Relation between coronary artery geometry and the distribution of early sudanophilic lesions

Relative Residence Time Can Account for Half of the Anatomical Variation in Fatty Streak Prevalence Within the Right Coronary Artery

PURPOSE

The patchy anatomical distribution of atherosclerosis has been attributed to variation in haemodynamic wall shear stress (WSS). The consensus is that low WSS and a high Oscillatory Shear Index (OSI) trigger the disease. We found that atherosclerosis at aortic branch sites correlates threefold better with transverse WSS (transWSS), a metric which quantifies multidirectional near-wall flow. Coronary artery disease has greater clinical significance than aortic disease but computation of WSS metrics is complicated by the substantial vessel motion occurring during each cardiac cycle. Here we present the first comparison of the distribution of atherosclerosis with WSS metrics computed for moving coronary arteries.

METHODS

Maps of WSS metrics were computed using dynamic geometries reconstructed from angiograms of ten non-stenosed human right coronary arteries (RCAs). They were compared with maps of fatty streak prevalence derived from a previous study of 1852 RCAs.

RESULTS

Time average WSS (TAWSS), OSI, transWSS and the cross-flow index (CFI), a non-dimensional form of the transWSS, gave non-significant or significant but low spatial correlations with lesion prevalence. The highest correlation coefficient (0.71) was for the relative residence time (RRT), a metric that decreases with TAWSS and increases with OSI. The coefficient was not changed if RRT was calculated using CFI, which captures multidirectional WSS only, rather than OSI, which encompasses both multidirectional and oscillatory WSS.

CONCLUSION

Contrary to our earlier findings in the aorta, low WSS in combination with highly multidirectional flow correlates best with lesion location in the RCA, explaining approximately half of its anatomical variation.

Relationship between Coronary Arterial Geometry and the Presence and Extend of Atherosclerotic Plaque Burden: A Review Discussing Methodology and Findings in the Era of Cardiac Computed Tomography Angiography

Coronary artery disease (CAD) represents a modern pandemic associated with significant morbidity and mortality. The multi-faceted pathogenesis of this entity has long been investigated, highlighting the contribution of systemic factors such as hyperlipidemia and hypertension. Nevertheless, recent research has drawn attention to the importance of geometrical features of coronary vasculature on the complexity and vulnerability of coronary atherosclerosis. Various parameters have been investigated so far, including vessel-length, coronary artery volume index, cross-sectional area, curvature, and tortuosity, using primarily invasive coronary angiography (ICA) and recently non-invasive cardiac computed tomography angiography (CCTA). It is clear that there is correlation between geometrical parameters and both the haemodynamic alterations augmenting the atherosclerosis-prone environment and the extent of plaque burden. The purpose of this review is to discuss the currently available literature regarding this issue and propose a potential non-invasive imaging biomarker, the geometric risk score, which could be of importance to allow the early detection of individuals at increased risk of developing CAD.

Influence of right coronary artery motion, flow pulsatility and non-Newtonian rheology on wall shear stress metrics

The patchy distribution of atherosclerosis within the arterial system is consistent with a controlling influence of hemodynamic wall shear stress (WSS). Patterns of low, oscillatory and transverse WSS have been invoked to explain the distribution of disease in the aorta. Disease of coronary arteries has greater clinical importance but blood flow in these vessels may be complicated by their movement during the cardiac cycle. Previous studies have shown that time average WSS is little affected by the dynamic geometry, and that oscillatory shear is influenced more. Here we additionally investigate effects on transverse WSS. We also investigate the influence of non-Newtonian blood rheology as it can influence vortical structure, on which transverse WSS depends; Carreau-Yasuda models were used. WSS metrics were derived from numerical simulations of blood flow in a model of a moving right coronary artery which, together with a subject-specific inflow waveform, was obtained by MR imaging of a healthy human subject in a previous study. The results confirmed that time average WSS was little affected by dynamic motion and that oscillatory WSS was more affected. They additionally showed that transverse WSS and its non-dimensional analogue, the Cross Flow Index, were affected still further. This appeared to reflect time-varying vortical structures caused by the changes in curvature. The influence of non-Newtonian rheology was significant with some physiologically realistic parameter values, and hence may be important in certain subjects. Dynamic geometry and non-Newtonian rheology should be incorporated into models designed to produce maps of transverse WSS in coronary arteries.

Paracrine Shear-Stress-Dependent Signaling from Endothelial Cells Affects Downstream Endothelial Function and Inflammation

Cardiovascular diseases (CVDs), mainly ischemic heart disease (IHD) and stroke, are the leading cause of global mortality and major contributors to disability worldwide. Despite their heterogeneity, almost all CVDs share a common feature: the endothelial dysfunction. This is defined as a loss of functionality in terms of anti-inflammatory, anti-thrombotic and vasodilatory abilities of endothelial cells (ECs). Endothelial function is greatly ensured by the mechanotransduction of shear forces, namely, endothelial wall shear stress (WSS). Low WSS is associated with endothelial dysfunction, representing the primary cause of atherosclerotic plaque formation and an important factor in plaque progression and remodeling. In this work, the role of factors released by ECs subjected to different magnitudes of shear stress driving the functionality of downstream endothelium has been evaluated. By means of a microfluidic system, HUVEC monolayers have been subjected to shear stress and the conditioned media collected to be used for the subsequent static culture. The results demonstrate that conditioned media retrieved from low shear stress experimental conditions (LSS-CM) induce the downregulation of endothelial nitric oxide synthase (eNOS) expression while upregulating peripheral blood mononuclear cell (PBMC) adhesion by means of higher levels of adhesion molecules such as E-selectin and ICAM-1. Moreover, LSS-CM demonstrated a significant angiogenic ability comparable to the inflammatory control media (TNFα-CM); thus, it is likely related to tissue suffering. We can therefore suggest that ECs stimulated at low shear stress (LSS) magnitudes are possibly involved in the paracrine induction of peripheral endothelial dysfunction, opening interesting insights into the pathogenetic mechanisms of coronary microvascular dysfunction.

Effect of Coronary Artery Bifurcation Angle on Atherosclerotic Lesion Localization Distance to the Bifurcation Site

Objectives

Although percutaneous coronary interventions become a common treatment modality for coronary artery diseases, lesion localization make these procedures more complex. As the lesion localizes near to the bifurcation site, more complex PCI procedures, overqualified equipments are needed and complication risk increases. Previous studies have demonstrated the strong correlation between wide angulation and significant coronary stenosis. However, a paucity of data exists about the association between bifurcation angle and lesion localization distance. In this study we analysed the effect of coronary bifurcation angle and left main coronary artery length on the atherosclerotic lesion localization.

Methods

Patients, who underwent coronary angiography between 01.01.2017- 31.12.2019 were scanned. Patients having atherosclerotic lesions causing more than 50% luminal narrowing and Medina classification score (0,0,0) were evaluated. After exclusion, 467 patients were included. 5 bifurcation subgroups (LAD-CX, LAD-Dx, CX-OM, RCA-RV, RPD-RPL) were formed. Distance of lesion to the bifurcation site, bifurcation angle and left main coronary artery length were analysed by 2 experienced cardiologists with invasive quantitaive coronary angiography (QCA) by using “extreme angio and cardiac pacs” software system.

Results

There was a strong inverse correlation between bifurcation angle and lesion localization distance to the bifurcation site (r = −0.706; p < 0.0001). There was a nonsignificant negative correlation between Left-main coronary artery length and lesion localization. Regression analysis revealed that bifurcation angle is an independent risk factor for predicting the localization of an atheroslerotic lesion in 5 mm length from the point of bifurcation site (β = −0.074, p < 0.0001). A cut-off value of 80.5° coronary bifurcation angle was found to have 84.1% sensitivity and 81.3% specificity in prediction of atherosclerotic lesion localization in 5 mm length from the point of bifurcation site.

Conclusion

In this study we showed that as the bifurcation angle increases, atherosclerotic lesions tend to approach to the bifurcation site. Since invertentions encompassing bifurcation sites are more complex, lesions with increased angulation may need extra care as they are more likely to present with further complications. Furthermore, bifurcation angle is an independent risk factor for lesion localization.

Widening and Rotation of Carotid Artery with Age: Geometric Approach

BACKGROUND

The anatomy and geometry of the carotid artery may play an important role in the pathogenesis of internal carotid artery (ICA) stenosis, but the exact nature of this role remains elusive. To clarify this issue, we sought to investigate age-related changes in the anatomy and geometry of the carotid artery in a Korean population.

MATERIALS AND METHODS

We conducted a retrospective study of 300 subjects who underwent carotid contrast-enhanced magnetic resonance angiography at our clinic between 2014 and 2016. The subjects were divided into 7 groups according to age. The carotid arteries were segmented using semiautomated methods to obtain various measurements of carotid anatomy and geometry, as suggested by Thomas et al. The various age groups were compared for different parameters, including ICA angle, bifurcation angle, and vessel volume and diameter.

RESULTS

Analysis of variance showed that with an increase in age, there were significant increases in vessel volume and diameter of the common carotid artery and ICA as well as the carotid bifurcation (P = .000). Significant age-related increase was also noted in the ICA angle and bifurcation angle (P = .000). The anatomical position of the ICA in subjects aged over 60 years was significantly higher than that in subjects below 40 years (12.5% versus .03%, P < .0001).

CONCLUSIONS

Age-related increases were noted in vessel volume and diameter of the carotid artery and ICA as well as the bifurcation angle. The widening and rotation of the carotid artery increased with age.

Mitochondrial Ca2+ transport in the endothelium: regulation by ions, redox signalling and mechanical forces

Calcium (Ca²⁺) transport by mitochondria is an important component of the cell Ca²⁺ homeostasis machinery in metazoans. Ca²⁺ uptake by mitochondria is a major determinant of bioenergetics and cell fate. Mitochondrial Ca²⁺ uptake occurs via the mitochondrial Ca²⁺ uniporter (MCU) complex, an inner mitochondrial membrane protein assembly consisting of the MCU Ca²⁺ channel, as its core component, and the MCU complex regulatory/auxiliary proteins. In this review, we summarize the current knowledge on the molecular nature of the MCU complex and its regulation by intra- and extramitochondrial levels of divalent ions and reactive oxygen species (ROS). Intracellular Ca²⁺ concentration ([Ca²⁺]_i), mitochondrial Ca²⁺ concentration ([Ca²⁺]_m) and mitochondrial ROS (mROS) are intricately coupled in regulating MCU activity. Here, we highlight the contribution of MCU activity to vascular endothelial cell (EC) function. Besides the ionic and oxidant regulation, ECs are continuously exposed to haemodynamic forces (either pulsatile or oscillatory fluid mechanical shear stresses, depending on the precise EC location within the arteries). Thus, we also propose an EC mechanotransduction-mediated regulation of MCU activity in the context of vascular physiology and atherosclerotic vascular disease.

Engineering design of artificial vascular junctions for 3D printing

Vascular vessels, including arteries, veins and capillaries, are being printed using additive manufacturing technologies, also known as 3D printing. This paper demonstrates that it is important to follow the vascular design by nature as close as possible when 3D printing artificial vascular branches. In previous work, the authors developed an algorithm of computational geometry for constructing smooth junctions for 3D printing. In this work, computational fluid dynamics (CFDs) is used to compare the wall shear stress and blood velocity field for the junctions of different designs. The CFD model can reproduce the expected wall shear stress at locations remote from the junction. For large vessels such as veins, it is shown that ensuring the smoothness of the junction and using smaller joining angles as observed in nature is very important to avoid high wall shear stress and recirculation. The issue is however less significant for capillaries. Large joining angles make no difference to the hemodynamic behavior, which is also consistent with the fact that most capillary junctions have large joining angles. The combination of the CFD analysis and the junction construction method form a complete design method for artificial vascular vessels that can be 3D printed using additive manufacturing technologies.

Global assays of hemostasis in the diagnostics of hypercoagulation and evaluation of thrombosis risk

Thrombosis is a deadly malfunctioning of the hemostatic system occurring in numerous conditions and states, from surgery and pregnancy to cancer, sepsis and infarction. Despite availability of antithrombotic agents and vast clinical experience justifying their use, thrombosis is still responsible for a lion’s share of mortality and morbidity in the modern world. One of the key reasons behind this is notorious insensitivity of traditional coagulation assays to hypercoagulation and their inability to evaluate thrombotic risks; specific molecular markers are more successful but suffer from numerous disadvantages. A possible solution is proposed by use of global, or integral, assays that aim to mimic and reflect the major physiological aspects of hemostasis process in vitro. Here we review the existing evidence regarding the ability of both established and novel global assays (thrombin generation, thrombelastography, thrombodynamics, flow perfusion chambers) to evaluate thrombotic risk in specific disorders. The biochemical nature of this risk and its detectability by analysis of blood state in principle are also discussed. We conclude that existing global assays have a potential to be an important tool of hypercoagulation diagnostics. However, their lack of standardization currently impedes their application: different assays and different modifications of each assay vary in their sensitivity and specificity for each specific pathology. In addition, it remains to be seen how their sensitivity to hypercoagulation (even when they can reliably detect groups with different risk of thrombosis) can be used for clinical decisions: the risk difference between such groups is statistically significant, but not large.

Comparison of morphological and rheological conditions between conventional and eversion carotid endarterectomy using computational fluid dynamics – a pilot study

Purpose:

To compare postoperative morphological and rheological conditions after eversion carotid endarterectomy versus conventional carotid endarterectomy using computational fluid dynamics.

Basic methods:

Hemodynamic metrics (velocity, wall shear stress, time-averaged wall shear stress and temporal gradient wall shear stress) in the carotid arteries were simulated in one patient after conventional carotid endarterectomy and one patient after eversion carotid endarterectomy by computational fluid dynamics analysis based on patient specific data.

Principal findings:

Systolic peak of the eversion carotid endarterectomy model showed a gradually decreased pressure along the stream path, the conventional carotid endarterectomy model revealed high pressure (about 180 Pa) at the carotid bulb. Regions of low wall shear stress in the conventional carotid endarterectomy model were much larger than that in the eversion carotid endarterectomy model and with lower time-averaged wall shear stress values (conventional carotid endarterectomy: 0.03–5.46 Pa vs. eversion carotid endarterectomy: 0.12–5.22 Pa).

Conclusions:

Computational fluid dynamics after conventional carotid endarterectomy and eversion carotid endarterectomy disclosed differences in hemodynamic patterns. Larger studies are necessary to assess whether these differences are consistent and might explain different rates of restenosis in both techniques.

Influences of Geometric Configurations of Bypass Grafts on Hemodynamics in End-to-Side Anastomosis

Background

Although considerable efforts have been made to improve the graft patency in coronary artery bypass surgery, the role of biomechanical factors remains underrecognized. The aim of this study is to investigate the influences of geometric configurations of the bypass graft on hemodynamic characteristics in relation to anastomosis.

Materials and Methods

The Numerical analysis focuses on understanding the flow patterns for different values of inlet and distal diameters and graft angles. The Blood flow field is treated as a two-dimensional incompressible laminar flow. A finite volume method is adopted for discretization of the governing equations. The Carreau model is employed as a constitutive equation for blood. In an attempt to obtain the optimal aorto-coronary bypass conditions, the blood flow characteristics are analyzed using in vitro models of the end-to-side anastomotic angles of 45°, 60° and 90°. To find the optimal graft configurations, the mass flow rates at the outlets of the four models are compared quantitatively.

Results

This study finds that Model 3, whose bypass diameter is the same as the inlet diameter of the stenosed coronary artery, delivers the largest amount of blood and the least pressure drop along the arteries.

Conclusion

Biomechanical factors are speculated to contribute to the graft patency in coronary artery bypass grafting.

Numerical investigation of pulsatile blood flow in a bifurcation model with a non-planar branch: the effect of different bifurcation angles and non-planar branch

INTRODUCTION

Atherosclerosis is a focal disease that susceptibly forms near bifurcations, anastomotic joints, side branches, and curved vessels along the arterial tree. In this study, pulsatile blood flow in a bifurcation model with a non-planar branch is investigated.

METHODS

Wall shear stress (WSS) distributions along generating lines on vessels for different bifurcation angles are calculated during the pulse cycle.

RESULTS

The WSS at the outer side of the bifurcation plane vanishes especially for higher bifurcation angles but by increasing the bifurcation angle low WSS region squeezes. At the systolic phase there is a high possibility of formation of a separation region at the outer side of bifurcation plane for all the cases. WSS peaks exist on the inner side of bifurcation plane near the entry section of daughter vessels and these peaks drop as bifurcation angle is increased.

CONCLUSION

It was found that non-planarity of the daughter vessel lowers the minimum WSS at the outer side of the bifurcation and increases the maximum WSS at the inner side. So it seems that the formation of atherosclerotic plaques at bifurcation region in direction of non-planar daughter vessel is more risky.

Effects of disturbed flow on vascular endothelium: pathophysiological basis and clinical perspectives

Vascular endothelial cells (ECs) are exposed to hemodynamic forces, which modulate EC functions and vascular biology/pathobiology in health and disease. The flow patterns and hemodynamic forces are not uniform in the vascular system. In straight parts of the arterial tree, blood flow is generally laminar and wall shear stress is high and directed; in branches and curvatures, blood flow is disturbed with nonuniform and irregular distribution of low wall shear stress. Sustained laminar flow with high shear stress upregulates expressions of EC genes and proteins that are protective against atherosclerosis, whereas disturbed flow with associated reciprocating, low shear stress generally upregulates the EC genes and proteins that promote atherogenesis. These findings have led to the concept that the disturbed flow pattern in branch points and curvatures causes the preferential localization of atherosclerotic lesions. Disturbed flow also results in postsurgical neointimal hyperplasia and contributes to pathophysiology of clinical conditions such as in-stent restenosis, vein bypass graft failure, and transplant vasculopathy, as well as aortic valve calcification. In the venous system, disturbed flow resulting from reflux, outflow obstruction, and/or stasis leads to venous inflammation and thrombosis, and hence the development of chronic venous diseases. Understanding of the effects of disturbed flow on ECs can provide mechanistic insights into the role of complex flow patterns in pathogenesis of vascular diseases and can help to elucidate the phenotypic and functional differences between quiescent (nonatherogenic/nonthrombogenic) and activated (atherogenic/thrombogenic) ECs. This review summarizes the current knowledge on the role of disturbed flow in EC physiology and pathophysiology, as well as its clinical implications. Such information can contribute to our understanding of the etiology of lesion development in vascular niches with disturbed flow and help to generate new approaches for therapeutic interventions.

Analysis of haemodynamic factors involved in carotid atherosclerosis using computational fluid dynamics

Atherosclerosis presents a massive healthcare burden in both the developing and developed world. There is mounting evidence relating to the involvement of haemodynamic factors in the pathogenesis of this process. This article aims to review the current understandings that have developed in this area, and to present a demonstrative case study obtained using state of the art computational fluid dynamics (CFD) methodology to model and analyse haemodynamic factors within the atheromatous carotid artery bifurcation.

Computational study of pulsatile blood flow in prototype vessel geometries of coronary segments

The spatial and temporal distributions of wall shear stress (WSS) in prototype vessel geometries of coronary segments are investigated via numerical simulation, and the potential association with vascular disease and specifically atherosclerosis and plaque rupture is discussed. In particular, simulation results of WSS spatio-temporal distributions are presented for pulsatile, non-Newtonian blood flow conditions for: (a) curved pipes with different curvatures, and (b) bifurcating pipes with different branching angles and flow division. The effects of non-Newtonian flow on WSS (compared to Newtonian flow) are found to be small at Reynolds numbers representative of blood flow in coronary arteries. Specific preferential sites of average low WSS (and likely atherogenesis) were found at the outer regions of the bifurcating branches just after the bifurcation, and at the outer-entry and inner-exit flow regions of the curved vessel segment. The drop in WSS was more dramatic at the bifurcating vessel sites (less than 5% of the pre-bifurcation value). These sites were also near rapid gradients of WSS changes in space and time - a fact that increases the risk of rupture of plaque likely to develop at these sites. The time variation of the WSS spatial distributions was very rapid around the start and end of the systolic phase of the cardiac cycle, when strong fluctuations of intravascular pressure were also observed. These rapid and strong changes of WSS and pressure coincide temporally with the greatest flexion and mechanical stresses induced in the vessel wall by myocardial motion (ventricular contraction). The combination of these factors may increase the risk of plaque rupture and thrombus formation at these sites.

Race-ethnic variation in carotid bifurcation geometry

BACKGROUND

Disturbances in local blood flow influenced by arterial geometry contribute to atherogenesis. Carotid bifurcation hemodynamics depend on the relative sizes of the common carotid artery (CCA), internal carotid artery (ICA), and external carotid artery (ECA), which vary considerably among individuals. The prevalence of carotid bifurcation atherosclerosis differs among race-ethnic groups and is generally lower in African Americans despite a more adverse vascular risk factor profile. We here examine whether there are race-ethnic differences in carotid bifurcation anatomy.

METHODS

The diameters of the CCA, carotid bulb, ICA, and ECA were measured from consecutive cerebral angiograms of African American, white, and Caribbean Hispanic patients. The bulb/CCA, ICA/CCA, ECA/CCA, ECA/ICA, and total cross-sectional outflow/inflow ratio ([ICA(2) + ECA(2)]/CCA(2)) were calculated.

RESULTS

The final analysis included 272 bifurcations of which 103 were among white, 87 Hispanic, and 82 African American patients. The mean age of the population was 59.8 +/- 15.8 years and 148 (54.4%) were men. African Americans had a lower ICA/CCA ratio (P < .001) and ICA/ECA ratio (P < .0001) and a higher ECA/CCA ratio (P < .0001) in comparison with the other two groups.

CONCLUSION

We found significant differences in the relative sizes of the ICA, ECA, and CCA among race-ethnic groups. African Americans had a proportionally smaller ICA and larger ECA in comparison with whites and Caribbean Hispanics.

Biomechanics of coronary artery and bypass graft disease: potential new approaches

The contribution of biomechanical factors to the incidence and distribution of coronary artery and bypass graft disease is underrecognized. This review examined the literature to determine which factors were relevant and the evidence for their importance. It identified two primary biomechanical factors that predispose to disease: (1) low-wall shear stress and (2) high-wall mechanical stress or strain. A range of secondary biomechanical factors have also been identified and include: vessel geometry; vessel movement; vessel wall characteristics and the presence of reflection waves. Potential surgical approaches for minimizing these effects are discussed.

Anatomical Variations of Coronary Ostia, Aortocoronary Angles and Angles of Division of the Left Coronary Artery of the Human Heart

Classic anatomical dissection of 150 heart specimens from adults aged 18 − 80 years was performed. Anatomical variations were studied in: (i) the position of the ostium of the left coronary artery; (ii) the angle between the proximal segment of the left coronary artery and the longitudinal axis of the aorta and between the circumflex and the anterior descending branches; (iii) the angle between the anterior descending artery and the diagonal branches, and between the diagonal and circumflex branches in trifurcation of the left coronary artery; (iv) the position of the ostium of the right coronary artery in the right coronary sinus of Valsalva; (v) the angle between the initial part of the right coronary artery and the logitudinal axis of the aorta; and (vi) the position of the initial part of the left coronary artery relative to the coronary groove. Knowledge of and the ability to recognize and identify the variety of sites of origin of coronary arteries, aortocoronary angles and angles of division of the left coronary artery of the human heart may help to overcome potential difficulties in cardiosurgical procedures, such as aortic valve replacement and reinsertion of coronary arteries.

Endothelial nitric oxide synthase and calcium production in arterial geometries: an integrated fluid mechanics/cell model

It is well known that atherosclerosis occurs at very specific locations throughout the human vasculature, such as arterial bifurcations and bends, all of which are subjected to low wall shear stress. A key player in the pathology of atherosclerosis is the endothelium, controlling the passage of material to and from the artery wall. Endothelial dysfunction refers to the condition where the normal regulation of processes by the endothelium is diminished. In this paper, the blood flow and transport of the low diffusion coefficient species adenosine triphosphate (ATP) are investigated in a variety of arterial geometries: a bifurcation with varying inner angle, and an artery bend. A mathematical model of endothelial calcium and endothelial nitric oxide synthase cellular dynamics is used to investigate spatial variations in the physiology of the endothelium. This model allows assessment of regions of the artery wall deficient in nitric oxide (NO). The models here aim to determine whether 3D flow fields are important in determining ATP concentration and endothelial function. For ATP transport, the effects of a coronary and carotid wave form on mass transport is investigated for low Womersley number. For the carotid, the Womersley number is then increased to determine whether this is an important factor. The results show that regions of low wall shear stress correspond with regions of impaired endothetial nitric oxide synthase signaling, therefore reduced availability of NO. However, experimental work is required to determine if this level is significant. The results also suggest that bifurcation angle is an important factor and acute angle bifurcations are more susceptible to disease than large angle bifurcations. It has been evidenced that complex 3D flow fields play an important role in determining signaling within endothelial cells. Furthermore, the distribution of ATP in blood is highly dependent on secondary flow features. The models here use ATP concentration simulated under steady conditions. This has been evidenced to reproduce essential features of time-averaged ATP concentration over a cardiac cycle for small Womersley numbers. However, when the Womersley number is increased, some differences are observed. Transient variations are overall insignificant, suggesting that spatial variation is more important than temporal. It has been determined that acute angle bifurcations are potentially more susceptible to atherogenesis and steady-state ATP transport reproduces essential features of time-averaged pulsatile transport for small Womersley number. Larger Womersley numbers appear to be an important factor in time-dependent mass transfer.

The association between right coronary artery morphology and endothelial function

BACKGROUND

Two distinct right coronary artery (RCA) morphologies have been previously described: sigma- and C-shaped RCAs. While the C-shaped RCA was significantly associated with atherosclerosis, the sigma-shaped was not. The association of RCA morphology and vascular endothelial function has not yet been assessed.

METHODS

To evaluate the association between the RCA's morphology and flow-mediated endothelium-dependent dilation (FMD) in patients without evidence of atherosclerotic lesions, we prospectively assessed FMD in 49 consecutive patients with non-specific chest pain, who were referred to our laboratory 30+/-10 days after corroboration of normal coronary arteries on coronary angiography. Endothelium-dependent brachial artery FMD and endothelium-independent nitroglycerin-mediated vasodilation (NTG) were assessed using high resolution (15 MHz) linear array ultrasound. The patients were divided into 2 groups according to their RCA morphology on coronary angiograms (sigma and C) which were analyzed by 2 independent readers who were blinded to the patients' FMD results.

RESULTS

The C-shaped group exhibited a significantly lower FMD compared to the sigma-shaped group (9.0+/-4.2% vs. 14.3+/-4.7%, p<0.04, respectively), while NTG was the same in both groups. There were no significant group differences in other variables.

CONCLUSIONS

Endothelium-dependent FMD in the brachial artery is significantly greater in sigma- compared to C-shaped RCA in coronary arteries without overt atherosclerotic lesions, suggesting a potential mechanism whereby C-shaped RCA are predisposed to atherosclerosis.

Are intramural suction-squeezing effects generated by the variations in radial wall stress during each heart beat the motor of atherosclerosis? A new concept

In the early sixties, the existence of predilection sites for atherosclerotic lesions inside the arterial circulation led to the concept that low wall shear stress (WSS) was responsible, together with systemic factors like high blood pressure, hypercholesterolemia, or diabetes, for the genesis and progression of atherosclerosis. It was found later that oscillating WSS and high WSS gradients could also be incriminated. Yet, this concept, which is broadly accepted today, fails to explains several facts, for instance that some arteries (e.g. epicardial coronary arteries) are more prone to become atherosclerotic than other ones exposed to the same systemic factors (e.g. hepatic and brain arteries). In this paper, we present a quite different concept. It is based on the fact that the increase in intravascular pressure and flow that occur in the arteries during systole generates, at the predilection sites of atherosclerotic lesions (bends, bifurcations, and branchings), an increase of radial wall stress in the outer layers of the arterial wall so that this stress becomes momentarily tensile. These cyclic stress increases have a suction effect that is likely to facilitate the diffusion of atherogenic cells and substances inside the wall. Furthermore, since arteries are not primarily structured to resist inversions of radial stress, they may also create damages (e.g. disruptions of cell membranes and elastic lamellae) followed by inflammations and micro-haemorrhages in the wall. This new concept may provide a complementary (or possibly alternative) explanation of atherosclerosis.

Effects of the shape of coronary arteries on the presence, extent, and severity of their disease

It is now known that a C-shaped right coronary artery (RCA) (C-RCA) is associated with more atherosclerotic disease than a sigma-shaped RCA (S-RCA). This study was designed to investigate the relationship between the shape of the RCA and the presence, extent, and severity of coronary artery disease (CAD) in all coronary arteries. Two hundred and forty-five patients were divided into C-RCA (group 1, n = 161) and S-RCA (group 2, n = 84) groups. The vessel score, extent score, and Gensini score were higher in group 1 than in group 2 (P < 0.001 for all comparisons). In multivariate analyses, age (P = 0.001), male sex (P < 0.001), smoking (P = 0.002), and C-RCA (P < 0.001) were independent predictors of significant CAD (presence of at least one lesion causing at least >50% stenosis). Multiple linear regression analysis to predict vessel score identified C-RCA (P < 0.001), age (P = 0.004), and male sex (P = 0.020), to predict extent score identified C-RCA (P < 0.001), age (P = 0.005), and male sex (P = 0.003), and to predict Gensini score identified C-RCA (P < 0.001), male sex (P = 0.009), and dyslipidemia (P = 0.049) as independently associated variables. Sensitivity and specificity of C-RCA for detecting significant CAD were 79% and 46%, respectively. This study showed that C-RCA was an independent predictor of significant CAD, which was independently associated with vessel score, extent score, and Gensini score. However, it was not clinically useful, because it was not specific or sensitive enough to identify patients with significant CAD.

Does shear stress modulate both plaque progression and regression in the thoracic aorta?

OBJECTIVES

The purpose of this study was to investigate the role of shear stress (SS) in plaque regression.

BACKGROUND

A condition favorable to the development of atherosclerotic lesions is low oscillating SS. In the descending thoracic aorta, the relationship between plaque distribution and SS has never been characterized. The regression of plaque as the result of lipid-lowering therapy is associated with reverse atherogenic mechanisms. Therefore, we investigated the role of SS in plaque regression. Magnetic resonance imaging (MRI) provides a unique opportunity to noninvasively study morphology and hemodynamics.

METHODS

Cross-sectional images of atherosclerotic plaques in the descending thoracic aorta of 10 asymptomatic, hypercholesteremic patients were acquired at baseline and 24 months after starting lipid-lowering therapy by using a black-blood sequence on a 1.5-T clinical MRI system (5 mm x 780 microm x 780 microm). Average wall thickness (WT) was derived per quadrant. The aorta was subdivided in segments 2 cm in length starting 1 cm from the aortic arch.

RESULTS

Average WT decreased with increasing distance from the arch (3.0 +/- 0.7 mm vs. 2.5 +/- 0.3 mm; p < 0.05) and showed a helical pattern from the proximal to distal segments. Phase-contrast MRI was performed in the thoracic aorta of eight healthy volunteers to derive typical average SS distribution. Shear stress predicted the location of WT (r(2) = 0.29, p < 0.05) but did not predict plaque regression. The best predictor of plaque regression was baseline WT.

CONCLUSIONS

Our data showing an association between WT and average low SS locations support the role of local hemodynamics in the development of atherosclerotic lesions in descending thoracic aorta. Furthermore, SS does not seem to be the major predictor for plaque regression by lipid-lowering interventions. Therefore, our data suggest that other mechanisms are involved in the lipid-reversal mechanism.

Wall Shear Stress Is Associated With Intima-Media Thickness and Carotid Atherosclerosis in Subjects at Low Coronary Heart Disease Risk

BACKGROUND AND PURPOSE

Systemic and local coronary heart disease (CHD) risk factors participate in atherogenesis. The role of wall shear stress, a major local risk factor, remains to be elucidated.

METHODS

Two hundred thirty-four subjects were carefully characterized for the presence of hypertension, hyperlipidemia, diabetes mellitus, obesity, and cigarette smoking and were divided into low- and high-risk groups. They underwent echo-Doppler examination of the carotid arteries. Atherosclerotic plaques and stenoses were detected, intima-media thickness (IMT) was measured, and wall shear stress was calculated.

RESULTS

One hundred eight subjects were classified as low-risk individuals. The prevalence of carotid atherosclerosis in this group was 18.5%. Wall shear stress was 24.23+/-7.21 dyne/cm(2) in individuals without atherosclerosis and 16.89+/-5.48 in those with atherosclerosis (P<0.000). In multiple regression analyses, wall shear stress, body mass index, and HDL cholesterol were inversely associated and total cholesterol was directly associated with the presence of atherosclerosis; only wall shear stress was associated with IMT. In the high-risk group the prevalence of atherosclerosis was 45.2%. Wall shear stress was 20.44+/-6.82 dyne/cm(2) in subjects without atherosclerosis and 17.84+/-6.88 dyne/cm(2) in those with atherosclerosis (P=0.037). Age was the only variable associated with both carotid atherosclerosis and IMT.

CONCLUSIONS

In subjects traditionally considered at low CHD risk, intima-media thickening and carotid atherosclerosis are significantly associated with low wall shear stress. In contrast, in subjects at high CHD risk, the contribution of wall shear stress seems to be masked, and age becomes the only factor significantly associated with both carotid atherosclerosis and IMT.

Degrees of severe stenoses in sigma-shaped versus C-shaped right coronary arteries

The right coronary artery (RCA) appears either C-shaped or sigma-shaped during standard angiography. The purpose of the present investigation was to assess whether C-shaped RCAs are associated with more atherosclerotic disease than sigma-shaped RCAs. The study sample comprised 120 consecutive patients who underwent coronary catheterization and multivariate analysis was conducted using several systemic risk factors for atherosclerosis. The proportion of sigma-shaped RCAs found in a group whose angiograms showed little or no obstruction (70%) was significantly higher than that found in the group with significant obstruction (33%, p <0.001). In conclusion, a C-shaped RCA is associated with atherosclerosis.

Geometry of the human common carotid artery. A vessel cast study of 86 specimens

The carotid artery is of special interest for the pathologist because of its frequent depositions, and for the fluidmechanician because of its complex flow properties. However, there is a distinct lack in current knowledge of its geometry. Therefore, a vessel cast study was undertaken. At post mortem, the arteries are excised and filled with a special resin at the proper transmural pressure. Eighty-six vessel casts of the carotid artery were performed, and some etiological factors of atherosclerosis, such as age, sex and disease, were collected. The following selected geometric parameters of these vessel casts were measured in this study: the diameters of the main branches of carotid bifurcation (common, internal and external arteries), and the angles between internal, external and common carotid arteries. The averaged geometric parameters and their variability over 86 vessel casts of the carotid artery were investigated. Furthermore, the relationship between these measured parameters and the etiological factors age, sex and disease was analyzed. The geometric parameters varied considerably, presumably contributing to a corresponding variability in the local hemodynamic and distribution of the atherosclerotic lesions.

Diastolic right ventricular filling vortex in normal and volume overload states

Functional imaging computational fluid dynamics simulations of right ventricular (RV) inflow fields were obtained by comprehensive software using individual animal-specific dynamic imaging data input from three-dimensional (3-D) real-time echocardiography (RT3D) on a CRAY T-90 supercomputer. Chronically instrumented, lightly sedated awake dogs (n = 7) with normal wall motion (NWM) at control and normal or diastolic paradoxical septal motion (PSM) during RV volume overload were investigated. Up to the E-wave peak, instantaneous inflow streamlines extended from the tricuspid orifice to the RV endocardial surface in an expanding fanlike pattern. During the descending limb of the E-wave, large-scale (macroscopic or global) vortical motions ensued within the filling RV chamber. Both at control and during RV volume overload (with or without PSM), blood streams rolled up from regions near the walls toward the base. The extent and strength of the ring vortex surrounding the main stream were reduced with chamber dilatation. A hypothesis is proposed for a facilitatory role of the diastolic vortex for ventricular filling. The filling vortex supports filling by shunting inflow kinetic energy, which would otherwise contribute to an inflow-impeding convective pressure rise between inflow orifice and the large endocardial surface of the expanding chamber, into the rotational kinetic energy of the vortical motion that is destined to be dissipated as heat. The basic information presented should improve application and interpretation of noninvasive (Doppler color flow mapping, velocity-encoded cine magnetic resonance imaging, etc.) diastolic diagnostic studies and lead to improved understanding and recognition of subtle, flow-associated abnormalities in ventricular dilatation and remodeling.

Vascular MRI in the diagnosis and therapy of the high risk atherosclerotic plaque

Disruption of a high risk plaque is known as the primary cause of cardiovascular events. Characterization of arterial wall components has become an essential adjunct in the identification of patients with plaques prone to rupture. Magnetic Resonance Imaging (MRI) has been revealed as one of the noninvasive tools possibly capable of identifying and characterizing high risk atherosclerotic plaque. MRI may facilitate diagnosis, and guide and serially monitor interventional and pharmacological treatment of atherosclerotic disease. In addition, it permits the simultaneous assessment of the anatomy, morphology, and hemodynamics for the study of flow-induced atherogenesis. It possibly will identify asymptomatic patients with subclinical atherosclerosis. This has potential significance for the improvement of strategies in primary and secondary prevention.

Haemodynamic factors and the important role of local low static pressure in coronary wall thickening

BACKGROUND/STUDY OBJECTIVES: The purpose of our study was to investigate the possible correlation between blood flow physical parameters and the wall thickening in typical human coronary arteries.

METHODS

Digitized images of seven transparent arterial segments prepared post-mortem were adopted from a previous study in order to extract the geometry for numerical analysis. Using the exterior outline, reconstructed forms of the vessel geometries were used for subsequent computational fluid dynamic analysis. Data was input to a pre-processing code for unstructured mesh generation. The flow was assumed to be two-dimensional, steady, laminar with parabolic inlet velocity profile. The vessel walls were assumed to be smooth, inelastic and impermeable. Non-Newtonian power law was applied to model blood rheology. The arterial wall thickening was measured and correlated to the wall shear stress, static pressure, molecular viscosity, and near wall blood flow velocity.

RESULTS

Wall shear stress, static pressure and near wall velocity magnitude exhibit negative correlation to wall thickening, while molecular viscosity exhibits positive correlation to wall thickening.

CONCLUSION

There is a strong correlation between the development of vessel wall thickening and the blood flow physical parameters. Amongst these parameters the role of local low wall static pressure is predominant.

Asymmetric redirection of flow through the heart

Through cardiac looping during embryonic development, paths of flow through the mature heart have direction changes and asymmetries whose topology and functional significance remain relatively unexplored. Here we show, using magnetic resonance velocity mapping, the asymmetric redirection of streaming blood in atrial and ventricular cavities of the adult human heart, with sinuous, chirally asymmetric paths of flow through the whole. On the basis of mapped flow fields and drawings that illustrate spatial relations between flow paths, we propose that asymmetries and curvatures of the looped heart have potential fluidic and dynamic advantages. Patterns of atrial filling seem to be asymmetric in a manner that allows the momentum of inflowing streams to be redirected towards atrio-ventricular valves, and the change in direction at ventricular level is such that recoil away from ejected blood is in a direction that can enhance rather than inhibit ventriculo-atrial coupling. Chiral asymmetry might help to minimize dissipative interaction between entering, recirculating and outflowing streams. These factors might combine to allow a reciprocating, sling-like, 'morphodynamic' mode of action to come into effect when heart rate and output increase during exercise.

Effects of coronary heart disease risk factors on atherosclerosis of selected regions of the aorta and right coronary artery. PDAY Research Group. Pathobiological Determinants of Atherosclerosis in Youth

We examined topographic distributions of atherosclerosis and their relation to risk factors for adult coronary heart disease in right coronary arteries and abdominal aortas of more than 2000 autopsied persons 15 through 34 years of age. We digitized images of Sudan IV-stained fatty streaks and of manually outlined raised lesions and computed the percent surface area involved by each lesion in each of 6 regions of each artery. In abdominal aortas of 15- to 24-year-old persons, fatty streaks involve an elongated oval area on the dorsolateral intimal surface and another oval area in the middle third of the ventral surface. Raised lesions in 25- to 34-year-old persons involve an oval area in the distal third of the dorsolateral intimal surface. In other areas of the abdominal aortas of older persons, fatty streaks occur but raised lesions are rare. In the right coronary arteries of 15- to 24-year-old persons, fatty streaks are most frequent on the myocardial aspect of the first 2 cm. Raised lesions follow a similar pattern in 25- to 34-year-old persons. High non-HDL cholesterol and low HDL cholesterol concentrations are associated with more extensive fatty streaks and raised lesions in all regions of both arteries. Smoking is associated with more extensive fatty streaks and raised lesions of the abdominal aorta, particularly in the dorsolateral region of the distal third of the abdominal aorta. Hypertension is not associated with fatty streaks in whites or blacks but is associated with more extensive raised lesions in blacks. Risk factor effects on arterial regions that are vulnerable to lesions are approximately 25% greater than risk factor effects assessed over entire arterial segments. These risk factor effects on vulnerable sites emphasize the need for risk factor control during adolescence and young adulthood to prevent or delay the progression of atherosclerosis.

Evaluation of common carotid hemodynamic forces. Relations with wall thickening

The localization of atherosclerotic lesions is influenced by hemodynamic factors, namely, shear stress and tensive forces. The present study investigated the relationships between shear stress and circumferential wall tension and between these hemodynamic factors and the intima-media thickness (IMT) of the common carotid artery in healthy men. Fifty-eight subjects were studied. Shear stress was calculated as blood viscosityxblood velocity/internal diameter. Circumferential wall tension was calculated as blood pressurexinternal radius. Blood velocity, internal diameter, and IMT were measured by high-resolution echo-Doppler. Mean shear stress was 12.6+/-3.3 dynes/cm(2) (mean+/-SD; range, 4.8 to 20.4) and was inversely related with age, blood pressure, and body mass index (BMI). Mean circumferential wall tension was 3.4+/-0.6x10(4) dynes/cm (range 2.4 to 5.6) and was directly associated with age and BMI. IMT was inversely associated with shear stress (r=0.55, P<0. 0001) and directly associated with circumferential wall tension (r=0. 43, P<0.0001). Shear stress and circumferential wall tension were inversely correlated (r=0.66, P<0.0001). In multiple regression analysis, shear stress and (marginally) cholesterol were independently associated with IMT, whereas circumferential wall tension, age, and BMI were not. These findings confirm that common carotid shear stress varies among healthy individuals and decreases as age, blood pressure, and BMI increase. Our findings also demonstrate that circumferential wall tension is directly associated with wall thickness, age, and BMI and that shear stress is associated with common carotid IMT independent of other hemodynamic, clinical, or biochemical factors.

Localizing role of hemodynamics in atherosclerosis in several human vertebrobasilar junction geometries

Atherosclerosis is a common finding in the vertebrobasilar junction and in the basilar artery. Several theories try to link the process of atherogenesis with the forces exerted by the flowing blood. An attractive relation has been found between the locations in vessels at which atherosclerotic plaques are often present and the locations in models where complicated flow patterns exist. Most of the studies provided data on bifurcations. Finding a similar relation in an arterial confluence would certainly add to the credibility of the (causal) relationship between hemodynamics and atherosclerosis. Further support can be provided if variations of the geometry result in changes of the location of the atherosclerotic lesions, corresponding to the changes of the flow force distribution. In our previous numerical and experimental work, the influence of geometric and hemodynamic parameters, such as asymmetrical inflow, confluence angle, and blunting of the apex, on the flow in vertebrobasilar junction models has been investigated in detail. Recirculation areas and distribution of the wall shear stress have been computed. In this anatomic study, the effect of modulation of these geometric and hemodynamic parameters on the flow pattern is compared with the size and location of plaques in human vertebrobasilar junctions and basilar arteries. In addition, a comparison is made between the preferential areas of atherosclerotic plaques in junctions and bifurcations to demonstrate the localizing role of hemodynamics in atherogenesis. The apex of the vertebrobasilar junction and the lateral walls of the basilar artery appeared to be prone to atherosclerosis. In 43 of 85 vertebrobasilar junctions, a plaque was found at the apex. Furthermore, the summed plaque thickness at both lateral walls differs significantly (paired t test, P=.03) from that at the walls facing the pons and the skull base. In contrast, several authors found that the lateral walls of the mother vessel and the apex in bifurcations are often spared. Modulation of the various parameters in the models changed the size of the regions with low wall shear stress and/or recirculation areas dramatically. A comparable effect was found in the occurrence of plaques in the human vertebrobasilar junction; eg, for an atherosclerotic plaque at the apex, a predicted probability larger than 0.5 was computed for blunted apexes and for sharp-edged apexes with a confluence angle exceeding 90 degrees. Apparently, two geometric risk factors for an atherosclerotic plaque at the apex can be distinguished: a blunted apex and a large confluence angle.

Relation between the structural asymmetry of coronary branch vessels and the angle at their origin

The relationship between the geometry of branch points on the left anterior descending coronary artery, and the morphometry of the proximal portions of the daughter vessels, was examined. The geometry at 23 branch points on 15 human hearts was derived from multiplane contrast angiograms, and the morphometry at 29 sites along the daughter vessels was obtained from transverse sections using computerized techniques. The angle of the branch at which the daughter originated was positively correlated with the maximum thicknesses of the intima and media, and with their circumferential asymmetry. The results suggest that large branch angles may favor eccentric intimal thickening, a phenomenon which may predispose to lipid accumulation and atherosclerosis.

Wall shear rate distribution in an abdominal aortic bifurcation model: effects of vessel compliance and phase angle between pressure and flow waveforms

The goal of this study was to determine how vessel compliance (wall motion) and the phase angle between pressure and flow waves (impedance phase angle) affect the wall shear rate distribution in an atherogenic bifurcation geometry under sinusoidal flow conditions. Both rigid and elastic models replicating the human abdominal aortic bifurcation were fabricated and the wall shear rate distribution in the median plane of the bifurcation was determined using the photochronic flow visualization method. In the elastic model, three phase angle conditions were simulated (+12, -17, -61 deg), and the results compared with those obtained in a similar rigid model. The study indicates a very low (magnitude close to zero) and oscillatory wall shear rate zone within 1.5 cm distal to the curvature site on the outer (lateral) wall. In this low shear rate zone, unsteadiness (pulsatility) of the flow greatly reduces the mean (time-averaged) wall shear rate level. Vessel wall motion reduces the wall shear rate amplitude (time-varying component) up to 46 percent depending on the location and phase angle in the model. The mean wall shear rate is less influenced by the wall motion, but is reduced significantly in the low shear region (within 1.5 cm distal to the curvature site on the outer wall), thus rendering the wall shear rate waveform more oscillatory and making the site appear more atherogenic. The effect of the phase angle is most noteworthy on the inner wall close to the flow divider tip where the mean and amplitude of wall shear rate are 31 and 23 percent lower, respectively, at the phase angle of -17 deg than at -61 deg. However, the characteristics of the wall shear rate distribution in the low shear rate zone on the outer wall that are believed to influence localization of atherosclerotic disease, such as the mean wall shear rate level, oscillation in the wall shear rate waveform, and the length of the low and oscillatory wall shear rate zone, are similar for the three phase angles considered. The study also showed a large spatial variation of the phase angle between the wall shear stress waveform and the circumferential stress waveform (hoop stress due to radial artery expansion in response to pressure variations) near the bifurcation (up to 70 deg). The two stresses became more out of phase in the low mean shear rate zone on the outer wall (wall shear stress wave leading hoop stress wave as much as 125 deg at the pressure-flow phase angle of -61 deg) and were significantly influenced by the impedance phase angle.

Influence of the Geometry of the Left Main Coronary Artery Bifurcation on the Distribution of Sudanophilia in the Daughter Vessels

Uptake of circulating albumin by the aortic wall is greater downstream than upstream of branches in immature rabbits, but the opposite pattern occurs in mature animals. We investigated the role of NO in determining these variations. Descending thoracic aortas of rabbits were cannulated using techniques that avoid depressurization, overstretching, and excessive fluid dynamic stresses at the endothelial surface. They were perfused in situ at a constant pressure and flow rate with oxygenated, protein-containing physiological buffer, with or without N w -monomethyl-L-arginine, an inhibitor of NO synthesis. Aortas were fixed 7 to 8 minutes after the addition of rhodamine-labeled albumin to this perfusate, and uptake of the tracer near intercostal ostia was measured by digital imaging fluorescence microscopy of sections through the wall. Despite the absence of pulsatile flow, blood cells, and many plasma components, patterns of transport in control experiments were the same as those occurring in vivo; uptake was greatest downstream of ostia in immature vessels and upstream in mature ones, although mean uptake was higher than previously reported. In the presence of the inhibitor, mean uptake in immature arteries was elevated threefold and the maximum tracer concentration occurred deeper in the wall, but there was no change in the fractional difference between regions. Conversely, the reverse of the control pattern of transport was observed in mature arteries exposed to the inhibitor, but there was no change in mean uptake. The reversal was almost entirely prevented by adding excess L-arginine to the perfusate and was largely stereospecific. Endogenous NO thus appears to determine the mature pattern of transport near branches and helps to maintain the barrier function of the immature wall. ( Arterioscler Thromb Vase Biol. 1997;17:1361-1368.)

The influence of the blunting of the apex on the flow in a vertebro-basilar junction model

The apex of human vertebro-basilar junctions can be sharp-edged or blunted. In the present study, the effect of blunted apex on the flow in vertebro-basilar junction models is investigated. We compared the flow phenomena in a series of junction models with blunted apices and confluence angles 45, 85, and 125 deg with the flow phenomena in a series of junction models with sharp-edged apices and the same range of confluence angles, studied in a previous paper (Ravensbergen et al., 1996b). The blunting of the apex appears to have an effect on the size of the local recirculation area near the apex and the prevailing low velocities. Large recirculation areas are found in the models with blunted apices, especially in those with small confluence angles. In addition, the blunting of the apex has no influence on the flow further downstream, nor on the structure and strength of the secondary flow field. Furthermore, a blunted apex appears to be a geometric risk factor for atherosclerosis. This supports the hypotheses that recirculation areas and low wall shear stress influence the development of atherosclerotic plaques.

Association between intima-media thickness and wall shear stress in common carotid arteries in healthy male subjects

BACKGROUND

Atherosclerotic lesions lie in regions of low wall shear stress. No relationship between wall shear stress and intima-media thickness in vivo has been reported. Aims of the present study were to verify the reproducibility of wall shear stress measurement in vivo and to evaluate its association with intima-media thickness in the common carotid artery in healthy subjects.

METHODS AND RESULTS

Wall shear stress was calculated according to the following formula: Shear Stress = Blood Viscosity x Blood Velocity/Internal Diameter. Blood viscosity was measured by use of a cone/plate viscometer. Blood velocity, internal diameter, and intima-media thickness were measured by high-resolution echo Doppler. Twenty-one healthy male subjects were investigated. Peak and mean shear stress values were 29.5 +/- 8.2 and 12.1 +/- 3.1 dynes/cm-2 (mean +/- SD), respectively. Peak shear stress was inversely related to intima-media thickness (r = .62), age (r = .77), systolic blood pressure (r = .61), and body mass index (r = .59) (P < .001 for all coefficients). Mean shear stress yielded similar results. The relationship between shear stress and intima-media thickness was independent of age, blood pressure, and body mass index. The reproducibility, calculated by Kendall's W test, was statistically significant.

CONCLUSIONS

Our results demonstrate that common carotid artery wall shear stress measurement in vivo is reproducible. It inversely relates to intima-media thickness, age, systolic blood pressure, and body mass index. These findings confirm in vivo the role of shear stress in intima-media thickening.

Relationship between the geometry and quantitative morphology of the left anterior descending coronary artery

The well established 'systemic' risk factors for atherosclerosis can explain only half of the variability in its occurrence. To account for some of the remaining variability, it was suggested that certain geometric features of atherosclerosis-prone segments ('geometric risk factors') can increase the likelihood of disease locally through their influence on the hemodynamic environment of the vessel wall. Since this mediation might elicit early morphological changes in the artery, relationships were sought between the histomorphometry and axial geometry of the left anterior descending (LAD) coronary arteries of 15 angiographically lesion-free human hearts obtained at autopsy. Geometric variables were quantified by image processing of multiplane angiograms of the hearts, and morphometry was obtained from transverse histologic sections at 91 sites. The results show that: (1) total intimal and medial area are negatively correlated with the distance from the site to the origin of the LAD; (2) the angle of the branch immediately proximal to the site is positively correlated with most of the intimal and medial variables, and appears to have a major influence on the intima; (3) the area ratio of the immediately proximal branch is correlated primarily with medial variables; and (4) local curvature is correlated only with the maximum thickness of the intima and media. These observations suggest that there are significant relationships between arterial geometry and vascular morphology prior to the development of frank disease.

The influence of the angle of confluence on the flow in a vertebro-basilar junction model

In earlier work, it was demonstrated that the flow in models of the vertebro-basilar junction is highly three-dimensional and the geometry exerts a strong influence on the hemodynamics. The morphology of the vertebro-basilar junction is very variable amongst individuals. In a study of 85 human vertebro-basilar junctions, the angle between the vertebral arteries varied between 10 and 160 degrees. To determine how the flow is influenced by this geometrical parameter, the flow is studied both experimentally, with laser Doppler velocimetry, and numerically, with a finite element package. A series of junction models is used with a range of confluence angles (45, 85 and 125 degrees). It appears that the angle of confluence has a strong influence on the structure and strength of the secondary flow field. The secondary velocities persist far downstream. Furthermore, near the apex, a region with low velocities is present. The larger the confluence angle is, the larger this region is, and even backflow may occur. In addition, the occurrence of atherosclerotic plaques in 85 human vertebro-basilar junctions is studied. Only one preferential location was found: the apex, the other plaques seem to be randomly distributed. The magnitude of the confluence angle of junctions with sharp-edged apices has a significant influence (p = 0.006) on the occurrence of a plaque at the apex. Apparently, a large confluence angle is a geometrical risk factor for atherosclerosis.