What are the most common causes of frequent clotting of arteriovenous fistulas and synthetic grafts in hemodialysis patients with end‑stage renal disease?

Causes of Frequent Arteriovenous Fistula and Graft Thrombosis

The primary cause of frequent clotting in both arteriovenous fistulas (AVFs) and grafts (AVGs) is unrecognized hemodynamically significant stenosis, which accounts for approximately 90% of access thromboses. 1

Primary Anatomic Cause: Venous Stenosis

Hemodynamically significant stenosis is the dominant mechanism driving thrombosis in dialysis access. The rate of thrombosis is largely determined by the presence of unrecognized stenotic lesions that progress undetected. 1

• Venous anastomotic stenosis is the most common site of stenosis in AVGs, caused by intimal hyperplasia at the graft-vein junction 2, 3, 4
• Each fistula type has characteristic stenosis locations related to significant angulation, which creates turbulent flow and intimal injury 2
• Early detection and treatment of stenosis reduces thrombosis frequency by up to 70% and significantly lowers access replacement rates 1

Secondary Non-Anatomic Causes

While stenosis accounts for 90% of cases, the remaining 10% of thromboses result from: 1

• Hypotension post-hemodialysis - reduced perfusion pressure leads to stasis and clot formation 1
• Hypercoagulable states - underlying thrombophilia increases clotting risk 1
• Decreased cardiac output - inadequate flow through the access circuit 1
• Access site infection - inflammatory changes promote thrombosis 1

Differential Risk Between Access Types

Synthetic grafts thrombose at twice the rate of native fistulas:

• Target thrombosis rate for AVGs: ≤0.5 episodes per patient-year 1
• Target thrombosis rate for AVFs: ≤0.25 episodes per patient-year (after initial 2-month maturation period) 1
• Native fistulas have inherently lower thrombosis rates compared to prosthetic grafts 1

Surveillance Strategy to Prevent Thrombosis

Systematic monitoring for stenosis is essential to prevent thrombosis:

• Physical examination should assess for adequate thrill, signs of stenosis, and abnormal findings at each dialysis session 5, 6
• Sequential flow measurements can detect deterioration of access function before thrombosis occurs 1
• Intra-access pressure monitoring identifies hemodynamically significant stenosis 6
• Doppler ultrasound confirms stenosis location and severity when clinical suspicion exists 6, 2

Critical Pitfall to Avoid

The most common error is failure to detect and treat stenosis before thrombosis occurs. Dialysis centers should maintain thrombosis rates below the guideline thresholds (0.5 for grafts, 0.25 for fistulas) through ongoing quality assurance programs that examine thrombosis rates and underlying causes. 1 When thrombosis occurs more than twice within a single month, or when correctable stenosis recurs, vascular surgery consultation for access revision should be obtained. 1

Hemodynamic Factors Contributing to Stenosis Formation

The arteriovenous anastomosis creates abnormal hemodynamic forces that drive stenosis development:

• Altered blood flow velocity and wall shear stress near the anastomosis site causes endothelial dysfunction 7
• Abnormal cyclic stretch promotes vascular smooth muscle cell proliferation 7
• These hemodynamic disturbances lead to intimal hyperplasia and adverse vascular remodeling 7, 3
• Significant angulation at characteristic stenosis sites creates turbulent flow and intimal injury 2