The relationship between shear stress and flow-mediated dilatation: implications for the assessment of endothelial function.

Endothelium-dependent flow-mediated dilatation (FMD) describes the vasodilatory response of a vessel to elevations in blood flow-associated shear stress. Nitric oxide (NO), one of many vasoactive substances released by the endothelium in response to shear stress, is of particular interest to researchers as it is an antiatherogenic molecule, and a reduction in its bioavailability may play a role in the pathogenesis of vascular disease. The goal of many human studies is to create a shear stress stimulus that produces an NO-dependent response in order to use the FMD measurements as an assay of NO bioavailability. The most common non-invasive technique is the 'reactive hyperaemia test' which produces a large, transient shear stress profile and a corresponding FMD. Importantly, not all FMD is NO mediated and the stimulus creation technique is a critical determinant of NO dependence. The purpose of this review is to (1) explain that the mechanisms of FMD depend on the nature of the shear stress stimulus (stimulus response specificity), (2) provide an update to the current guidelines for FMD assessment, and (3) summarize the issues that surround the clinical utility of measuring both NO- and non-NO-mediated FMD. Future research should include (1) the identification and partitioning of mechanisms responsible for FMD in response to various shear stress profiles, (2) investigation of stimulus response specificity in coronary arteries, and (3) investigation of non-NO FMD mechanisms and their connection to the development of vascular disease and occurrence of cardiovascular events.