Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.     

Effects of varied lipid core volume and fibrous cap thickness on stress distribution in carotid arterial plaques

The rupture of atherosclerotic plaques is known to be associated with the stresses that act on or within the arterial wall. The extreme wall tensile stress is usually recognized as a primary trigger for the rupture of the plaque. The present study used one-way fluid-structure interaction simulation to investigate the impacts of fibrous cap thickness and lipid core volume to the wall tensile stress value and distributions on the fibrous cap. Von Mises stress was employed to represent the wall tensile stress (VWTS). A total of 13 carotid bifurcation cases were manipulated based on a base geometry in the study with varied combinations of fibrous cap thickness and lipid core volume in the plaque. Values of maximum VWTS and a stress value of VWTS_90, which represents the cut-off VWTS value of 90% in cumulative histogram of VWTS possessed at the computational nodes on the luminal surface of fibrous cap, were used to assess the risk of plaque rupture for each case. Both parameters are capable of separating the simulation cases into vulnerable and more stable plaque groups, while VWTS_90 is more robust for plaque rupture risk assessment. The results show that the stress level on the fibrous cap is much more sensitive to the changes in the fibrous cap thickness than the lipid core volume. A slight decrease of cap thickness can cause a significant increase of stress. For all simulation cases, high VWTS appears at the fibrous cap near the lipid core (plaque shoulder) regions.     

Multicolor fluorescence technique to detect apoptotic cells in advanced coronary atherosclerotic plaques

Apoptosis occurring in atherosclerotic lesions has been suggested to be involved in the evolution and the structural stability of the plaques. It is still a matter of debate whether apoptosis mainly involves vascular smooth muscle cells (vSMCs) in the fibrous tissue or inflammatory (namely foam) cells, thus preferentially affecting the cell-poor lipid core of the atherosclerotic plaques. The aim of the present investigation was to detect the presence of apoptotic cells and to estimate their percentage in a series of atherosclerotic plaques obtained either by autopsy or during surgical atherectomy. Apoptotic cells were identified on paraffin-embedded sections on the basis of cell nuclear morphology after DNA staining and/or by cytochemical reactions (TUNEL assay, immunodetection of the proteolytic poly (ADP-ribose) polymerase-1 [PARP-1] fragment); biochemical procedures (identifying DNA fragmentation or PARP-1 proteolysis) were also used. Indirect immunofluorescence techniques were performed to label specific antigens for either vSMCs or macrophages (i.e., the cells which are most likely prone to apoptosis in atherosclerotic lesions): the proper selection of fluorochrome labeling allowed the simultaneous detection of the cell phenotype and the apoptotic characteristics, by multicolor fluorescence techniques. Apoptotic cells proved to be less than 5% of the whole cell population, in atherosclerotic plaque sections: this is, in fact, a too low cell fraction to be detected by widely used biochemical methods, such as agarose gel electrophoresis of low-molecular-weight DNA or Western-blot analysis of PARP-1 degradation. Most apoptotic cells were of macrophage origin, and clustered in the tunica media, near or within the lipid-rich core; only a few TUNEL-positive cells were labeled for antigens specific for vSMCs. These results confirm that, among the cell populations in atherosclerotic plaques, macrophage foam-cells are preferentially involved in apoptosis. Their death may decrease the cell number in the lipid core and generate a possibly defective apoptotic clearance: the resulting release of matrix-degrading enzymes could contribute to weakening the fibrous cap and promote the plaque rupture with the risk of acute ischemic events, while increasing the thrombogenic pultaceous pool of the plaque core.     

Effect of calcification on the mechanical stability of plaque based on a three-dimensional carotid bifurcation model

Background

This study characterizes the distribution and components of plaque structure by presenting a three-dimensional blood-vessel modelling with the aim of determining mechanical properties due to the effect of lipid core and calcification within a plaque. Numerical simulation has been used to answer how cap thickness and calcium distribution in lipids influence the biomechanical stress on the plaque.

Method

Modelling atherosclerotic plaque based on structural analysis confirms the rationale for plaque mechanical examination and the feasibility of our simulation model. Meaningful validation of predictions from modelled atherosclerotic plaque model typically requires examination of bona fide atherosclerotic lesions. To analyze a more accurate plaque rupture, fluid-structure interaction is applied to three-dimensional blood-vessel carotid bifurcation modelling. A patient-specific pressure variation is applied onto the plaque to influence its vulnerability.

Results

Modelling of the human atherosclerotic artery with varying degrees of lipid core elasticity, fibrous cap thickness and calcification gap, which is defined as the distance between the fibrous cap and calcification agglomerate, form the basis of our rupture analysis. Finite element analysis shows that the calcification gap should be conservatively smaller than its threshold to maintain plaque stability. The results add new mechanistic insights and methodologically sound data to investigate plaque rupture mechanics.

Conclusion

Structural analysis using a three-dimensional calcified model represents a more realistic simulation of late-stage atherosclerotic plaque. We also demonstrate that increases of calcium content that is coupled with a decrease in lipid core volume can stabilize plaque structurally.     

Inflammatory markers: linking unstable plaques to coronary event, an interventional perspective

Abundant data links inflammatory mechanisms to atheromatous plaque destabilization leading to plaque rupture and coronary events. The discovery of inflammatory cells and inflammatory mediators within atherosclerotic plaques prone to rupture led to a series of studies demonstrating an association between various markers of inflammation and future coronary events. Inflammatory markers have also been used in patients undergoing coronary angioplasty in an attempt to predict restenosis and risk for post-procedural coronary events. This review article provides an overview on the potential use of inflammatory markers in the context of coronary interventions.     

Stress analysis of carotid plaque rupture based on in vivo high resolution MRI

Atheromatous carotid plaque rupture is responsible for the majority of ischaemic strokes in the developed world. Plaque rupture has been associated with plaque morphology, plaque components' properties, inflammation and local stress concentration. High resolution multi-spectral magnetic resonance imaging (MRI) has allowed the plaque components to be visualized in vivo. This study combined the recent advances in finite element analysis (FEA) and MRI, and performed stress analysis of five vulnerable carotid plaques based on the geometry derived from in vivo MRI. Image segmentation was based on multi-spectral MRI and co-registered with histology for plaque characterization. Plaque fibrous cap, lipid pool and vessel wall were modelled as isotropic, incompressible hyperelastic materials undergoing large deformation under pulse pressure loading. High stress concentrations were predicted at the shoulders and the thinnest fibrous cap regions of the plaque, and the mean maximal stresses were found to be higher in the ruptured plaques (683.3 kPa) than those in the unruptured plaques (226.9 kPa). The effect of the relative stiffness of fibrous cap to lipid pool on the stress within the cap itself was studied. It was shown that larger relative stiffness of fibrous cap to lipid pool resulted in higher stress within the cap. Thus, it is likely that high stress concentrations in vulnerable plaque may cause plaque rupture and lead to acute ischaemic sequelae. A combination of in vivo high resolution MRI and FEA could potentially act as a useful tool to assess plaque vulnerability and risk stratify patients with carotid atheroma.     

Carotid arterial plaque stress analysis using fluid–structure interactive simulation based on in-vivo magnetic resonance images of four patients

The rupture of atherosclerotic plaques is known to be associated with the stresses that act on or within the arterial wall. The extreme wall tensile stress (WTS) is usually recognized as a primary trigger for the rupture of vulnerable plaque. The present study used the in-vivo high-resolution multi-spectral magnetic resonance imaging (MRI) for carotid arterial plaque morphology reconstruction. Image segmentation of different plaque components was based on the multi-spectral MRI and co-registered with different sequences for the patient. Stress analysis was performed on totally four subjects with different plaque burden by fluid–structure interaction (FSI) simulations. Wall shear stress distributions are highly related to the degree of stenosis, while the level of its magnitude is much lower than the WTS in the fibrous cap. WTS is higher in the luminal wall and lower at the outer wall, with the lowest stress at the lipid region. Local stress concentrations are well confined in the thinner fibrous cap region, and usually locating in the plaque shoulder; the introduction of relative stress variation during a cycle in the fibrous cap can be a potential indicator for plaque fatigue process in the thin fibrous cap. According to stress analysis of the four subjects, a risk assessment in terms of mechanical factors could be made, which may be helpful in clinical practice. However, more subjects with patient specific analysis are desirable for plaque-stability study.     

3D computational parametric analysis of eccentric atheroma plaque: influence of axial and circumferential residual stresses

Plaque rupture plays a role in the majority of acute coronary syndromes. Rupture has usually been associated with stress concentrations, which are mainly affected by the plaque geometry and the tissue properties. The aim of this study is to evaluate the influence of morphology on the risk of plaque rupture, including the main geometrical factors, and to assess the role of circumferential and axial residual stresses by means of a parametric 3D finite element model. For this purpose, a 3D parametric finite element model of the coronary artery with eccentric atheroma plaque was developed. Healthy (adventitia and media in areas without atheroma plaque) and diseased (fibrotic and lipidic) tissues were considered in the model. The geometrical parameters used to define and design the idealized coronary plaque anatomy were the lipid core length, the stenosis ratio, the fibrous cap thickness, and the lipid core ratio. Finally, residual stresses in longitudinal and circumferential directions were incorporated into the model to analyse the influence of the important mechanical factors in the vulnerability of the plaque. Viewing the results, we conclude that residual stresses should be considered in the modelling of this kind of problems since they cause a significant alteration of the vulnerable plaque region limits. The obtained results show that the fibrous cap thickness and the lipid core length, in combination with the lipid core width, appear to be the key morphological parameters that play a determinant role in the maximal principal stress (MPS). However, the stenosis ratio is found to not play a significant role in vulnerability related to the MPS. Plaque rupture should therefore be observed as a consequence, not only of the cap thickness, but as a combination of the stenosis ratio, the fibrous cap thickness and the lipid core dimensions.     

Noninvasive detection of macrophages using a nanoparticulate contrast agent for computed tomography

Sudden fibrous cap disruption of 'high-risk' atherosclerotic plaques can trigger the formation of an occlusive thrombus in coronary arteries, causing acute coronary syndromes. High-risk atherosclerotic plaques are characterized by their specific cellular and biological content (in particular, a high density of macrophages), rather than by their impact on the vessel lumen. Early identification of high-risk plaques may be useful for preventing ischemic events. One major hurdle in detecting high-risk atherosclerotic plaques in coronary arteries is the lack of an imaging modality that allows for the identification of atherosclerotic plaque composition with high spatial and temporal resolutions. Here we show that macrophages in atherosclerotic plaques of rabbits can be detected with a clinical X-ray computed tomography (CT) scanner after the intravenous injection of a contrast agent formed of iodinated nanoparticles dispersed with surfactant. This contrast agent may become an important adjunct to the clinical evaluation of coronary arteries with CT.     

The roles of myeloperoxidase in coronary artery disease and its potential implication in plaque rupture

Atherosclerosis is the main pathophysiological process underlying coronary artery disease (CAD). Acute complications of atherosclerosis, such as myocardial infarction, are caused by the rupture of vulnerable atherosclerotic plaques, which are characterized by thin, highly inflamed, and collagen-poor fibrous caps. Several lines of evidence mechanistically link the heme peroxidase myeloperoxidase (MPO), inflammation as well as acute and chronic manifestations of atherosclerosis. MPO and MPO-derived oxidants have been shown to contribute to the formation of foam cells, endothelial dysfunction and apoptosis, the activation of latent matrix metalloproteinases, and the expression of tissue factor that can promote the development of vulnerable plaque. As such, detection, quantification and imaging of MPO mass and activity have become useful in cardiac risk stratification, both for disease assessment and in the identification of patients at risk of plaque rupture. This review summarizes the current knowledge about the role of MPO in CAD with a focus on its possible roles in plaque rupture and recent advances to quantify and image MPO in plasma and atherosclerotic plaques.     

IL-7R blockade reduces post-myocardial infarction-induced atherosclerotic plaque inflammation in ApoE−/- mice

Modulating inflammation by targeting IL-1β reduces recurrent athero-thrombotic cardiovascular events without lipid lowering. This presents an opportunity to explore other pathways associated with the IL-1β signaling cascade to modulate the inflammatory response post-myocardial infarction (MI). IL-7 is a mediator of the inflammatory pathway involved in monocyte trafficking into atherosclerotic plaques and levels of IL-7 have been shown to be elevated in patients with acute MI. Recurrent athero-thrombotic events are believed to be mediated in part by index MI-induced exacerbation of inflammation in atherosclerotic plaques. The objective of the study was to assess the feasibility of IL-7R blockade to modulate atherosclerotic plaque inflammation following acute MI in ApoE^?/- mice. Mice were fed Western diet for 12 weeks and then subjected to coronary occlusion to induce an acute MI. IL-7 expression was determined using qRT-PCR and immuno-staining, and IL-7R was assessed using flow cytometry. Plaque inflammation was evaluated using immunohistochemistry. IL-7R blockade was accomplished with monoclonal antibody to IL-7R. IL-7 mRNA expression was significantly increased in the cardiac tissue of mice subjected to MI but not in controls. IL-7 staining was observed in the coronary artery. Plaque macrophage and lipid content were significantly increased after MI. IL-7R antibody treatment but not control IgG significantly reduced macrophage and lipid content in atherosclerotic plaques. The results show that IL-7R antibody treatment reduces monocyte/macrophage and lipid content in the atherosclerotic plaque following MI suggesting a potential new target to mitigate increased plaque inflammation post-MI.     

Quantifying effects of plaque structure and material properties on stress distributions in human atherosclerotic plaques using 3D FSI models

BACKGROUND: Atherosclerotic plaques may rupture without warning and cause acute cardiovascular syndromes such as heart attack and stroke. Methods to assess plaque vulnerability noninvasively and predict possible plaque rupture are urgently needed. METHOD: MRI-based three-dimensional unsteady models for human atherosclerotic plaques with multi-component plaque structure and fluid-structure interactions are introduced to perform mechanical analysis for human atherosclerotic plaques. RESULTS: Stress variations on critical sites such as a thin cap in the plaque can be 300% higher than that at other normal sites. Large calcification block considerably changes stress/strain distributions. Stiffness variations of plaque components (50% reduction or 100% increase) may affect maximal stress values by 20-50%. Plaque cap erosion causes almost no change on maximal stress level at the cap, but leads to 50% increase in maximal strain value. CONCLUSIONS: Effects caused by atherosclerotic plaque structure, cap thickness and erosion, material properties, and pulsating pressure conditions on stress/strain distributions in the plaque are quantified by extensive computational case studies and parameter evaluations. Computational mechanical analysis has good potential to improve accuracy of plaque vulnerability assessment.     

Calcifications in atherosclerotic plaques and impact on plaque biomechanics

The catastrophic mechanical rupture of an atherosclerotic plaque is the underlying cause of the majority of cardiovascular events. The infestation of vascular calcification in the plaques creates a mechanically complex tissue composite. Local stress concentrations and plaque tissue strength properties are the governing parameters required to predict plaque ruptures. Advanced imaging techniques have permitted insight into fundamental mechanisms driving the initiating inflammatory-driven vascular calcification of the diseased intima at the (sub-) micron scale and up to the macroscale. Clinical studies have potentiated the biomechanical relevance of calcification through the derivation of links between local plaque rupture and specific macrocalcification geometrical features. The clinical implications of the data presented in this review indicate that the combination of imaging, experimental testing, and computational modelling efforts are crucial to predict the rupture risk for atherosclerotic plaques. Specialised experimental tests and modelling efforts have further enhanced the knowledge base for calcified plaque tissue mechanical properties. However, capturing the temporal instability and rupture causality in the plaque fibrous caps remains elusive. Is it necessary to move our experimental efforts down in scale towards the fundamental (sub-) micron scales in order to interpret the true mechanical behaviour of calcified plaque tissue interactions that is presented on a macroscale in the clinic and to further optimally assess calcified plaques in the context of biomechanical modelling.     

Study of carotid arterial plaque stress for symptomatic and asymptomatic patients

Stroke is one of the leading causes of death in the world, resulting mostly from the sudden ruptures of atherosclerosis carotid plaques. Until now, the exact plaque rupture mechanism has not been fully understood, and also the plaque rupture risk stratification. The advanced multi-spectral magnetic resonance imaging (MRI) has allowed the plaque components to be visualized in-vivo and reconstructed by computational modeling. In the study, plaque stress analysis using fully coupled fluid structure interaction was applied to 20 patients (12 symptomatic and 8 asymptomatic) reconstructed from in-vivo MRI, followed by a detailed biomechanics analysis, and morphological feature study. The locally extreme stress conditions can be found in the fibrous cap region, 85% at the plaque shoulder based on the present study cases. Local maximum stress values predicted in the plaque region were found to be significantly higher in symptomatic patients than that in asymptomatic patients (200 ± 43 kPa vs. 127 ± 37 kPa, p=0.001). Plaque stress level, defined by excluding 5% highest stress nodes in the fibrous cap region based on the accumulative histogram of stress experienced on the computational nodes in the fibrous cap, was also significantly higher in symptomatic patients than that in asymptomatic patients (154 ± 32 kPa vs. 111 ± 23 kPa, p<0.05). Although there was no significant difference in lipid core size between the two patient groups, symptomatic group normally had a larger lipid core and a significantly thinner fibrous cap based on the reconstructed plaques using 3D interpolation from stacks of 2D contours. Plaques with a higher stenosis were more likely to have extreme stress conditions upstream of plaque throat. The combined analyses of plaque MR image and plaque stress will advance our understanding of plaque rupture, and provide a useful tool on assessing plaque rupture risk.     

胆固醇结晶激活动脉粥样硬化斑块巨噬细胞NLRP3炎症体的途径

动脉粥样硬化既是胆固醇在血管壁聚集的疾病,也是发生在动脉壁的一种低强度慢性炎症形式。近年来有研究证实胆固醇结晶在动脉粥样硬化发生发展中具有重要作用。新的显微技术证实,胆固醇结晶在动脉粥样硬化斑块形成的早期即已出现,并与早期炎症有关。胆固醇结晶通过诱发局部炎症,促进大的脂质核心形成;刺破纤维帽,导致斑块破裂进而促进动脉粥样硬化斑块的进展。在影响斑块进程中,NLRP3炎症体的激活对此发挥了重要的作用。NLRP3炎症体是研究最多最明确的炎症体,其与非炎症性疾病的发生发展密切相关。以胆固醇结晶激活NLRP3炎症体的途径作为研究靶点,为动脉粥样硬化的诊断和治疗提供了新的思路和方法。该文就胆固醇结晶在动脉粥样硬化斑块中激活巨噬细胞NLRP3炎症体的两种途径做一综述。     

Molecular mechanisms of plaque instability

PURPOSE OF REVIEW: Coronary artery thrombosis superimposed on a disrupted atherosclerotic plaque initiates abrupt arterial occlusion and is the proximate event responsible for 60-80% cases of acute coronary syndromes. This article provides a concise update on the evolving concepts in the pathophysiology of plaque rupture and thrombosis. RECENT FINDINGS: Over the past several years, the critical role of plaque composition rather than plaque size or stenosis severity, in plaque rupture and thrombosis have been recognized. The necrotic lipid core and plaque inflammation appear to be key factors. Extracellular matrix loss in the fibrous cap, a prelude to rupture, is attributed to matrix degrading enzymes as well as to death of matrix synthesizing smooth muscle cells; inflammation appears to play a critical role in both these processes. Inflammatory cell derived tissue factor is a key contributor to plaque thrombogenicity. Inflammation has also been implicated in plaque neovascularity, intraplaque hemorrhage and plaque expansion. Recent observations have also highlighted the important modulatory role of immune system in atherosclerosis and plaque composition. SUMMARY: Improved understanding of mechanisms causing plaque instability should provide novel insights into prevention of athero-thrombotic cardiovascular events.     

Mechanical stresses in carotid plaques using MRI-based fluid-structure interaction models

Risk assessment in patients with carotid atherosclerosis relies on the degree of luminal stenosis. Incorporating morphological information on plaque composition obtained noninvasively through the use of magnetic resonance imaging (MRI) could include other variables besides the degree of stenosis into carotid plaque risk assessment. Knowledge of the morphologic composition of the plaque allows determination of mechanic stresses exerted on the protective fibrous cap, which may be of importance in the assessment of plaque vulnerability. Based on image processing of transverse MRI scans, longitudinal 2D fluid-structure interaction (FSI) simulations of carotid atherosclerotic plaques were performed facilitating in-vivo estimation of longitudinal internal fibrous cap stresses. The FSI simulation combined finite element analysis (FEA) with computational fluid dynamics (CFD) simulations of blood-flow variables. Preliminary results from two symptomatic patients revealed longitudinal stress levels (max. 254.1 and 143.2 kPa) approaching established criteria for plaque rupture at known predilection sites of plaque rupture. Determination of longitudinal fibrous cap stresses may prove useful in assessing plaque vulnerability and improve risk stratification in patients with carotid atherosclerosis.     

小型猪动脉粥样硬化斑块稳定性模型研究

目前已有的动物模型在研究动脉粥样硬化斑块破裂、破裂的可控性及量化研究方面均不能满足研究的需要.为了建立类似于人类动脉粥样硬化病变的斑块模型,体外研究斑块稳定性,应用传统的高脂高胆固醇膳食诱导建立了小型猪动脉粥样硬化模型,并从血脂水平和斑块病理形态学特征方面加以了证实.该模型中斑块与人类成熟斑块的高度相似性使其成为研究斑块稳定性和斑块破裂的较好模型.从量化比较这一出发点着手,建立了一个体外可控可量化诱导斑块破裂模型,方法简单易行,是一个较好的量化研究斑块破裂和破裂相关因素间关系的实验模型.     

Initial stress in biomechanical models of atherosclerotic plaques

Rupture of atherosclerotic plaques is the underlying cause for the majority of acute strokes and myocardial infarctions. Rupture of the plaque occurs when the stress in the plaque exceeds the strength of the material locally. Biomechanical stress analyses are commonly based on pressurized geometries, in most cases measured by in-vivo MRI. The geometry is therefore not stress-free. The aim of this study is to identify the effect of neglecting the initial stress state on the plaque stress distribution. Fifty 2D histological sections (7 patients, 9 diseased coronary artery segments), perfusion fixed at 100 mmHg, were segmented and finite element models were created. The Backward Incremental method was applied to determine the initial stress state and the zero-pressure state. Peak plaque and cap stresses were compared with and without initial stress. The effect of initial stress on the peak stress was related to the minimum cap thickness, maximum necrotic core thickness, and necrotic core angle. When accounting for initial stress, the general relations between geometrical features and peak cap stress remain intact. However, on a patient-specific basis, accounting for initial stress has a different effect on the absolute cap stress for each plaque. Incorporating initial stress may therefore improve the accuracy of future stress based rupture risk analyses for atherosclerotic plaques.     

Oxysterols and symptomatic versus asymptomatic human atherosclerotic plaque

Atherosclerosis is the most common cause of mortality in the Western world, contributing to about 50% of all deaths. Atherosclerosis is characterized by deposition of lipids onto the coronary or carotid arterial wall and formation of an atherosclerotic plaque. Atherosclerotic plaques are categorized into two groups: symptomatic and asymptomatic. The symptomatic plaques tend to be unstable and prone to rupture, and are associated with an increase in ischemic events. Oxysterols, products of cholesterol oxidation, are cytotoxic materials. Their level and type may be associated with plaque formation, development and stability. Oxysterols stimulate the formation of foam cells, advance atherosclerotic plaque progression, and contribute to plaque vulnerability and instability due to their cytotoxicity and their ability to induce cell apoptosis. Studies indicate that plasma 7β-OH CH level can be used as a biomarker for detecting carotid and coronary artery disease. Further clinical studies are needed to evaluate the potential of oxysterols for use as biomarkers for plaque vulnerability and instability. The identification of biomarkers in the blood that can distinguish between symptomatic and asymptomatic plaques remains an unresolved issue.