Flow-mediated dilation

Flow-mediated dilation (FMD) is a non-invasive ultrasound method used to measure the endothelial function of arteries. It is a critical indicator of vascular health, reflecting the ability of blood vessels to dilate in response to increased blood flow. This dilation is primarily mediated by the endothelium, the inner lining of blood vessels, through the release of nitric oxide (NO), a potent vasodilator. FMD is widely used in clinical and research settings to assess the risk of cardiovascular diseases (CVD), including atherosclerosis, hypertension, and coronary artery disease.

Overview

Flow-mediated dilation is based on the principle that increased blood flow to an artery causes a temporary increase in the diameter of the artery. This phenomenon occurs in response to shear stress, a mechanical force resulting from blood flow that stimulates the endothelial cells to release NO. The measurement of FMD typically involves the use of high-resolution ultrasound to observe changes in the diameter of the brachial artery (a major blood vessel in the upper arm) following a period of induced blood flow increase, usually achieved by inflating and then deflating a blood pressure cuff placed around the forearm.

Procedure

The standard procedure for measuring FMD involves several steps: 1. The subject is asked to rest in a supine position for at least 5 minutes to ensure baseline physiological conditions. 2. A baseline scan of the brachial artery is performed using ultrasound to measure its diameter. 3. A blood pressure cuff placed around the forearm is inflated to a pressure above systolic blood pressure to occlude blood flow for approximately 5 minutes. 4. The cuff is then rapidly deflated, causing a sudden increase in blood flow through the brachial artery. 5. Ultrasound measurements are taken immediately after cuff deflation to record the peak dilatory response of the artery. 6. The FMD is calculated as the percentage change in arterial diameter from baseline to peak dilation.

Clinical Significance

Flow-mediated dilation is a valuable tool for assessing endothelial function, which is crucial for vascular health. Impaired FMD is associated with several cardiovascular risk factors, such as smoking, diabetes, hypercholesterolemia, and hypertension. It is also predictive of future cardiovascular events, making it an important parameter in the evaluation of patients at risk for CVD.

Limitations

While FMD is a useful indicator of endothelial function, it has some limitations. The technique is operator-dependent, requiring skilled personnel to perform the ultrasound measurements accurately. Variability in measurement techniques and patient preparation can also affect the reproducibility of results. Furthermore, FMD does not provide information on the endothelium-independent dilation, which is another aspect of vascular function.

Related Pages

• Endothelium
• Nitric oxide
• Atherosclerosis
• Hypertension
• Coronary artery disease
• Cardiovascular diseases