Case Studies In Small Animal

Cardiovascular Medicine

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Case 28

Arteriovenous Fistula Section from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

Arteriovenous Shunts (Fistulas)

An arteriovenous (AV) shunt (fistula) is an abnormal vascular communication that allows blood to flow from an artery to a vein without traversing a capillary bed. Extracardiac arteriovenous shunts may be congenital, which occur as single or multiple communications, or acquired, which are typically single communications. The most common form of a congenital arteriovenous fistula in dogs and cats is a patent ductus arteriosus. Acquired AV shunts are usually the result of blunt or penetrating trauma or may result from neovascularization associated with neoplastic growths. While they typically involve the extremities, AV shunts can occur anywhere in the body.

The local and systemic effects of AV shunts result from the shunting of blood from the high resistance systemic arterial system into the low resistance systemic venous system. Regionally, venous blood is hyperoxygenated and may cause proliferation in the affected vessels and so increase the number of AV communications. The effected region may feel warmer than surrounding areas, may be edematous and swollen, and may be painful. Dermatologic abnormalities such as necrosis, ulceration, and abnormal pigmentation in the skin overlying the region have also been reported. Turbulence within the communication results in a continuous bruit (murmur) and fremitus (palpable thrill).

Systemically, the continuous shunting of blood, if of significant volume, may lead to a generalized volume overload with increased blood volume and eccentric hypertrophy of all cardiac chambers. Typically systolic function is maintained and is often hyperdynamic. With very large shunts, the increase in end-diastolic pressure may be enough to produce signs of congestive heart failure. Cardiac output is typically elevated, such that the clinical syndrome may be called high-output cardiac failure. Ultimately, systolic myocardial failure may develop further exacerbating the signs of congestive heart failure. Signs of right heart failure may predominate, although, pulmonary edema is also usually present.

The ideal therapy is to surgically ligate the communicating vessels. However, surgical correction may be limited by multiple microscopic connections and the physical location of the shunt. In patients with congestive heart failure the use of diuretics may be palliative.

  

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.