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Measure of Cariovascular risk : Endothelial Function & Dysfunction (Flow Mediated Dilation - FMD)

Endothelial dysfunction is considered to be the first stage of atherosclerosis. Impaired endothelial function is recognized as an early and modulating process in the pathophysiology of cardiovascular disease. The assessment and characterization of endothelial function in the diagnosis of cardiovascular diseases is a topic of both clinical and research importance.

Endothelial function is often quantified by flow-mediated dilation (FMD), which represents the endothelium-dependent relaxation of a conduit artery-typically the brachial artery due to an increased blood flow. Brachial artery reactivity is a frequently used non-invasive ultrasonographic assessment of FMD indicating endothelium-dependent response to shear stress. This measure is a marker of increased cardiovascular risk, and correlates with impaired endothelium-dependent relaxation in the coronary arteries.

However, complementary computerized image analysis techniques are still very desirable to give accuracy and objectivity to the measurements. So FLOMEDI proposes a system (FMD-I) for the automatic measurement of flow-mediated dilation (FMD) and intima-media thickness (IMT) for the assessment of the arterial function.


Method

Flow mediated dilation (FMD) describes the increase in diameter of a blood vessel, generally the brachial artery, in answer to a sharp increase in the shear associated to blood flow. The mechanical stimulus (shear stress : forearm occlusion with a pneumatic cuff) on the endothelium generates liberation of nitric oxide (NO) responsible for the dilation. The answer of the arterial diameter to the change of flow can be measured in a non invasive way by means of ultrasound, caused by a temporary forearm occlusion before and after an ischemia. The dilation magnitude, expressed as the percentage change of the dilation diameter between 30 and 90 seconds of having liberated the occlusion and the basal diameter (FMD in %), is traditionally used to evaluate endothelium response.

The software FMD-I will allow you to continuously monitor (in Real-Time with ultrasound or Off-line on video recording) the evolution of dilation of the artery.

Process

Step 1 : baseline

Before, place a pneumatic cuff on the left forearm (the wrist).
The brachial artery is imaged 10-15 cm below the shoulder at rest for 30 seconds to acquire the baseline diameter with FMD-I and an ultrasound device in B-mode (7,5Mhz linear probe).

Step 2 : occlusion

After baseline known, the pneumatic cuff positioned around the wrist is then inflated to 220mmHg (suprasystolic pressure) during 5 min.

In detail, the endothelium responds to changes in intravascular shear stress by releasing several compounds which determining relaxation of smooth muscle cells and, subsequently, vasodilation. Upon release of the cuff, the sudden increase in blood flow (and shear stress) that follows reperfusion is a powerful stimulus for endothelium-dependent vasorelaxation, which can be observed using ultrasounds. In sum, the endothelium shows measurable responses to flow changes, determining endothelium dependent, flow-mediated dilation (FMD).

Step 3 : maximum dilation

After the interval of 5 min, the cuff is deflated to achieve reactive hyperemia. The artery is imaged by ultrasound device and the dilation of artery is followed by FMD-I during 2 min. The FMD value is returned after the selection of maximum diameter(dilation).

FMD is defined by maximum diameter after occlusion divided by the baseline diameter (in this example FMD = 6,31).