Flow-Mediated Dilation (FMD)

Flow-mediated dilation (FMD) is the widening of an artery in response to increased blood flow. The term also names a noninvasive ultrasound test used to assess this response, usually in the brachial artery of the upper arm. It provides information about the function of the endothelium, the thin layer of cells lining blood vessels .¹

In a common test protocol, an inflated cuff temporarily stops blood flow through the forearm. Releasing it produces a brief increase in flow. Ultrasound tracks how much the artery widens, and the result is commonly expressed as the percentage increase from its resting diameter. The measured change is in artery diameter, rather than simply the amount of blood passing through .²

Faster flow increases shear stress, the frictional force of blood moving along the vessel lining. This stimulates signals that relax the surrounding smooth muscle, allowing the artery to widen. Under standard testing conditions, nitric oxide accounts for much of this response, but FMD is not a direct measurement of nitric oxide concentration .¹

The result depends on the stimulus as well as the artery's response. Starting diameter, cuff placement, and the way images are analyzed can affect the reported percentage. Standardized methods therefore matter when comparing measurements between people or before and after an intervention .²