FIG 74.10 Reactive hyperemia peripheral arterial tonometry recordings of individuals with normal and abnormal responses (From Bonetti PO, Pumper GM, Higano ST, et al Noninvasive identification of patients with early coronary atherosclerosis by assessment of digital reactive hyperemia J Am Coll Cardiol 2004;44:2137–2141.) The magnitude of reactive hyperemia in response to induced shear stress on the endothelium of conduit arteries is also regarded as a measure of peripheral microvascular function.284 This is commonly quantified by the velocity-time integral of hyperemic flow adjusted for heart rate, which has been shown to be associated with cardiovascular risk factors and outcomes.285,286 Circulating Biomarkers The circulating pool of nitric oxide as a measure of its bioavailability can be assessed by measuring plasma nitroso compounds and nitrite.287–289 Although the exact chemical species of this pool are unclear and their values may be confounded by diet,290 studies have shown that plasma nitrite reflects regional endothelial nitric oxide synthase activity.289 Plasma nitrite reserve during reactive hyperemia288 and plasma nitroso compounds287 have been found to be reduced in the presence of endothelial dysfunction Reduced nitric oxide availability can also be inferred from elevated levels of dimethylarginine, the naturally occurring antagonist of nitric oxide synthase,291 which has been shown to be associated with cardiovascular disease and mortality in adults.292 Activation of endothelial cells upregulates the expression of adhesion molecules, resulting in increased circulating levels of intercellular adhesion molecule 1, vascular cell adhesion molecule 1, E-selectin, and P-selectin The levels of inflammatory markers—including high-sensitivity C-reactive protein, interleukin-6, tumor necrosis factor-α, and markers of oxidative stress, such as oxidized low-density lipoprotein and 8-iso-prostaglandin F2α—can be used to reflect ongoing endothelial activation and vascular inflammation.293,294 Disturbance of the protective role of the endothelium against thrombosis in endothelial dysfunction is reflected by the imbalance between endotheliumderived tissue plasminogen activator and plasminogen activation inhibitor-1295 and release into circulation of von Willebrand factor.296 Measurement of the dynamic release of tissue-plasminogen activator by agonists such as bradykinin, substance P, methacholine, and desmopressin has been used to assess endothelial function,297–300 although data on its relationship with cardiovascular risk factors and prognostic values are limited Damage to endothelial cells results in detachment into the circulation in their entirety or as microparticles Endothelial microparticles have been found to be increased in vasculitis and atherosclerotic diseases.301–304 There is evidence to suggest that endothelial microparticles may contribute directly to endothelial dysfunction.302,305 The quantity of circulating endothelial cells and microparticles is therefore a potentially useful marker of endothelial function With regard to the repair of damaged endothelium, apart from the proliferation of local mature endothelial cells, the importance of bone marrow–derived endothelial stems cells and endothelial progenitor cells is increasingly recognized.306–308 Mobilization of endothelial progenitor cells is in part nitric oxide–dependent.309 Furthermore, the number of progenitor cells has been shown to correlate negatively with the cardiovascular risk score and positively with brachial artery flow-mediated dilation in adults.310 The usefulness of endothelial microparticles and endothelial progenitor cells as markers of endothelial function in the pediatric population awaits further clarification MicroRNAs (miRNAs) are small noncoding single-stranded RNAs that form complementary pairs with specific target messenger RNAs to negatively regulate their expression through translational repression or degradation The understanding of miRNAs in cardiac and vascular biology has significantly expanded in the past decade.311,312 The role of circulating miRNAs as diagnostic and prognostic markers in cardiovascular disease is currently being actively explored Recently, in obese children, three circulating miRNAs—including hsamiR-125a-5p, hsa-miR-342-3p, and hsa-miR-365b-3p—have been shown to be potential biomarkers of endothelial dysfunction.313 The mechanistic link between miRNA expression and endothelial dysfunction remains to be explored, however Arterial Structure Functional alteration of the arterial system with endothelial dysfunction and arterial stiffening is regarded as an early event in the development of cardiovascular disease Early detection of subclinical alteration of the arterial structure in the young may provide a window for assessment of cardiovascular risk.314–316 High-resolution ultrasonography has enabled the interrogation of structural alterations of superficial arteries The intima-media thickness of the carotid artery is commonly measured and regarded as a surrogate marker of subclinical atherosclerosis.315,316 In adults, the associations between carotid intima-media thickening and traditional cardiovascular risk factors and coronary artery disease are well documented.317–320 Its predictive value of future cardiovascular events has also been shown.321–323 In clinical trials, carotid intima-media thickness is used as an endpoint to assess the impact on cardiovascular risk reduction of pharmacologic interventions.324–326 There is overwhelming evidence to suggest that atherosclerosis begins in childhood.327,328 The extent of atherosclerotic changes in children and young adults has been found to correlate with cardiovascular risk factors similar to those identified in adults (see Chapter 25) Hence development and application of robust noninvasive methods to assess early atherosclerotic changes of the arteries in the young may improve risk stratification and enable early institution of interventions Measurement of Intima-Media Thickness Recommendations for the standardization of measurement of intima-media thickness in children and adolescents have been summarized in statements made by the American Heart Association315 and the Association for European ... endothelial microparticles and endothelial progenitor cells as markers of endothelial function in the pediatric population awaits further clarification MicroRNAs (miRNAs) are small noncoding single-stranded RNAs that form