Acute and critical limb ischemia: When time is limb
ABSTRACTAcute and critical limb ischemia post significant rates of morbidity and death, and need to be promptly recognized and treated to avoid amputation. Perfusion should be thoroughly assessed using multiple methods, and patients should be considered for revascularization (angioplasty or bypass surgery) to restore blood perfusion. Underlying conditions that need to be assessed and treated include cardiovascular disease, diabetes, and infection.
KEY POINTS
- In assessing peripheral artery disease, perform a thorough history and physical examination, paying close attention to the onset and characteristics of pain, activity level, history, and pulses, and the condition of the feet.
- Acute limb ischemia is a sudden decrease in limb perfusion, potentially threatening limb viability. Patients who have acute cessation of blood flow, sensation, or motor function need immediate revascularization to avoid amputation.
- Critical limb ischemia ranges from rest pain to gangrene and must be addressed with a multidisciplinary approach.
- The ankle-brachial index is a noninvasive, inexpensive test that can be done in the office with a hand-held Doppler device to assess the presence and severity of peripheral artery disease.
ASSESSING WOUND PERFUSION
Ankle- and toe-brachial indices assess perfusion
The ankle-brachial index19 is a good superficial assessment of perfusion. Multiple epidemiologic studies have shown the prognostic value of the ankle brachial index beyond the traditional risk factors and even the Framingham risk score.19 Values:
- Normal 1.1–1.30 (> 1.31 is abnormal and consistent with calcified vessels, and is an unreliable measure)
- Low normal 0.91–1.00
- Mild disease 0.71–0.90
- Moderate disease 0.41–0.70
- Severe disease ≤ 0.40.
However, the ankle-brachial index assesses perfusion only to the ankle, and many patients have ulcers in the toes and distal foot. The toe-brachial index must be specifically ordered in most institutions (if the first toe has an ulcer, the second toe should be assessed). The toe-brachial index is also important if the ankle-brachial index cannot be obtained because of calcified, noncompressible arteries in the ankle. A normal toe-brachial index is greater than 0.7.
The segmental blood pressure examination compares blood pressure measurements at multiple sites in the lower extremity. A drop of more than 20 mm Hg between segments indicates obstruction at that location. The test is simple and noninvasive and often can replace computed tomography.20
Transcutaneous oximetry
Transcutaneous oximetry measures the Tcpo2 from 1 to 2 mm deep in the skin from local capillaries. Measured adjacent to an ulcer, it is useful to predict wound healing and to assess the response to hyperbaric oxygen therapy.21 The values are:
- Normal > 70 mm Hg
- Impaired wound healing < 40 mm Hg
- Critical limb ischemia < 30 mm Hg.
Although most agree that a Tcpo2 below 40 mm Hg requires revascularization, low values can arise from many causes other than peripheral artery disease, including high altitude, pulmonary disease, heart failure, edema, inflammation, callus, and skin diseases such as scleroderma.
Skin perfusion pressure better predicts healing
Skin perfusion pressure is a measure of the capillary opening pressure after occlusion and is another way to assess perfusion. This test is not routinely done and must be specially requested.
The test is performed by inflating a blood pressure cuff on the leg until blood flow is occluded, then using laser Doppler to determine reactive hyperemia, ie, the gradual return of blood flow during controlled pressure release. The pressure at which movement is detected is the skin perfusion pressure.22
The laser Doppler probe emits and detects light scattered in the tissue. Light hitting moving blood cells undergoes a change in frequency, ie, a Doppler shift. An algorithm converts the optical information in the skin perfusion pressure by capturing the onset of capillary flow return and determining the pressure at which flow returns. Categories of results:
- > 50 mm Hg—normal
- 40–50 mm Hg—mild ischemia (wound healing probable)
- 30–40 mm Hg—moderate ischemia (wound healing uncertain)
- < 30 mm Hg—critical limb ischemia (wound healing unlikely).
Skin perfusion pressure testing has the advantages of not being affected by vessel calcification, thickened skin, or edema. It can be used on the plantar aspect of the foot and on digits. Recent small studies indicate that it is more sensitive for predicting wound healing than Tcpo2 measures.
On the other hand, skin perfusion pressure testing is not useful for predicting response to hyperbaric oxygen therapy. Also, blood flow occlusion by the cuff may be painful.
Intraoperative fluorescence angiography
Intraoperative fluorescence angiography is used to assess flap viability during reconstructive surgery and is being studied to determine its usefulness for assessing tissue viability in limb ischemia.
The test provides real-time assessment of capillary perfusion, determining surface tissue viability. The imaging head contains a digital camera, a laser light source, and a distance sensor. The test requires intravenous administration of indocyanine green, which binds to plasma proteins and is cleared through the liver, making it safe for patients with renal dysfunction. It cannot be used in patients with allergies to iodine contrast, penicillin, or sulfa.23
PREVENTION TARGETS CARDIOVASCULAR RISK FACTORS
Preventive measures are the same as for cardiovascular disease, ie, aggressive risk-factor modification: quitting smoking, lowering low-density lipoprotein cholesterol, reducing blood pressure, controlling diabetes, and managing heart failure.
Dual antiplatelet therapy should be instituted with aspirin and clopidogrel (Plavix) in patients undergoing revascularization. One can also consider cilostazol (Pletal); however, the role of this agent in patients with critical limb ischemia is less defined.
BYPASS OR ANGIOPLASTY?
The Bypass Versus Angioplasty in Severe Ischaemia of the Leg (BASIL) trial24 randomly assigned 452 patients with severe limb ischemia due to infrainguinal atherosclerosis to receive either surgery-first or angioplasty-first care and followed them for 5.5 years.
No significant differences between the two groups were found in amputation-free survival, deaths, or health-related quality of life. However, hospital costs associated with the surgery-first strategy were about one-third higher. As expected, more patients in the surgery group developed a wound infection, and more patients in the angioplasty group required bypass surgery at some point.
The conclusion that can be reached from this study is that patients presenting with severe limb ischemia due to infrainguinal atherosclerotic occlusive disease who are suitable for both surgical and interventional procedures can be treated with either method. However, most experts consider endovascular therapy as the first option in many patients. The National Institutes of Health recently funded a study to compare contemporary endovascular therapy vs surgery in patients with critical limb ischemia.
TAKE-HOME POINTS
In the last decade, significant endovascular advances have been made. New devices and techniques have enhanced our ability to treat high-risk patients who have critical limb ischemia. The combination of risk factor modification, accurate diagnosis, and aggressive revascularization should prevent limb loss in many of these patients. For the primary care physician, a low threshold for assessing perfusion in patients with critical limb ischemia is important using a screening ankle-brachial index and toe-brachial index. These patients should promptly be referred to a vascular specialist for further evaluation and treatment.
