Endovascular Treatment in the Management of Arteriovenous Access
Arteriovenous access surgery provides a lifeline for those patients with end-stage renal disease in need of hemodialysis. The high flow arteriovenous anastomosis results in a durable, easily accessed, reliable means of needle cannulation for high flux hemodialysis. Despite the obvious benefits of modern arteriovenous access surgery, numerous challenges make management of these accesses difficult. First and foremost, arteriovenous access is probably the most predictable situation in vascular surgery for the development of intimal hyperplasia and resultant vascular stenosis. Virtually every arteriovenous access has some element of intimal hyperplasia-induced vascular stenosis.
Second, any vascular stenosis that develops because of high flow arteriovenous access can occur at any location along the course of the access. While the most predictable location for intimal hyperplasia is at the venous anastomosis of an arteriovenous graft, stenoses can occur anywhere. Although it was once thought that central venous stenosis associated with arteriovenous access was mainly related to prior subclavian catheter use, the current rarity of subclavian access for dialysis catheters would suggest that intimal hyperplasia in the subclavian venous out-flow may be related to anatomic flow issues unique to this location ( Fig. 39.1 ). Although subclavian venous lines or pacemaker leads can certainly result in venous out-flow stenosis, the cause of subclavian venous out-flow stenosis is unknown in many cases and multifactorial in most (see Chapter 40 ). Despite this, stenosis within the central veins remains a constant issue in vascular access surgery.
Endovascular treatment has revolutionized arteriovenous access management, making multiple repetitive interventions often possible. While management of arteriovenous access previously necessitated repeating open operations, endovascular treatments brought a more minimally invasive option that could be done in an outpatient setting with minimal resources, often in an office-based setting. While endovascular management was initially controversial secondary to the perception of limited durability, over time it became apparent that the results of endovascular management have several advantages. Nonetheless, endovascular treatment particularly with bare-metal stents may provide an impediment for subsequent treatments because of the ongoing scarring process and debris associated with placement of these devices ( Fig. 39.2 ). The scarring process often limits further interventions as a result of the physical barrier resulting from stents or scarring within the vein.
Arteriovenous access is critical for patients requiring hemodialysis for end-stage renal disease. Continuous arteriovenous access is important in avoiding catheter use and prolonging complication-free hemodialysis. While there are both open and endovascular techniques for managing arteriovenous access, endovascular treatment has many advantages. First, any open operative revision invariably leads to loss of autogenous venous out-flow because of surgical scarring in the out-flow conduit. Every patient will need multiple accesses and revisions during the course of their dialysis lifespan, and preserving venous out-flow is therefore critical to providing adequate duration of dialysis access. Although endovascular revision of a dialysis access does not tend to be as durable as some open revisions, the fact that endovascular treatment can be repeated more than compensates for any limitation in durability. Second, open hemodialysis access revision is an operative or re-operative procedure that is inherently traumatic, painful, and requires more recovery than any endovascular treatment. Some of these open procedures may even necessitate an overnight or longer stay in the hospital. While creation of a new dialysis access often requires an open operative procedure, thrombectomy and endovascular revision are generally minimally invasive procedures that do not require overnight hospitalization and can often be performed in an outpatient setting.
Considerations for Creation of Access
Open operative treatment is necessary to create the initial arteriovenous access. This generally involves either anastomosis of an artery to a vein or the insertion of a conduit between an artery and a vein to create a high-flow arteriovenous access that can be used repetitively for dialysis access. Once the access is established then the management necessary to maintain the access can often be provided using endovascular techniques.
What Are the Requirements for Arteriovenous Access?
In order to perform hemodialysis effectively, any arteriovenous access used will need relatively high flow. Typically, pump flows in the hemodialysis machine will be 600 to 900 mL per minute. To avoid recirculation of previously filtered blood into the hemodialysis machine, flow in the fistula or graft should generally be at least twice the pump speed. Any flow less than that will lead to recirculation and less efficient cleansing of the blood. This may result in prolonged dialysis sessions, and it is for this reason the Kt/V is (clearance) (time)/Volume is routinely measured for each dialysis session. Any trend toward decreasing dialysis efficiency would suggest a stenosis, most commonly associated with the venous out-flow.
Additionally, out-flow venous stenosis can be inferred from two aspects of the dialysis access: first, pulsatility of the access implies venous out-flow stenosis; and second, prolonged decannulation bleeding after needle removal similarly implies high venous pressures. While there are many factors that must be taken into account, in its simplest form all hemodialysis access monitoring focuses on assessment for venous out-flow stenosis.
Complications primarily related to access stenosis with or without thrombosis ( Fig. 39.3 ) are the most common complications necessitating endovascular access treatment. Endovascular treatment is advantageous in this setting because it limits the loss of venous conduit or out-flow in the revised arteriovenous access. Although each endovascular treatment clearly increases the likelihood of restenosis, the time interval between restenosis events does not necessarily decrease. If the patient suffers a restenosis after balloon angioplasty, in many cases bare-metal stenting of the venous out-flow may be appropriate. If stenosis recurs after a self-expanding bare-metal stent, covered stent or a drug-coated balloon may provide additional durability. In many cases, if not all, endovascular treatment for failure of prior endovascular treatment is the standard of care.
Endovascular Treatment of Arteriovenous Access
The presence of a stenosis limiting the duration or quality of dialysis requires treatment to restore the efficiency of the dialysis session. There are many options for treatment of a failing or failed dialysis access to restore this efficiency. First, treatment of stenosis by balloon angioplasty can restore nearly normal flow through the conduit. Second, as the stenosis recurs the scarring process may return more rapidly despite adequate venous out-flow caused by activation of the scarring process and early restenosis. In this setting, a mechanical barrier to the restenosis is sometimes needed. Bare-metal stents have been used in the setting but allow the recurrence of stenosis through the interstices of the stent. After this recurrence there are two alternatives: first, mechanically altering the rate of recurrence by trying to block the in-growth of scar tissue and intimal hyperplasia; or second, by altering the scarring process using topical drug application from a drug-coated balloon. Which is chosen depends on the underlying pathophysiology in the patient’s history. Longer lesions may be better treated by drug-coated balloon although this remains only a hypothesis. Mechanical recoil of a short segment may still benefit from bare-metal stent placement or, after recurrence, may necessitate a covered stent placement.
Access Stenosis versus Thrombosis
Invariably, an access becomes stenotic before it occludes ( Fig. 39.4 ). The timing of the diagnosis of the problems with a given arteriovenous access determines whether the treatment will be management of a stenosis or an occlusion. For all practical purposes, an occlusion is simply the continuation of the process started by an arteriovenous access stenosis. As the flow becomes more restricted and the out-flow vessel more damaged, the risk of access thrombosis goes up. For this reason, treatment of an access thrombosis is often quite similar to that used for treatment of an access stenosis. In the setting of thrombosis, crossing the lesion is similar to that of a stenotic access; nonetheless, the presence of thrombus within the access requires that thrombus be removed either mechanically or pharmacologically. Both may need to be used in any given patient. Although clinically significant effects from any clot that may be embolized are rare, the clot underlying the thrombosis invariably gets released, most commonly into the pulmonary vascular bed. With a higher clot burden, thrombolysis becomes more attractive because it allows the clot to be liquefied and flow more easily out of the access.