Flowmediated

Flow-mediated dilation (FMD) is a noninvasive ultrasound-based technique used to assess endothelial function. It measures the dilation of a conduit artery, typically the brachial artery, in response to increased shear stress after a brief period of arterial occlusion (reactive hyperemia).

Mechanism and procedure: Endothelium-derived nitric oxide mediates the vasodilatory response to shear stress, though other mediators may contribute. Baseline diameter is recorded, the forearm cuff is inflated to suprasystolic pressure for about 5 minutes, then released. Following release, increased flow causes the artery to dilate; peak diameter is usually observed within 30–60 seconds. The percent change from baseline is used as the index of endothelial function. Factors such as age, sex, smoking, hyperlipidemia, diabetes, medications, and temperature affect FMD.

Interpretation and applications: FMD is used in clinical research to evaluate cardiovascular risk and to monitor responses to lifestyle modification or pharmacologic therapy. Lower FMD has been associated with higher cardiovascular risk. Results are lab-dependent and not diagnostic; they should be interpreted in context and compared against standardized laboratory procedures.

Limitations and considerations: Technique requires trained operators, standardized protocols, and high-quality ultrasound. Results show inter- and intra-operator variability and are influenced by technical factors such as cuff placement, limb length, and baseline vessel size, as well as patient preparation (fasting, caffeine, smoking). Proper standardization and quality control are essential for reliable interpretation.