Chronic Critical Limb Ischemia

Chronic critical limb ischemia is manifested by pain at rest, nonhealing 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. 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.

 

 

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. 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.

 

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.

 

 

 

 

Diagnosis

 

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 nonhealing 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.

FIGURE 1C. The patient underwent operative debridement and began a regimen of dressing changes (gauze dampened with normal saline) three times a day. He also began wearing a shoe that allowed ambulation without direct pressure on the ulcer. He was followed weekly in the outpatient clinic.

FIGURE 1D. The ulcer shows good progress in healing after three weeks of conservative therapy.

FIGURE 1E. After six weeks of outpatient treatment, the ulcer is well healed.

 

Another widely used parameter is the ankle-brachial index, which is a ratio of the systolic pressure at the dorsalis pedis or posterior tibial artery divided by the systolic pressure at the brachial artery. Patients with claudication typically have an ankle-brachial index of 0.5 to 0.8, while patients with critical limb ischemia usually have an ankle-brachial index of 0.4 or less

Vascular laboratories also use Doppler probes to measure the pulse volume waveform at segmental locations in the leg arteries. A change in the Doppler waveform from triphasic to biphasic to monophasic and then stenotic waveforms can identify sites of arterial blockage.

 

Differential Diagnosis

 

Ischemic rest pain may be confused with night cramps, arthritis or diabetic neuropathy. Night cramps occur in the calf muscles; they usually awaken the patient from sleep and are relieved by massaging the muscle, by walking or by using antispasmodic agents. Patients with arthritis of the metatarsal bones may have pain in the foot. This pain is often experienced at night and may be relieved by standing. The distinguishing characteristic of arthritic pain is that it usually occurs intermittently and at sporadic intervals, whereas ischemic rest pain consistently occurs after a specific interval of recumbency.

 

Diabetic neuropathy may also present with pain in the foot and is occasionally associated with diminished pulses and trophic skin changes. This pain, however, is not steadfastly associated with recumbency. The other features of diabetic neuropathy, such as loss of light touch (i.e., the monofilament test) and decreased vibratory sense, can also serve as distinguishing characteristics.

 

Ischemic Rest Pain

 

Patients with ischemic rest pain should be given pain medication as necessary, and any underlying systemic cause of inadequate blood flow, such as cardiac failure, should be corrected. If pain persists after four to eight weeks of conservative therapy with pain medication and interventions to optimize the patient's overall condition, the possibility of operative intervention should be explained to the patient, including the risks and benefits of the procedure.

 

Surgical intervention includes revascularization and amputation. If the patient wants to undergo revascularization and is an acceptable operative candidate, arteriography is often performed for further evaluation and planning of revascularization. Some centers are utilizing magnetic resonance angiography as an alternative or supplement to arteriography to minimize the risk of dye exposure

 

Nonhealing Wounds

 

Patients with nonhealing wounds or gangrene should be evaluated for the presence of infection. Infected wounds require antibiotic therapy, surgical debridement, or both. Conservative therapy includes teaching the patient ways to avoid trauma to the wound site, including the wearing of properly fitting shoes. Dressings should be changed frequently; the patient should be seen weekly until the wound heals .

 

Further intervention may be required if conservative therapy does not lead to improvement, as indicated by increasing wound size, persistent or spreading infection or no evidence of healing after four to eight weeks. Progressive gangrene, rapidly enlarging wounds and continuous ischemic rest pain unrelieved by dependency are each unstable conditions that can rapidly lead to limb loss and require urgent intervention. However, many patients with critical limb ischemia have a stable or slowly progressive presentation. Review of the data reveals that patients with chronic critical limb ischemia have a three-year limb loss rate of about 40 percent This suggests that a substantial proportion of patients with critical ischemia are not at risk of imminent limb loss.