Arteriovenous Fistula
An arteriovenous fistula is an abnormal channel between an artery and a vein.
Normally, blood flows from arteries into capillaries and then into veins. When an arteriovenous fistula is present, blood flows directly from an artery into a vein, bypassing the capillaries. A person may be born with an arteriovenous fistula (congenital fistula), or a fistula may develop after birth (acquired fistula).
Congenital arteriovenous fistulas are uncommon. Acquired arteriovenous fistulas can be caused by any injury that damages an artery and a vein that lie side by side. Typically, the injury is a piercing wound, as from a knife or bullet. The fistula may appear immediately or may develop after a few hours. The area can swell quickly if blood escapes into the surrounding tissues.
Some medical treatments, such as kidney dialysis, require that a vein be pierced for each treatment. With repeated piercing, the vein becomes inflamed and clotting can develop. Eventually, scar tissue may develop and destroy the vein. To avoid this problem, doctors may deliberately create an arteriovenous fistula, usually between an adjoining vein and artery in the arm. This procedure widens the vein, making needle insertion easier and enabling the blood to flow faster. Faster flowing blood is less likely to clot. Unlike some large arteriovenous fistulas, these small, intentionally created fistulas do not lead to heart problems, and they can be closed when no longer needed.
Symptoms and Diagnosis
When congenital arteriovenous fistulas are near the surface of the skin, they may appear swollen and reddish blue. In conspicuous places, such as the face, they appear purplish and may be unsightly.
If a large acquired arteriovenous fistula is not treated, a large volume of blood flows under high pressure from the artery into the vein network. Vein walls are not strong enough to withstand such high pressure, so the walls stretch and the veins enlarge and bulge (sometimes resembling varicose veins). In addition, blood flows more freely into the enlarged veins than it would if it continued its normal course through the arteries. As a result, blood pressure falls. To compensate for this fall in blood pressure, the heart pumps more forcefully and more rapidly, thus greatly increasing its output of blood. Eventually, the increased effort may strain the heart, causing heart failure. The larger the fistula, the more quickly heart failure can develop.
With a stethoscope placed over a large acquired arteriovenous fistula, doctors can hear a distinctive "to-and-fro" sound, like that of moving machinery. This sound is called a machinery murmur. Doppler ultrasonography is used to confirm the diagnosis and to determine the extent of the problem. For fistulas between deeper blood vessels (such as the aorta and vena cava), magnetic resonance imaging (MRI) is more useful.
Treatment
Small congenital arteriovenous fistulas can be cut out or eliminated with laser coagulation therapy. This procedure must be performed by a skilled vascular surgeon, because the fistulas are sometimes more extensive than they appear to be on the surface. Arteriovenous fistulas near the eye, brain, or other major structures can be especially difficult to treat.
Acquired arteriovenous fistulas are corrected by a surgeon as soon as possible after diagnosis. Before the surgery, a radiopaque dye, which can be seen on x-rays, may be injected to outline the fistula more clearly in a procedure called angiography (see Section 3, Chapter 21). If the surgeon cannot reach the fistula easily (for example, if it is in the brain), complex injection techniques that cause clots to form may be used to block blood flow through the fistula. For example, coils or plugs may be inserted into the fistula at the various points where the vein and the artery meet. This procedure is performed using x-rays for guidance and does not require open surgery.
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