NO Bioavailability
A key and quantifiable feature of endothelial dysfunction is the inability of arteries and arterioles to dilate fully in response to an appropriate stimulus that stimulates release of vasodilators from the endothelium like nitric oxide (NO). Endothelial dysfunction is commonly associated with decreased NO bioavailability, which is due to impaired NO production by the endothelium and/or increased inactivation of NO by reactive oxygen species.
This can be tested by a variety of methods including iontophoresis of acetylcholine, direct administration of various vasoactive agents to segments of blood vessels, localised heating of the skin and temporary arterial occlusion by inflating a blood pressure cuff to high pressures. Testing can also take place in the coronary arteries themselves but this is invasive and not normally conducted unless there is a clinical reason for intracoronary catheterisation.
Of all the current tests employed in the research setting, flow-mediated dilation is the most widely used non-invasive test for assessing endothelial function. This technique measures endothelial function by inducing reactive hyperemia via temporary arterial occlusion and measuring the resultant relative increase in blood vessel diameter via ultrasound. As people with endothelial dysfunction have low NO bioavailability, their blood vessels have a decreased capacity to dilate in response to certain stimuli, compared to those with normal endothelial function.
Because NO has anti-inflammatory and anti-proliferative effects and therefore helps inhibit atherosclerosis, it is easy to see how endothelial dysfunction may contribute to future adverse cardiovascular events. Unfortunately the variability in such tests (e.g. due to time of day, food, menstrual cycle, temperature, etc.) means that no technique has yet been identified that would allow endothelial testing to attain routine clinical significance, although there are some tests under clinical evaluation such as measuring of arterial stiffness.
Read more about this topic: Endothelial Dysfunction