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.
CAUSES OF ACUTE LIMB ISCHEMIA
Thrombosis accounts for about 50% of cases. Underlying causes of the thrombosis are artherosclerosis (native or bypass), aneurysm, trauma, vasculitis (eg, in a rheumatologic disease such as lupus), and hypercoagulable states (particularly in patients with cancer).
Embolism accounts for about 30% of cases. Emboli usually arise from plaque rupture in atherosclerotic arteries or a clot breaking off from an aneurysm or from within the heart in patients with atrial fibrillation or another underlying heart disease. Paradoxical embolism, caused by an embolism crossing the heart through an opening such as a patent foramen ovale, is rare.
Uncommon causes include arterial dissection following trauma, adventitial cystic disease, popliteal artery entrapment, ergotism (from consuming fungus-contaminated grains), and human immunodeficiency virus arteriopathy.
The physical examination provides clues to the origin: livedo reticularis (purple discoloration in a mottled pattern) and blue nail beds indicate that an embolus is likely. Tests, including electrocardiography, echocardiography, and computed tomography of the chest and abdomen to look for an aneurysm, can help identify the cause. Ultrasonography of the popliteal arteries should also be considered to search for an aneurysm.
CRITICAL LIMB ISCHEMIA
Critical limb ischemia is more likely than acute limb ischemia to be seen in a general practice. Many aspects need to be addressed simultaneously, by different specialists: vascular and endocrine systems, infection, and wound care. The most successful management strategy is a dynamic approach using every piece of information.10
The Rutherford classification of peripheral artery disease has six categories based on the clinical presentation, with categories I through III being mild to severe claudication. We discuss here only the more severe categories: IV (pain at rest), V (tissue loss), and VI (gangrene).
Strong indicators of pain at rest are that the patient has to get up at night to dangle the leg over the bed or walk a few steps, or sleeps in a chair, or refuses to elevate the leg because of pain. The affected leg tends to appear red when the patient is standing (dependent rubor), but pale when the foot is elevated (elevation pallor).
Confirming that a patient has dependent rubor can be challenging, especially in people with dark skin. Classically, redness is seen when the leg is down and disappears with elevation, but in cellulitis, redness can also be reduced by elevating the leg. A foot that is hot to the touch is an indication of infection and not lack of perfusion alone.
The hemodynamic definition of critical limb ischemia is11:
- Ankle-brachial pressure index less than 0.4
- Reduced toebrachial pressure index, ie, less than 0.7
- Reduced transcutaneous pressure of oxygen (Tcpo2), ie, less than 40 mm Hg.
From 15% to 20% of patients with claudication will progress to critical ischemia over their lifetime, and in patients with claudication who also have diabetes, the risk is nearly 10 times higher. Without revascularization, the risk of amputation within 1 year is 73% for patients in Rutherford class IV and 95% for patients in class V or VI.
Revascularization and limb preservation
Preserving the limb is a prime goal. For patients who have an amputation, the mortality rate is 40% within 2 years.8 These patients tend to be elderly, and after an amputation, most will not learn to use a prosthesis and resume their previous level of activity. Other treatment objectives are to relieve pain, reduce cardiovascular risk, and minimize procedural complications.
Although limb preservation is not a controversial goal, best practices to preserve limbs are not universally available. Goodney et al12 studied variation in the United States in the use of lower-extremity vascular procedures for critical limb ischemia. They defined “low-intensity” to “high-intensity” regions of the country depending on the proportion of patients who underwent a vascular procedure in the year before amputation. They found considerable variation, but even in the region of highest intensity, more than 40% of patients did not have a vascular procedure in the year before amputation.
Similarly, Jones et al13 mapped amputation rates by US state and found significant variation even after adjusting for risk factors such as tobacco use and obesity.
Controversy surrounds the specifics of revascularization treatment, as in many fields in vascular medicine. However, most experts agree that improved perfusion is the goal.
The Trans-Atlantic Inter-Society Consensus for the Management of Peripheral Artery Disease recommends revascularization as the best treatment for patients with critical limb ischemia.8 In addition, the American College of Cardiology and American Heart Association Guidelines for the Management of Patients With Peripheral Arterial Disease (Lower Extremity, Renal, Mesenteric, and Abdominal Aortic) state that the tibial or pedal artery that is capable of providing continuous and uncompromised outflow to the foot should be used as the site of distal anastomosis.14 These guidelines do not yet mention endovascular therapy.
Angiosomes guide revascularization
In the past few years, the ability to facilitate healing of foot ulcers has improved. Angiosomes—regions of vascularization supplied by specific arteries—can be mapped on the skin, similar to the way dermatomes are mapped for neural innervation (Figure 1). The foot and lower leg region has six angiosomes perfused by three arteries that branch off the popliteal artery after it passes behind the knee:
- The anterior tibial artery supplies the dorsum of the foot and the front of the lower limb.
- The posterior tibial artery supplies the plantar surface of the foot via three branches—the medial plantar, lateral plantar, and calcaneal branches.
- The peroneal artery supplies the lateral part of the foot with collaterals to the anterior and posterior tibial arteries if they are compromised.
Studies have compared angiosome-based treatment vs revascularizing the best available artery (thus depending on collateral flow to compensate to surrounding areas). They have found that regardless of whether an endovascular or bypass method of revascularization was used, an angiosome-based approach led to significantly higher amputation-free survival rates.15–17
Patients typically do not have blockage of only a single tibial artery. Graziani et al18 assessed the vascular lesions in 417 patients with critical limb ischemia and found that multiple below-knee arteries were frequently involved. This makes it difficult to decide where to target revascularization efforts, and the angiosome concept helps with that.
