High-Flow AV Graft
AV grafts are widely used for hemodialysis access, but some grafts produce access flows well above what dialysis requires — and that excessive flow can increase cardiac workload in susceptible patients.
The AV graft is a workhorse of hemodialysis access — reliable, technically straightforward to place, and quickly usable for dialysis. It is also, by design, a low-resistance shunt between the arterial and venous systems. In most patients, graft flow falls into a clinically acceptable range; in a subset of patients, however, the graft delivers far more blood per minute than dialysis requires. That excess flow is not a problem for the dialysis machine itself; it is a continuous load on the heart.
What is a high-flow AV graft?
A high-flow AV graft is a dialysis access graft whose dialysis access flow is significantly above what is needed for adequate hemodialysis. Historically, the field has described high-flow using fixed thresholds such as 1.5 to 2.0 L/min, but recent clinical discussion — including at the Vascular Access Society of the Americas (VASA) — has emphasized that the more important standard is the physiologic effect on the patient, not the access-flow number alone.
A given graft flow may be well tolerated in one patient and impose meaningful cardiac stress in another. Cardiac reserve, age, valve disease, pre-existing pulmonary hypertension, and other comorbidities all shape how much continuous high-flow AV access an individual heart can absorb without consequence.
Why graft flow matters
The volume of blood moving through an AV graft — the graft flow — is the most direct driver of the cardiac load the access creates. The dialysis machine pulls a few hundred milliliters per minute through the dialyzer during a treatment session; everything beyond that, present 24 hours a day, is continuous extra flow the heart has to support.
Graft flow is therefore both essential and a variable to be watched. Surveillance of access flow alongside cardiac function is more informative than a single threshold, because what looks like a tolerable number on a flow study may be poorly tolerated by a heart with limited reserve.
High-flow grafts and cardiac burden
The continuous extra flow created by a high-flow graft increases cardiac workload, because the heart must pump a larger volume per minute to perfuse the rest of the body. Sustained increases in cardiac output demand can produce changes in cardiac structure and function over time. Cardiac remodeling is most often discussed in this context, along with pulmonary hypertension, which can develop alongside the elevated venous return that high-flow AV access creates.
In some patients, the cumulative load contributes to high-output heart failure — a clinical state in which the heart pumps more blood than usual yet still cannot meet the body's needs. The relationship is not deterministic: many patients tolerate AV grafts, including some with significant access flow, without developing these complications. For deeper background, see cardiac burden of AV fistulas and AV grafts and high-output heart failure and dialysis access.
Symptoms and clinical concerns
Symptoms sometimes associated with a high-flow AV graft and the cardiac burden it can create include:
- Shortness of breath, especially with exertion or when lying flat
- Fatigue beyond what is typical for the patient
- Exercise intolerance or reduced functional capacity
- Signs of pulmonary hypertension on imaging or right-heart catheterization
- Heart-failure symptoms such as fluid retention or worsening overall function
These symptoms can have many causes and are not diagnostic of high-output heart failure by themselves. Any new or worsening symptoms in a hemodialysis patient with an AV graft warrant evaluation by a physician familiar with the patient's history. This page is informational and is not medical advice.
Why fixed thresholds may not tell the whole story
A single number — for example, 1.5 L/min — is convenient as a definition of high-flow, but it can miss patient-specific risk. Two issues with fixed thresholds are increasingly discussed in the field:
- Some patients show objective cardiac changes at graft flows below conventional high-flow cutoffs.
- The same graft flow may be well tolerated in a patient with strong cardiac reserve and poorly tolerated in a patient with reduced cardiac reserve, valve disease, or pre-existing pulmonary hypertension.
A patient-centered framing — judging high-flow by its physiologic effect on the heart, not by the access-flow number alone — better captures the cardiac burden any given graft creates in any given patient.
Access flow reduction
Access flow reduction is a clinical strategy that lowers the volume of blood moving through a high-flow AV graft while preserving the access for hemodialysis. Approaches may include banding, surgical revision, or other flow-reduction procedures, depending on the patient and the graft anatomy. The intent is to reduce continuous cardiac burden without sacrificing the access. Access flow reduction has historically been pursued reactively, after symptoms or complications appear; the underlying clinical question is the same regardless of timing.
Flow-control AV graft design
A flow-control AV graft is built around the recognition that the dialysis machine and the heart have different flow needs. The machine pulls a few hundred milliliters per minute through the dialyzer during a treatment session; everything beyond that is continuous extra flow the heart has to support. A flow-control AV graft is designed with built-in geometry that delivers the flow needed during dialysis while limiting that continuous excess — making flow-control a property of the access from the start rather than a reactive intervention.
This is conceptually different from a graft optimized for maximum patency at maximum flow. The design question becomes "how much flow does dialysis need, and how much continuous extra flow can the patient's heart safely tolerate?" rather than "how much flow can the graft deliver?"
VascX and elastic flow-control grafts
For high-flow AV grafts, the most direct path to flow-control is at the design stage — before the access becomes a continuous load on the heart. VascX's elastic flow-control grafts are a flow-control AV graft intended to be placed using standard surgical workflows, with built-in geometry that delivers dialysis-grade flow during treatment while limiting continuous between-session access flow. The graft sits alongside elastic flow-control stents in VascX's broader flow-control dialysis access platform; the graft addresses new access creation, the stent addresses high-flow circuits already in place.
VascX products are investigational. The company does not claim that its devices are proven to treat or prevent cardiac remodeling, pulmonary hypertension, high-output heart failure, hospitalization, mortality, or access failure. The platform is designed to address the underlying physiology — dialysis access flow — that the field increasingly views as relevant to cardiac outcomes in hemodialysis patients.
Frequently asked questions
What is a high-flow AV graft?
A high-flow AV graft is an arteriovenous graft whose dialysis access flow is significantly above what is needed for adequate hemodialysis. Historically, the field has described high-flow using fixed thresholds such as 1.5 to 2.0 L/min, but recent clinical discussion suggests that the relevant standard is the physiologic effect on the patient — particularly on the heart — not the access-flow number alone. A given graft flow may be tolerated in one patient and impose a meaningful cardiac burden in another.
Why can an AV graft increase cardiac workload?
An AV graft creates a low-resistance arteriovenous shunt through a synthetic conduit, diverting blood from the arterial system directly into the venous system. To maintain perfusion to the rest of the body, the heart must pump a larger total volume per minute. This added cardiac output demand is continuous — present 24 hours a day for as long as the graft is in place — and can increase cardiac workload over time, particularly when graft flow is high.
Can a high-flow graft contribute to high-output heart failure?
In some patients, yes. A high-flow AV graft can contribute to high-output heart failure by adding a continuous, non-physiologic flow load on top of any underlying cardiovascular disease. Cardiac remodeling and pulmonary hypertension are commonly discussed in this context as well, and can precede or accompany high-output heart failure in susceptible patients. The relationship is not deterministic — many patients tolerate AV grafts without progressing to heart failure — but the field increasingly recognizes the graft circuit itself as a relevant cardiac variable.
What symptoms may be associated with a high-flow AV graft?
Symptoms that may be associated with a high-flow AV graft and the cardiac burden it can create include shortness of breath, fatigue, exercise intolerance, and signs of heart failure such as fluid retention or reduced functional capacity. Some patients may also show signs of pulmonary hypertension. These symptoms have many possible causes and are not specific to high-flow grafts. Patients experiencing these symptoms should consult their physician for evaluation.
What is access flow reduction for a high-flow graft?
Access flow reduction in a high-flow AV graft is a clinical strategy that lowers the volume of blood moving through the graft while preserving its function for hemodialysis. Approaches may include banding, surgical revision, or other flow-reduction procedures, depending on the patient and the graft anatomy. The goal is to reduce continuous cardiac burden without sacrificing the access. Treatment decisions should be made by the patient's care team; this page is not medical advice.
What is a flow-control AV graft?
A flow-control AV graft is an arteriovenous graft designed with built-in geometry that limits the volume of blood flowing through it. Rather than relying on a separate access flow reduction procedure later, the graft itself is intended to deliver dialysis-grade flow during treatment while limiting the continuous high-flow burden on the heart between sessions. The design intent is to incorporate flow control into the access from the start rather than as a reactive intervention.
How do elastic flow-control grafts relate to high-flow AV grafts?
Elastic flow-control grafts are an investigational form of flow-control AV graft. They are designed to deliver the dialysis-grade flow needed during treatment while limiting continuous access flow between sessions, with the goal of preserving dialysis adequacy and reducing the cardiac burden associated with high-flow AV grafts. The elastic design is intended to accommodate standard access management, including thrombectomy, and return to its calibrated flow-control profile afterward. VascX's elastic flow-control grafts are in development and have not been cleared or approved by the U.S. Food and Drug Administration.
Are VascX products FDA-cleared?
VascX products are currently in development and are not yet cleared or approved by the U.S. Food and Drug Administration.