An arteriovenous fistula (AVF) is a direct connection created between an artery and a vein, bypassing the network of tiny blood vessels called capillaries. This unusual passageway alters the normal flow of blood within the circulatory system. Normally, blood moves from high-pressure arteries, through the capillaries for exchange of oxygen and nutrients, and then into the low-pressure veins for return to the heart. The AVF disrupts this sequence, causing high-pressure arterial blood to rush directly into the vein, changing the pressure and volume dynamics.
Defining the Arteriovenous Fistula
Arteries are muscular vessels designed to handle the high pressure generated by the heart pumping oxygenated blood outward. Veins are thinner-walled, low-pressure vessels that carry deoxygenated blood back toward the heart. When an AVF is present, high-pressure arterial blood is immediately diverted into the low-pressure venous system.
This sudden influx of high-pressure, high-volume flow causes the receiving vein to undergo “arterialization,” a structural remodeling. The vein walls thicken, and the vessel diameter enlarges significantly to accommodate the increased blood flow. This enlarged vessel can handle high flow rates, but the rapid shunting of blood increases the total volume returning to the heart, potentially forcing the heart to work harder.
Bypassing the capillary network means tissues downstream from the fistula may receive less blood, oxygen, and nutrients. The severity of the physiological effects depends on the size and location of the fistula. A large fistula, such as one involving the abdominal aorta, can drastically decrease overall circulatory resistance, requiring the heart to increase its output to maintain adequate blood flow elsewhere.
Distinguishing Between Surgical and Pathological Fistulas
Arteriovenous fistulas fall into two categories based on their origin: those created intentionally and those that form unintentionally. The surgical AVF is a deliberate connection made by a vascular surgeon for a specific medical purpose. These are generally created in the limbs, often the arm, to provide reliable, long-term access to the bloodstream.
Pathological fistulas occur due to an unintended cause. They can be congenital, developing during fetal development due to an abnormality in how arteries and veins connect. Acquired fistulas result from external trauma, such as a penetrating injury damaging both an adjacent artery and vein. They can also form following medical procedures if a needle or catheter punctures both vessel types, or due to the erosion of an arterial aneurysm into a nearby vein.
The Role of AVFs in Hemodialysis Access
The most common reason for a surgically created AVF is to establish a safe and durable access point for patients undergoing hemodialysis for end-stage kidney disease. The AVF is the preferred type of vascular access over synthetic grafts or central venous catheters. This preference is due to its superior long-term patency, lower infection rates, and reduced need for maintenance procedures.
The surgical procedure involves creating an anastomosis, or direct connection, between an artery and a vein. This is often done using the radial artery and cephalic vein in the wrist or the brachial artery and cephalic vein in the upper arm. Once established, the vein must go through a period of “maturation” before it can be used for dialysis.
Maturation typically takes several weeks (one to four months), during which the vein arterializes, increasing in diameter and wall thickness. A mature AVF provides the necessary characteristics for effective hemodialysis, including the high flow rate (often at least 600 milliliters per minute) required to filter the blood efficiently. The thickened vein wall can also withstand the repeated needle insertions, known as cannulation. Failure of the fistula to mature requires further intervention or the creation of a new access point.
Treatment Options for Non-Surgical Fistulas
Pathological AVFs that cause health problems must be treated because the shunting of blood can lead to complications such as heart failure, limb swelling, or bleeding. Small, asymptomatic fistulas, particularly those in the limbs, may only require careful monitoring, as some minor ones can close spontaneously. Treatment is necessary when the fistula is large or severely disrupts normal blood circulation.
One common treatment method is endovascular embolization, a minimally invasive procedure. This involves inserting a catheter into a blood vessel and guiding it to the fistula’s location. The connection is then blocked from the inside using materials such as coils or specialized agents to reroute blood flow back through the normal capillary bed. For large or complex fistulas that cannot be closed with a catheter, a traditional surgical approach may be necessary. This involves an open operation to separate the artery and vein and surgically repair the vessel walls.