Page 126 - Libro vascular I

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DUPLEX ASSESSMENT OF LOWER LIMB ARTERIAL DISEASE
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Figure 9.5 The appearance of critical lower limb
The position of the obstruction can be variable. There are two main causes of acute ischemia.
First, acute thrombosis of an existing arterial lesion, a so-called acute-on-chronic occlusion, can occur when the blood flow across a diseased seg- ment of an artery is so slow that it spontaneously thromboses. Long segments of an artery may occlude in this situation. Acute ischemia is more likely to occur if the collateral circulation around the disease is poorly developed. Occasionally, patients have predisposing coagulation disorders that lead to spontaneous arterial thrombosis.
Second, an embolus may be released from other areas of the body, such as the heart or from an aneurysm, which then blocks an artery in the extremity. An embolus frequently obstructs bifurca- tions such as the common femoral bifurcation or distal popliteal artery and tibioperoneal trunk. Another example is obstruction of the aortic bifur- cation by an embolus projecting down both CIA origins, referred to as a saddle embolus. The body has very little time to develop collateral circulation around embolic occlusions, and the limb may be very ischemic.
The symptoms of acute ischemia are of rapid onset, and the patient classically presents with a cold, painful, pulseless, paresthetic leg. In this situa- tion, emergency intervention by surgical embolec- tomy, bypass surgery or thrombolysis should be performed, provided that the patient is fit enough for treatment. Left untreated, acute ischemia can lead to muscle death or necrosis. This can cause swelling of the calf muscle, and eventually the sac, or fascia, surrounding the muscles will restrict any further swelling, leading to a pressure increase within the muscle compartments. This is known as a compartment syndrome, and the increased intra- compartmental pressure can further exacerbate the muscle ischemia. If limb salvage is possible, surgical splitting of the fascia, called a fasciotomy, may be required to release the excess pressure.
Severe muscle ischemia can produce toxins caus- ing systemic symptoms that can lead to organ failure and death. An urgent amputation is usually per- formed if there is no viable option to restore blood flow to the limb. Acute ischemia can also occur due to microembolization to the foot, leading to occlu- sion of the small vessels. The microemboli can origi- nate from the heart, from atherosclerotic plaques or
ischemia with gangrene of the small toe.
(Fig. 9.5). The European Working Group (1992) on critical limb ischemia (CLI) defined CLI as:
... persistently recurring ischaemic rest pain requir- ing regular analgesia for more than two weeks, with an ankle systolic pressure of 50mmHg and/or a toe systolic pressure of 30 mmHg; or ulceration or gangrene of the foot or toes, with ankle systolic pressure of 50 mmHg and/or a toe systolic pressure of 30 mmHg.
This may be a strict definition of CLI, as patients with ulceration are frequently seen in the vascular laboratory with ankle pressures above 50 mmHg. The treatment of lower limb ischemia includes angioplasty or arterial bypass grafting. Unfortunately some patients are not suitable can- didates for any form of limb salvage, and amputa- tion is the inevitable outcome.
Acute ischemia
Acute ischemia, as the name suggests, is due to sud- den arterial obstruction in the lower limb arteries.