An arteriovenous shunt, also referred to as an arteriovenous fistula, is a connection between an artery and a vein that allows blood to flow between the two without first going through the capillaries. There are a number of reasons that one can form. It can be congenital, develop as a side effect of surgery or trauma, or can be purposely created as a part of a medical treatment.
In normal anatomy, oxygenated blood travels through the arteries, which branch into smaller and smaller vessels, eventually reaching the capillaries. These tiny vessels are only large enough for red blood cells to squeeze through one at a time, and allow for the transfer of oxygen from the blood into the cells. The capillaries then carry blood cells into veins, which bring blood back to the heart and lungs. If a patient has an arteriovenous shunt, however, the capillary system never receives blood cells, or receives a reduced number of them because the artery and the vein are directly connected through an aberrant blood vessel.
In most cases, a shunt does not create a major problem for a patient. Though the blood flow into capillaries is bypassed and the cells in the bypassed region do not receive as much oxygenated blood as normal, these types of shunts usually occur only between small arteries and veins and do not create much damage. If there are large arteries and veins involved, however, the connection can require surgery. Restoring blood flow may be necessary if some tissue is at risk due to lack of oxygen, which can cause the tissue to die.
An arteriovenous shunt can cause a number of symptoms, including swelling and redness, if it is causing a problem for the patient. Veins may swell up, as can the tissue around the shunt, which can look similar to varicose veins. This swelling is most common in the legs and can occur as a result of an injury or trauma. Occasionally, a life threatening fistula can appear in the lungs and require prompt medical treatment.
Patients with certain medical conditions, including kidney failure, may receive a surgically created arteriovenous shunt as a part of their medical treatment. With kidney failure, the shunt makes it easier for a patient to receive dialysis treatment, which is necessary while the patient is waiting for a kidney transplant. Connecting a vein to an artery widens the vein, increases blood flow, and makes it easier for the patient to receive dialysis.