Highly localized arterial remodeling in patients with coronary atherosclerosis: an intravascular ultrasound study

Effect of stenosis and dilatation on the hemodynamic parameters associated with left coronary artery

BACKGROUND AND OBJECTIVE

The main objective of the work is to examine the curvature effects of stenosis/dilatation region pertaining to left coronary artery. The hemodymamic features during the cardiac cycle is thoroughly examined.

METHODS

A numerical fluid structure interaction model incorporating multi- layered elastic artery wall, non-Newtonian blood viscosity and pulsating boundary conditions is developed. The composite arterial wall consists of a thin layer tunica intima, atheroma and a thick wall. Higher stiffness of atheroma is captured by using higher Young's modulus. The CFD and FSI models are validated with available experimental and analytical data. Computations are done with five different non-Newtonian models and arterial wall with various elasticity levels. The local and time averaged WSS, velocity contours downstream of stenosis, wall pressure and pressure drop during various phases of cardiac cycle are provided in detail.

RESULTS

The influence of non-Newtonian effects of blood viscosity is found to be significant especially at stenosis regions. The flexible wall caused wall deformation and the associated flow and pressure wave propagation affecting WSS and pressure drop compared to the rigid wall. Flow recirculation is noticed at stenosis downstream locations and its strength increases with increased severity of the stenosis. A stenosis is characterised by a sudden drop in wall pressure and a slower two stage recovery during peak velocity periods of the cardiac cycle.

CONCLUSIONS

The pressure drop, local WSS at stenosis centre, and radial velocity increase are significantly higher for stenosis cases and the effect is severe during peak diastole. The variation in hemodynamic parameters is found to be less significant for dilatation. Significantly lower WSS is noticed for the recirculation regions downstream of stenosis which can enhance the tendency for monocytes to attach to the endothelium. The radius of curvature of the stenosis is found to be the most sensitive parameter affecting the hemodynamic characteristics rather than the detailed geometry of the stenosis. The main effect of variation of artery wall stiffness is noted at recirculation regions present downstream of stenosis. The results from the study may be useful for predicting wall shear stress signatures associated with stenosis/dilatation changes and the management of specific cases.

THE MECHANICAL FACTORS INFLUENCING THE ASSESSMENT OF INTERMEDIATE STENOSIS SEVERITY EXPLAINED THROUGH FRACTIONAL FLOW RESERVE

Assessment of intermediate coronary lesions with diameter stenosis of 40% to 70% severity is being a challenge for cardiologist to identify potentially ischemic stenosis for revascularization and nonculprit stenosis which can be deferred from stenting. An invasive coronary angiography and intravascular ultrasound provide anatomic information of stenosis severity whereas an invasive fractional flow reserve index (FFR) provides the functional significance of the stenosis severity. The measurement of functional significance of stenosis severity minimizes the procedural complications such as coronary dissection, in stent restenosis etc. rather than anatomical significance measure. The FFR cutoff value of [Formula: see text]0.8 is used to distinguish ischemic and nonischemic stenosis. The FFR is clinically well validated even though it is influenced by the mechanical factors such as hyperemic flow and guide wire insertion. In recent times, noninvasive coronary computed tomography (CCTA) modality has become popular in the diagnosis of coronary artery disease. The CCTA permits the assessment of cross-sectional parameters such as minimum lumen area and lumen diameter, lesion length and plaque morphology. However, the CCTA provides limited information on the functional significance of stenotic lesions as compared to FFR. The purpose of this review is to discuss the mechanical factors influencing the invasive FFR while assessing the functional significance of intermediate stenosis severity. In addition, the hidden mechanical factors influencing the noninvasive CCTA assessment of stenosis severity will be discussed from the critical information obtained from FFR which could be beneficial for the clinician particularly in the assessment of intermediate stenosis severity.

Buckling Reduces eNOS Production and Stimulates Extracellular Matrix Remodeling in Arteries in Organ Culture

Artery buckling alters the fluid shear stress and wall stress in the artery but its temporal effect on vascular wall remodeling is poorly understood. The purpose of this study was to investigate the early effect of artery buckling on endothelial nitric oxide synthase (eNOS) expression and extracellular matrix remodeling. Bilateral porcine carotid arteries were maintained in an ex vivo organ culture system with and without buckling while under the same physiological pressure and flow rate for 3-7 days. Matrix metalloproteinase-2 (MMP-2), MMP-9, fibronectin, elastin, collagen I, III and IV, tissue inhibitor of metalloproteinase-2 (TIMP-2), and eNOS were determined using Western blotting and immunohistochemistry. Our results showed that MMP-2 expression level was significantly higher in buckled arteries than in the controls and higher at the inner curve than at the outer curve of buckled arteries, while collagen IV content showed an opposite trend, suggesting that artery buckling increased MMP-2 expression and collagen IV degradation in a site-specific fashion. However, no differences for MMP-9, fibronectin, elastin, collagen I, III, and TIMP-2 were observed among the outer and inner curve sides of buckled arteries and straight controls. Additionally, eNOS expression was significantly decreased in buckled arteries. These results suggest that artery buckling triggers uneven wall remodeling that could lead to development of tortuous arteries.

Numerical investigation of the effect of stenosis geometry on the coronary diagnostic parameters

The present study deals with the functional severity of a coronary artery stenosis assessed by the fractional flow reserve (FFR). The effects of different geometrical shapes of lesion on the diagnostic parameters are unknown. In this study, 3D computational simulation of blood flow in three different geometrical shapes of stenosis (triangular, elliptical, and trapezium) is considered in steady and transient conditions for 70% (moderate), 80% (intermediate), and 90% (severe) area stenosis (AS). For a given percentage AS, the variation of diagnostic parameters which are derived from pressure drop across the stenosis was found in three different geometrical shapes of stenosis and it was observed that FFR is higher in triangular shape and lower in trapezium shape. The pressure drop coefficient (CDP) was higher in trapezium shape and lower in triangular model whereas the LFC shows opposite trend. From the clinical perspective, the relationship between percentage AS and FFR is linear and inversely related in all the three models. A cut-off value of 0.75 for FFR was observed at 76.5% AS in trapezium model, 79.5% in elliptical model, and 82.7% AS for the triangular shaped model. The misinterpretation of the functional severity of the stenosis is in the region of 76.5%-82.7 % AS from different shapes of stenosis models.

Effect of Pulsatile Flow Waveform and Womersley Number on the Flow in Stenosed Arterial Geometry

The salient hemodynamic flow features in a stenosed artery depend not only on the degree of stenosis, but also on its location in the circulatory system and the physiological condition of the body. The nature of pulsatile flow waveform and local Womersley number vary in different regions of the arterial system and at different physiological state, which affects the local hemodynamic wall parameters, for example, the wall shear stress (WSS) and oscillatory shear index (OSI). Herein, we have numerically investigated the effects of different waveforms and Womersley numbers on the flow pattern and hemodynamic parameters in an axisymmetric stenosed arterial geometry with 50% diametral occlusion. Temporal evolution of the streamlines and hemodynamic parameters are investigated, and the time-averaged hemodynamic wall parameters are compared. Presence of the stenosis is found to increase the OSI of the flow even at the far-downstream side of the artery. At larger Womersley numbers, the instantaneous flow field in the stenosed region is found to have a stronger influence on the flow profiles of the previous time levels. The study delineates how an approximation in the assumption of inlet pulsatility profile may lead to significantly different prediction of hemodynamic wall parameters.

Effect of hypertension on coronary remodeling patterns in angiographically normal or minimally atherosclerotic coronary arteries: an intravascular ultrasound study

Whether there is any particular role of hypertension in remodeling process has not been completely understood yet. The aim of this study was to assess the association between hypertension and remodeling patterns in normal or minimally atherosclerotic coronary arteries. Seventy-nine patients who were free of significant coronary atherosclerosis were divided into two groups according to the absence (n = 39) or presence (n = 40) of hypertension; and standard intravascular ultrasound examination was performed in 145 segments. To determine the remodeling pattern in early atherosclerotic process, patients were also analyzed according to the level of plaque burden at the lesion site after the analysis of remodeling patterns. Positive remodeling was more prevalent in the hypertensive group (52.5% vs. 12.8%; P < .001) whereas negative remodeling was more common in diabetic patients (53.6% vs. 27.4%; P = .03). Mean remodeling index was 1.04 for hypertensives and 0.96 for normotensives (P = .03). There were no correlations between remodeling patterns and other risk factors such as age, family history, and hypercholesterolemia. Early atherosclerotic lesions (< 30%) exhibited more negative remodeling characteristics while intermediate pattern was observed more frequently in patients with high plaque burden (P = .006 and .02, respectively). Positive remodeling showed no association in this context (P = .07). This study demonstrated that minimal atherosclerotic lesions in hypertensives had a tendency for compensatory arterial enlargement. Positive remodeling may result from local adaptive processes within vessel wall or hemodynamic effects of blood pressure itself.

Brachial artery diameter measurement: a tool to simplify non-invasive vascular assessment

AIM

The mechanisms of vascular remodeling have attracted great interest since it is a phenomenon related to cardiovascular diseases. We would like to examine studies that contributed to clarify the remodeling mechanisms, to explore the different faces of atherosclerosis process.

DATA SYNTHESIS

A number of invasive and non-invasive vascular assessment methods were developed, to detect the early sign of atherosclerosis. It became clear that the invasive tests were not applicable to large-scale studies. Consequently, a non-invasive test was developed. Studies showed that the endothelial function evaluation is a predictor of future cardiac events in individuals at cardiovascular risk and in those with established disease. However, analyzing several works, an interesting concept emerged, i.e., the inverse relation between endothelium-dependent dilation and vessel size, since large vessel tend not to dilate significantly. This notion emphasized the role of basal diameter on vascular response. In particular, as brachial artery diameter is the measure on which FMD is based, it could add more information in clinical evaluation, simplifying the assessment. Several studies showed that morphological change of brachial artery is a better indicator of the extent of coronary disease rather than FMD. Other studies showed that brachial diameter has predictive significance in the stratification of cardiovascular risk.

CONCLUSION

Brachial diameter is a useful and simple tool. It should be incorporated into the overall assessment of cardiovascular risk but further studies are warranted to determine the final place of brachial diameter assessment in routine clinical setting.

In-vivo assessment of the natural history of coronary atherosclerosis: vascular remodeling and endothelial shear stress determine the complexity of atherosclerotic disease progression

PURPOSE OF REVIEW

Atherosclerotic disease progression is determined by localized plaque growth, which is induced by systemic and local hemodynamic factors, and the nature of the wall remodeling response. The purpose of this review is to summarize the processes underlying the heterogeneity of coronary atherosclerosis progression in relation to the local hemodynamic and arterial remodeling environment.

RECENT FINDINGS

Multiple competing biological processes in the extracellular matrix define the extent of vascular remodeling and disease progression. The remodeling phenomenon is not consistent but is characterized by great phenotypical heterogeneity which reflects the complex effect of systemic, genetic and hemodynamic factors on the arterial wall response to plaque formation and progression. The exaggeration of expansive remodeling (i.e., excessive expansive remodeling) likely contributes to the transformation of an initially favorable action into an excessive course of vessel expansion, continued disease progression and plaque instability. Extremely low endothelial shear stress and excessive expansive remodeling establish a vicious cycle which leads to the formation of severe plaques with high-risk characteristics.

SUMMARY

The dynamic interplay between the local hemodynamic environment and the wall remodeling behavior determines the complexity of the natural history of atherosclerosis and explains the development of localized plaque vulnerability.

Small calcified coronary atherosclerotic plaque simulation model: minimal size and attenuation detectable by 64-MDCT and MicroCT

Zero calcium score may not reflect the absence of calcifications as small calcifications could be missed. This study aimed to evaluate minimal size and minimal attenuation of coronary calcifications detectable by computed tomography (CT) and to determine the minimal spatial resolution required for detecting calcification onset. Using open source CT simulation software, CTSim^©, several 50%-stenotic coronary artery phantoms were designed with 5 μm resolution, realistic morphology and tissue-specific Hounsfield Unit (HU) values. The plaque had an attenuation resembling fibrous plaque and contained a single calcification. X-ray projections were simulated with settings resembling non-contrast-enhanced 64 multi detector-row CT (64-MDCT). Scanning and reconstruction were simulated with spatial resolution of a 64-MDCT (0.4mm) and of a MicroCT (48 μm). Starting from a single calcium granule, the calcification was simulated to grow in size and attenuation until it could be detected using clinically accepted calcium determination scheme on MicroCT and 64-MDCT images. The smallest coronary calcifications detectable at MicroCT and 64-MDCT, which had a realistic attenuation (−1,024 to 3,072 HU), were of 25 μm and 215 μm diameter, respectively. The area was overestimated 7.7 and 8.8 times, respectively. Calcifications with smaller size need to have an unrealistically high attenuation to be detectable by 64-MDCT. In conclusion, 64-MDCT is only able to detect coronary calcifications with minimal diameter of 215 μm. Consequently, early onset of calcification in coronary plaque will remain invisible when using CT and a zero calcium score can not exclude the presence of coronary calcification.

An intravascular ultrasound analysis in women experiencing chest pain in the absence of obstructive coronary artery disease: a substudy from the National Heart, Lung and Blood Institute-Sponsored Women's Ischemia Syndrome Evaluation (WISE)

AIMS

Using intravascular ultrasound (IVUS), we sought to characterize coronary morphology in women with chest pain without major epicardial obstructive coronary artery disease (CAD). We have previously observed an unexpectedly high rate of adverse outcomes among women with chest pain and normal or insignificant obstructive CAD. Information about the presence and characteristics of coronary atherosclerosis in these women could provide insight into the mechanisms related to increased risk, as well as improved diagnosis, prevention, and treatment.

METHODS

Women (n = 100) with suspected ischemia without obstructive CAD (>50% stenosis) underwent IVUS of a left coronary segment with measurements by a core lab masked to clinical and angiographic findings.

RESULTS

Angiograhic core lab analysis found 69.6% of patients had no (≤20%) and 30.4% had minimal (20-<50%) CAD. IVUS segmental images were interpretable by the core lab in 92 women, with 19 (21%) having no atherosclerosis (intimal-medial thickness <0.5 mm). In the remaining 73 women (79%), percent atheroma volume was 27 ± 8% and mean maximum plaque thickness was 0.53 ± 0.22 mm. Thirty-eight women with atherosclerosis (53%) had ≥30% of interrogated vessel involved. The average vessel involvement was 40%, and the maximum plaque thickness was 1.27 mm. The number of risk factors strongly correlated with percent atheroma volume (r = 0.53, P < 0.0001) and percent vessel involvement (r = 0.51, P < 0.0001), with the strongest independent predictor of both being age. Remodeling was assessed in 59/73 women (81%), and 73% had evidence of positive remodeling.

CONCLUSIONS

In symptomatic women without significant luminal obstructive CAD, we observed a high prevalence of atherosclerosis with positive remodeling and preserved lumen size. These findings may help explain increased risk and emphasize need for improved diagnostic and treatment options for women with concealed CAD.

An angular compounding technique using displacement projection for noninvasive ultrasound strain imaging of vessel cross-sections

Strain is considered to be a useful indicator of atherosclerotic plaque vulnerability. This study introduces an alternative for a recently introduced strain imaging method that combined beam steered ultrasound acquisitions to construct radial strain images of transverse cross-sections of superficial arteries. In that study, axial strains were projected in the radial direction. Using the alternative method introduced in this study, axial displacements are projected radially, followed by a least squares estimation of radial strains. This enables the use of a larger projection angle. Consequently, fewer acquisitions at smaller beam steering angles are required to construct radial strain images. Simulated and experimentally obtained radio-frequency data of radially expanding vessel phantoms were used to compare the two methods. Using only three beam steering angles (-30°, 0° and 30°), the new method outperformed the older method that used seven different angles and up to 45° of beam steering: the root mean squared error was reduced by 38% and the elastographic signal- and contrast-to-noise ratios increased by 1.8 dB and 4.9 dB, respectively. The new method was also superior for homogeneous and heterogeneous phantoms with eccentric lumens. To conclude, an improved noninvasive method was developed for radial strain imaging in transverse cross-sections of superficial arteries.

MRI-based biomechanical imaging: initial study on early plaque progression and vessel remodeling

The goal of the study is to develop a noninvasive magnetic resonance imaging (MRI)-based biomechanical imaging technique to address biomechanical pathways of atherosclerotic progression and regression in vivo using a 3D fluid-structure interaction (FSI) model. Initial in vivo study was carried out in an early plaque model in pigs that underwent balloon-overstretch injury to the left carotid arteries. Consecutive MRI scans were performed while the pigs were maintained on high cholesterol (progression) or normal chow (regression), with an injection of a plaque-targeted contrast agent, Gadofluorine M. At the end of study, the specimens of carotid arterial segments were dissected and underwent dedicated mechanical testing to determine their material properties. 3D FSI computational model was applied to calculate structure stress and strain distribution. The plaque structure resembles early plaque with thickened intima. Lower maximal flow shear stress correlates with the growth of plaque volume during progression, but not during regression. In contrast, maximal principle structure stress/stain (stress-P1 and strain-P1) were shown to correlate strongly with the change in the plaque dimension during regression, but moderately during progression. This MRI-based biomechanical imaging method may allow for noninvasive dynamic assessment of local hemodynamic forces on the development of atherosclerotic plaques in vivo.

CFD analysis of pulsatile blood flow in an atherosclerotic human artery with eccentric plaques

Atherosclerosis is a slow vascular degeneration. It thickens the internal walls of a blood vessel locally depositing an atherosclerotic plaque. Such reduced lumen increases the resistance to blood flow. Plaques can be punctual (eccentric, here considered) or circumferential (symmetrical). Stenoses do not have a typical shape: we hypothesised here a reference geometry (trapezium) with its possible evolutions (semi-ellipse, triangle). Two criteria (Equivalent Area and Equivalent Dimensions) were then defined to compare the results among the 35 case studies numerically analysed with a Computational Fluid Dynamics code (Comsol Multiphysics 3.3). Blood was considered a Cassonian fluid with modified viscosity equation. The artery was cylindrical, rigid and straight, interested by a pulsatile blood flow. Among the variables: shape and dimensions of the stenoses; number of stenoses (single or coupled pathologies); mutual locations (3 possibilities). The main results were that the length of the consequent flow disturbance is due to the stenotic shape and height; blood flow recirculation, downstream of the pathology, is due to the slope of the stenotic walls; and the peak velocities depend on the shape and height of stenosis. The differences from case to case diminish in diastole.

Early characterization of atherosclerotic coronary plaques with multidetector computed tomography in patients with acute coronary syndrome: a comparative study with intravascular ultrasound

We compared 16-slice computed tomography (CT) with intravascular ultrasound (IVUS) in their ability to identify the culprit lesion, and to assess plaque characterization and vascular remodelling in acute coronary syndrome (ACS). Twenty patients were prospectively studied. Coronary plaque identification and characterization were compared using 16-slice CT and 40-MHz catheter-based IVUS. Minimum lumen area (MLA), cross-sectional vessel area (CVA) and vessel remodelling were determined for each comparable lesion. One hundred and sixty-nine segments were compared and 84 plaques analysed. Sixteen-slice CT detected 95% of culprit lesions (19/20). No feature suggestive of plaque rupture was detected by 16-slice CT. Attenuation measurements within all lesions revealed different values for hypoechoic (38 +/- 33 HU), hyperechoic (94 +/- 44 HU), and calcified plaques (561 +/- 216 HU), (P < 0.001). Agreement between 16-slice CT and IVUS on measuring MLA and CVA was evaluated using Bland-Altman analysis. Pearson and intra-class coefficient (ICC) were 0.81 and 0.70 for MLA, and 0.81 and 0.36 for CVA, for 16-slice CT and IVUS, respectively. Agreement between both techniques for vessel positive remodelling was moderate (kappa = 0.54, P < 0.001). Sixteen-slice CT has shown moderate accuracy in quantifying and characterizing coronary plaques compared with IVUS. Spatial resolution of 16-slice CT remains a major limitation, however, to accurately assess the complex lesions involved in ACS.

Relationship between the number of coronary risk factors and coronary atherosclerosis assessed by high-frequency transthoracic echocardiography

Atherosclerosis is a diffuse process and can be evaluated in the distal part of the left anterior descending coronary artery (LAD). We evaluated whether 2-dimensional high-frequency transthoracic echocardiography could detect changes in coronary wall thickness and size associated with coronary risk factors. In all, 312 patients without a history of coronary artery disease and 40 patients with LAD disease proven by coronary angiography underwent transthoracic echocardiography evaluation of the distal LAD using high-frequency transducer. LAD wall thickness, luminal diameter, external diameter, and coronary flow velocity were measured, and these values were compared according to the number of coronary risk factors (hypertension, hypercholesterolemia, diabetes, smoking, and family history). The feasibility of measuring coronary thickness and coronary flow velocity was 86% and 90%, respectively. LAD wall thickness and external diameter significantly increased according to the increase in the number of risk factors. Patients with 3 or more coronary risk factors showed similar LAD wall thickness (0.82 +/- 0.2 vs 0.89 +/- 0.2 mm) and external diameter (3.4 +/- 0.8 vs 3.3 +/- 0.9 mm) compared with those obtained in patients with LAD disease. High-frequency transthoracic echocardiography is feasible to measure coronary artery wall thickness and external diameter in the distal LAD. This approach has a potential for noninvasive evaluation of the arterial remodeling and the changes in these variables after risk factor modification and intensive drug therapy.

In-vivo coronary flow profiling based on biplane angiograms: influence of geometric simplifications on the three-dimensional reconstruction and wall shear stress calculation

Background

Clinical studies suggest that local wall shear stress (WSS) patterns modulate the site and the progression of atherosclerotic lesions. Computational fluid dynamics (CFD) methods based on in-vivo three-dimensional vessel reconstructions have recently been shown to provide prognostically relevant WSS data. This approach is, however, complex and time-consuming. Methodological simplifications are desirable in porting this approach from bench to bedside. The impact of such simplifications on the accuracy of geometry and wall shear stress calculations has to be investigated.

Methods

We investigated the influence of two methods of lumen reconstruction, assuming circular versus elliptical cross-sections and using different resolutions for the cross-section reconstructions along the vessel axis. Three right coronary arteries were used, of which one represented a normal coronary artery, one with "obstructive", and one with "dilated" coronary atherosclerosis. The vessel volume reconstruction was performed with three-dimensional (3D) data from a previously validated 3D angiographic reconstruction of vessel cross-sections and vessel axis.

Results

The difference between the two vessel volumes calculated using the two evaluated methods is less than 1 %. The difference, of the calculated pressure loss, was between 2.5% and 8.5% for the evaluated methods. The distributions of the WSS histograms were nearly identical and strongly cross-correlated (0.91–0.95). The good agreement of the results was confirmed by a Chi-square test.

Conclusion

A simplified approach to the reconstruction of coronary vessel lumina, using circular cross-sections and a reduced axial resolution of about 0.8 mm along the vessel axis, yields sufficiently accurate calculations of WSS.

Influence of lumen shape and vessel geometry on plaque stresses: possible role in the increased vulnerability of a remodelled vessel and the "shoulder" of a plaque

OBJECTIVE

To use finite element modelling to look at the impact of lumen shape and vessel geometry on stress distribution in a vessel.

METHODS

A finite element model of an atherosclerotic plaque in a coronary artery was created and a non-linear analysis with Ogden strain energy function was done. The three most common shapes seen in an artery with an eccentric plaque--namely an elliptical lumen inside a circular vessel (A), a circular lumen inside an elliptical vessel, typical of a vessel with positive remodelling (B), and a circular lumen inside a circular vessel (C)--were modelled with and without lipid. Stress was analysed in the region of the fibrous cap separating the lumen from the plaque and the region of maximum stress along the circumference of the lumen was noted.

RESULTS

In a normal circular shaped coronary artery, the haemodynamic stresses were uniformly distributed all around the circle. However, if the circle was changed to an ellipse, the stresses were redistributed along the major axis and dropped substantially along the minor axis. The stresses in a positively remodelled vessel (B) were significantly greater than in A and C, by almost 100%. Moreover, the haemodynamic stresses increased significantly towards the major axis or the shoulder in A and B, due to lumen shape and vessel geometry alone, even in the absence of lipid in the model. The stresses also had a direct relation with the thickness of the lipid pool and an inverse relation with cap thickness and lumen stenosis.

CONCLUSIONS

The increased vulnerability of the shoulder region of a plaque and a remodelled coronary artery are due, apart from other factors, to increased biomechanical stresses as a result of lumen shape and vessel geometry.

Insights into the pathogenesis of vein graft disease: lessons from intravascular ultrasound

The success of coronary artery bypass grafting (CABG) is limited by poor long-term graft patency. Saphenous vein is used in the vast majority of CABG operations, although 15% are occluded at one year with as many as 50% occluded at 10 years due to progressive graft atherosclerosis. Intravascular ultrasound (IVUS) has greatly increased our understanding of this process. IVUS studies have shown that early wall thickening and adaptive remodeling of vein grafts occurs within the first few weeks post implantation, with these changes stabilising in angiographically normal vein grafts after six months. Early changes predispose to later atherosclerosis with occlusive plaque detectable in vein grafts within the first year. Both expansive and constrictive remodelling is present in diseased vein grafts, where the latter contributes significantly to occlusive disease. These findings correlate closely with experimental and clinicopathological studies and help define the windows for prevention, intervention or plaque stabilisation strategies. IVUS is also the natural tool for evaluating the effectiveness of pharmacological and other treatments that may prevent or slow the progression of vein graft disease in clinical trials.

Imaging of the vulnerable plaque: new modalities

Atherosclerosis is currently considered to be an inflammatory and thus a systemic disease affecting multiple arterial beds. Recent advances in intravascular imaging have shown multiple sites of atherosclerotic changes in coronary arterial wall. Traditionally, angiography has been used to detect and characterize atherosclerotic plaque in coronary arteries, but recently it has been found that plaques that are not significantly stenotic on angiography cause acute myocardial infarction. As a result, newer imaging and diagnostic modalities are required to predict which of the atherosclerotic plaque are prone to rupture and hence distinguish "stable" and "vulnerable" plaques. Intravascular ultrasound can identify multiple plaques that are not seen on coronary angiography. Thermography has shown much promise and is based on the concept that the inflammatory plaques are associated with increased temperature and can also identify "vulnerable patients." Of all these newer modalities, magnetic resonance imaging has shown the most promise in identification and characterization of vulnerable plaques. In this article, we review the newer coronary artery imaging modalities and discuss the limitations of traditional coronary angiography.

Coronary arterial remodeling in differing clinical presentations of unstable angina pectoris--an intravascular ultrasound study

BACKGROUND

Coronary arterial remodeling influences the clinical presentation of ischemic heart disease; however, there is little information on the relationship between coronary arterial remodeling and the type of angina pectoris that patients manifest.

HYPOTHESIS

The study was undertaken to determine the difference of coronary arterial remodeling in patients with different types of angina pectoris.

METHODS

We analyzed 100 patients with ischemic heart disease using intravascular ultrasound (IVUS). Intracoronary IVUS images of proximal reference (PR), distal reference (DR), and target lesion were recorded, and intraluminal area (LA) and external elastic membrane (EEM) were measured. We defined a remodeling index as 100 x (lesion EEM - [PR-EEM + DR-EEM]/2) / ([PR-EEM + DR-EEM]/2). Cases were classified into three groups according to the clinical history (Group 1a: de novo unstable angina pectoris, Group 1b: accelerating unstable angina pectoris, and Group 2; stable angina pectoris).

RESULTS

The remodeling index in Group 1a was significantly larger than that in Groups 1b and 2 (18.6 +/- 28.5 vs. 5.3 +/- 27.1 and 18.6 +/- 28.5 vs. -2.7 +/- 17.6, p = 0.0347 and p = 0.0005, respectively), but there was no statistical difference in remodeling index between Groups 1b and 2.

CONCLUSIONS

Our results indicate that positive coronary arterial remodeling is more prevalent in patients with new onset of angina pectoris. The specific type of coronary arterial remodeling may affect the clinical presentation of patients with coronary artery disease.

Left anterior descending coronary artery wall thickness measured by high-frequency transthoracic and epicardial echocardiography includes adventitia

High-frequency, 2-dimensional transthoracic echocardiography (HR-2DTTE) measurements of the left anterior descending (LAD) coronary artery wall thickness are larger than measurements obtained by intravascular ultrasound. We hypothesize that this difference is due to inclusion of the third vascular layer, which may represent adventitia by HR-2DTTE, and that this layer must be increasing in thickness with the development of atherosclerosis. We evaluated the contribution of this third layer to the wall thickness of the normal and atherosclerotic LAD artery imaged by HR-2DTTE using high-frequency epicardial echocardiography (HFEE) as the reference standard. Eighteen patients (10 men, mean age 62 years), 13 with coronary atherosclerosis and 5 with normal coronary arteries, referred for open-heart surgery, underwent preoperative HR-2DTTE evaluation of the LAD artery (SONOS 5500; 3- to 8-MHz transducer) and intraoperative HFEE of the LAD artery (SONOS 5500; 6- to 15-MHz transducer). Wall thickness was greater in patients with coronary atherosclerosis than in those with normal coronary arteries by both HR-2DTTE (1.9 +/- 0.3 vs 1.0 +/- 0.1 mm, p = <0.001) and HFEE (1.8 +/- 0.2 vs 1.0 +/- 0.2 mm, p = <0.001). On HFEE, the average intima plus media thickness was greater in patients with coronary atherosclerosis than in those with normal coronary arteries (0.78 +/- 0.3 vs 0.34 +/- 0.1 mm, p = 0.005). The average thickness of adventitia was also greater in patients with coronary atherosclerosis than in those with normal coronary arteries (0.92 +/- 0.2 vs 0.54 +/- 0.2 mm, p = 0.0005). HR-2DTTE and HFEE measurements of the wall thickness correlated well (r = 0.83 [reader 1], p <0.001; r = 0.61 [reader 2], p <0.01). A third vascular layer, which likely included adventitia, represents a significant portion of the LAD wall thickness imaged by HR-2DTTE and HFEE, and it significantly increases in thickness with the development of atherosclerosis.

Effect of disease eccentricity on compensatory remodeling of coronary arteries: evidence from intravascular ultrasound before interventions

BACKGROUND

Compensatory remodeling occurs to maintain lumen area in human coronary vessels. However, few data exist regarding the relationship between vessel remodeling and plaque distribution. Therefore, we studied coronary sites with or without remodeling by intravascular ultrasound and correlated with disease distribution.

METHODS AND RESULTS

A total of 90 coronary sites with significant stenosis (>50%) from 80 patients were examined before interventions. For identifying the vessel remodeling, external elastic membrane (EEM) area was measured at the stenotic sites and the adjacent proximal and distal sites. The reference EEM area was calculated by averaging proximal and distal EEM areas, and percent enlargement of the EEM area was calculated by the formula: [(stenosis EEM area-reference EEM area)/reference EEM area]x100. Plaque area was determined by reducing the lumen from EEM areas. The maximal (max) and minimal (min) distances from the center of the lumen to the EEM were also measured, and the disease eccentricity index was calculated by the formula: [(max-min)/max]. The lesion was defined as eccentric if the index was >0.5 and as concentric if <or=0.5. There were 39 eccentric and 51 concentric lesions. The enlargement remodeling was observed at 32 lesions with the enlargement of EEM area of 28.0+/-16.0% (5.5 to 71.3%). Enlargement was more frequently observed in the eccentric than in concentric lesions (P<0.05). However, within 32 stenoses, which showed enlargement, there was no difference in enlargement between eccentric (n=19, 25.6+/-21.0%) and concentric lesions (n=13, 21.5+/-12.0%). Also, there was no statistical correlation between the increase in plaque area and % enlargement of EEM area irrespective of plaque morphology.

CONCLUSIONS

These data demonstrate that in advanced coronary disease compensatory enlargement occurs more frequently at the eccentric than concentric coronary lesions. However, the EEM area was limited to expand regardless of the disease morphology.

Shear stress and vascular remodeling: study of cardiac allograft coronary artery disease as a model of diffuse atherosclerosis

BACKGROUND

The assessment of remodeling in diffuse atherosclerosis by intravascular ultrasound is hampered by the lack of an appropriate reference segment. Transplant coronary artery disease is an accepted model of diffuse atherosclerosis. Flow-dependent remodeling is regulated by shear stress. Thus, normal levels of shear stress at baseline flow reflect adequate regulation and provide a functional assessment of flow-dependent remodeling.

METHODS

The approach was evaluated in 91 patients with transplant coronary artery disease and in 9 non-transplanted controls and 16 control transplant recipients. Quantitative coronary angiography, intracoronary ultrasound and intracoronary velocity studies were performed at baseline and after pharmacologic intervention. Calculated shear stress was compared between these groups and a sub-group with coronary angioplasty at follow-up (8 of 60 patients with control angiography after 23 +/- 8 months). The relation of shear stress to flow, diameter, flow/area ratio and endothelial function was analyzed.

RESULTS

Normal shear stress was found in non-transplanted controls and transplant recipients without coronary artery disease. Patients with coronary angioplasty at follow-up had elevated shear stress and enhanced endothelial dysfunction on the initial investigation. Shear stress was not correlated with flow (r = 0.062, non-significant), but with diameter (r = -0.654), flow/area ratio (r = 0.814) and endothelial dysfunction (r = 0.722).

CONCLUSION

Calculated local shear stress appears to be useful for the assessment of the adequacy of flow-dependent macrovascular remodeling in diffuse atherosclerosis. Elevated blood flow/area ratio is a potential clinical marker of increased shear stress that reflects inadequate flow-dependent remodeling.

Factors predicting compensatory vascular remodelling of the carotid artery affected by atherosclerosis

OBJECTIVE

To investigate factors predicting the development of outward remodelling of the carotid artery in patients with atherosclerosis.

DESIGN

130 patients with carotid artery stenosis (15-85% of the vessel diameter) were divided into two groups, based on the presence or absence of outward remodelling of the sclerotic carotid segment on high resolution ultrasonography. Logistic regression analysis was used to evaluate the contribution of haemodynamic, laboratory, and clinical measurements on the development of remodelling, including age, sex, type of stenosis, extent of plaque, per cent diameter stenosis, underlying disease, selected drug treatment, and plasma concentrations of total cholesterol, high density lipoprotein cholesterol, triglyceride, and uric acid.

RESULTS

64 patients (49%) had outward remodelling. Multivariate regression analysis showed that hypertension, the type of plaque, the thickness of the plaque, and the extent of stenosis were independent factors predicting remodelling. The odds ratios of hypertension, unstable shape of plaque, thickness of plaque, and the extent of the stenosis were 6.70, 3.02, 2.04, and 1.05, respectively. Other measurements did not contribute significantly to the estimation of remodelling.

CONCLUSIONS

Compensatory enlargement of the vessel occurs in about 50% of carotid artery segments with a diameter stenosis of 15-85%. Hypertension and the shape of the plaque are major determinants of the development of outward remodelling.

Vascular remodeling in atherosclerotic coronary arteries is affected by plaque composition

BACKGROUND

Narrowing of lumen in atherosclerotic lesions is determined not solely by accumulation of plaque but also by constrictive or expansive vascular remodeling. Underlying mechanisms and determinants of these bidirectional processes are not known.

OBJECTIVES

To elucidate the response of vascular remodeling to progressive atherosclerosis by analyzing its potential association with composition of plaque.

METHODS

Seventy patients with 77 de-novo coronary artery lesions underwent intravascular ultrasound imaging before coronary intervention. Target lesions were defined as soft, fibrous/mixed, and calcified plaques. Quantitative measurements of area of lumen (A(L)), total area of vessel (A(TV)) and area of plaque (A(P) = A(TV)-A(L)) were performed at the lesion site and at the proximal and distal reference sites. Remodeling was determined by using a remodeling index [I(R) = (stenosis of A(TV)/mean reference A(TV)) x 100].

RESULTS

Overall vascular remodeling was balanced with a mean remodeling index of 100.2+/-19.3% and a high interlesion range (60.2-152.4%). The remodeling index for soft lesions was significantly higher than those for fibrous/mixed and calcified lesions (110+/-18.8 versus 96.2+/-14.4 and 85.9+/-15.1%, P < 0.01). Calcified lesions exhibited lower remodeling indexes than did uncalcified lesions (85.9+/-15.1 versus 104.6+/-18.4%, P < 0.01).

CONCLUSIONS

Processes involved in vascular remodeling are affected by composition of plaque insofar as there is a higher prevalence of constrictive remodeling among calcified plaques and a higher prevalence of expansive remodeling among soft lesions. These findings indicate that constrictive remodeling is a late manifestation in atherogenesis. Future studies are warranted in order to enhance the understanding of progression of atherosclerosis, and of mechanisms of vascular remodeling and their impacts on interventional therapy.

Review: Clinical aspects of vascular remodeling

Vascular remodeling represents a spectrum of structural changes whereby the vascular wall responds to changes in its hemodynamic environment. Such changes may be classified as vessel enlargement (outward remodeling), diminution (inward remodeling), alternatively as adaptive (compensatory, appropriate to the hemodynamic stimulus), or maladaptive (dysfunctional, inappropriate). The direction and scale of remodeling are coordinated by endothelial production of growth factors, proteases, and cellular adhesion molecules in response to sensed changes in blood flow. In early atherosclerosis, outward remodeling preserves lumen size. Although protective in the long-term, the matrix degradation involved in this process may predispose atherosclerotic plaques to rupture, hence increasing the risks of acute coronary syndromes. Inward remodeling also occurs in advanced atherosclerotic lesions, whereby the vessel shrinks rather than enlarging, exacerbating rather than ameliorating stenosis. In transplant coronary artery disease, early inward remodeling may be a more important component of vessel stenosis than intimal thickening, while inappropriate inward remodeling appears to be as least as important as excessive intimal growth in the development of restenosis after angioplasty. Increased awareness of vascular remodeling, and in particular its malaptive forms, may provide new therapeutic insights for the future.

[Methods for coronary functional assessment]

Functional evaluation of coronary vasomotion encompasses the assessment of dynamic changes in coronary lumen, vessel wall, blood flow, intracoronary pressure and myocardial perfusion in response to specific pharmacologic stimuli. These parameters are obtained to characterize mechanisms of physiologic regulation and to evaluate pathophysiologic processes and potential therapeutic strategies, especially with regard to the development of coronary atherosclerosis. To this end, a variety of direct (invasive) and indirect (non-invasive) diagnostic tools are employed. Among the invasive methods are registration of intracoronary Doppler flow, coronary pressure measurements, quantitative coronary angiography and intravascular ultrasound. The non-invasive modalities consist of coronary Doppler echocardiography, positron emission tomography, myocardial scintigraphy and magnetic resonance imaging. Because of the different technical and physiological principles involved, these methods are complementary by providing independent access to different aspects. The combined invasive functional testing as employed in the cardiac catheterization laboratory allows for a simultaneous synopsis of high-resolution coronary imaging and direct measurement of physiologic parameters during local application of defined pharmacologically active substances. However, the demands in terms of equipment, time and operator skills are high and limit this combined invasive approach to specialized centers. Besides these research purposes, a number of functional methods has entered the clinical arena. They are employed to evaluate the hemodynamic significance of coronary lesions and to assess functional outcome of therapeutic interventions in the catheterization laboratory. The underlying principles and applications of the different methods are described and an overview of selected results is presented.