Arteriovenous fistulas, often called "malformations," may be congenital or acquired. Abnormal communications betweenarteries and veins occur in many diseases and affect vessels of all sizes and in many locations. In congenital fistulas, thesystemic effect is often not great, because although the communications may be multiple, they are small. The effects ofAV fistulas depend upon their size and location—for example, when a limb is involved, extensive A-V communications maexist with increased flow and increased muscle mass and bone length.

Acquired fistulas are usually the result of trauma, violent or iatrogenic. Spontaneous fistulas are uncommon. Thesecommunications can have considerable flow and high-output heart failure can occur. Surgically created fistulas forhemodialysis access are a unique class of AF fistulas.

Arteriovenous malformations in the gastrointestinal tract may cause hemorrhage. Osler-Weber-Rendu syndrome is anautosomal dominant disorder characterized by gastrointestinal bleeding and epistaxis due to large arteriovenous anomaliein the gastrointestinal tract and lungs. Pulmonary lesions cause recirculation with lower P02, polycythemia, clubbing, anccyanosis.

Penetrating injuries either from trauma or iatrogenic ones from arterial punctures are the most common causes of acquiretfistulas. Blunt trauma, erosion of an atherosclerotic or mycotic arterial aneurysm into adjacent veins, communication withan arterial prosthetic graft, or neoplastic invasion can all cause AV fistulas as well. When large vessels are involved, thepresentation is dramatic. For example, an aortic aneurysm can rupture into the inferior vena cava. These large fistulasenlarge rapidly and result in cardiac dilation and failure when shunting is excessive.