Mechanical Devices and Acute Limb Ischemia

This increasingly common disorder responds well to alternative treatment approaches.

By Gary M. Ansel, MD; Charles F. Botti, Jr, MD; and Mitchell J. Silver, DO

As our population ages, the incidence of acute limb ischemia is increasing. These acute occlusions may be secondary to thrombus formation or may be due to embolization of material from a distant site. The most common etiologies include cardiac arrhythmia and cardiomyopathy. With the increasing use of endovascular procedures for the treatment of arterial obstructive lesions, iatrogenic causes such as thrombus from a sheath or debris embolization during balloon angioplasty are becoming more frequent. Although males usually experience more arterial thrombosis due to their predilection for atherosclerotic disease, embolization has no such sex predilection.

Emboli generally lodge at arterial bifurcations such as the common femoral bifurcation, or at sites of arterial stenosis. The severity of symptoms depends primarily on the location of the obstruction and the status of the collateral vascular supply. Although patients with pre-existing nonocclusive arterial disease have often developed collateral vessels and therefore may present with less acute ischemic symptoms, embolization into an arterial bed without previous collateral development may produce profound ischemia.

Historically, approaches to acute limb ischemia have included surgical embolectomy or bypass, thrombolysis, and amputation. Amputation should be reserved for limbs that have experienced such prolonged ischemia that irreversible major tissue loss has occurred. In such cases, both arterial and venous Doppler signals will be absent on examination. Patients with acute profound ischemia with immediate limb threat, especially secon-dary to embolization, will exhibit motor function and neurosensory changes.

Patients with immediate limb threat are traditionally treated with emergency surgery. These revascularizations often proceed without the time delay of angiography if the site of occlusion is obvious by physical exam. On examination, arterial Doppler signals are usually absent whereas venous Doppler signals are preserved. Subacute or more chronic limb ischemia with limb viability is usually treated with either thrombolytic therapy or surgery.

Acute limb ischemia should be treated in an organized fashion that allows for the fastest restoration of arterial flow while minimizing the risk of treatment. Due to the urgency of treatment, common comorbidities, such as cardiac disease, may not be able to be adequately addressed. Amputation rates are usually proportional to the delay in relieving the occlusion.

If possible, intervention or surgery should always be preceded by anticoagulation with therapeutic levels of heparinization.1 Heparin may prevent propagation of thrombus or further embolization while preparing for more definitive therapy. The treatment of acute limb ischemia does not stop with the restoration of arterial flow, since this restoration may itself result in profound acidosis and hyperkalemia. The patient is at significant risk for arrhythmia, rhabdomyolysis, myocardial infarction, pulmonary embolus, stroke, and death. Local effects of revascularization include free oxygen radical production that may lead to cellular membrane damage, which often precedes intracellular and extracellular leakage with subsequent edema development. Severe limb ischemia treated with revascularization may lead to such severe vascular leakage that compartment syndrome develops. Fasciotomy may be necessary to prevent further neurovascular injury and limb loss.

Beyond the limb risk, acute vascular occlusion is associated with a mortality rate of at least 10% to 20%, even with successful surgical revascularization.2,3 Open surgical treatment usually includes Fogarty embolectomy or surgical bypass of the affected arterial bed. Although often successful, Fogarty thromboembolectomy has several limitations: residual thrombus is frequently left, branch vessel occlusions are not cleared, and the act of dragging the debris out with the balloon causes endothelial injury that may be thrombogenic and vasospastic.

Due to these surgical shortcomings, thrombolytic therapy has been used when the patient’s clinical condition allows for delay in restoration of blood flow. The rationale for this approach includes the advantages of slowly restoring flow, identifying the underlying arterial lesion, converting any surgical procedure to an elective procedure, restoring collateral circulation, and avoiding trauma to the vessel endothelium. Several randomized trials have compared surgical and thrombolytic therapy in various clinical settings. Using thrombolytic therapy and avoiding emergency surgery appears to be associated with a mortality advantage for patients with acute limb ischemia. Thrombolytic therapy, however, especially with prolonged infusions over 24 hours, is associated with life-threatening hemorrhage in more than 12% of patients and may not provide rapid enough restoration of flow to be used as a sole modality when significant neuromuscular symptoms are present. For patients with more chronic limb ischemia, surgery appears to offer superior limb salvage.4-6

As the field of endovascular therapy has developed and matured, devices have begun to alter these traditional approaches to limb ischemia. With a multifaceted approach that includes mechanical thrombectomy devices to debulk the thrombus, pharmacologic agents to help break up the thrombus, and stenting of resistant debris, many patients can now be successfully treated without open surgery, even in cases of acute profound ischemia. In our experience, endovascular treatment of acute limb ischemia is better tolerated than emergency surgery or prolonged thrombolysis, especially in the elderly and in patients with comorbidities.

The first, yet still commonly used, percutaneous technique for acute thrombus removal is catheter aspiration. This method involves embedding a 6 to 8F guiding catheter in the thrombus and creating a vacuum with a syringe. This technique is most effective for patients with a small amount of fresh thrombus.7

New mechanical devices
The thrombus burden of an acutely occluded artery is frequently significant, especially with surgical bypass grafts. Simple catheter aspiration is inadequate, which is why mechanical devices for continuous active aspiration were developed. The most extensively studied device is the rheolytic thrombectomy catheter (AngioJet, Possis Inc., Minneapolis, MN). Several reports have shown good procedural success with low amputation and mortality rates.8-11 These studies illustrate the need for adjunctive treatment with low-dose thrombolysis in 18% to 58% of patients; however, with recent improvements in this device (ie, new, more efficient 6F catheter designs), we have seen a decrease in the need for postprocedure thrombolysis (Figure 1).

The major treatment limitation common to all of the mechanical thrombectomy devices is a lack of effect on the more organized fibrous ends of the occlusion. These areas become more resistant to removal as the occlusion ages and often require treatment with a stent or thrombolysis. Similarly, embolic material may be too firm or organized for complete removal. Downstream embolization may also prolong the ischemia time during endovascular therapy. Embolization of soft thrombus is easily treated by advancing the device distally, but occasionally more organized thrombus embolizes. These organized thrombus emboli may be treated with surgical embolectomy or by stenting the resistant debris against the arterial wall to restore flow. Even with these limitations, a large, multicenter registry of 99 patients with arterial thrombus demonstrated a 30-day limb salvage rate of 96% with a mortality rate of only 7.1%. Interestingly, the majority of deaths in this study were secondary to progressive cancer and not related to the arterial occlusion.11

In our experience, other percutaneous mechanical aspiration devices have been less impressive than the currently available AngioJet. The excimer laser (Spectranetics Corporation, Indianapolis, IN), however, has been shown to have thrombolytic qualities. It is effective in treating fresh coronary thrombus as well as long chronic superficial femoral artery occlusions, although its role awaits further delineation.12,13

Isolated Thrombolysis
The Trellis (Bacchus Vascular, Inc., Santa Clara, CA) is a novel drug infusion device designed for isolated thrombolysis. A thrombus is isolated between two occlusion balloons while the thrombolytic is mechanically dispersed with an oscillating wire and then aspirated. Theoretically, isolated thrombolysis enables single-setting thrombolysis by delivering a high concentration of thrombolytic agent while preventing systemic dispersion. Adjunctive procedures may be performed in the same single setting. Data from a voluntary company registry for isolated thrombolysis have indicated no reported bleeding complications and lower hospital costs due to reduced ICU stay and lytic dosing;14 a future registry is planned. The Trellis is approved for use in the peripheral vasculature.
In summary, endovascular treatment of acute limb ischemia is maturing and may offer a safe, efficacious alternative to open surgery. The success of this multifaceted approach usually depends on experience with a multitude of endovascular devices. The elderly and paients with significant comorbidities are prime candidates for this less invasive approach.

Gary M. Ansel, MD, FACC, is Director of Peripheral Vascular Intervention at Midwest Research Foundation in Columbus, Ohio. He serves on the Advisory Boards of Possis, Inc., and Bacchus Vascular, Inc. Dr. Ansel may be reached at (614) 262-6772;
Charles F. Botti, Jr, MD, FACC, is an investigator at MidWest Cardiology Research Foundation and a staff cardiovascular interventionalist at MidOhio Cardiology & Vascular Consultants in Columbus, Ohio. Dr. Botti may be reached at (614) 262-6772.

Mitchell J. Silver, DO, FACC, is Director of Coronary and Peripheral Intervention at Doctors Hospital West and Director of the Non-Invasive Vascular Lab at MidOhio Cardiology & Vascular Consultants in Columbus, Ohio. Dr. Silver may be reached at (614) 262-6772.

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Endovascular Today is a publication dedicated to bringing you comprehensive coverage of all the latest technology, techniques, and developments in the endovascular field. Our Editorial Advisory Board is composed of the top endovascular specialists, including interventional cardiologists, interventional radiologists, vascular surgeons, neurologists, and vascular medicine practitioners, and our publication is read by an audience of more than 22,000 members of the endovascular community.