Current Concepts In Revascularization For Limb Salvage

Offering emerging insights on arterial revascularization as well as the treatment of venous disease, these authors discuss pertinent diagnostic tools and provide pearls on procedures ranging from bypass procedures and atherectomy to stents and vein ablation.

Historically, the objective of vascular surgery has been arterial revascularization of ischemic tissue, which allows for wound healing and relieves pain due to arterial insufficiency. Surgeons have relied on the physical exam as a primary diagnostic modality and have generally considered palpable distal pulses to be indicative of a sufficient blood supply for wound healing in the leg in general and the foot specifically.

Those basic principles are still true. However, the more modern approach would have the goal of vascular surgery being optimization of distal tissue perfusion. Accordingly, vascular surgery is involved in restoring the arterial component of perfusion in patients with peripheral artery disease (PAD) as well as the venous component of tissue perfusion, or venous drainage in the setting of venous outflow obstruction or reflux (i.e. blood inflow and outflow). The cardiac component of perfusion as well as addressing local autoregulatory pathways for blood flow generally remain in the realm of medical management, specifically vascular medicine specialists.

Physicians have traditionally considered the arterial component of perfusion to be the most important and treatable piece of the vascular insufficiency puzzle. Although there are no definitive ways to “put a number” on optimal perfusion, various tools of estimation are in routine use in practice. The most commonly used tests are ankle-brachial index/pulse volume recording (ABI/PVR) studies and transcutaneous oxygen tension (TcPO₂ or TCOM).

Pulse volume recording is a non-invasive and low-tech study, which provides a decent idea about overall leg perfusion, and may suggest the level of blockage when PAD exists. One can further augment the value of this test by immersing the foot in warm or cold water, thereby relieving or causing arterial vasospasm, which is measurable. In some cases, a resting ABI/PVR may not note a blockage in the artery.

To improve the yield in normal or borderline studies, one may perform an exercise stress test, performing the ABI/PVR before and immediately after exercise. Generally, a normal value (and the target for achieving wound healing) is an ABI greater than 0.95. The values may be artificially inflated (ABI greater than 1.05) in patients with calcified arterial walls (e.g. diabetes or renal failure). Waveform analysis and comparison to the ABI/PVR in the other leg as well as measuring toe pressures may assist in judging whether adequate arterial perfusion is present. For a patient with an open wound or anticipated foot surgery, an ABI measurement of less than 0.95 or greater than 1.05 and/or abnormally dampened waveforms should prompt a consultation by a vascular surgeon.

Skin oxygen tension measurements frequently gauge the effectiveness of hyperbaric therapy and can estimate the effectiveness of revascularization or an appropriate level of amputation. Ischemic tissue demonstrates a TcPO₂ of <40 mmHg while the patient is breathing room air (with normal being over 70 mmHg). A good predictive value of the presence of macrovascular disease (i.e. treatable by a vascular surgeon) would be an inadequate response to breathing 100 percent oxygen. Specifically, when the TcPO₂ value does not rise more than 10 mmHg after oxygen challenge and in particular, if it remains below 30 mmHg, macrovascular PAD likely exists. Therefore, when TcPO₂ at the site of an ulcer is less than 40 mmHg with poor augmentation after a 100 percent oxygen challenge, prompt evaluation by a vascular surgeon is required.

There are various imaging studies that indicate the location and degree of blockage. They vary in accuracy, price, radiation and contrast exposure. These studies include ultrasound, computed tomography (CT) angiogram, magnetic resonance (MR) angiogram and diagnostic angiogram. While these imaging studies are not intended to screen patients, one would obtain them prior to vascular intervention. Unlike perfusion studies (PVR and TcPO₂), vascular imaging studies cannot predict wound healing or the level of amputation (unless the imaging study comes back completely normal), but they can provide roadmaps for intervention.

Assessing The Pros And Cons Of Open Procedures
There are many ways to increase arterial flow, whether it is by re-opening a blocked vessel or performing a bypass procedure. The desired end result is pulsatile flow to a foot wound or surgical site. The interventions are usually either open or percutaneous. As a rule of thumb, only vascular surgeons perform open operations. These operations have higher morbidity but remain open longer than less invasive procedures. Conversely, various specialists do minimally invasive procedures. While patients tolerate these procedures better, there is reduced patency although vascular surgeons can repeat them multiple times if needed. Vascular surgeons frequently combine both open and endovascular approaches to achieve reconstructions with longer patency and less morbidity.

Open surgery procedures include endarterectomy (removal of the plaque from the vessel) and bypasses (sewing a conduit around the blockage).

Lower extremity endarterectomy is now limited to common femoral artery disease and remains the procedure of choice for this condition. One can perform this surgery as a “same-day” procedure with the patient having local anesthesia with sedation. There is no adequate endovascular alternative at this point.

Surgeons usually employ general anesthesia for bypasses, requiring hospital stays of at least three days. Bypass procedures get their names by the inflow and outflow vessel, connected by a conduit (e.g. aortobifemoral bypass with PTFE graft or a popliteal artery to posterior tibial artery bypass with a reversed saphenous vein). The primary benefit of bypass is long patency, especially when using the patient’s own vein as a conduit.

Examining The Benefits Of Stents And Atherectomies
When it comes to endovascular procedures, vascular surgeons can deliver various balloons, stents and atherectomy devices via a sheath (a tube that maintains an opening in the vessel of approximately 2 mm) to treat the diseased vessel. The first order of business in these procedures is obtaining an image or angiogram. Then one crosses the blocked segment with a wire and performs the procedure through the sheath and over the wire. It may include a combination of balloon angioplasty (including medicated balloons), stents (bare metal, drug-eluting or covered stents), atherectomy devices (designed to physically remove or modify plaque) and medications (clot busters or vasoactive agents).

The original endovascular treatment was based on angioplasty, stretching the narrowing in the vessel. When surgeons perform balloon angioplasty, the plaque become fractured and the vessel wall stretches. Unfortunately, the gains with simple balloon angioplasty are often short-lived as elastic recoil brings the vessel walls back together, causing a re-narrowing while the trauma of fracturing leads to a cascade of inflammatory processes, causing restenosis of the artery. Patency from simple angioplasty generally depends on multiple factors. Restenosis occurs quicker with smaller arteries so they may remain open for only a few weeks while bigger vessels take longer to reocclude and may stay open for several months.

It is expected that at three months’ follow-up after angioplasty, one-third of tibial vessels will close again, one-third will be more than 50 percent narrower and one-third will remain open.¹ However, that timeframe may be sufficient for healing of the wound.

Bare metal stents offer no advantage over simple angioplasty in tibial vessels but the drug-eluting stents extend the effectiveness of angioplasty.² The newer technologies, such as drug-coated balloons, bioabsorbable stents and various other modalities that inhibit restenosis, may offer longer patency in the tibial arteries in the future.

When dealing with larger size vessels such as iliac and femoral arteries, bare metal stents significantly extend the patency of the vessel after angioplasty by abrogating elastic recoil of the vessel. The chronic outward pressure of the stent may increase the neointimal proliferation but it takes longer to close the vessel down. At one year, only one-third to one-half of superficial femoral arteries remain patent after simple balloon angioplasty while more than 80 percent patency is apparent in most studies after placement of the bare metal stent.³ Beyond the first year, however, the patency of a bare metal stent declines rapidly. Using drug-eluting stents or fabric-covered stents extends the longevity of treatment.^3-5

Immunosuppressive drugs that coat the drug-eluting stents block the inflammatory response to fracturing the artery and prevent the restenosis process. Covered stents, on the other hand, block cell migration and thereby completely abrogate restenosis within the stent. As appealing as they sound, covered stents have their own drawbacks in addition to a higher cost than bare metal stents. Drug-eluting stents may cause acute thrombosis and require an aggressive anti-platelet regimen, usually consisting of aspirin and Plavix, for a few months after the procedure. Covered stents, on the other hand, block side branches (i.e. potential collaterals) and cause severe ischemia when they thrombose.

Another strategy, utilized by vascular specialists, is using various types of atherectomies for opening diseased arteries. Atherectomy is a procedure for removing or modifying plaque. This procedure facilitates angioplasty, extends the patency of the treated vessel and reduces the risk of restenosis.

When it comes to plaque removal, vascular surgeons can use a rotational burr, which sands down the plaque, or a directional cutter, which shaves off and takes out slivers of plaque. Plaque removal may allow angioplasty with lower pressures, which reduces barotrauma to the vessel and may reduce the rate of restenosis. Removing plaque also facilitates significant luminal gain, frequently obviating the need for stent placement.⁶ Another goal of atherectomy is plaque modification to reduce the stiffness of arteries, which allows angioplasty with lower pressures, which in turn causes less barotrauma to the vessel wall.
Surgeons may combine plaque removal and modification to achieve maximal luminal gain while possibly suppressing neointimal hyperplasia. Additionally, a hybrid of open and endovascular approaches can frequently achieve long-term patency while reducing surgical risk.

Keys To Addressing PAD
In summary, here are a few keys that vascular surgeons consider with the current management of PAD.

• Asymptomatic patients do not need surgical treatment while patients with open wounds or those anticipating foot surgery will need as close to inline pulsatile flow as technically possible.
• Bypass surgeries have significantly higher morbidity while offering longer lasting revascularization. Open surgeries are also “the last resort” after failed minimally invasive procedures.
• The smaller size of the vessel, the less durable the results that patients can expect after endovascular intervention. Therefore, though iliac artery stenting may treat minimal symptoms, surgeons should only perform tibial artery interventions for a patient with open distal wounds. In the latter case, the treating medical team should expect arterial patency for just a few months and aggressively work at healing any open wounds.

Expert Pearls On Treating Venous Disease
Vascular surgeons have not treated venous return as aggressively as they have in the past and physicians frequently overlook it. Blood in the veins returns against gravity toward the heart. When a patient is upright, the venous pressure has to be greater than the hydrostatic pressure of the hydrostatic column of fluid from the lowest point (i.e. the feet) to the level of the right atrium. That amount of hydrostatic back pressure in the tissues due to diseased veins causes congestion and decreases tissue perfusion overall. Therefore, conservative therapy focuses on decreasing that pressure gradient via leg elevation and compressing veins externally through the use of elastic stockings.

Inadequate venous blood return is usually attributed to two main factors: vein obstruction and valvular insufficiency. The main causes of vein blockage are: acute deep vein thrombosis (DVT); vein scarring after DVT (postphlebitic syndrome); external vein compression by overlying artery (May-Thurner syndrome); or a congenital septum.

The best initial imaging study of the leg veins is ultrasound by a skilled technician. The study should include evaluation of the deep veins to rule out obstruction as well as the superficial veins to evaluate the presence and severity of any reflux (i.e. retrograde flow in veins). If the physician suspects iliac vein or inferior vena cava obstruction, the non-invasive modalities may include computed tomography (CT) venogram or magnetic resonance (MR) venogram although the most accurate imaging modality is percutaneous venogram with intravascular ultrasound.

Emerging Approaches To Venous Pathology
Lately, vascular specialists have started to aggressively treat acute deep venous thrombosis (DVT), reducing the edema and risk of pulmonary embolism. In addition to any immediate benefit, removing fresh thrombus abrogates the development of chronic venous hypertension by preventing vein narrowing due to scarring (i.e. postphlebitic syndrome).

There are several devices available that facilitate the delivery of a thrombolytic agent into the DVT. (One such device, the EKOS catheter (EKOS Corp.), uses ultrasound waves.) Other devices assist this delivery and remove thrombus and products of lysis. Near complete removal of the acute DVT from deep vein generally requires thrombolysis overnight. This approach provides prompt and durable relief of swelling while preventing the development of chronic venous hypertension. One should consider thrombolysis to be the new standard of treatment for iliofemoral DVT.⁷

Vein compression of the central veins (iliac vein and inferior vena cava) has been recognized as a cause of both venous hypertension (i.e. venous ulcers, swelling, skin changes and thickening) and an increased risk of DVT in the leg.^8-10 Physicians can now treat this condition by stenting iliac veins. Central veins are large enough and maintain reasonable flow rates so when one places stents in those veins, they tend to stay patent. Development of such ancillary technology as intravascular ultrasound has facilitated progress in vein stenting, providing the necessary tool to evaluate the size of the vein and place the stent in a precise location.

Although vein obstruction is a more significant cause of venous hypertension than valvular reflux, treating superficial vein reflux plays a role in preventing venous ulcer recurrence and produces good results with very low procedural risk.⁸ Since there is no apparent reason for the existence of superficial veins, the objective of treatment is either to remove or shut down superficial veins with leaking valves. Vascular surgeons rarely perform vein removal (stripping) now as this approach has been replaced by less invasive means such as vein ablation, whether it is heating the wall of the vein, heating the blood and causing vein wall damage indirectly (laser ablation), or using various chemical solutions to induce permanent vein closure.

Unfortunately, all attempts to create artificial valves for treating deep vein reflux have been unsuccessful to date. Deep veins in the leg provide all the blood return from the extremity and are very prone to thrombosis with any manipulation. Deep vein valve reconstruction or transplantation has met with limited success due to sclerosis or thrombosis. (There is no positive data as all attempts so far have been unsuccessful with the exception of native valve transplants, but this surgery does not seem to justify the risk of surgery.) Therefore, the strategy regarding diseased deep veins of the leg is to maintain patency with treatment of reflux yet to be realized.

In summary, there are some key considerations that vascular surgeons weigh in the current treatment of venous pathology.

• One may consider acute iliofemoral DVT for thrombolysis at least within the first two weeks of the occurrence of thrombosis.
• Symptomatic patients with central vein compression or occlusion from prior thrombosis may benefit from stenting.
• Vein ablation offers minimal risk and durable results in the treatment of superficial vein reflux, and physicians should consider this first in cases of suspected venous hypertension.

Dr. Kucher is in private practice with the Vascular Experts at Southern Connecticut Vascular Center. He is board certified in general and vascular surgery, and is a Fellow of the American College of Surgeons.

Dr. Gagne is in private practice with the Vascular Experts at Southern Connecticut Vascular Center. He is a Fellow of the American College of Surgeons.

References

1.     Schmidt A, Ulrich M, Winkler B, et al. Angiographic patency and clinical outcomes after balloon-angioplasty for extensive infrapopliteal arterial disease. Catheter Cardiovasc Interv. 2010; 76(7):1047-54.
2.     Vogel TR, Dombrovskiy TY, Haser PB, et al. PS120. Are outcomes of tibial artery atherectomy or stenting superior to tibial angioplasty alone over time in the US Medicare population? J Vasc Surg. 2011; 53(6Suppl):61S.
3.     Dake M. The Zilver PTX randomized trial of treating femoropopliteal artery disease: 5-year results. Presented at Vascular Interventional Advances (VIVA), November 4-7, 2014, Las Vegas, NV.
4.     Golchehr B, Kruse R, van Walraven LA, et al. Three-year outcome of the heparin-bonded Viabahn for superficial femoral artery occlusive disease. J Vasc Surg. 2015; epub June 6.
5.     Lammer J, Zeller T, Hausegger KA, et al. Sustained benefit at 2 years for covered stents versus bare-metal stents in long SFA lesions: the VIASTAR trial. Cardiovasc Intervent Radiol. 2015; 38(1):25-32.
6.     McKinsey J, Zeller T, Rocha-Singh KJ, et al. Lower extremity revascularization using directional atherectomy: 12 month prospective results of the DEFINITIVE LE study. JACC Cardiovasc Interv. 2014; 7(8):923-33.
7.     Enden T, Haig Y. Long-term outcome after additional catheter-directed thrombolysis versus standard treatment for acute ilio-femoral deep vein thrombosis (the CaVenT study): a randomized controlled trial. Lancet. 2012; 379(9810):31-8.
8.     Raju S, Neglen P. High prevalence of nonthrombotic iliac vein lesions in chronic venous disease: a permissive role in pathogenicity. J Vasc Surg. 2006; 44(1):136-43.
9.     Neglén P, Oglesbee M, Olivier J, Raju S. Stenting of chronically obstructed inferior vena cava filters. J Vasc Surg. 2011;54(1):153-61.
10.     Neglén P, Hollis KC, Olivier J, Raju S. Stenting of the venous outflow in chronic venous disease: long-term stent-related outcome, clinical, and hemodynamic result. J Vasc Surg. 2007;46(5):979-990.