Methods to assess endothelium-dependent vasodilation in peripheral arteries. (A) Noninvasive determination of flow-mediated vasodilation by high-resolution ultrasound. A high-resolution ultrasound probe is used to measure the longitudinal diameter of the brachial artery proximal to the antecubital fossa in a resting patient. Reliability of this method requires that the ultrasound image be recorded and analyzed using an image analysis software after digitalization of the images and transfer to a computer. After the baseline measurement, a blood pressure cuff is inflated to suprasystolic pressure in order to induce ischemia of the arm that is maintained during 5 minutes. The pressure in the cuff is to be controlled repeatedly in order to ensure complete stasis of blood, which can be controlled and documented by using the Doppler function of the ultrasound probe. After 5 minutes, the cuff is released rapidly. Accumulated ischemic metabolites in the forearm (like lactate, ADP, hypoxia, etc.) lead to peripheral vasodilation of resistance vessels in the forearm—the phenomenon well known as hyperemia. Hyperemia causes increased flow velocity in the brachial artery in order to compensate the increased demand for oxygen and nutrients in the arm; this increase in flow results in shear stress at the brachial artery endothelium, which in turn causes the release of nitric oxide (NO) and results in endothelium-dependent vasodilation of the brachial artery. The maximum dilation during hyperemia usually occurs at 60 seconds after cuff release; brachial artery diameter is recorded again at this time, and the difference between basal and hyperemic diameter is expressed as percent flow-mediated vasodilation (FMD). After 30 minutes of rest, another baseline diameter is recorded, and 0.8 μg of glycerol trinitrate (GTN) is applied sublingually. At 3 minutes after GTN maximal vasodilation is usually reached, arterial diameter is measured once again. The difference between the second baseline diameter recording and the GTN-induced diameter is called the endothelium-independent vasodilation. This is an important control feature, as GTN-induced vasodilation is also mediated via NO, but it does not test the endothelial capacity to release biologically active NO. Thus, changes in smooth muscle responsiveness to NO can be identified, which may blur interpretation of FMD when this test is repeatedly performed, like before and after pharmacotherapeutic intervention with some kind of vasoactive medication. (B) Venous occlusion plethysmographic determination of acetylcholine-induced forearm vasodilation. An indwelling needle is placed into the brachialartery under local anesthesia for the infusion of acetylcholine, GTN, and potentially other drugs. A cuff around the wrist cuts off perfusion of the hand during the measurement period. Acetylcholine induces endothelial activation of NO synthase; release of NO leads to vasodilation of small arterioles in the forearm. The venous occlusion plethysmograph detects the dilation of an elastic strain gauge placed around the forearm during occlusion of venous blood flow at the upper arm. Occlusion pressure is increased to supravenous, but infradiastolic pressure transiently and repeatedly; the dilation of the strain gauge is continuously recorded; the slope of the resulting curve is a measure of forearm vasodilation. This method is more invasive than the method described in (A); however, it has the advantage that inhibitors of NO synthase, antioxidants, or other pharmacologically active substances can be co-infused thus allowing to gain a more detailed insight into the pathophysiological mechanisms underlying endothelial dysfunction. Source: The Endothelium in Health and Disease, Peripheral Arterial Disease Citation: Dieter RS, Dieter RA, Jr., Dieter RA, III. Peripheral Arterial Disease; 2016 Available at: http://accesscardiology.mhmedical.com/DownloadImage.aspx?image=/data/Books/1789/dieperi_ch5_f007b.png&sec=123045326&BookID=1789&ChapterSecID=123045275&imagename= Accessed: October 03, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved