What does flow-mediated dilation measure?

It measures the widening of a conduit artery in response to a standardised increase in blood flow, a response that depends largely on endothelial release of nitric oxide and is used as a non-invasive index of endothelial function.

Why does increased blood flow during exercise cause vessels to dilate?

Faster flow raises the frictional shear stress on the vessel lining, which stimulates the endothelium to release nitric oxide and other relaxing factors that dilate the vessel and lower resistance.

Vascular Responses and Endothelial Function

Blood vessels are not passive tubes: their smooth muscle constricts and relaxes, and the endothelial lining senses blood flow and chemical signals to set vessel calibre. During exercise the rise in blood flow increases the frictional shear stress on the endothelium, prompting release of vasodilators such as nitric oxide. This shear-mediated and metabolic vasodilation lowers resistance in active tissue, and the responsiveness of the endothelium is widely used as an index of vascular health.

Definition

Vascular responses are the constriction and dilation of blood vessels that set vascular resistance, and endothelial function is the capacity of the vascular endothelium to regulate vessel tone, principally by releasing nitric oxide and other vasoactive mediators in response to shear stress and chemical stimuli.

Scope

The topic covers vascular smooth-muscle and endothelial control of vessel tone, the role of shear stress and nitric oxide in exercise vasodilation, flow-mediated dilation as a non-invasive measure of endothelial function, and how the endothelium contributes to matching flow to demand. It is reference physiology and not a basis for clinical testing or treatment decisions.

Core questions

How do vascular smooth muscle and the endothelium together set vessel diameter?
How does increased blood flow during exercise produce shear-mediated vasodilation?
What is flow-mediated dilation and what does it measure?
How does endothelial function relate to the vascular response to exercise?

Key concepts

Vascular smooth muscle tone
Endothelium-derived nitric oxide
Shear stress
Flow-mediated dilation
Endothelial dysfunction
Conduit versus resistance vessels

Mechanisms

When blood flow rises during exercise, the increased shear stress on the endothelium stimulates endothelial nitric oxide synthase, releasing nitric oxide that relaxes adjacent smooth muscle and dilates the vessel; other endothelial mediators and metabolic signals contribute to the overall fall in resistance (Joyner & Casey, 2015). This flow-mediated, largely nitric-oxide-dependent dilation can be assessed non-invasively in conduit arteries by measuring the diameter response to a standardised increase in flow, a technique codified in methodological guidelines (Thijssen et al., 2011; Thijssen et al., 2020). Impaired endothelial vasodilation, termed endothelial dysfunction, reflects reduced nitric-oxide bioavailability and is interpreted as a marker of vascular health (Vita & Keaney, 2002).

Clinical relevance

Endothelial function and its non-invasive assessment are widely studied as indices of vascular health and are used in research to characterise how vessels respond to flow. This entry describes the underlying physiology for reference; it does not recommend specific tests, thresholds, or treatments for individuals.

Evidence & guidelines

Standardised methodology for measuring flow-mediated dilation is set out in expert guideline statements. Thijssen and colleagues' methodological and physiological guideline, and its later restatement, define how the measure should be obtained and interpreted, while reviews discuss endothelial function as a barometer of cardiovascular risk.

History

Recognition in the 1980s that the endothelium releases a diffusible relaxing factor, later identified as nitric oxide, transformed understanding of vascular control. Non-invasive flow-mediated dilation of conduit arteries was subsequently developed as a window onto endothelial function, and consensus guidelines standardised its measurement.

Debates

How standardised must flow-mediated dilation be to be interpretable?: Because the measurement is sensitive to technique, the field has debated how strictly protocols must be followed for results to be comparable, with expert statements arguing that adherence to detailed guidelines is essential for reproducible interpretation.

Key figures

Dick Thijssen
Daniel Green
Joseph Vita
Paul Vanhoutte

Seminal works

thijssen-2011
vita-keaney-2002
joyner-casey-2015

Frequently asked questions

What does flow-mediated dilation measure?: It measures the widening of a conduit artery in response to a standardised increase in blood flow, a response that depends largely on endothelial release of nitric oxide and is used as a non-invasive index of endothelial function.
Why does increased blood flow during exercise cause vessels to dilate?: Faster flow raises the frictional shear stress on the vessel lining, which stimulates the endothelium to release nitric oxide and other relaxing factors that dilate the vessel and lower resistance.