Assessing the Endothelium

pp 429-439

Abstract

Vascular endothelial cells form a continuous monolayer that covers the inner surface of the en-tire vascular system, thereby representing the primary anatomical site that separates the blood compartment from the body's interstitium. Due to its position, the endothelium is permanently exposed to hemodynamic stress exerted by blood flow, blood pressure, and wall distention. In addition to these mechanical stimuli, the endothelium receives a variety of chemical signals, both blood-borne and tissue-derived, which may elicit endothelial responses acting on the vessel wall itself or on more distant target sites. Because of its key localization and numerous physiologic and pathologic functions, the vascular endothelium is recognized as a regulatory organ that plays an important role in cardiovascular function in health and disease. The endothelium modulates vascular smooth-muscle tone and mediates hemostasis, cellular proliferation, and inflammatory and immune mechanisms in the vessel wall. The endothelial cell modulates the tone of underlying vascular smooth-muscle cells by the production of relaxing and constricting factors in response to physiologic or pathologic stimuli. The normal endothelium maintains a nonadhesive luminal surface and has anticoagulant, fibrinolitic, and antithrombotic properties. Injury or activation of the endothelium disrupts its normal regulatory properties and results in abnormal endothelial cell function (endothelial dysfunction). Clinically, endothelial cell dysfunction can be manifested as vasoespam, thrombus formation, atherosclerosis, or restenosis. Endothelial cells undergo morphologic and functional alterations in response to cytokines, which may contribute to the pathogenesis of vasculitis and atherosclerosis. In this review, a noninvasive technique for assessment of endothelial function is presented, that is basically a noninvasive translation of the approach used in coronary circulation, contrasting vascular responses to endothelium-dependent and endothelium-independent stimuli and using high-resolution external ultrasound, and restricted to the study of large and medium-sized superficial arteries such as the brachial, femoral and radial arteries.