Chronic Critical Limb Ischemia

Chronic critical limb ischemia is manifested by pain at rest, non-healing wounds and gangrene. Ischemic rest pain is typically described as a burning pain in the arch or distal foot that occurs while the patient is recumbent but is relieved when the patient returns to a position in which the feet are dependent. Objective hemodynamic parameters that support the diagnosis of critical limb ischemia include an ankle-brachial index of 0.4 or less, an ankle systolic pressure of 50 mm Hg or less, or a toe systolic pressure of 30 mm Hg or less. Intervention may include conservative therapy, revascularization or amputation. Progressive gangrene, rapidly enlarging wounds or continuous ischemic rest pain can signify a threat to the limb and suggest the need for revascularization in patients without prohibitive operative risks. Bypass grafts are usually required because of the multilevel and distal nature of the arterial narrowing in critical limb ischemia. Patients with diabetes are more likely than other patients to have distal disease that is less amenable to bypass grafting. Compared with amputation, revascularization is more cost-effective and is associated with better perioperative morbidity and mortality. Limb preservation should be the goal in most patients with critical limb ischemia.

Atherosclerosis underlies most peripheral arterial disease. Narrowed vessels that cannot supply sufficient blood flow to exercising leg muscles may cause claudication, which is brought on by exercise and relieved by rest. (For a review of the diagnosis and management of claudication, see the article by Santilli, et al., in the March 1996 issue of American Family Physician.1) As vessel narrowing increases, critical limb ischemia can develop when the blood flow does not meet the metabolic demands of tissue at rest. While critical limb ischemia may be due to an acute condition such as an embolus or thrombosis, most cases are the progressive result of a chronic condition, most commonly atherosclerosis.
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An ankle systolic pressure of 50 mm Hg or less or a toe systolic pressure of 30 mm Hg or less suggests the presence of critical limb ischemia.
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Chronic critical limb ischemia is defined not only by the clinical presentation but also by an objective measurement of impaired blood flow. Criteria for diagnosis include either one of the following (1) more than two weeks of recurrent foot pain at rest that requires regular use of analgesics and is associated with an ankle systolic pressure of 50 mm Hg or less, or a toe systolic pressure of 30 mm Hg or less, or (2) a nonhealing wound or gangrene of the foot or toes, with similar hemodynamic measurements.2 The hemodynamic parameters may be less reliable in patients with diabetes because arterial wall calcification can impair compression by a blood pressure cuff and produce systolic pressure measurements that are greater than the actual levels.

Risk Factors:
Chronic critical limb ischemia is the end result of arterial occlusive disease, most commonly atherosclerosis. In addition to atherosclerosis in association with hypertension, hypercholesterolemia, cigarette smoking and diabetes,3,4 less frequent causes of chronic critical limb ischemia include Buerger's disease, or thromboangiitis obliterans, and some forms of arteritis.5
Figure 1a
FIGURE 1A. Right heel ulcer in a 56-year-old patient with diabetes. The ulcer failed to heal after three months of conservative treatment.

Diabetes is a particularly important risk factor because it is frequently associated with severe peripheral arterial disease. Atherosclerosis develops at a younger age in patients with diabetes and progresses rapidly. Moreover, atherosclerosis affects more distal vessels in patients with diabetes; the profunda femoris, popliteal and tibial arteries are frequently affected, while the aorta and iliac arteries are minimally narrowed. These distal lesions are less amenable to revascularization. Atherosclerosis in distal arteries in combination with diabetic neuropathy contributes to the higher rates of limb loss in diabetic patients compared with nondiabetic patients.6,7

Clinical Presentation:
The development of chronic critical limb ischemia usually requires multiple sites of arterial obstruction that severely reduce blood flow to the tissues.7,8 Critical tissue ischemia is manifested clinically as rest pain, nonhealing wounds (because of the increased metabolic requirements of wound healing) or tissue necrosis (gangrene).

Ischemic rest pain is classically described as a burning pain in the ball of the foot and toes that is worse at night when the patient is in bed. The pain is exacerbated by the recumbent position because of the loss of gravity-assisted flow to the foot. Ischemic rest pain is located in the foot, where tissue is farthest from the heart and distal to the arterial occlusions.1 Patients with ischemic rest pain often need to dangle their legs over the side of the bed or sleep in a recliner to regain gravity-augmented blood flow and relieve the pain. Patients who keep their legs in a dependent position for comfort often present with considerable edema of the feet and ankles.

Non-healing wounds are usually found in areas of foot trauma caused by improperly fitting shoes or an injury. A wound is generally considered to be nonhealing if it fails to respond to a four- to 12-week trial of conservative therapy such as regular dressing changes, avoidance of trauma, treatment of infection and debridement of necrotic tissue.

Gangrene is usually found on the toes. It develops when the blood supply is so low that spontaneous necrosis occurs in the most poorly perfused tissues.

Diagnosis:
Patients with diabetes develop atherosclerotic lesions in the more distal leg vessels, which are less amenable to revascularization.
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The presence of rest pain can sometimes be difficult to discern in patients with other chronic leg pain, such as that caused by peripheral neuropathy. Labeling a wound as non-healing can also be a subjective assessment. However, a number of physical findings and objective hemodynamic parameters can be used to substantiate a diagnosis of chronic critical limb ischemia. Typical physical findings include absent or diminished pedal pulses, shiny smooth skin of the feet and legs, and muscle wasting of the calves.

An objective measurement of blood flow is easily accomplished with the use of a hand-held Doppler probe and a blood pressure cuff.1 The cuff is inflated until the pulse distal to the cuff is no longer heard by Doppler. The cuff is then slowly deflated until the pulse is again detected. This measurement is recorded as the systolic pressure. As previously mentioned, an ankle systolic pressure of 50 mm Hg or less or a toe systolic pressure of 30 mm Hg or less suggests the presence of critical limb ischemia.

JAMIE D. SANTILLI, M.D., and STEVEN M. SANTILLI, M.D., PH.D.
University of Minnesota School of Medicine
Minneapolis, Minnesota