Arterial inflow, in the most technical sense, refers to flow to the level of the inguinal ligament, or in other words, specifies flow within the aorta and iliac vessels. However, inflow is often used to designate flow into a graft, and it will be used as such in this chapter. It makes intuitive sense that the patency of a bypass graft will be compromised if flow through vessels proximal to a bypass graft is compromised due to existing arterial occlusive disease. An obvious case in point would be the situation of an occluded external iliac artery ipsilateral to an occluded superficial femoral artery. In this situation, it is clear that a femoral-topopliteal bypass procedure could not be done without first addressing the iliac occlusion. However, at other times, a stenosis proximal to the planned origin of a bypass graft is present that is either overlooked, or if noted, has questionable hemodynamic significance. It is in these circumstances that careful evaluation must be undertaken to carry out the correct operation.

A critical stenosis is defined as the amount of narrowing needed to cause a decrease in flow or pressure. A reduction of the diameter of a vessel by 50% is equivalent to a 75% reduction in cross-sectional area. The relationship between percent stenosis and flow is complex. Flow depends not only on the degree of stenosis but on the resistance distal to the stenosis as well. With decreasing resistance, such as would occur with the placement of a bypass graft distal to a stenosis, the flow curve is shifted to the left. Therefore, a stenosis that may not have had an associated pressure drop at rest may become hemodynamically significant with the placement of a bypass graft distal to the stenosis, because the flow through the graft will decrease resting resistance. If such a stenosis proves to be hemodynamically significant when distal resistance is lowered, it must be addressed before a graft is placed, or the patient is at risk for early graft failure.

The surgeon has a number of tools that can be used to determine if a stenosis is hemodynamically significant. Arteriography is used to plan operative intervention and is often relied on to identify stenotic segments of vessels. Because atherosclerotic plaque often forms on the posterior wall of the vessel, a single anterior-posterior view may underestimate or fail to identify significant narrowing of a vessel. For this reason, liberal use of oblique views (i.e., right anterior oblique, RAO; left anterior oblique, LAO), in addition to the anterior-posterior orientation, should be obtained. Often the experienced clinician can detect that the pulse distal to a stenosis is abnormally decreased. However, clinical examination is subjective and has limitations. For example, in obese patients, normal pulses may appear to be diminished; conversely, in very thin patients, there may be a proximal stenosis despite palpating pulses that seem to be normal. The noninvasive laboratory can provide objective information beyond physical examination. Segmental pressures can localize hemodynamically significant lesions at multiple levels, although in some patients with heavily calcified vessels, the blood pressure cuff cannot occlude flow, leading to unreliable segmental pressures. The Doppler-derived analogue waveform can be helpful in detecting an abnormality in these patients. A normal waveform is triphasic, indicating that flow is normal to that level in the arterial tree with a high degree (95%) of certainty. With a biphasic waveform, 85% of patients will have normal arterial flow. The percent of patients with normal flow drops to 50% or lower when a monophasic waveform is present. Despite the usefulness of noninvasive evaluation, there are situations in which these studies may be normal or equivocal, yet a stenosis is suspected on an angiogram. In these cases the surgeon needs to determine
if the lesion will become hemodynamically significant when the outflow resistance is decreased. The obvious concern is that fashioning a graft in such a situation would place the graft at risk for thrombosis. These situations are especially common for lesions found in the iliac system. Pull-back pressures have been advocated and are often done at the time of the angiogram. Any drop in pressure over a stenosis should be viewed with concern and should be addressed prior to constructing a bypass graft that will use this vessel for inflow. However, as discussed above, there may not be any observable pressure gradient until the distal resistance drops. Injecting a vasodilator such as papaverine (i.e., papaverine test) can decrease distal resistance. The papaverine test can be done either at the time of the diagnostic angiogram or intra-operatively after exposure of the vessels. An adequate dose of the papaverine (usually 30 mg to 60 mg) must be injected in order to double blood flow. Using a continuous-wave Doppler, the peak frequency is used to approximate flow. After injection of the vasodilator, there should be at least a doubling of the frequency, which in turn suggests a doubling of flow. Pressure is transduced distal to the stenosis before and after administration of the vasodilator. Because there may be a systemic effect with injection of the vasodilator, the systemic pressure, as measured from a radial arterial line or a brachial cuff, must be monitored and compared to the pressure transduced from the artery. The ratio of the artery to systemic pressure is calculated before papaverine administration. After injection of papaverine, the ratio is again measured when any systemic effect has had its maximum effect. A drop in the artery to systemic ratio of greater than 15% after the injection of vasodilator suggests that the stenosis will be hemodynamically significant and should be corrected before using this vessel as the inflow site for the bypass. The specific ways to address such a stenosis are discussed in other chapters but could include endarterectomy, bypass, or angioplasty with or without stent.

A second factor that affects lower-extremity revascularization outcome is the choice of conduit. A number of conduits can be considered for lower-extremity revascularization. In general, there are three types of grafts: synthetic, biologic, or composite. The type of graft chosen for a specific operation depends on the specific operation being done, presence or absence of infection or bacterial contamination, and the published patency of the graft type for the particular bypass being considered.

There are a number of characteristics that would be desirable in the ideal prosthetic graft. These would include such things as durability, biocompatibility with the host, resistance to infection, ease of manufacturing, availability in various sizes, low cost, ability to store, imperviousness to blood, and resistance to thrombus formation. Essentially all synthetic grafts have some degree of porosity. Porosity is felt to be advantageous in that it allows fibroblast migration into the graft interstices and fibrin attachment, i.e., healing of the graft. Only a few synthetic grafts are currently in use, including Dacron, polytetrafluoroethylene (Teflon or ePTFE), polyurethane, and bioresorbable grafts. Of these, Dacron and ePTFE grafts are by far the most common. Bioresorbable grafts have been used experimentally and will not be further discussed in this chapter.

Dacron grafts are a type of textile graft and as such can be constructed by either weaving or knitting the Dacron yarn. Woven grafts have a lower porosity, are stiff, and are very strong. However, they have poorer handling characteristics, tend to fray at the cut edges, and because of the tight weave, have decreased tissue incorporation. Knitted grafts, on the other hand, are more flexible, making them easier to handle. Knitted grafts are also more porous and so require preclotting before implantation. An advantage of the increased porosity is improved tissue ingrowth and healing of the graft. Textile grafts are often modified by adding a velour finish to the graft surfaces. The velour is created by loops of yarn extending out at right angles from the graft surface. The velour improves the elasticity and handling characteristics of the graft and provides a lattice for fibrin deposition and fibroblast adherence. The inner velour is believed to provide a better surface for deposition of the fibrinous material that initially lines the graft surface when exposed to blood and results in a relatively thromboresistant flow surface. In order to take advantage of the benefits of the knitted graft and to obviate the need for preclotting, grafts are treated with either collagen or gelatin. This treatment prevents bleeding through the graft wall after implantation, but it is quickly resorbed, allowing tissue in-growth.

ePTFE grafts are extruded rather than woven or knitted. By visual inspection, it would appear that the grafts are “solid.” However, microscopic inspection reveals that the grafts are porous with solid nodes connected by fine fibrils. The commercially available grafts have an intranodal diameter of 30 microns. Some feel that the advantages of ePTFE grafts are that they do not require preclotting, do not dilate over time, may have better resistance to infection, and if they thrombose are easier to thrombectomize than textile grafts. A significant disadvantage is that when graft failure occurs, it is often due to intimal hyperplasia that forms at the distal anastomosis. This intimal hyperplasia frequently involves the native artery distal to the anastomosis. As a result, simple graft thrombectomy will not restore long-term patency, and extension of the graft to a more distal location is often needed. An additional consideration is that ePTFE grafts are also more expensive than textile grafts.

Biologic grafts include allografts (arterial homografts, venous allografts, umbilical vein), xenografts (bovine), and autogenous conduits. Allografts and xenografts are immunoreactive and must be treated to prevent rejection. These grafts have a propensity for aneurysmal formation over time. The patency of umbilical vein grafts when carried below the knee tends to be inferior as compared to results obtained when using saphenous vein. Umbilical vein grafts can be considered when there are no autogenous options and/or in the face of infection. The usual autogenous graft used for lower-extremity bypass is the greater saphenous vein. In patients who have had the vein removed (prior coronary bypass, previous vein stripping) or who have inadequate vein due to prior superficial thrombophlebitis or inadequate diameter, other venous conduits should be considered, such as the lesser saphenous vein, arm vein (cephalic and basilic vein), and the superficial femoral vein. When using vein as the conduit, outcomes are better when one continuous venous conduit of good quality is used, as opposed to splicing together multiple venous segments.

Bypass grafts above the inguinal ligament are usually performed using a synthetic graft. The vessels proximal to the inguinal ligament are large and with high arterial flow; as a result, patency of prosthetic grafts is excellent, approximating 90% at 5 years for aortobifemoral bypass grafts. In selected circumstances, as in the presence of bacterial contamination or the need to replace an infected graft, the use of a venous conduit that has a large diameter, such as the superficial femoral vein, can be

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