| Background Acute coronary syndromes are thought to result from atherosclerotic plaque disruption and intramural thrombus formation. In nearly 60%-70% of all patients presenting with acute ischemic syndromes, a coronary angiogram performed weeks or months before the acute events had shown the culprit lesion site to have mildly stenosis (diameter stenosis between 20%-60%). Therefore, it is probably important for preventing the acute cardiovascular events to stabilize vulnerable plaque in mildly stenosis. Pathologic studies have demonstrated that ruptured plaques have been shown to have several histomorphologic features including a large plaque, a large lipid core, a thin fibrous cap with inflammatory cell infiltration and more neovascularity. In addition, plaque disruption is triggered by intrinsic plaque changes, such as increased pressure in atheroma and by extrinsic stress. Prior investigators have suggested that several kinds of stress, including shear stress and circumferential and localized wall stress, play an important role in plaque disruption. Therefore, we presume that the plaque vulnerability of mildly lesions will be related to its intrinsic structural features and biomechanical characteristics. However, very little is known about their relationship between structural features and plaque vulnerability and about effect of biomechanical characteristics and plaque behavior on vulnerable plaque. It is well known that coronary angiography (CAG) can not accurately determine lesion morphology because it only shows the silhouette of the contrast materiel passing through the stenotic lesions. In recent years, intravascular ultrasound (IVUS) has evolved as a valuable adjunct to angiography. IVUS allows precise tomographic measurement of lumen area and plaque size, distribution and, to some extent, composition. Therefore, IVUS provide us likelihood for study on structural features and biomechanics characteristics in angiographic mildly stenosis in vivo.Clinical decision making in patients with intermediate coronary stenosis(40%-60% diameter stenosis) is still challenging. CAG and IVUS are often used to evaluate coronary stenosis severity, but provide a little information in assessment of the functional significance of the lesion. The later is essential in clinic decision making. Myocardial fractional flow reserve (FFRmyo) is a special index of the functional severity of coronary stenosis. Although the application of FFRmyo to assess intermediate coronary lesion is widely performed in some centers in developed countries, its use in China was lagged. Because it is relatively expensive to measure FFRmyo, it will be beneficial to save the expenses and to short operation time if CAG and IVUS criteria could be clinically used as tools to discriminate functional significant of intermediate stenosis. Objective The purposes of present study are as follow: 1) to investigate the vulnerable mechanism of mildly lesion by evaluating the effect of structure and biomechanical characteristics on plaque; 2) to investigate the structural characteristics of angiographic contrast materiel drain-lagged segments; 3) to assess the functional significance of the mildly lesion and to find the relation between FFRmyo and CAG indexes (percent diameter stenosis and minimal lumen diameter) and IVUS indexes (percent area stenosis and minimal lumen cross-sectional area) of lesion severity.Methods 1) In 20 patients of angiographic contrast materiel drain-lagged and in 62 patients of angiographic mildly coronary stenosis (diameter stenosis 20%-60%), IVUS imaging (2.9F catheter, 30-MHz transducer) was performed and intracoronary pressure was recorded. The characteristics of the plaques and reference segments were analyzed and the biomechanical properties (distensibility index and stiffness) of coronary artery were calculated and the plaque behavior during cardiac cycle was determined. The plaques were classified as either unstable plaques group or stable plaques group by IVUS image. The difference of biomechanical properties and plaque...
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