Description and Special Anatomic Considerations

Vascular bypass surgery involves placement of a conduit to serve as an alternative vascular pathway to a diseased or obstructed arterial bed. Vascular surgical conduits are anatomically classified on the basis of the locations of the proximal and distal anastomoses. The most common infrainguinal bypass is the femoropopliteal bypass, between the common femoral and the popliteal arteries (Fig. 114-1A,B). The distal anastomosis may be to either the above-knee or the below-knee segment of the popliteal artery (Fig. 114-1C). Conduits are further defined according to the material from which they are constructed. They may be native, such as an autogenous vein or artery, or they may be prosthetic, such as expanded polytetrafluoroethylene (ePTFE) or Dacron. The native greater saphenous vein is preferred for bypass surgery in the lower extremity because it performs better than any other conduit choice. However, it may not always be an available option because donor veins may be diseased or may have been previously harvested for other vascular procedures, such as coronary artery bypass surgery. Other autogenous veins used as vascular conduits include the short saphenous vein, the femoral vein (also known as the superficial femoral vein) within the thigh, and the basilic and cephalic veins of the upper extremity.

FIGURE 114-1 Femoropopliteal bypass graft. Images (A to C, cranial to caudal) from a right leg arteriogram show the proximal anastomoses (A, arrow) of a femoropopliteal bypass graft and the course of the graft (B, arrow) in the medial right thigh. The distal anastomosis of this femoropopliteal bypass graft is to the below-knee segment of the popliteal artery (C, arrow). Note the focal bulges (C, arrowhead) in the graft where venous valves were present in this in situ saphenous vein graft. These valves would have been excised with a valvulotome at the time of bypass surgery.

The autogenous greater saphenous vein conduits can be further subdivided into in situ and reversed vein grafts. Use of an in situ vein graft involves mobilization of only the proximal and distal ends of the vessel while allowing most of the vein to remain within its vascular bed. The venous valves must be incised, and venous tributaries arising from the in situ graft must be ligated. Proximal and distal anastomoses to the artery are then created at the mobilized ends of the in situ graft. A reversed saphenous vein graft must first be carefully harvested from the thigh, with ligation of all tributary vein branches. The vein is reversed during the bypass procedure, which allows unobstructed flow through the venous valves. Because of the reversal of the vein, the smaller distal end of the harvested vein is anastomosed to the larger caliber proximal artery, a situation that has generated a variety of surgical strategies to deal with the mismatch. Some surgeons use a harvested saphenous vein in a nonreversed fashion after incising the valves.

Prosthetic grafts typically are used for aortobifemoral and extra-anatomic bypass surgery, such as axillofemoral or cross-femoral bypass graft surgery; when they are used for femoropopliteal bypass surgery, long-term patency is significantly improved when the distal anastomosis is to the above-knee rather than the below-knee segment of the popliteal artery. There are relatively poor results for distal revascularization with prosthetic grafts. If bypass to the below-knee popliteal arterial segment is necessary, an autogenous vein graft is indicated; if the greater saphenous vein is not an option as a conduit, other autogenous veins may be used. Composite grafts that use a prosthetic above the knee coupled with an autogenous graft to cross the joint and to anastomose to the below-knee segment are also used.

Prosthetic grafts have certain advantages, including ease of use, shortened surgical times, and less extensive operative dissection. The disadvantages relative to autogenous grafts include higher frequencies of intimal hyperplasia, thrombosis, and anastomotic stenoses. There are also higher rates of graft infections, material deterioration, and anastomotic pseudoaneurysms than in their native counterparts.

The lack of a completely satisfactory prosthetic substitute for the greater saphenous vein has led to the use of other biologic conduits, such as human umbilical vein, arterial or venous homografts, and xenografts. There are certain inherent problems, such as aneurysmal degeneration (Fig. 114-2), and long-term patency issues that are unique to these biografts, and results remain mixed compared with prosthetic grafts.⁷

FIGURE 114-2 A and B, This bovine xenograft used for distal bypass to the dorsalis pedis artery (arrow) underwent severe degeneration, as seen in this arteriogram. There are multiple stenoses and aneurysms present in the graft as a result of the degeneration. This is an inherent problem in both xenografts and homografts. C, Gray-scale ultrasound image of the bovine xenograft at knee level demonstrates that there is marked aneurysmal degeneration with considerable laminar thrombus (T) and an eccentric patent lumen (asterisk).

Extra-anatomic bypass refers to grafts that are constructed in anatomic locations that are significantly different from the normal location of the diseased arteries that are being bypassed. Typical examples are the cross-femoral (femorofemoral) and axillofemoral bypass grafts (Figs. 114-3 to 114-5). These were originally designed for patients too ill to undergo direct aortofemoral bypass or to replace grafts that were infected; these are still the primary indications. In addition, they now often serve as adjuncts to endovascular repair of abdominal aortic aneurysms (EVAR), particularly in the category of aorto–uni-iliac EVAR. These grafts are usually constructed of prosthetic material and generally have somewhat lower long-term patency than more traditional vascular bypass grafts, such as the aortobifemoral bypass graft. The obturator bypass (Fig. 114-6) was developed to replace femoropopliteal bypass surgery in patients with groin infections involving the native arteries or previously placed grafts and in patients with other complicating circumstances in the groin, such as trauma or previous radiation treatment. This bypass can be constructed with prosthetics or with autogenous vein.⁸

FIGURE 114-3 Axillofemoral bypass graft. Digital subtraction angiography (DSA) images (A to C, cranial to caudal) show the proximal limb (A, arrow) of an axillofemoral bypass graft, the thoracic course (B, arrow) of the axillofemoral bypass graft, and the distal anastomoses to the profunda femoral artery (C, arrow). A femorofemoral bypass graft (not shown) was also constructed in this patient.

FIGURE 114-4 Conventional x-ray arteriogram shows a right-to-left femorofemoral bypass graft (white arrow). The study also reveals an inflow stenosis (black arrow) in the right external iliac artery stent graft and occlusion of the heavily calcified native superficial femoral artery (arrowheads) in this patient who had also previously undergone a femoropopliteal bypass graft. This patient will require correction of the inflow abnormality and either thrombolysis of the bypass graft or surgical revision.

FIGURE 114-5 CTA shows a right-to-left femorofemoral bypass graft constructed in a patient who had previously undergone endovascular repair of an abdominal aortic aneurysm and subsequently had occlusion of the left limb of the graft.

FIGURE 114-6 CTA shows that there has been placement of an obturator bypass (arrow) in this patient who has previously had placement of an axillofemoral and right-to-left femorofemoral bypass, followed by a left axillofemoral bypass graft. Both procedures eventually failed because of chronic infection in the left inguinal region, necessitating the obturator bypass.

Indications

Patients with lower extremity peripheral arterial disease that is manifested as claudication are typically managed medically, with an emphasis on lifestyle and risk factor modification coupled with an exercise regimen. Although many patients will show symptomatic improvement, a large number will have progression of disease. In addition, compliance of patients with such a management strategy is generally poor. Disabling, lifestyle-limiting claudication or progression to critical limb ischemia, characterized by rest pain or tissue loss, may eventually occur and thus require either infrainguinal bypass surgery or endovascular revascularization.

Patients with peripheral arterial disease may be classified by both a clinical description of the symptoms and objective testing criteria by the Rutherford categories of chronic limb ischemia (Table 114-1). These aid in prognosis and treatment planning.

TABLE 114-1 Rutherford Categories of Chronic Limb Ischemia

The anatomic location of the peripheral arterial disease affects the choice of a surgical or endovascular procedure. Arterial bypass remains the standard for revascularization and is indicated in patients with long-segment chronic total occlusion of the superficial femoral artery, chronic total occlusion of the popliteal artery and proximal trifurcation vessels, diffuse, severe multiple stenoses or occlusions that involve the entirety of the superficial femoral artery, and recurrent stenoses or occlusions after two or more prior endovascular treatments.

Contraindications

Arterial bypass surgery is contraindicated if there is insufficient inflow or outflow to maintain patency of the vascular conduit. Before bypass, therefore, some type of imaging (CT angiography [CTA], MR angiography [MRA], or catheter angiography) is generally performed for delineation of arterial anatomy and to identify an appropriate target artery for distal anastomoses. If there is no target vessel or there is uncorrected inflow disease, arterial bypass surgery will have a poor outcome.

A conduit should not be constructed in an actively infected tissue bed. An infection should be aggressively treated to resolution; if vascularity is insufficient to allow healing, extra-anatomic bypass or amputation should be considered.

Outcomes and Complications

Long-term graft patency, limb salvage, and mortality are the primary reported endpoints for revascularization procedures. Relief of symptoms is subjective and thus more difficult to accurately quantify and assess.

Graft patency is described as primary, assisted primary, and secondary. Primary patency indicates that no additional procedures have been performed that involve the vascular conduit, including any graft extensions that may be required for progression of disease distally. Assisted primary patency includes any minor revisions or endovascular treatments of lesions that threaten graft patency. If the graft has thrombosed and patency is restored by thrombolysis, thrombectomy and revision, or other means, this is considered secondary patency.

Complications and lesions that threaten vascular conduit longevity include infection, development of anastomotic stenoses (Fig. 114-7) or pseudoaneurysms, progression of disease distal to the graft resulting in inadequate outflow, failure to incise all valves or to ligate all venous side branches within an in situ bypass graft, intimal hyperplasia, poor conduit quality, and degeneration of the graft. Such complications will require open surgical or endovascular correction.

FIGURE 114-7 A, DSA reveals a stenosis (arrow) of an in situ femoropopliteal bypass graft at the distal anastomoses to the below-knee popliteal artery. B, Magnified DSA arteriogram delineates the high-grade distal anastomotic stenosis more clearly. C, The anastomotic stenosis was treated with balloon angioplasty, successfully eliminating the stenosis and maintaining graft patency. This is an example of assisted primary patency.

Local complications, including hemorrhage, infection, and graft thrombosis, may occur at the time of the initial surgery. Hemorrhage is usually related to an anastomotic problem, such as a suture line disruption, or to a poorly ligated side branch (arterial or venous). Graft infections commonly result from hospital-acquired organisms in the early period and are increased in the presence of hematoma, lymphocele, or wound infection. Graft infections occurring more than 3 months after the bypass are usually due to microorganisms such as normal skin flora. Infections are manifested on imaging studies as perigraft fluid collections and may be confirmed with CT- or ultrasound-guided aspiration followed by culture and sensitivity testing. Special culture techniques may be required.

The outcomes of various infrainguinal bypass procedures with use of currently available vascular conduits are summarized in Tables 114-2 to 114-4. With regard to extra-anatomic bypass grafts, the long-term patency rates are typically lower than for the anatomically positioned grafts. The obturator bypass graft for infrainguinal occlusive disease has reported patency rates of 73% and 57% at 1 and 5 years, respectively, which are somewhat lower than with conventional femoropopliteal bypass.⁸

TABLE 114-2 Above-Knee Femoropopliteal Grafts

TABLE 114-3 Below-Knee Femoropopliteal Grafts

TABLE 114-4 Infrapopliteal Grafts

Imaging Findings

Description and Special Anatomic Considerations

Both percutaneous transluminal angioplasty and intravascular stent placement have become widely accepted as primary treatment of infrainguinal peripheral arterial disease so that endovascular treatments now have an extremely important role in its management. Application of endovascular therapies remains a dynamic process as currently available technology evolves and new treatment devices and options are introduced (e.g., atherectomy, covered stents, cryoplasty, drug-eluting stents, lasers, biodegradable stents).

The most frequently employed endovascular treatment options are angioplasty and intravascular stent placement. Angioplasty may be used as a stand-alone primary therapy (Figs. 114-8 and 114-9) or may be combined with stent placement. Stents provide an intravascular scaffold for the vessel lumen and are available in a variety of materials, configurations, and delivery systems. Stents may be constructed from stainless steel, platinum, Elgiloy, and nitinol and may be combined with ePTFE or Dacron to produce a covered stent endoprosthesis or “stent graft.” Noncovered stents may be constructed with “open” or “closed cell” designs, which influence stent flexibility, conformability, radial strength, fracture resistance, and restenosis rates. In addition, there are balloon-mounted and self-expanding stents available in both the noncovered and covered groups (Figs. 114-10 and 114-11). Balloon-mounted stents are typically sized to correspond to the desired diameter of the vessel lumen, whereas self-expanding stents are usually oversized and may require secondary angioplasty to achieve a satisfactory diameter.

FIGURE 114-8 Angioplasty balloons are available in various lengths and diameters and have varying burst pressures, depending on the material from which they are constructed. This is a noncompliant balloon, meaning that the balloon will retain a tubular shape and will not conform to the vessel luminal characteristics by expanding beyond the manufacturer’s predetermined maximum diameter.

FIGURE 114-9 A, DSA image shows two stenoses (arrows) in the mid and distal superficial femoral artery. B, After angioplasty, there is restoration of a normal luminal caliber in the treatment areas (arrows). C, Magnification view of the distal treatment zone in B after angioplasty shows small linear contrast collections (arrows) paralleling the vessel lumen. These represent “cracks” or fissures within the plaque that have resulted from the angioplasty. The mechanism is that of a controlled fracture of the plaque, often accompanied by stretching of the muscular media. Over time, this usually remodels and assumes a more normal appearance.

FIGURE 114-10 Noncovered stents are available as either self-expanding (A) or balloon mounted (B). The self-expanding stent is typically oversized relative to the native artery to be treated, whereas the balloon-mounted stent is more precisely sized to the native vessel diameter.

FIGURE 114-11 Covered stents (stent grafts or vascular endoprostheses) are available as either self-expanding (A) or balloon mounted (B).

Given the decreased restenosis rates in the coronary arteries that have resulted from use of drug-eluting stents, there has been considerable enthusiasm for extending the application to treatment of the lower extremity arteries. Initial results have been mixed, but clinical trials remain ongoing.

Atherectomy involves a catheter-based atherosclerotic plaque excision system consisting of two components: a low-profile monorail excision catheter and a palm-sized power drive unit. A tiny rotating blade housed near the catheter tip is exposed when activated, removing and capturing thin shavings of plaque from the arterial wall into a collection chamber. Atherectomy thus permits “debulking” of atheroma from the lesion and may be used as a primary therapy or in combination with other endovascular treatment options. Catheters vary in diameter and tip length to accommodate various lesions (Figs. 114-12 and 114-13).

FIGURE 114-12 Graphic of the FoxHollow SilverHawk atherectomy catheter shows the cutting blade (arrowhead) within the cutting window engaging the atheroma (black arrow). As thin shavings of atheroma are excised, they are collected in the nose cone (white arrow) at the catheter tip.

FIGURE 114-13 A, Image from an atherectomy procedure shows a focal stenosis in the superficial femoral artery (arrow) before intervention. B, A guide wire has been advanced across the stenosis, and the atherectomy catheter (arrow) has been introduced. C, After the atherectomy, there is an almost completely normal appearance to the artery at the site of the previous stenosis (arrow).

Cryoplasty is an angioplasty-based technology that uses liquid nitrous oxide as the balloon inflation medium, which lowers the balloon surface temperature to −10° C. Theoretically, cryoplasty causes an altered plaque response in which, as a result of freezing, microfractures form and weaken the plaque, contributing to a more uniform vessel dilation and less injury to the media. There may also be less elastic recoil and an induction of cellular apoptosis through freezing (Fig. 114-14).^9,10

FIGURE 114-14 A, Image from a cryoplasty procedure shows a high-grade superficial femoral artery stenosis (arrowhead) on the diagnostic DSA arteriogram. B, Fluoroscopic spot film shows that the cryoplasty balloon (arrows) is inflated within the stenotic area; note the atherosclerotic calcification adjacent to the cryoplasty balloon. C, After cryoplasty, there is a normal caliber to the arterial lumen in the treatment zone (arrows).

Another angioplasty-based technology is cutting balloon angioplasty, in which multiple small atherotomes (microsurgical blades) are fixed longitudinally on the outer surface of a noncompliant balloon. These expand radially during balloon inflation, delivering longitudinal incisions into the plaque and the vessel. Theoretically, there should be advantages to cutting balloon angioplasty through reduction of vascular injury by scoring of the vessel and the plaque longitudinally rather than by causing an uncontrolled disruption of the atherosclerotic plaque. However, in a randomized trial of 1385 coronary lesions, there was no significant difference between cutting and standard angioplasty at 6-month follow-up in angiographic and clinical results. The primary endpoint of angiographic restenosis at 6 months was 31.4% in the cutting balloon angioplasty group versus 30.4% in the standard group. This trial showed that cutting balloon angioplasty is equivalent in safety and efficacy endpoints to standard angioplasty, but it did not prove superiority for the general pool of percutaneous coronary intervention patients.¹¹

There are varying opinions as to the optimal endovascular treatment strategy for infrainguinal lesions with regard to angioplasty alone, angioplasty accompanied by primary stenting, and the use of noncovered versus covered stents. Treatment options depend on the route of arterial access; the type, length, and location of the lesions; the presence or absence of inflow disease; the quality of the runoff vessels distal to the lesions; the overall clinical status of the patient; and the skills and long-term success rates of the operator. The cost associated with endovascular procedures is also a consideration; generally, these technologies are substantially more expensive than open surgical revascularization procedures.

Infrainguinal lesions may be treated either with an ipsilateral antegrade femoral artery puncture or from the contralateral approach, with the catheter and wire advanced across the aortic bifurcation to gain access to the arteries of the affected limb. The successful use of the latter approach depends on the distal aortic and pelvic arterial anatomy; tortuous or stenotic iliac arteries or an acutely angled aortic bifurcation may significantly complicate the negotiation of the catheter into the treatment area. This approach may be necessary for treatment of lesions that involve the common femoral or the proximal superficial or profunda femoral arteries.

Indications

Determination of which lesions are appropriate for endovascular treatment versus traditional surgical revascularization continues to be an evolving and sometimes controversial process. Addressing these concerns has resulted in multidisciplinary attempts to categorize lesions, to stratify the risks and benefits of endovascular treatment, and to propose treatment guidelines. In 1994, the American Heart Association proposed percutaneous transluminal angioplasty guidelines for endovascular versus surgical treatment. These have since been revised as longer follow-up data have become available and as intravascular stents have been incorporated into endovascular treatment regimens.

The TransAtlantic Inter-Society Consensus Working Group (TASC) developed another classification system in 2000¹² and addressed both aortoiliac and infrainguinal occlusive disease, with the latter guidelines limited to femoropopliteal disease. These guidelines were updated in 2007¹³ and reflect the expanded role of endovascular therapy as a primary option in the treatment of vascular occlusive disease (Table 114-5).

TABLE 114-5 TransAtlantic Inter-Society Consensus (TASC II) Recommendations (Femoropopliteal Arterial Disease)

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Dec 26, 2015 | Posted by admin in CARDIOVASCULAR IMAGING | Comments Off on Lower Extremity Operations and Interventions

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Lesion Category	Lesion Characteristics	Treatment Recommendation
Type A	Single stenosis ≤10 cm in length Single occlusion ≤5 cm in length	Endovascular treatment
Type B	Multiple lesions (stenoses or occlusions), each ≤5 cm Single stenosis or occlusion ≤15 cm not involving the infrageniculate popliteal artery Single or multiple lesions in the absence of continuous tibial vessels to improve inflow for a distal bypass Heavily calcified occlusion ≤5 cm in length Single popliteal stenosis