Flow-mediated dilation (FMD)

Flow-mediated dilation (FMD) is a non-invasive ultrasound test of how well your blood-vessel lining (the endothelium) works. It measures the percentage that your brachial (arm) artery widens after a surge of blood flow. The protocol: a blood-pressure cuff on your forearm is inflated above systolic pressure for five minutes, then released. The rush of blood that follows puts shear stress on the endothelium. That triggers an enzyme (eNOS) to release nitric oxide (NO). NO relaxes the vessel's muscle and briefly widens it. A high-resolution ultrasound measures that widening. FMD declines with age, as eNOS activity drops, oxidative stress rises, and NO availability falls. So it is a sensitive readout of vascular aging. A meta-analysis (Ras et al., 2013) pooled prospective cohorts. It found that each 1% higher FMD went with about 8% lower cardiovascular event risk (RR 0.92; 95% CI 0.88 to 0.95). That makes FMD an independent predictor beyond standard risk factors. Despite that, FMD is not in standard clinical risk guidelines. There are a few reasons. The measurement depends heavily on the operator. There are no universal reference ranges. And experts still debate whether the cuff method fully isolates NO-dependent widening from other mediators (Green et al., 2011).

Last reviewed: June 22, 2026

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