An arteriovenous fistula is an abnormal channel or passage between an artery and a vein. An arteriovenous fistula is a disruption of the normal blood flow pattern. Normally, oxygenated blood flows to the tissue through arteries and capillaries. Following the release of oxygen in the tissues, the blood returns to the heart in veins. An arteriovenous fistula is an abnormal connection of an artery and a vein. The blood bypasses the capillaries and tissues, and returns to the heart. Arterial blood
has a higher blood pressure than veins and causes swelling of veins involved in a fistula. Although both the artery and the vein retain their normal connections, the new opening between the two will cause some arterial blood to shunt into the vein because of the blood pressure difference.
Arteriovenous fistulae (AVF) are characterized by abnormal shunting of blood between the arterial system and the venous system, without the presence of a normal intervening capillary bed. Because the capillary bed represents the source of resistance to blood flow in the circulatory system, AVF are low resistance, high flow lesions. They represent a group of disorders that are characterized by abnormal shunting from the ECA, ICA, vertebrobasilar system, or spinal arteries into normal or abnormal venous channels. The primary treatment is often endovascular because of the feasibility of catheterizing the lesion directly and placing embolic material directly at the site of the abnormal arteriovenous connection. For purposes of this practice guideline, Arteriovenous malformations (AVMs) will be considered as a separate entity and are discussed in a separate guideline. Additionally, spinal lesions and lesions contained entirely in the extracranial space will be separately addressed, although many principles of the treatment of fistulae with cerebral components also apply to the treatment of all fistulae.
There are two types of arteriovenous fistulas, congenital and acquired. A congenital arteriovenous fistula is one that formed during fetal development. It is a birth defect. In congenital fistulas, blood vessels of the lower extremity are more frequently involved than other areas of the body. Congenital fistulas are not common. An acquired arteriovenous fistula is one that develops after a person is born. It usually occurs when an artery and vein that are side-by-side are damaged and the healing process results in the two becoming linked. After catheterizations, arteriovenous fistulas may occur as a complication of the arterial puncture in the leg or arm. Fistulas also form without apparent cause. In the case of patients on hemodialysis, physicians perform surgery to create a fistula. These patients receive many needle sticks to flush their blood through dialysis machines and for routine blood analysis testing. The veins used may scar and become difficult to use. Surgery is used to connect an artery and vein so that arterial blood pressure and flow rate widens the vein and decreases the chance of blood clots forming inside the vein.
The main symptoms of arteriovenous fistulas near the surface of the skin are bulging and discolored veins. In some cases, the bulging veins can be mistaken for varicose veins. Other fistulas can cause more serious problems depending on their location and the blood vessels involved. 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.
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. 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.