Anatomy/Physiology

The legs are supplied with arterial blood via the aorta which bifurcates at the level of the umbilicus into the left and right common iliac arteries (CIA) which themselves divide into the internal iliac arteries that supply the muscles of the buttock and the pelvic organs and the external iliac arteries (EIA) that run around the pelvic brim to enter the top of the leg.

The common femoral artery (CFA) enters the thigh just posterior to the inguinal ligament near its mid point and at this point the artery is easily palpable by compression against the underlying bones. The CFA bifurcates in the upper thigh into two unequal branches. The smaller branch is the profunda femoris artery (PFA) which passes posteriorly and supplies the muscles of the thigh; the larger branch is the superficial femoral artery (SFA) which supplies the calf and foot by spiralling more superficially around the medial side of the thigh to pierce the adductor muscles above the knee. From here it continues as close to the posterior surface of the femur as the popliteal artery where it can be palpated just above the knee joint with the knee slightly flexed and relaxed.

The popliteal artery crosses the knee joint deep between the femoral condyles and divides posterior to the tibia in the upper calf to form the three distal arteries: the anterior tibial (AT), the posterior tibial (PT) and the peroneal. The AT passes between the tibia and fibula and runs deep in the anterior compartment to emerge on the dorsum of the foot as the dorsalis pedis (DP) artery where it is easily palpable just lateral to the extensor hallucis longus tendon. The posterior tibial artery runs deep in the posterior compartment of the calf, entering the foot posterior to the medial malleolus where it is easily palpable half way between the medial malleolus and the calcaneus.

During lower limb exercise the blood flow in the femoral vessels may increase many fold and the normal arteries are able to carry this increase flow with ease. Occlusive arterial disease can affect any and many sites in the leg and very often affects the SFA where it pierces the adductor muscles. An arterial stenosis causes the flow of blood to become disordered and less efficient and, if severe enough, a single arterial stenosis can restrict the blood flow during exercise causing relative ischaemia of the muscles which is felt as pain.

The cross-sectional area of the normal SFA must be reduced by more than 70% before these symptoms are produced. A further reduction in area has relatively little effect because of the compensatory effect of small collateral arteries that link the profunda femoris with the popliteal artery. However, if occlusive disease develops at more than one site in the iliac, femoral and distal vessels, the effects become additive. If this happens the severity of the symptoms will increase and the distance that a patient can walk before symptoms develop will decrease. Eventually the compensatory physiological reserve is exhausted such that pain is present even at rest (critical ischaemia) and at this point there is a high risk of ulceration, infection, gangrene and limb loss.

Symptoms/Signs

The cardinal early symptom of occlusive arterial disease is muscle pain that develops during exercise and that is relieved by rest (intermittent claudication). The speed of onset and severity of the pain is related to the degree of exercise and is reproducible from day to day.

The site of the pain gives clues as to the site of the arterial disease:

• Isolated SFA and popliteal disease cause calf muscle claudication



• External iliac, common femoral and profunda femoris disease cause thigh and calf claudication



• Aortic or common iliac disease produce buttock, thigh and calf claudication.

The more sites that are affected the more severe the claudication up to the point of critical ischaemia and rest pain. Rest pain is often worse at night and may prevent sleeping, and the only relief maybe to sleep in a chair with the feet dependent.

The cardinal signs of occlusive arterial disease are due to a reduction in the blood pressure downstream of a segment of an affected artery and this can be detected as a weakening of the peripheral pulses at the groin, knee or in the feet. This low peripheral blood pressure is exacerbated by exercise and this is a useful test in patients with isolated iliac disease. It may cause symptoms during exercise but thay may have subjectively normal pulses at rest.

The objective measurement of the systolic pressure at the ankle is made using a sphygmomanometer and a Doppler ultrasound probe to detect the flow in the foot vessels. Normally the systolic pressure at the ankle is compared with the systolic pressure in the brachial artery to create a ratio called the ankle brachial pressure index (ABPI).

In normal subjects at rest the ABPI is greater than 0.9; values of 0.5-0.9 are consistent with intermittent claudication and patients with critical ischaemia usually have values less than this. In severe ischaemia a more sensitive test is to elevate the leg until the foot blanches (or the Doppler flow in the foot artery disappears) and then measure the height above the bed using 3 cm of elevation as equivalent to 2 mmHg of perfusion pressure. If this is test is positive then placing the foot in a dependent position will produce a reactive hyperaemia and a deep red colour in the foot (Berguer's Sign).

All patients with suspected critical ischaemia should be examined closely for evidence of infection, ulcers and gangrene of the toes, foot and heel. These patients are at high risk of developing pressure sores on the heel which are extremely painful and very difficult to heal.

Complications

Intermittent claudication represents a loss of functional reserve that may threaten a patients' lifestyle, but rest pain and critical ischaemia indicate that the limb is threatened and urgent action is needed to relieve symptoms and, if possible, prevent limb loss.

Critical ischaemia, and to a lesser degree intermittent claudication, are indicators of systemic occlusive arterial disease with a higher than average risk of potentially life threatening cardiac and cerebrovascular complications.

Investigation

The primary investigation for lower limb occlusive artery disease is colour flow Doppler ultrasound (duplex). The duplex scan is non-invasive and can be performed in the outpatient clinic. On a duplex scan the operator can indicate regions of narrowing and occlusion and can record functional information in the form of arterial flow waveforms.

The effect of disease at different sites is associated with characteristic flow waveform patterns and this together with the symptoms, examination findings and ABPI measurements gives a good overall assessment of the peripheral arterial tree.

For more detailed functional information there are a range of tests that involve measurements during or after exercise.

For more detailed anatomical information, in particular the condition of the smaller arteries in the thigh, calf and foot, a diagnostic angiogram is required. Most vascular surgeons will request an up-to-date angiogram if reconstructive surgery is anticipated. However, angiography provides little direct functional information and predicting the functional severity of arterial stenoses from their appearance on an angiogram suffers from a high degree of inter and intra observer variation.

Magnetic resonance angiography (MRA) is a new non-invasive technique that is likely to replace much of the diagnostic angiography.

Treatment

There are three broad approaches to treatment that are usually employed in combination depending on the site and severity of the arterial disease.

1. Conservative treatment aims to minimise the risk factors that cause progression of the occlusive disease and to allow the maximum benefit of the normal physiological compensation mechanisms. Simply put, patients with intermittent claudication are advised to "stop smoking and keep walking". Medical treatment of any co-existing risk factors such as diabetes mellitus, hyperlipidaemia and prophylactic anti-platelet agents are also recommended. This strategy works well for isolated SFA disease because of the good collateral pathway offered by the PFA and its branches. Conservative treatment is less effective for iliac disease and intervention for symptomatic iliac disease is usually required.



2. Endovascular treatment is targeted treatment of isolated arterial lesions performed remotely under local anaesthetic using x-ray or ultrasound guidance. Most commonly this involves a specially trained radiologist manipulating a guide wire across the offending stenosis and dilating the lesion with a special catheter fitted with an inflatable balloon (percutaneous transluminal angioplasty or PTA). The haemodynamic behaviours of an arterial stenosis is such that, if the stenosis severity is reduced to less than 50% of the area of the normal artery then the functional impairment of the stenosis effectively disappears. Short arterial occlusions can often be re-opened by passing a guide wire across though the occluded segment and performing a balloon angioplasty to restore the lumen.



3. An open surgical bypass operation is the definitive treatment for extensive occlusive arterial disease. Arterial bypass procedures are major operations that carry significant risks, particularly as the patients are usually elderly and have co-existing cardiorespiratory disease. Any bypass operation must be planned carefully to ensure that an acceptable balance between risk and benefit is achieved, and for this reason open bypass procedures are only employed if the simpler techniques outlined above are not possible or have not produced the required improvement. The principle of arterial bypass surgery is simple: all functionally significant occlusive disease should be bypassed in order that the bypass graft has adequate inflow and outflow. The objective is to increase the perfusion pressure of the affected tissues so that the normal tissue control of blood flow (autoregulation) is restored.

Outcome/Prognosis

Patients undergoing treatment for symptomatic occlusive arterial disease represent a higher risk group than an age matched control group in terms of potentially life-threatening cardiovascular events. Despite this, a successful arterial bypass is more cost effective and provides a greater improvement in quality of life than a major amputation.

The long-term prognosis for these patients is closely related to the severity of the vascular disease. For patients undergoing bypass procedures for claudication, at 5 years about one third are well, one third have required further surgical intervention and one third have died, usually from cardiac events. If the patient has a bypass for critical ischaemia the prognosis is worse. Despite these risks, the likelihood of having a major amputation for occlusive arterial disease is relatively low.