Vascular Graft Thrombosis

Vascular graft thrombosis is when a blood clot forms inside an artificial blood vessel that has been surgically placed in the body, blocking blood flow and causing the graft to fail.

Table of contents

• What is Vascular Graft Thrombosis?
• Causes and Risk Factors
• When Graft Thrombosis Occurs
• How Common is Graft Thrombosis?
• Treatment Options

What is Vascular Graft Thrombosis?

Vascular graft thrombosis happens when a blood clot blocks an artificial blood vessel that has been surgically implanted to restore blood flow in the body. Vascular grafts (also called vascular prostheses) are tubes made from synthetic materials or processed natural tissue that are used to replace or bypass diseased blood vessels^[1]. These grafts are commonly used for several purposes, including creating access points for kidney dialysis, repairing damaged blood vessels after injury, fixing weakened bulging arteries, and performing heart bypass surgery^[1].

When a graft becomes blocked by a clot, it represents a direct failure of the treatment and is considered one of the most challenging problems in vascular surgery^[5]. The clot prevents blood from flowing through the graft, which can lead to serious complications depending on where the graft is located in the body.

Causes and Risk Factors

Thrombosis is a major cause of poor performance in synthetic vascular grafts, especially in smaller diameter vessels measuring less than 6 millimeters^[3]. Several factors contribute to the formation of blood clots in grafts. The most important factor is that synthetic grafts do not have a natural inner lining of cells called endothelium, which normally prevents blood clots from forming in healthy blood vessels^[3]. Without this protective layer, the graft surface can trigger platelet activation (the clumping together of blood cells) and start the clotting process.

In arteriovenous grafts (grafts connecting an artery to a vein for dialysis), thrombosis commonly occurs due to intimal hyperplasia, which is excessive tissue growth at the connection point between the graft and vein or in the vein that carries blood away from the graft^[2]. This tissue buildup narrows the opening, slows blood flow, and increases the chance of clot formation.

Other contributing factors include problems with blood flow into the graft, issues with the graft material itself, poor blood flow out of the graft, and conditions that make a person’s blood clot more easily^[5]. The size and blood flow through a graft also matter—grafts that are larger and have more blood flowing through them are less likely to develop clots^[5].

When Graft Thrombosis Occurs

The timing of graft failure helps doctors understand what caused the problem and decide on the best treatment. Graft thrombosis can be divided into three time periods: early (1 to 30 days after surgery), intermediate (30 days to 2 years), and late (more than 2 years)^[5].

Early graft failure almost always results from technical problems during surgery. These can include poorly constructed connections between the graft and blood vessels, twists or kinks in the graft, or problems with the inner lining of the blood vessel at the connection point^[5]. Issues with the graft material itself or existing blood clotting disorders can also play a role, though technical errors are the most common cause.

Intermediate graft failure most often happens because of intimal hyperplasia, where smooth muscle cells from the blood vessel wall migrate and multiply inside the vessel, depositing material that narrows the opening and can lead to clotting^[5]. This typically occurs where the graft connects to a natural blood vessel.

Late graft failure that occurs more than two years after placement is usually caused by atherosclerosis (hardening and narrowing of arteries due to fatty deposits) either in the graft itself or in the blood vessels connected to it^[5].

How Common is Graft Thrombosis?

The frequency of graft thrombosis varies considerably depending on several factors, including where in the body the graft is placed, what type of material it is made from, and the patient’s overall health^[5]. Early graft thrombosis is reported to occur in 2% to 20% of cases, depending on the location^[5].

For grafts placed below the groin, the outlook is particularly challenging. Despite advances in surgical techniques, graft materials, and medications, approximately half of all grafts placed below the groin will fail within 5 years^[5]. Synthetic grafts placed below the knee have an especially high failure rate, with nearly 80% developing thrombosis within 5 years^[5].

Grafts placed in larger blood vessels like the aorta (the body’s main artery) have much lower rates of thrombosis, though they can still fail with serious consequences^[5]. Late graft thrombosis rarely affects less than 10% of grafts and can exceed 80% for reconstructions in the lower leg^[5].

Treatment Options

When a vascular graft becomes blocked by a clot, doctors have several treatment options. The approach depends on factors such as how long ago the clot formed, where the graft is located, and the patient’s overall condition.

For arteriovenous grafts used in dialysis, timing matters somewhat but is not urgent within the first few days. There is no difference in success rates between treating a clot on the same day it forms versus waiting 48 to 72 hours^[2]. However, after a week or more, treatment becomes more difficult because the clot adheres more firmly to the blood vessel wall^[2]. Success rates drop from the high 90% range to the mid 80% range after 1 to 2 weeks^[2].

Thrombectomy procedures remove the clot from the graft. This can be done surgically through an incision or using minimally invasive techniques that involve inserting special devices through small punctures^[7]. During graft thrombectomy, doctors always find what is called an arterial plug—a clot at the connection between the graft and artery—which must be removed as part of the procedure^[2].

Manual aspiration thrombectomy, where doctors use catheters to physically suction out the clot, has been successfully used to treat blocked grafts^[7]. Other devices that mechanically break up and remove clots are also available. After removing the clot, doctors often perform additional procedures like balloon angioplasty (stretching narrowed areas with a balloon) to address the underlying problem that caused the clot.

In some cases, especially when there is persistent narrowing at connection points, doctors may place stent grafts (expandable tubes covered with fabric) to hold the vessel open^[4]. One case study reported that placing such a device at a problem area maintained graft function for 60 months without further episodes of clotting^[4].

Medications also play a role in treating graft thrombosis. In one reported case involving a saphenous vein graft used for heart bypass, treatment with rivaroxaban (an anticoagulant medication that prevents blood clotting) led to complete resolution of a large clot within 4 weeks^[9]. This suggests that anticoagulant medications may be useful for certain types of graft thrombosis, particularly when the clot formed due to slow blood flow rather than a blockage.