Arteriovenous fistula is an abnormal communication between an artery and vein bypassing the capillary network. The true incidence is unknown, although A-V fistulas are reported uncommonly in dogs and rarely in cats. Centrally located anomalies may occur between the cardiac chambers (e.g. ventricular septal defect) or between the great vessels (e.g. aorticopulmonary window, patent ductus arteriosus) and are reviewed in posts. In this section, only peripheral A-V malformations (arteriovenous fistulas) are discussed.
Congenital A-V fistulas are rare and are caused by arrest or misdirection of embryologic vascular differentiation. The persistence of primitive equipotential capillaries and the subsequent failure of the existing anastomotic embryologic channels to differentiate into arteries or veins are responsible for their abnormal communications. Terminology applied to congenital arteriovenous fistulas has been a source of confusion (eg. hemangioma racemosum, Park-Weber syndrome, heman-gioma cavernosum, strawberry birthmark, nevus angiectasis, cirsoid aneurysm, congenital A-V aneurysm).
The most common cause of an acquired arteriovenous fistula is blunt or penetrating trauma. Iatrogenic trauma is often related to venipuncture and accidental perivascular injection of irritating medicaments. Additional causes include neovascularized tumors (carotid body tumor, thyroid tumor, hemangiosarcoma), ruptured aneurysm, mass ligature of arteries and veins, gunshot wounds, and erosion of contiguous vessel walls by infection or arteriosclerosis. Most A-V fistulas involve the extremities, but they can occur anywhere in the body including the neck, spinal cord, body wall, head, brain, abdomen, liver, and lung. They can also occur as a complication of mass ligation of arteries and veins during closed castration.
Irrespective of cause and location, arteriovenous fistulas cause similar altered blood flow dynamics. The basic principle is that blood follows the path of least resistance. The A-V malformation is an abnormal connection between a high-pressure arterial system and a low-pressure venous system with a higher capacity. Due to the difference in resistance, blood flows preferentially through the fistula rather than continuing via the artery to the capillary bed. This shunting results in a reduction of flow in the artery distal to the fistula and increased flow in the venous system. In addition to stealing blood from the distal capillary bed, these changes lead to the development of collateral circulation around the fistula. The communicating shunt increases venous pressure, venous blood oxygen saturation, and the diameter and number of anastomotic arteries and veins. Eventually an ectasia of the feeding artery and aneurysmal sacs in the venous area of the A-V communications may develop. Histologic changes occur in affected vessels so that arteries begin to look more like veins (venification) and veins like arteries (arterialization). The rapid runoff of blood through the arteriovenous fistula into the capacitant venous circulation causes turbulence and, potentially, sound (bruit).
Shunted blood flow through a large fistula can be so great that retrograde flow from a distal artery into the fistula occurs. Arterial blood supply to the tissues distal to the A-V fistula may become compromised from competition with the fistula and secondary venous hypertension, resulting in stagnation of venous blood flow in the dilated veins, or even reversal of blood flow in a distal rather than proximal direction. Local edema and ischemia foDowed by tissue necrosis, ulceration, or organ dysfunction may ensue. In large and mostly centrally located fistulas, the central blood dynamics also become affected. Large flow volumes across the A-V fistula compromise the blood supply to other regions of the body by shifting blood into the capacitance venous circulation. Compensatory responses to increased blood volume (cardiac preload) include elevated heart rate, cardiac contractility, and eccentric cardiac hypertrophy. Augmented cardiac output and gradual expansion of the blood volume partially restore the blood supply to deprived tissues. Eventually, however, the increased venous return and the elevated cardiac workload may induce high-output heart failure.
Clinical history and signs vary according to the location, size, duration, etiopathology, and topography of the fistula. Arteriovenous fistulas of the extremities may appear as painless, easily compressible, warm bulges or varicosities. Sometimes they are detected incidentally during routine clinical examination. With medium-sized or large fistulas of the extremities, a continuous palpable thrill and pulsation, along with a machinery murmur, can be detected. The leg or region distal to the fistula may be swollen, warmer, or colder than the proximal area (sometimes with severe local ischemia), painful, and affected by pitting edema. Lameness, cyanosis, therapy-resistant toe ulcers, scab formation, or gangrene can occasionally occur distal to the fistula with severe local ischemia. In the dilated superficial veins, a faint pulsation may be felt. In the feeding arteries a water-hammer pulse is often present. Animals with high-output heart failure may develop a cardiac murmur of mitral regurgitation and moist lung sounds, indicating pulmonary edema.
When firm pressure is applied proximal to the arteriovenous fistula, or if the feeding artery is compressed, the thrill and bruit disappear and the pulse, and heart rate may drop as a result of diminished venous return and reduced cardiac output. This is referred to as Branham’s bradycardia sign, and the maneuver is called Branham’s or Nicoladoni-Branham’s test. Together with the local thrill or fremitus and the bruit, the Branham’s sign is pathognomonic for arteriovenous fistulas.
Other signs have been reported and vary with respect to location and hemodynamic alterations caused by the arteriovenous fistula. An A-V fistula in the orbit was reported to cause exoph-thalmos. Recurrent bleeding from the mouth was observed in an A-V fistula of the tongue. Restless behavior and lethargy followed by progressive seizures and hemiparesis were reported in a 10-year-old mixed breed dog with a subcortical cerebral A-V malformation. An arteriovenous fistula of the spinal cord at the thoracolumbar junction in a 10-month-old female Australian shepherd dog caused a deteriorating hindlimb ataxia, difficulty in getting up and down, hypersensitivity over the lumbosacral region, and urinary incontinence. Cardiomegaly and pulmonary overcirculation were detected radiographically in a dog with a postsurgical AV fistula, and these changes regressed after the fistula was surgically closed. An A-V fistula was found surgically in the jejunum of an 8-month-old dog with persistent anemia and melena. A-V fistulas in tumors have been recognized as a strong pulsation and fremitus detectable during palpation of the tumor mass Auscultation of the tumor mass may reveal a machinery-type murmur or bruit.
Congenital hepatic arteriovenous fistulas have been reported in young dogs. The hepatic artery connects to the portal vein and causes portal hypertension or reversal of the blood flow in the portal vein. There can be a single fistula or multiple fistulas that can be distinctly visualized by abdominal color Doppler ultrasonography and coeliac arteriography. Clinical signs include lethargy, vomiting, diarrhea, anorexia, weight loss, ascites, polyuria, and neurologic signs related to hepatic encephalopathy.
Pulmonary arteriovenous fistulas can lead to respiratory distress and cyanosis. Survey thoracic radiographs of animals with large fistulas may indicate cardiomegaly, a prominent aortic arch, a hypervascular lung field, and an increased interstitial or patchy pulmonary radiodensity, indicating pulmonary congestion or edema, respectively.
Diagnostic ultrasonography, especially color Doppler imaging, has been clinically useful for identifying and localizing A-V fistulas. Findings included the presence of focal intra-arterial flow at the fistula site throughout the cardiac cycle, turbulent pulsatile venous flow at the fistula site, and nonpulsatile venous flow proximal to the fistula. Angiography may be used to define fistula anatomy and size more accurately and to plan management strategies. The absence of a normal capillary phase and premature outlining of the veins are typical arteriographic findings in shunting lesions. The differential diagnoses of arteriovenous fistulas include neoplasms, varices, abscesses, cystic skin lesions, aneurysms, lymphedema, and scarring.
Management of A-V fistulas has historically required surgical intervention. This usually involves careful ligation of all proximal and distal arteries feeding the fistula, ligation of the draining veins, and complete excision of the A-V collaterals with the venous aneurysmal sacs (so called quadruple ligation). If relapse occurs or if multiple fistulas are present, a wide excision of the involved areas or limb amputation may be the only alternative.
Surgical approaches can often be technically difficult, mutilating, hazardous, and can have a high failure rate with residual clinical symptoms. Consequently, arterial embolization techniques using special embolization wire coils have begun to be used to occlude arteriovenous fistulas, owing to the minimally invasive nature of these procedures. In a cat with a forelimb fistula, cyanoacrylate embolization was used to obliterate the fistula followed by en bloc surgical resection of vessels. Transcatheter embolization using vasoocclusive coils was described in three cats and two dogs with peripheral A-V fistulas.
The prognosis for animals with small arteriovenous fistulas is often good. Morbidity is highest with centrally located A-V fistulas, with fistulas that cause organ dysfunction, with large fistulas that induce a high cardiac output, and with fistulas that cannot be successfully occluded using embolization techniques.