Peripheral Vascular Disease (cont.)
How does atherosclerosis cause disease?
There are two ways atherosclerosis causes disease; 1)
atherosclerosis can limit the ability of the narrowed arteries to increase
delivery of blood and oxygen to tissues during periods of increased oxygen
demand such as during exertion, or 2) complete obstruction of an artery by a
thrombus or embolus
(thrombus and embolus are forms of blood clots; see below) resulting in tissue
necrosis (death of
tissue). Exertional angina and intermittent claudication are
two examples of insufficient delivery of blood and oxygen to meet tissue demand;
whereas strokes and heart attacks are examples of death of tissue caused by
complete artery obstruction by blood clots.
There are many similarities between coronary artery
diseases (atherosclerosis involving the arteries of the heart) and peripheral
artery disease. For example, patients with exertional angina typically have no
symptoms at rest. But during
exertion the critically narrowed coronary arteries are incapable of increasing
blood and oxygen delivery to meet the increased oxygen needs of the heart
muscles. Lack of blood and oxygen causes
chest pain (exertional
angina). Exertional angina typically subsides when the patient rests. In
patients with intermittent claudication, the narrowed arteries in the lower
extremities (for example, a narrowed artery at the groin) cannot increase blood
and oxygen delivery to the calf muscles during walking. These patients experience pain in
the calf muscles that will only subside after resting.
Patients with unstable angina have critically narrowed
coronary arteries that cannot deliver enough blood and oxygen to the heart
muscle even at rest. These patients have chest pain at rest and are at imminent
risk of developing heart attacks. Patients with severe artery occlusion in the
legs can develop rest pain (usually in the feet). Rest pain represents such
severe occlusion that there is insufficient blood supply to the feet even at
rest. They are at risk of developing foot ulcers and gangrene.
When the arteries are narrowed as a result of
atherosclerosis, blood tends to clot in the narrowed areas, forming a so-called
thrombus (plural thrombi). Sometimes pieces of the thrombi break off and travel
in the bloodstream until they are trapped in a narrower point in the artery
beyond which they cannot pass. A thrombus or piece of thrombus that travels to
another point is called an embolus (pleural emboli). Thrombi and emboli can cause
sudden and complete artery blockage, leading to tissue necrosis (death of
tissue).
For example, complete blockage of a coronary artery by a thrombus
causes heart attack, while complete blockage of a carotid or cerebral artery
causes ischemic stroke. Emboli originating form atherosclerosis in the aorta
(the main artery delivering blood
to the body) can obstruct small arteries in the feet, resulting in painful and
blue (cyanotic) toes, foot ulcers, and even gangrene.
What are collaterals?
Sometimes, despite the presence of a severe blockage in
an artery, the involved area does not become painful or ischemic due to the
presence of collateral circulation, meaning that the particular area is supplied
by more than one artery to an extent that blockage of a single vessel does not result in
a severe degree of ischemia. Collateral circulation can develop over time to
help provide oxygenated blood to an area where an artery is narrowed. Doctors
believe that regular supervised exercise can stimulate the growth and development of collateral circulation
and relieve symptoms of intermittent claudication.
Next: What are potential complications of peripheral
artery disease? »
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