascular access suitable for cannulation and not just to create an AV anastomosis.
Some hemodynamic considerations ought to be considered. Both the artery and the vein must be able to undergo dilatation, the prerequisite of the high blood flow needed for a successful hemodialysis treatment. The radial artery normally offers a flow of about 20 to 30 ml/min. Immediately after creation of the anastomosis, the flow increases up to a level of 200 to 300 ml/min. After maturation, a fistula with a flow of 600 to 1200 ml/min will be established  . This means that a small-lumen, thick-walled, calcified radial artery, as is found in a majority of diabetic and hypertensive patients, cannot provide a good fistula function as needed for hemodialysis. Early thrombosis and insufficient arterial fistula flow are observed frequently in such patients. An AV fistula in the elbow region with arterialization of the cephalic and/or the basilic vein will be a better choice in these patients despite the availability of a suitable peripheral vein. Also, the limited life expectancy of these patients will make the decision to start with an elbow fistula easier. They will benefit favorably from a well constructed elbow fistula instead of suffering from the complications of a peripheral anastomosis.
A variant of an AV fistula in the elbow region, the Gracz fistula is worthy of mention. Here, the perforating vein, originating from various sites of the venous triangle, is anastomosed to the brachial or the beginning of the radial artery  . In contrast to earlier suggestions, the deep venous system should not be interrupted. The length of the anastomosis is limited by the diameter of the perforating vein. Thus, the risk of peripheral ischemia is reduced significantly. Furthermore, the total of the venous system is arterialized and suitable for cannulation. The anastomosis buried in the depth of the elbow is protected from accidental arterial puncture while placing the dialysis cannulae.
An elbow and upper arm AV fistula should be considered in all patients with an exhausted peripheral venous network. An arterialized and superficialized basilic vein can provide an excellent long-term vascular access  as also the cephalic vein  . In some patients with upper arm fistulae and history of multiple prior operations, the change to the contralateral arm with creation of a new anastomosis is preferable.
Thrombosis is reported to be the most frequent complication of the Brescia-Cimino AV fistula. Commonly, thrombosis is a result of morphological lesions such as stenoses, aneurysms, para-vascular hematomas or rarely, infection. Early diagnosis of fistula dysfunction with immediate elective surgical or interventional repair are required to prolong fistula function before onset of thrombosis. Most thrombotic events are avoidable. Any thrombosis is an emergency often requiring central venous catheterization and hospitalization and incur high costs. The therapy of choice is the surgical approach combining thrombectomy and correction of the underlying anatomic lesion, or the creation of a new AV anastomosis proximally. Thrombectomy alone cannot re-establish reliable long-term fistula function. Correction of stenosis requires surgery and a great variety of surgical techniques are available. Resection of a short venous segment with a new end-toend-anastomosis, replacement of a lengthy stenotic segment by graft material or various patch techniques are the procedures currently available to the surgeons.
A special aspect is the creation of AV fistulae in children. Nowadays, microsurgical techniques are absolutely required. Clamping of the tiny blood vessels should be avoided by using preventive hemostasis with an inflatable tourniquet. Thereby, the frequency of arterial and venous spasm can be reduced considerably  .
Peripheral ischemia represents a special challenge for the vascular surgeon. Various techniques have been reported to combat this. They include ligation of the anastomosis, ligation of the proximal arterial limb, arterio-arterial bypass insertion crossing the region of the anastomosis, several banding procedures or a new arterial supply to the vein by interposition of an expanded polytetrafluoroethylene (ePTFE) prosthetic graft originating from a more centrally placed arterial anastomosis. The procedure of choice is decided by the patient's individual situation. Digital necrosis will necessitate ligation of the anastomosis in most patients. Dialysis can be continued with a permanent venous catheter or a change over to peritoneal dialysis.
In summary, the AV fistula is the gold standard for blood access. Consequently, preservation of veins during the pre-dialysis period and the meticulous use of the venous network for access surgery is a demanding challenge. High technical skill, experience and patience are needed. These procedures should be performed, or at least supervised by the most experienced member of the surgical team. Planning an access operation is like playing chess. The next two or three interventions must be taken into account. Elective repair of dysfunctional AV fistulae will reduce the frequency of thrombotic events, thereby avoiding the need for central venous catheters, hospitalization and increased costs.
| Prosthetic Grafts|| |
Because some patients lack suitable veins for development of an AV fistula or present with complications, various graft materials have come into use as a bridge between artery and vein. In the field of access surgery, ePTFE has proven to be the best currently available graft material. Recent developments are represented by the DIASTAT graft (W.L. Gore & Assoc, Flagstaff, USA), the Carboflo graft (Impra, Inc., Phoenix, USA) and by the hybrid PTFE graft (Atrium Medical Corp., USA). The Gore DIASTAT graft was designed for immediate cannulation, thus allowing for urgent dialysis. The middle section of the graft contains a cannulation segment consisting of a stretch ePTFE base tube surrounded by flat and round ePTFE fibers that are secured by a thin, perforated ePTFE cover.
The creation of the anastomoses with the DIASTAT grafts evidently does not differ from the commonly used Gore stretch grafts. Tapered versions are available. The problem seems to be the cannulation segment of the graft. On the one hand, early cannulation can be performed with a significantly decreased incidence of hematomas; cannulation must be performed against a certain resistance caused by the structure of the graft wall  . On the other hand, the construction of the middle section offers special problems with segmental replacement in case of infection or pseudoaneurysmal enlargement. There are unproven statements (personal communications) that this multi-filament structure favors the development and maintenance of infections.
Thus, the real advantages of this new prosthesis in comparison to the well established Gore grafts remains questionable.
The Carboflo impra ePTFE prosthetic graft with carbonization of the inner graft surface was introduced to reduce thrombogenicity. Preliminary results seem to confirm this idea although comparative studies are not available at the present time. The most recently available hybrid PTFE graft is promising as far as surgical handling and kinking resistance is concerned. An outer layer with 60 u pores, an inner layer with 20 µ pores and the so called throupores are said to favor a good incorporation into the perigraft tissue and the growth of a constant neo-intima. There are no publications available on this material in the field of access surgery.
A tendency observed during the last decade has been an increasing use of grafts even in new hemodialysis patients. Many authors, mainly from the United States, have reported a 60 to 90% incidence of ePTFE bridge grafts in the construction of first fistulae  . In contrast, European access surgeons have an overall use of ePTFE bridge fistulae of less than 10%  . The well known differences in patient characteristics (a higher proportion of diabetics, hypertensive patients and drug addicts in the US dialysis population in comparison to the European patients) cannot explain this dramatic difference. The higher complication rate of ePTFE grafts is beyond any doubt  . Prof. Blagg from Seattle, U.S., most recently emphasized that "vascular surgeons need to be educated that an AV-fistula is always preferable to a graft and, most importantly, that establishment of permanent blood access in a dialysis patient requires skill and attention to detail if it is truly to be permanent"  .
Although grafts constitute a substantial and indispensable component of the vascular access surgeon's repertoire, the indications should be restricted to "last resort" venous areas in the arms, to insertion in brachiocephalic  or thigh positions or to other locations of second or third choice. Some technical aspects may be reconsidered. The feeding artery and the draining vein should have a minimal inner diameter of 3 or 4 mm; the greater, the better. In the case of too tiny vessels, Keoghane suggested to create first an AV fistula; after maturation, that means arterial as well as venous dilatation, a graft can be placed advantageously  . The most frequent complication of ePTFE grafts in an AV position is thrombosis due to intimal fibromuscular hyperplasia occurring at the graft vein anastomosis. An excellent review on this demanding problem was recently given by Sukhatme  . A surgical trick to minimize these run off stenoses consists of constructing the graft vein anastomosis in a length between 20 and 30 mm; the graft must be placed in an almost parallel position to the draining vein, thus avoiding increased shear stress and turbulences. The use of a tunneler is mandatory.
In summary, ePTFE grafts are necessary to provide vascular access in patients with loss of suitable veins. The stricter the selection criteria of candidates the better the results will be. Some technical aspects should be observed carefully.
| Permanent Catheters|| |
Central venous catheterization was introduced as temporary vascular access replacing the Scribner shunt and the subclavian, internal jugular and the femoral approaches were used. Various single or double-lumen catheters were developed and the Seldinger insertion technique was routinely chosen. Over many years central venous catheters were placed via the subclavian route. Phlebographic studies have revealed a 50% stenosis/ occlusion rate in patients with history of subclavian vein catheterization  , resulting in edema of the arm, especially when an AV fistula was also present. Thus, time has come to abandon subclavian cannulas in patients with endstage renal failure  . Consequently, catheter insertion for temporary vascular access should preferably be done by the internal jugular or femoral route. It is worth remembering that permanent catheters placed in the internal jugular vein carry a minor risk of stenosis/occlusion of the innominate vein.
Placement of permanent catheters is indicated in patients with exhaustion of peripheral access sites, those with severe cardiac failure, peripheral ischemia due to arterial steal and hypotension from various causes. Soft, cuffed, silastic catheters placed either percutaneously or by surgical cut down via the internal jugular vein are the best choice. Single-lumen catheters may be more beneficial due to a reduced rate of thrombotic events in comparison with double-lumen cannulas.
Recently, a technique of stripping malfunctioning central venous catheters caused by a fibrin sheath has been described. After exclusion of catheter kinking or malposition, a threefold stripping procedure is performed via the femoral vein using an Amplatz Gosseneck snare  . Also, endoluminal thrombolytic therapy and percutaneous mechanical techniques can extend the mean survival of permanent catheters up to five years. A newly available double-lumen silastic catheter has been introduced by the Toronto group, Canada. One lumen of this Uldall catheter is collapsible, permitting percutaneous insertion of the 16F catheter into the internal jugular vein through a 13F sheath. The results obtained have been similar to those with other catheter types. An excellent, comprehensive overview on catheter-related problems was published by Twardowski recently  .
| Interventional Techniques|| |
Percutaneous transluminal angioplasty (PTA) has become a major tool for maintenance of angioaccess for hemodialysis. The other interventional techniques include intra-vascular stent placement and various percutaneous thrombectomy techniques combined with fibrinolysis. Indications are dependent on the personal experiences of the institution concerned and the availability of the various techniques. For the treatment of central vein stenosis or occlusion, where surgery is difficult and risky, stent placement yields definitely better results than PTA alone, especially in cases with elastic recoil. If intimal hyperplasia recurs within the stented segment, the stenosis can be relieved by simple PTA. Sometimes however, atherectoray or additional stent placement may become necessary  . The challenge is to prevent re-stenosis.
Thrombosis due to intimal-fibromuscular hyperplasia occurring at the graft-vein anastomosis in patients with ePTFE grafts is treated by a combination of fibrinolysis and PTA. The advantage of being able to combine the procedure for treating the symptom (thrombosis) with that of detecting and treating the disease (stenosis) is considerable  . But, the need for repeat dilatations (up to one re-intervention every four months) involves additional costs and patient discomfort, so that surgery is still preferable in many cases. The results of an aggressive surgical approach are more reliable and show better patency rates. However, some critically ill patients may still benefit from interventional procedures. It must always be remembered that the dilated, stented segment of the vein is only functionally, but not morphologically, cured.
Percutaneous mechanical declotting of thrombosed ePTFE bridge grafts without urokinase, a technique published by Trerotola, et al  , consists of pushing the thrombus after fragmentation into the central circulation, thus causing a clinically irrelevant pulmonary embolism. The procedure is in crucial discussion  . The preferred treatment of occluded ePTFE grafts remains an open question. An institution with a highly experienced interventional radiologist and the availability of advanced technology will probably favor interventional procedures; the "aggressive surgical approach" will presumably be chosen in the presence of an experienced vascular surgeon.
Angioplasty is preferred when there is venous stenosis of an AV fistula in the upper arm, where surgery is expected to be difficult. Stents should not be placed along cannulation areas. PTA in post-anastomotic stenoses of wrist fistulae is less successful. Surgical correction of these stenoses gives more reliable results.
| Access Monitoring|| |
Early diagnosis of fistula dysfunction in the dialysis units is mandatory. Well trained and attentive nephrologists and their staff should be aware of any change in arterial and venous pressure, cannulation problems, decreased arterial blood flow, signs of peripheral ischemia or venous hypertension. Recirculation studies can support the assumed diagnosis. If careful clinical examination is unable to provide an exact diagnosis, ultrasonographic and angiographic techniques are needed. At this point, the vascular surgeon should be involved. Transcutaneous Doppler/Duplex investigations appear quite accurate for measuring fistula flow and is achieved easier in ePTFE grafts with a constant inner diameter than in AV fistulae. Color Doppler examinations can visualize characteristics of blood flow caused by stenoses or aneurysms. Ultrasonographic techniques should be integrated into a routinely performed screening program  . The gold standard for the surgeon is still angiographic findings. Preferred technique is the digital subtraction angiography. If this technique is not available, most of the problems that concern the arterialized veins can be solved with conventional equipment provided that the radiologist is asked to be as exact as possible. Transfemoral arterial catheterization for angiography should not be adopted in dialysis patients. Whenever peripheral angiography is performed, the central veins should also be visualized to exclude central vein stenosis. It is a frustrating experience to have achieved peripheral repair of an AV fistula or an ePTFE graft and not get the desired result because of an undetected central venous stenosis.
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