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The arms and legs rely on a good blood supply in order to function – in particular, the leg muscles have a high requirement for oxygen and do not tolerate a reduction in blood supply. If the blood supply to a limb is slowly blocked off over a matter of months or years, so-called 'collateral' blood vessels may be able to take on the work, thus allowing the muscles some degree of blood supply. This manifests as intermittent claudication, where muscle pain is experienced above a certain level of exercise. However, if the blood supply is suddenly cut off due to complete blockage of the main vessel, there is no time for collateral vessels to develop. The result is acute limb ischaemia: a sudden onset of a pale, painful, cold limb that requires urgent treatment.
What Causes It?
Atherosclerosis is a disease of the inner lining of blood vessels whereby a plaque of fatty material accumulates and harden on the artery wall. This is the cause of most heart attacks, as the coronary arteries are gradually blocked off by plaques, and is also the cause of some strokes due to disease of the blood vessels supplying the brain. The aorta can be affected, causing weakening of its wall and thus the fomration of an atherosclerotic aneurysm. Unsurprisingly, this disease actually affects all arteries, including those in the arms and legs. As atherosclerosis is accelerated by an unhealthy diet and smoking, it provides a good advertisement for a healthy diet.
Thus, the majority of cases of intermittent claudication and acute limb ischaemia are due to narrowing of the limb arteries due to fatty plaque formation. As with the heart and brain, acute ischaemia of the limb occurs if the plaque suddenly ruptures, blocking the vessel.
It is also possible for a free-floating clot (embolus) to drift along an artery and become stuck in the narrowing. These clots are usually from the left atrium of the heart – clot formation here occurs if the heart is beating either weakly or in the wrong rhythm, as this allows blood to sit and stagnate in the atrium.
An acutely ischaemic limb requires urgent treatment, else it will eventually become all of the six 'P's: pale, painful, pulseless, paraesthetic (pins and needles), paralysed and perishingly cold. A dose of anti-clotting medication should be given provided there is no underlying bleeding risk. When a limb has lost both feeling and movement, surgery is required urgently to restore blood supply. If this does not occur, surgery will instead be required to amputate the affected part. Sometimes restoration of blood supply causes the limb to swell up, requiring further surgery to relieve the pressure inside the muscle compartments.
In the case of atherosclerotic arteries, it is necessary to create a bypass around the obstruction using either veins borrowed from elsewhere, or synthetic graft material. In severe intermittent claudication, such surgery is preceded by a scan using a contrast material to show where the artery is obstructed. However, in acute limb ischaemia there is little time, and the contrast will only show the start of the blockage as there are no collateral vessels leading to the other end.
An embolus blocking the artery is sometimes so inaccessible as to require treatment with a clot-busting drug (which comes with a risk of bleeding and bruising), but can usually be removed from the artery surgically. Originally this was done by opening the artery at the site of the obstruction and pulling out the embolus by hand. However, an American surgeon by the name of Thomas J Fogarty became distressed by the poor survival rate following such procedures. The result was the Fogarty catheter: a tube with an inflatable balloon at the end. The catheter can enter the artery through a small cut at the groin and be fed down past the clot; once the balloon has passed through the blockage, it is inflated and then pulled back out, bringing the embolus with it. It is estimated that this apparently simple idea has saved millions of lives and limbs.
Fatty plaques will cause the arteries to narrow slowly over time and cause intermittent pain on exercise, but a plaque rupture or embolus of a clot from the heart can fully obstruct an artery suddenly, leading to a limb that is cold, pale, painful and liable to be lost unless the blood supply can be restored quickly.
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