Arteriovenous fistulas, often called “malformations,” may be congenital or acquired. Abnormal communications between arteries and veins occur in many diseases and affect vessels of all sizes and in many locations. In congenital fistulas, the systemic effect is often not great, because although the communications may be multiple, they are small. The effects of AV fistulas depend upon their size and location—for example, when a limb is involved, extensive A-V communications may exist 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. These communications can have considerable flow and high-output heart failure can occur. Surgically created fistulas for hemodialysis access are a unique class of AF fistulas.
Arteriovenous malformations in the gastrointestinal tract may cause hemorrhage. Osler-Weber-Rendu disease or syndrome (also termed hereditary hemorrhagic telangiectasia) is an autosomal disorder characterized by gastrointestinal bleeding and epistaxis due to large arteriovenous anomalies in the gastrointestinal tract and lungs. Pulmonary lesions cause recirculation with lower PO2, polycythemia, clubbing, and cyanosis.
Penetrating injuries either from trauma or iatrogenic ones from arterial punctures are the most common causes of acquired fistulas. Blunt trauma, erosion of an atherosclerotic or mycotic arterial aneurysm into adjacent veins, communication with an arterial prosthetic graft, or neoplastic invasion can all cause AV fistulas as well. When large vessels are involved, the presentation is dramatic. These large fistulas enlarge rapidly and result in cardiac dilation and failure when shunting is excessive.
AV Fistula for Hemodialysis
A successful AV fistula for hemodialysis access requires a large vein that lies close to the skin. The cephalic vein is ideal for this purpose, and radial artery to cephalic vein AV fistula (Cimino fistula) is the classic hemodialysis access fistula. If no suitable vein is available for an autogenous fistula, prosthetic grafts are used, most commonly of PTFE. These are most commonly placed subcutaneously in a loop configuration. The poor patency rates of these grafts, 40% at 2 years, and potential for infection, have driven national guidelines to encourage a higher rate of autogenous fistula formation. To maximize autogenous vein utilization, current practice includes transposing deep veins such as the basilic vein in the upper arm to the subcutaneous tissue. All veins used for access require “arterialization” of the wall, which takes at least 6 weeks prior to cannulization for dialysis. Transposed veins may take even longer to mature.
Flow rates of 300 cc/min or greater are necessary for efficient dialysis. Patients with a newly created AV access should be watched carefully for arterial steal and distal ischemia. Diabetics are the most vulnerable to this complication due to calcified proximal arteries or intrinsic arterial lesions of the hand.
Symptoms and Signs of Arteriovenous Fistulas
A typical continuous machinery murmur can be heard over most acquired arteriovenous fistulas and is often associated with a palpable thrill and locally increased skin temperature. Proximally, the arteries and veins dilate and the pulse distal to the lesion diminishes. There may be signs of venous insufficiency, coolness, and hypertrophy distal to the communication on the involved extremity. Tachycardia occurs in some patients as a feature of increased cardiac output. When the fistula is occluded, the pulse rate slows (Branham’s sign).
In contrast, venous malformations rarely produce hemodynamic effects. Because flow rates are low, bruits and thrills are absent.
MRI has become the imaging study of choice for the follow-up of peripheral arteriovenous malformations, but CTA also gives excellent anatomic information. Precise delineation of arteriovenous fistulas can be done with selective arteriograms.
Treatment of Arteriovenous Fistulas
Not all arteriovenous connections require treatment. Most venous malformations should be treated conservatively. In addition, small peripheral fistulas may be observed and will often remain asymptomatic. Some are surgically inaccessible.
The indications for intervention include hemorrhage, local expansion, severe venous or arterial insufficiency, cosmetic deformity, and heart failure.
Most fistulas are now managed by embolization under radiographic control. The embolic material used includes blood clot, glass beads, Gelfoam, and muscle. Arteriovenous malformations of the head and of the pelvis appear particularly well suited for this form of therapy. Direct injection of sclerosant compounds into venous malformations under fluoroscopic control also has been successful.
Surgical options are generally reserved for large acquired fistulas. Covered stent grafts are now being used for a variety of traumatic fistulas. Surgery should be avoided in most cases of congenital arteriovenous malformations, as cure requires en bloc resection of all tissue involved. When the fistulous connections involve substantial portions of an extremity, local ligation is invariably followed by recurrence, and only temporary palliation can be expected.
The results of therapy vary according to the extent, location, and type of fistula. In general, traumatic fistulas have the most favorable prognosis. Congenital fistulas are more difficult to eradicate, due to the numerous arteriovenous connections usually present.