**4.1 FMD**

Flow-mediated dilation represents an endothelium-dependent, largely NO-mediated dilatation of conduit arteries in response to an imposed increase in blood flow and shear stress first described in 1992 [20].

#### *4.1.1 FMD procedure*

FMD is typically assessed in brachial artery with a standardized diameter of 3−5 mm. Through a high-resolution B-mode ultrasound, images of the brachial artery are taken, usually with an ultrasound probe of 7.5–12 MHz [21]. An approach by tangential scanning is a common mistake and results in underestimation of the true brachial artery diameter (**Figure 4**). Recent studies, which adopt H-shaped, probe capturing two short-axis and one long-axis for automatic probe position correction may overcome this previous limitation [22].

A simultaneous evaluation of pulse-wave Doppler velocity is recommended, given the importance of shear stress as the eliciting stimulus for dilatation. The recommended isonation angle is <60° for optimal data acquisition, which should be kept constant [23, 24].

To ensure an optimal image throughout the hole FMD procedure, a probe-holding device is recommended. A stereotactic adjustable probe-holding device allows adjustment of probe position during the test, allowing to maintain the same scan in the study [25].

Many subject-related factors can influence FMD such as alcohol, smoking, food, supplements, drugs, physical activity, and mental stress. Some factors directly stimulated NO-release, but others, such as acute physical exercise and mental stress, modify baseline vasomotor tone [26, 27].

### *4.1.2 Clinical evidence*

In a study, brachial FMD has associated whit intima-media thickness progression in a population free of CVD, and in hypertensive, postmenopausal women [28]. A follow-up study in hypertensive patients with FMD predicted target organ damage progression for 3 years, even adjusted for known CVRF [29].

One meta-analysis described a significant 8−13% lower risk of CVD per percentage point increase in brachial artery FMD (e.g. from 7–8% dilatation). This reduction

#### **Figure 4.**

*FMD representation with cuff positioned in the forearm. Through ultrasound assessment, brachial artery diameter is measured before and 5 minutes after the ischemia. FMD can oscillate depending on if a hypertensive drug is used before the procedure such as nitrates. Original graphic created with BioRender.com.*

was present in high and low-risk population but appeared larger in patients with established CVD [30, 31].

The clinical value of long-term changes in FMD may have a prognostic implication. For interventional trials FMD could represent a surrogate endpoint, especially since FMD is a tool with a rapid response effect to therapies, allowing recognition and identification of new bioactive substances or drugs able to modify FMD [32].
