Acute Lower Extremity Ischemia

Acute lower extremity ischemia (ALI) is a serious condition with substantial morbidity and mortality. ALI refers to recent onset of lower extremity pain; long-standing lower extremity ischemia is dealt with elsewhere in this book. If ALI is severe and prolonged, the patient may develop progressive paresthesia, motor dysfunction, and eventually tissue infarction in the lower extremity. The rapidity plus severity of the onset of symptoms depend on the location, extent, and degree of collateral pathways around the obstruction. The estimated incidence of ALI is approximately 1.4–2.6 cases per 10,000 patients per year. ALI accounts for approximately 20% of inpatient peripheral vascular cases and amputation rates of 10%–30%. After the presenting event, the 1-year mortality rates of ALI are approximately 15%–20% along with low amputation-free survival rates of approximately 50%–65%. The two principal causes of ALI are (1) embolic, in which an artery may become occluded when an embolus becomes dislodged from a proximal source, and (2) thrombotic, in which occlusion of a native artery occurs in the setting of severe atherosclerotic disease.

A thorough clinical workup, which includes a physical examination, noninvasive imaging, and laboratory work is paramount when evaluating ALI. The physical examination should start with visual inspection of the extremity in question to ascertain the level of the ischemia (e.g., foot, ankle, knee, above the knee). A mottled versus a pink extremity are at opposite ends of the risk spectrum. The examination is continued with evaluation of the sensory and motor systems, which can further reveal the severity and duration of the ischemia and guide clinicians to appropriate treatment. Additionally, palpation and/or Doppler examination of the peripheral pulses can help the clinician target the level of the occlusion. The clinical presentation of ALI has classically been stated as the six Ps: pain, pallor, pulselessness, paresthesia, paralysis, and poikilothermia (inability to regulate body temperature).

After a thorough physical examination, noninvasive imaging is performed to supplement the evaluation. Available modalities include pulsed volume recording (PVR), duplex ultrasonography, computed tomographic angiography, and magnetic resonance angiography. In our practice, we start with duplex ultrasound and PVR, adding computed tomographic and magnetic resonance angiography as needed.

Laboratory evaluation is also critical in the evaluation and workup of ALI. Serum urea nitrogen and creatinine values provide baseline renal function and can guide clinicians on the appropriate therapy. Prothrombin time, partial thromboplastin time (PTT), and international normalized ratio are obtained for a coagulation baseline.

An acute, profoundly ischemic limb is a surgical emergency. Cell death begins within 4 hours of total ischemic time and is irreversible after 6 hours. Revascularization in this setting may result in sudden tissue reperfusion and lead to a systemic acid–base disorder and impaired cardiopulmonary function.

Once the diagnosis of ALI has been made, medical management with systemic anticoagulation using a heparin or an argatroban (in patients with heparin-induced thrombocytopenia) infusion is initiated. It is thought that prompt anticoagulation limits propagation of clot and decreases microvascular thrombosis.

Over the years, the treatment options for ALI have evolved to include medical management, surgical revascularization, and most recently endovascular revascularization. Current endovascular options include catheter-directed thrombolysis (CDT), percutaneous mechanical/pharmacomechanical thrombectomy (PMT), and vacuum-assisted thrombectomy (VAT).

In the 1970s, intravenous thrombolytic agents (e.g., streptokinase) were shown to restore patency in acutely occluded arteries in 75% of cases. However, systemic infusion of thrombolytic agents carries a high risk of remote bleeding complications and has largely been abandoned in favor of CDT. Local intraarterial thrombolytic therapy has gained popularity over the past 20 years and in many cases, barring contraindications, is the initial intervention of choice. Thrombolytic agents are infused directly into the occluding thrombus in a concentrated fashion so systemic drug concentration and the total dose required for the procedure are reduced. As flow is restored, the underlying culprit lesion is usually unmasked and managed by interventional or surgical techniques.

In recent years, PMT and VAT have emerged as either an adjunct or stand-alone therapeutic option in ALI. In PMT, the clot is endovascularly macerated and removed, reestablishing flow in the artery. VAT, on the other hand, relies on vacuum-assisted aspiration of clot from the artery.

Indications

A classification system was devised by the Society for Vascular Surgery (SVS, United States) and updated by the Trans-Atlantic Inter-Society Consensus (TASC) II guidelines in 2007 for describing the degree of acute limb ischemia ( Table 11.1 ). The main objective of this categorization is to stratify the acuity of limb ischemia into defined groups for decision-making purposes. The TASC II guidelines also suggest an algorithm for the diagnosis and treatment of acute limb ischemia ( Fig. 11.1 ).

TABLE 11.1

Society for Vascular Surgery Clinical Categories of Acute Limb Ischemia

Modified from Rutherford RB, Baker JD, Ernst C, et al. Recommended standards for reports dealing with lower extremity ischemia: revised version. J Vasc Surg 1997;26:517–538.

Category	Definition	Prognosis	Physical Examination		Doppler Signals
			Sensory Loss	Muscle Weakness	Arterial	Venous
I	Viable	Not immediately threatened	None	None	+	+
II	Threatened
II-a	Marginally	Salvageable with prompt treatment	Minimal (toes)	None	Occasional +, frequently −	+
II-b	Immediately	Salvageable with immediate treatment	More than toes, pain at rest	Mild to moderate	Rare +, usually −	+
III	Irreversible	Major permanent tissue loss	Anesthetic	Paralysis	−	−

Endovascular therapy is indicated in patients who have acute-onset claudication (SVS I, the limb is generally viable) or limb-threatening ischemia (generally SVS II-a, but also SVS II-b) due to thrombotic or embolic arterial occlusion.