What is FMD?

Brachial artery scanning to measure FMD.

Flow-Mediated Dilatation is the increase in calibre of a blood vessel in response to increased blood flow. This was first described by Schretzenmayr. FMD is endothelium dependent, wherein the endothelium acts as a mechanotransducer that senses changes in shear stress and then determines the release of dilatators. Several dilator factors have been proposed to be involved in FMD: prostaglandins , ATP or an endothelium-derived hyperpolarizing factor, and nitric oxide (NO). An increased release of NO in response to increases in shear stress causes dilatation of underlying smooth muscle of conduit arteries.

Why is FMD significant?

A decreased FMD reflects endothelial dysfunction and may predict coronary artery disease in susceptible patients. The brachial artery FMD can be measured by ultrasound scan and acts as surrogate for the coronary vasculature which can only be assessed by more invasive methods.

How is FMD related to platelet reactivity?

Another effect of NO released by the endothelium under increased shear stress is decrease of platelet reactivity. Does it follow, therefore, that if decreased FMD determines endothelial dysfunction and decreased NO release, it also determines an increased platelet reactivity?

A study by Shechter, et.al. explores the association between platelet reactivity and brachial artery FMD in controls (without established cardiovascular disease) and in patients with acute myocardial infarction. Platelet reactivity was measured by conventional aggregometry and by the IMPACT-R machine. The study concludes that FMD is inversely correlated to platelet reactivity in both controls and AMI patients. An abstract of the said study can be found here.

One of the risk factors for cardiovascular disease is Diabetes. In fact, there is a 2-4 fold increase in the risk of dying from heart disease if the patient is diabetic. One of the reasons is increased production of thromboxane which is a vasoconstrictor and at the same time promotes platelet aggregation.

Acetylsalicylic acid (Aspirin).

Aspirin, or acetylsalicylic acid is a non-steroidal anti-inflammatory drug that has many effects. It is an anti-pyretic, an analgesic and, an anti-inflam-matory drug. It also acts as an anti-platelet because it irreversibly inhibits the  formation of thromboxane through the cyclooxygenase I pathway.

The American Diabetes Association and the American Heart Association have endorsed the following guidelines: “Aspirin may be used in women over 60 and men over 50 who have diabetes mellitus who have other risk factors for heart attack and stroke.”

A study by Spectre, et.al. showed that twice daily dosing of aspirin improved the laboratory outcomes in high-risk DM II patients. The laboratory parameters used were impedance aggregometry (WBA) and the IMPACT-R (CPA). An abstract of the study can be found here.