The circulatory system relies on a precise and orderly flow of blood. Arteries carry oxygen-rich blood from the heart under high pressure into a network of tiny capillaries. Here, the exchange of oxygen and nutrients for waste products occurs in the body’s tissues. Finally, low-pressure veins collect this deoxygenated blood and return it to the heart.
An arteriovenous fistula (AVF) is an abnormal, direct connection between an artery and a vein that disrupts this circuit. This linkage allows blood to bypass the capillary network, creating a “short-circuit” in the circulatory system. This causes blood to flow from a high-pressure artery into a low-pressure vein, altering blood flow and affecting the heart.
The Different Origins of Arteriovenous Fistulas
An arteriovenous fistula can be congenital, acquired through injury, or intentionally created for medical purposes. These abnormal connections can develop from several distinct causes and exist almost anywhere in the body.
Congenital fistulas are present from birth due to errors during fetal development when blood vessels do not properly differentiate. These are uncommon and may not show symptoms until later in life, as a low-flow fistula can evolve into a high-flow lesion.
Acquired fistulas most often result from traumatic injuries, like a deep cut or puncture wound. Such an injury can damage an adjacent artery and vein, causing them to heal together improperly and form a connection. This type of traumatic AVF represents about 4% of such cases.
Iatrogenic fistulas are created surgically for a medical need, most frequently to create a durable access point for hemodialysis in patients with kidney failure. The surgically created AVF provides a high-flow vessel that can withstand repeated needle insertions, allowing the dialysis machine to efficiently clean the blood.
Cardiovascular Impact
An AVF establishes a low-resistance pathway that alters cardiovascular dynamics. By acting as a shortcut, it shunts arterial blood directly into the venous system, bypassing the high-resistance capillaries. This diversion causes a sharp increase in blood returning to the heart, a condition known as increased venous return or preload.
To manage this surge in blood volume, the heart must work harder. It compensates by pumping more forcefully and rapidly, leading to a substantial increase in cardiac output. Over time, this chronic volume overload places strain on the heart, causing the chambers to stretch and enlarge, a change that can begin within days.
This overwork can lead to high-output heart failure. In this condition, the heart pumps an exceptionally large volume of blood, yet it may fail to meet the body’s metabolic needs. A significant fraction of blood is caught in a non-productive loop, cycling through the fistula instead of delivering oxygen to tissues. This inefficiency leads to symptoms like shortness of breath and fatigue.
The fistula’s location influences the degree of cardiac strain. Fistulas created more proximally, such as in the upper arm, have much higher flow rates than those located distally in the wrist. The higher flow from proximal fistulas places a greater burden on the heart, increasing the risk of high-output heart failure.
Diagnostic Process
A physical examination can reveal characteristic signs of an AVF. A clinician may feel a palpable, buzzing vibration over the fistula, a sensation known as a “thrill.” Using a stethoscope, a doctor can also listen for a continuous, whooshing sound called a “bruit,” caused by turbulent blood flow.
If a fistula is suspected, non-invasive imaging is used to confirm the diagnosis. Duplex ultrasound is the most common initial test, using sound waves to image the fistula and measure blood flow. This allows clinicians to assess the size and significance of the shunt.
For more detailed mapping, especially when planning a procedure, advanced imaging like CT angiography (CTA) or MR angiography (MRA) may be used. These tests use a contrast dye to create detailed, three-dimensional images, revealing the fistula’s precise location and structure.
In some cases, an invasive test called conventional angiography is performed. This procedure, often considered the gold standard, involves inserting a catheter into an artery and guiding it to the fistula. A contrast dye is then injected, and a series of X-ray images provides a real-time map of the blood flow for planning treatments.
Management and Treatment Approaches
The management strategy for an AVF depends on its size, location, symptoms, and impact on heart function. For small, asymptomatic fistulas that do not strain the heart, watchful waiting with regular monitoring is often sufficient.
For fistulas requiring intervention, endovascular repair is a common, minimally invasive approach. Using catheter-based techniques, a specialist guides instruments to the fistula to deploy materials like metal coils or a covered stent graft, which seals the abnormal connection from within.
Open surgery is another option, particularly for complex fistulas or when endovascular methods are not suitable. A surgeon makes an incision to directly access the vessels and can either tie off the connection (ligation) or reconstruct the artery and vein.
For dialysis fistulas that cause high-output heart failure, the goal is to reduce flow rather than close the connection completely. A surgeon can perform a “banding” procedure, placing a band around the fistula to narrow its diameter. This reduces the shunting blood flow and the heart’s workload while preserving the fistula for dialysis.