Flow-mediated dilation (FMD) is a non-invasive assessment used to determine the health and flexibility of a person’s blood vessels. It provides insight into the function of the endothelium, the delicate layer of cells lining the interior of all arteries. The FMD test measures how well an artery widens in response to a temporary increase in blood flow, reflecting the vessel’s ability to regulate its diameter. It employs high-resolution ultrasound technology to monitor diameter changes in an artery, most commonly the brachial artery in the upper arm. FMD is utilized in clinical research as a safe, repeatable method for evaluating vascular health.
The Endothelial Mechanism of Dilation
The endothelium regulates vascular tone, controlling the constriction or relaxation of the vessel wall. The mechanism underlying FMD begins with an increase in shear stress, the frictional force exerted by flowing blood against the endothelial cell surface. When blood flow rapidly increases, the resulting friction signals the endothelial cells to produce and release chemical messengers.
The primary messenger is nitric oxide (NO), a gaseous molecule that acts as a powerful vasodilator. Nitric oxide diffuses from the endothelium into the underlying smooth muscle cells that form the artery wall. Once there, NO initiates a biochemical cascade that causes these muscle cells to relax, leading to the artery’s dilation.
This NO-mediated response increases the vessel’s internal diameter, allowing heightened blood flow to pass through with less resistance. Although nitric oxide is the primary driver of FMD, other molecules like prostacyclin and endothelial-derived hyperpolarizing factors (EDHFs) also contribute to the overall dilatory response. The magnitude of the resulting dilation is directly proportional to the health and responsiveness of the endothelial layer.
How Flow-Mediated Dilation is Measured
The FMD test is a standardized, non-invasive procedure focusing on the brachial artery in the upper arm. The initial step involves measuring the artery’s diameter at rest using a high-resolution Doppler ultrasound device. A blood pressure cuff is then placed on the forearm or upper arm and inflated above the patient’s systolic pressure.
The cuff remains inflated for about five minutes, temporarily restricting blood flow and inducing ischemia in the downstream tissues. Upon rapid deflation, the sudden release of pressure causes a swift, temporary surge of blood flow into the oxygen-starved limb, a phenomenon known as reactive hyperemia. This intense flow creates the high shear stress stimulus necessary to prompt the endothelium to release its vasodilators.
The ultrasound device then continuously measures the artery’s diameter for several minutes following the cuff release. The FMD result is calculated as the maximum percentage increase in the artery’s diameter compared to the initial resting diameter. This percentage change quantifies the degree of endothelium-dependent dilation.
FMD as a Marker of Vascular Health
The FMD score serves as a direct index of endothelial function, an early indicator of overall vascular health. A high FMD percentage signifies a healthy, responsive endothelium capable of producing sufficient nitric oxide to relax the artery wall. Conversely, a lower FMD score suggests endothelial dysfunction, indicating the artery’s lining is impaired and less responsive to the shear stress stimulus.
Endothelial dysfunction is an early measurable event in the development of atherosclerosis, the process where plaque builds up inside arteries. This impairment often appears long before any visible structural changes occur in the vessel walls. Low FMD results are viewed as a prognostic indicator, predicting an increased long-term risk for future cardiovascular events.
A decrease in FMD is statistically associated with a higher likelihood of experiencing events such as cardiac death, myocardial infarction, and stroke. For example, one meta-analysis showed that every one percent reduction in FMD is associated with an increased risk of future cardiovascular events. Measuring FMD helps clinicians assess risk, even in individuals who appear healthy or are free of clinically evident cardiovascular disease.
Lifestyle and Medical Influences on Endothelial Function
Endothelial function, as reflected by FMD scores, is highly sensitive to external factors and can be improved or worsened by daily habits and underlying medical conditions. Negative influences promote inflammation and oxidative stress that damage the endothelium. These factors include:
- Diabetes mellitus.
- Hyperlipidemia.
- Obesity.
- Smoking, which compromises FMD by reducing nitric oxide availability.
Positive changes in lifestyle can lead to measurable improvements in endothelial function over time. Regular aerobic exercise is beneficial because the repeated increase in blood flow and shear stress during activity stimulates the long-term production of nitric oxide within the endothelium. Even low-to-moderate intensity exercise, as well as high-intensity interval training, can significantly improve FMD in overweight and obese adults.
Dietary choices also play a significant role, with the intake of trans fatty acids known to adversely affect FMD. Conversely, diets rich in antioxidants, such as those that include cocoa or chocolate, have been shown to increase FMD. Certain medications, such as statins, are also prescribed for their ability to improve FMD and overall endothelial health.