Endothelial dysfunction  In vascular diseases, endothelial dysfunction is a systemic pathological state of the endothelium. Along with acting as a semi-permeable membrane, the endothelium is responsible for maintaining vascular tone and regulating oxidative stress by releasing mediators, such as nitric oxide, prostacyclin and endothelin, and controlling local angiotensin-II activity. Endothelial dysfunction may be involved in the development of atherosclerosis and may predate vascular pathology. Nitric oxide (NO) suppresses platelet aggregation, inflammation, oxidative stress, vascular smooth muscle cell migration and proliferation, and leukocyte adhesion. A feature of endothelial dysfunction is the inability of arteries and arterioles to dilate fully in response to an appropriate stimulus, such as exogenous nitroglycerine, that stimulates release of vasodilators from the endothelium like NO. Endothelial dysfunction is commonly associated with decreased NO bioavailability, which is due to impaired NO production by the endothelium or inactivation of NO by reactive oxygen species. In the coronary circulation, angiography of coronary artery responses to vasoactive agents may be used to test for endothelial function, and venous occlusion plethysmography and ultrasonography are used to assess endothelial function of peripheral vessels in humans. A non-invasive method to measure endothelial dysfunction is % Flow-Mediated Dilation (FMD) as measured by Brachial Artery Ultrasound Imaging (BAUI). Current measurements of endothelial function via FMD vary due to technical and physiological factors. Furthermore, a negative correlation between percent flow mediated dilation and baseline artery size is recognised as a fundamental scaling problem, leading to biased estimates of endothelial function. A non-invasive, FDA-approved device for measuring endothelial function that works by measuring Reactive Hyperemia Index (RHI) is Itamar Medical's EndoPAT™. It has shown an 80% sensitivity and 86% specificity to diagnose coronary artery disease when compared against the gold standard, acetylcholine angiogram. This results suggests that this peripheral test reflects the physiology of the coronary endothelium. Since NO maintains low tone and high compliance of the small arteries at rest, a reduction of age-dependent small artery compliance is a marker for endothelial dysfunction that is associated with both functional and structural changes in the microcirculation. Small artery compliance or stiffness can be assessed simply and at rest and can be distinguished from large artery stiffness by use of pulsewave analysis.