What is the definition of a high-flow Arteriovenous (AV) fistula?

From the Guidelines

A high-flow arteriovenous (AV) fistula is defined as an abnormal connection between an artery and vein that carries an excessive volume of blood flow, typically exceeding 500 mL/min, which can lead to serious complications such as cardiac overload, steal syndrome, venous hypertension, and aneurysm formation. This condition can occur naturally due to trauma or congenital malformations, or it may be surgically created for hemodialysis access. According to the KDOQI clinical practice guideline for vascular access: 2019 update 1, defining high-flow AV access, its hemodynamic impact, and treatment options are crucial for patient management.

Key Considerations

• High-flow AV fistulas can cause significant morbidity and mortality if left untreated, emphasizing the need for early recognition and management 1.
• Diagnosis typically involves clinical examination, ultrasound with Doppler studies to measure flow rates, and sometimes angiography for detailed imaging.
• Treatment depends on the cause and severity but may include surgical revision, endovascular procedures like embolization or stent placement, or fistula banding to reduce flow.
• For dialysis fistulas specifically, flow reduction procedures aim to maintain adequate dialysis access while preventing complications from excessive flow.

Management

• The KDOQI guideline suggests that it is reasonable to closely monitor and prophylactically manage AV access with high flows to avoid serious or irreversible complications, based on the patient’s individual circumstances and the clinician’s best judgment and discretion 1.
• Close monitoring refers to physical examination and history on routine dialysis rounds and determination of Qa/CO every 6-12 months, or more frequently as needed.
• The choice of treatment should be based on the patient's individual circumstances, the clinician's expertise, and the availability of resources, with the goal of preventing long-term cardiovascular complications and tissue damage.

From the Research

Definition of High-Flow Arteriovenous (AV) Fistula

• A high-flow AV fistula is defined by excessive blood flow that disrupts overall circulatory dynamics, posing a significant challenge, particularly in patients with cardiovascular compromise 2.
• The exact threshold to define high-flow access is not universally accepted, but AVF with a Qa of 1-1.5 L/min or cardio-pulmonary recirculation (Qa/CO) >20% are considered at risk 3.
• High-flow AVF is associated with complications like high output heart failure, pulmonary hypertension, massively dilated fistula, central vein stenosis, dialysis associated steal syndrome or distal hypoperfusion ischemic syndrome 4.
• Indexation of AVF blood flow should be considered in defining high-flow fistula, as the effect of Qa may differ in individuals of different sizes, with a Qa value ≥ 603 ml/min/m2.7 being suggested as a potential threshold 5.

Key Characteristics

• High-output cardiac failure is a well-known phenomenon of high-flow fistula in hemodialysis patients 4.
• The pathophysiology contributing to this disease process is the shunting of blood from the high-resistance arterial system into the lower resistance venous system, increasing the venous return up to cardiac failure 4.
• Accurate and well-timed diagnosis of high flow arteriovenous hemodynamics by monitoring of blood flow of fistula and cardiac function is required to stop this process prior to cardiac failure 4.

Diagnostic Criteria

• A vascular access flow rate of 1 to 1.5 l/min has been suggested as a potential threshold for defining high-flow AVF 3, 4.
• Indexation of AVF blood flow, such as Qa ≥ 603 ml/min/m2.7, may be a more accurate definition of high-flow fistula 5.
• Echocardiographic parameters, such as left ventricular mass, left ventricular diastolic volume, and left atrial volume, may also be used to identify patients at higher risk for high-output cardiac failure 5.