Flow-mediated dilation (%FMD) is a test that assesses how well an artery can dilate in response to increased blood flow. It is a direct way to measure the function of the endothelium — the inner lining of the vessels.
A healthy endothelium releases nitric oxide and dilates the vessel; a damaged one does not. So a low %FMD is an early, still reversible sign of atherosclerosis, long before plaques.
How the test is performed
A cuff is placed on the upper arm and the blood flow is compressed for a few minutes. After the cuff is released, blood rushes along the artery, and a healthy endothelium responds by dilating it. Ultrasound measures the artery's diameter before and after — the increase in percent is the %FMD.
The greater the dilation, the better the vascular function. The method is non-invasive and safe, but it is sensitive to technique, so it is more often used in research and specialized centers.
Why it matters
%FMD reflects the earliest stage of vascular damage — endothelial dysfunction, which can still be improved through lifestyle. A low %FMD is associated with increased cardiovascular risk.
The good news: endothelial function is improved by aerobic exercise, quitting smoking, a Mediterranean-style diet, and control of blood pressure, cholesterol and blood sugar — the same things that protect the vessels overall.
Assess your vascular age
%FMD reflects endothelial function — assess the “age” of your vessels:
Frequently asked questions
What does %FMD show?
The ability of an artery to dilate in response to increased blood flow — that is, endothelial function. A high percentage of dilation means healthy, responsive vessels, while a low one means endothelial dysfunction, an early sign of atherosclerosis and increased cardiovascular risk.
Can %FMD be improved?
Yes, especially at an early stage. Endothelial function is improved by regular aerobic exercise, quitting smoking, a Mediterranean-style diet, and control of blood pressure, cholesterol and blood sugar. This is one of the reasons lifestyle so strongly affects vascular health.
Where is this test done?
%FMD is more often used in scientific research and specialized vascular centers, because the result is sensitive to measurement technique. In routine practice vascular function is more often assessed indirectly — by risk factors, blood pressure and arterial stiffness.
Related terms
Sources
- Flow-mediated dilation and endothelial function (review). NCBI/PMC
- Endothelial function and cardiovascular disease. NCBI/PMC
- American Heart Association — What Is Atherosclerosis?