Collapse of diseased arteries with eccentric cross section

Buckling of Arteries With Noncircular Cross Sections: Theory and Finite Element Simulations

The stability of blood vessels is essential for maintaining the normal arterial function, and loss of stability may result in blood vessel tortuosity. The previous theoretical models of artery buckling were developed for circular vessel models, but arteries often demonstrate geometric variations such as elliptic and eccentric cross-sections. The objective of this study was to establish the theoretical foundation for noncircular blood vessel bent (i.e., lateral) buckling and simulate the buckling behavior of arteries with elliptic and eccentric cross-sections using finite element analysis. A generalized buckling equation for noncircular vessels was derived and finite element analysis was conducted to simulate the artery buckling behavior under lumen pressure and axial tension. The arterial wall was modeled as a thick-walled cylinder with hyper-elastic anisotropic and homogeneous material. The results demonstrated that oval or eccentric cross-section increases the critical buckling pressure of arteries and having both ovalness and eccentricity would further enhance the effect. We conclude that variations of the cross-sectional shape affect the critical pressure of arteries. These results improve the understanding of the mechanical stability of arteries.

Effect of Axial Stretch on Lumen Collapse of Arteries

The stability of the arteries under in vivo pressure and axial tension loads is essential to normal arterial function, and lumen collapse due to buckling can hinder the blood flow. The objective of this study was to develop the lumen buckling equation for nonlinear anisotropic thick-walled arteries to determine the effect of axial tension. The theoretical equation was developed using exponential Fung strain function, and the effects of axial tension and residual stress on the critical buckling pressure were illustrated for porcine coronary arteries. The buckling behavior was also simulated using finite-element analysis. Our results demonstrated that lumen collapse of arteries could occur when the transmural pressure is negative and exceeded a critical value. This value depends upon the axial stretch ratio and material properties of the arterial wall. Axial tensions show a biphasic effect on the critical buckling pressure. The lumen aspect ratio of arteries increases nonlinearly with increasing external pressure beyond the critical value as the lumen collapses. These results enhance our understanding of artery lumen collapse behavior.

Cap buckling as a potential mechanism of atherosclerotic plaque vulnerability

Plaque rupture in atherosclerosis is the primary cause of potentially deadly coronary events, yet about 40% of ruptures occur away from the plaque cap shoulders and cannot be fully explained with the current biomechanical theories. Here, cap buckling is considered as a potential destabilizing factor which increases the propensity of the atherosclerotic plaque to rupture and which may also explain plaque failure away from the cap shoulders. To investigate this phenomenon, quasistatic 2D finite element simulations are performed, considering the salient geometrical and nonlinear material properties of diverse atherosclerotic plaques over the range of physiological loads. The numerical results indicate that buckling may displace the location of the peak von Mises stresses in the deflected caps. Plaque buckling, together with its deleterious effects is further observed experimentally in plaque caps using a physical model of deformable mock coronary arteries with fibroatheroma. Moreover, an analytical approach combining quasistatic equilibrium equations with the Navier-Bresse formulas is used to demonstrate the buckling potential of a simplified arched slender cap under intraluminal pressure and supported by foundations. This analysis shows that plaque caps - calcified, fibrotic or cellular - may buckle in specific undulated shapes once submitted to critical loads. Finally, a preliminary analysis of intravascular ultrasonography recordings of patients with atherosclerotic coronary arteries corroborates the numerical, experimental and theoretical findings and shows that various plaque caps buckle in vivo. By displacing the sites of high stresses in the plaque cap, buckling may explain the atherosclerotic plaque cap rupture at various locations, including cap shoulders.

Compliant model of a coupled sequential coronary arterial bypass graft: effects of vessel wall elasticity and non-Newtonian rheology on blood flow regime and hemodynamic parameters distribution

We have recently developed a novel design for coronary arterial bypass surgical grafting, consisting of coupled sequential side-to-side and end-to-side anastomoses. This design has been shown to have beneficial blood flow patterns and wall shear stress distributions which may improve the patency of the CABG, as compared to the conventional end-to-side anastomosis. In our preliminary computational simulation of blood flow of this coupled sequential anastomoses design, the graft and the artery were adopted to be rigid vessels and the blood was assumed to be a Newtonian fluid. Therefore, the present study has been carried out in order to (i) investigate the effects of wall compliance and non-Newtonian rheology on the local flow field and hemodynamic parameters distribution, and (ii) verify the advantages of the CABG coupled sequential anastomoses design over the conventional end-to-side configuration in a more realistic bio-mechanical condition. For this purpose, a two-way fluid-structure interaction analysis has been carried out. A finite volume method is applied to solve the three-dimensional, time-dependent, laminar flow of the incompressible, non-Newtonian fluid; the vessel wall is modeled as a linearly elastic, geometrically non-linear shell structure. In an iteratively coupled approach the transient shell equations and the governing fluid equations are solved numerically. The simulation results indicate a diameter variation ratio of up to 4% and 5% in the graft and the coronary artery, respectively. The velocity patterns and qualitative distribution of wall shear stress parameters in the distensible model do not change significantly compared to the rigid-wall model, despite quite large side-wall deformations in the anastomotic regions. However, less flow separation and reversed flow is observed in the distensible models. The wall compliance reduces the time-averaged wall shear stress up to 32% (on the heel of the conventional end-to-side model) and somewhat increases the oscillatory nature of the flow. It is found that the effects of wall compliance and non-Newtonian rheology are not independent, and they interact with each other. In spite of the modest influence of wall compliance and non-Newtonian rheology on the hemodynamic parameters distribution, the inclusion of these properties has unveiled further advantages of the coupled sequential anastomoses model over the conventional end-to-side anastomosis which had not been revealed in the previous study with the rigid-wall and Newtonian fluid models. Hence, the inclusion of wall compliance and non-Newtonian rheology in flow simulation of blood vessels can be essential in quantitative and comparative investigations.

Effects of Geometric Variations on the Buckling of Arteries

Arteries often demonstrate geometric variations such as elliptic and eccentric cross sections, stenosis, and tapering along the longitudinal axis. Effects of these variations on the mechanical stability of the arterial wall have not been investigated. The objective of this study was to determine the buckling behavior of arteries with elliptic, eccentric, stenotic, and tapered cross sections. The arterial wall was modeled as a homogenous anisotropic nonlinear material. Finite element analysis was used to simulate the buckling process of these arteries under lumen pressure and axial stretch. Our results demonstrated that arteries with an oval cross section buckled in the short axis direction at lower critical pressures compared to circular arteries. Eccentric cross-sections, stenosis, and tapering also decreased the critical pressure. Stenosis led to dramatic pressure variations along the vessel and reduced the buckling pressure. In addition, tapering shifted the buckling deformation profile of the artery towards the distal end. We conclude that geometric variations reduce the critical pressure of arteries and thus make the arteries more prone to mechanical instability than circular cylindrical arteries. These results improve our understanding of the mechanical behavior of arteries.

Determination of the critical buckling pressure of blood vessels using the energy approach

The stability of blood vessels under lumen blood pressure is essential to the maintenance of normal vascular function. Differential buckling equations have been established recently for linear and nonlinear elastic artery models. However, the strain energy in bent buckling and the corresponding energy method have not been investigated for blood vessels under lumen pressure. The purpose of this study was to establish the energy equation for blood vessel buckling under internal pressure. A buckling equation was established to determine the critical pressure based on the potential energy. The critical pressures of blood vessels with small tapering along their axis were estimated using the energy approach. It was demonstrated that the energy approach yields both the same differential equation and critical pressure for cylindrical blood vessel buckling as obtained previously using the adjacent equilibrium approach. Tapering reduced the critical pressure of blood vessels compared to the cylindrical ones. This energy approach provides a useful tool for studying blood vessel buckling and will be useful in dealing with various imperfections of the vessel wall.

The theoretical foundation for artery buckling under internal pressure

The stability of blood vessels under the lumen blood pressure is essential to the maintenance of normal arterial function. Buckling equations have been established recently for linear and nonlinear elastic artery models with assumed sinusoidal mode shapes. However, the theoretical base for the assumption is not clear. This study established differential equations of artery buckling and then proved that straight arteries bifurcated into sinusoidal mode shapes when buckling occurs. These results set the buckling equation on a solid theoretical foundation.

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.

Nonlinear buckling of blood vessels: a theoretical study

Tortuosity and kinking often occur in arteries and veins but the underlying mechanisms are poorly understood. It has been suggested recently that long arteries may buckle and become tortuosity due to reduced axial tension or hypertensive pressure, but very few studies have been done to establish the biomechanical basis for artery buckling. Here we developed the arterial buckling equation using a nonlinear elastic thick-walled cylindrical model with residual stress. Our results demonstrated that arteries may buckle due to high blood pressure or low axial tension and that residual stress in the arteries increases the buckling pressure. These results are in general agreement with the previous linear elastic model. The buckling equation provides a useful tool for studying artery tortuosity and kinking.

A biomechanical model of artery buckling

The stability of arteries under blood pressure load is essential to the maintenance of normal arterial function and the loss of stability can lead to tortuosity and kinking that are associated with significant clinical complications. However, mechanical analysis of arterial bent buckling is lacking. To address this issue, this paper presents a biomechanical model of arterial buckling. Using an elastic cylindrical arterial model, the mechanical equations for arterial buckling were developed and the critical buckling pressure was found to be a function of the wall stiffness (Young's modulus), arterial radius, length, wall thickness, and the axial strain. Both the model equations and experimental results demonstrated that the critical pressure is related to the axial strain. Arteries may buckle and become tortuous due to reduced (subphysiological) axial strain, hypertensive pressure, and a weakened wall. These results are in accordance with, and provide a possible explanation to the clinical observations that hypertension and aging are the risk factors for arterial tortuosity and kinking. The current model is also applicable to veins and ureters.

Hemodynamic factors and atheromatic plaque rupture in the coronary arteries: from vulnerable plaque to vulnerable coronary segment

Coronary plaque disruption with superimposed thrombosis is the underlying pathology in the acute coronary syndromes and sudden death. Coronary plaques are constantly stressed by a variety of mechanical and hemodynamic forces that may precipitate or 'trigger' disruption of vulnerable or, at extreme conditions, even stable plaques. This paper reviews the exciting new evidence on the hemodynamic factors that may play a role in this process and provides the rationale for the introduction of the concept of the vulnerable coronary segment in the study of acute coronary syndromes.

Microvascular stress analysis

PURPOSE OF THE STUDY

To develop a finite element model (FEM) to study the effect of the stress and strain, in microvascular anastomoses that result from the geometrical mismatch of anastomosed vessels.

MATERIAL AND METHODS

FEMs of end-to-end and end-to-side anastomoses were constructed. Simulations were made using finite element software (NISA). We investigated the angle of inset in the end-to-side anastomosis and the discrepancy in the size of the opening in the vessel between the host and recipient vessels. The FEMs were used to predict principal and shear stress and strain at the position of each node.

RESULTS

Two types of vascular deformation were predicted during different simulations: longitudinal distortion, and rotational distortion. Stress values ranged from 151.1 to 282.4MPa for the maximum principal stress, from -122.9 to -432.2MPa for the minimum principal stress, and from 122.1 to 333.1MPa for the maximum shear stress. The highest values were recorded when there was a 50% mismatch in the diameter of the vessels at the site of the end-to-end anastomosis.

CONCLUSION

The effect of the vessel's size discrepancy on the blood flow and deformation was remarkable in the end-to-end anastomosis. End-to-side anastomosis was superior to end-to-end anastomosis. FEM is a powerful tool to study vascular deformation, as it predicts deformation and biomechanical processes at sites where physical measurements are likely to remain impossible in living humans.

Transient integral boundary layer method to calculate the translesional pressure drop and the fractional flow reserve in myocardial bridges

Background

The pressure drop – flow relations in myocardial bridges and the assessment of vascular heart disease via fractional flow reserve (FFR) have motivated many researchers the last decades. The aim of this study is to simulate several clinical conditions present in myocardial bridges to determine the flow reserve and consequently the clinical relevance of the disease. From a fluid mechanical point of view the pathophysiological situation in myocardial bridges involves fluid flow in a time dependent flow geometry, caused by contracting cardiac muscles overlying an intramural segment of the coronary artery. These flows mostly involve flow separation and secondary motions, which are difficult to calculate and analyse.

Methods

Because a three dimensional simulation of the haemodynamic conditions in myocardial bridges in a network of coronary arteries is time-consuming, we present a boundary layer model for the calculation of the pressure drop and flow separation. The approach is based on the assumption that the flow can be sufficiently well described by the interaction of an inviscid core and a viscous boundary layer. Under the assumption that the idealised flow through a constriction is given by near-equilibrium velocity profiles of the Falkner-Skan-Cooke (FSC) family, the evolution of the boundary layer is obtained by the simultaneous solution of the Falkner-Skan equation and the transient von-Kármán integral momentum equation.

Results

The model was used to investigate the relative importance of several physical parameters present in myocardial bridges. Results have been obtained for steady and unsteady flow through vessels with 0 – 85% diameter stenosis. We compare two clinical relevant cases of a myocardial bridge in the middle segment of the left anterior descending coronary artery (LAD). The pressure derived FFR of fixed and dynamic lesions has shown that the flow is less affected in the dynamic case, because the distal pressure partially recovers during re-opening of the vessel in diastole. We have further calculated the wall shear stress (WSS) distributions in addition to the location and length of the flow reversal zones in dependence on the severity of the disease.

Conclusion

The described boundary layer method can be used to simulate frictional forces and wall shear stresses in the entrance region of vessels. Earlier models are supplemented by the viscous effects in a quasi three-dimensional vessel geometry with a prescribed wall motion. The results indicate that the translesional pressure drop and the mean FFR compares favourably to clinical findings in the literature. We have further shown that the mean FFR under the assumption of Hagen-Poiseuille flow is overestimated in developing flow conditions.

Detection of cardiac cycle from intracoronary ultrasound

In this paper, we describe a method automatically to determine the phase of a cardiac cycle for each video frame of an intravascular ultrasound (IVUS) video recorded in vivo. We first review the principle of IVUS video and demonstrate the general applicability of our method. We show that the pulsating heart leads to phasic changes in image content of an IVUS video. With an image processing method, we can reverse this process and reliably extract the heart-beat phase directly from IVUS video. With the phase information, we demonstrate that we can build 3-D (3D) time-variant shapes and measure lumen volume changes within a cardiac cycle. We may also measure the changes of IVUS imaging probe off-center vector within a cardiac cycle, which serves as an indicator of vessel center-line curvature. The cardiac cycle extraction algorithm requires one scan of the IVUS video frames and takes O(n) time to complete, n being the total number of the video frames. The advantage of this method is that it requires no user interaction and no hardware set-up and can be applied to coronary scans of live beating hearts. The extracted heart-beat rate, compared with clinical recordings, has less than 1% error.

Finite element modelling and simulations in cardiovascular mechanics and cardiology: A bibliography 1993–2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.

Single-wire pressure and flow velocity measurement to quantify coronary stenosis hemodynamics and effects of percutaneous interventions

BACKGROUND

Lack of high-fidelity simultaneous measurements of pressure and flow velocity distal to a coronary artery stenosis has hampered the study of stenosis pressure drop-velocity (DeltaP-v) relationships in patients.

METHODS AND RESULTS

A novel 0.014-inch dual-sensor (pressure and Doppler velocity) guidewire was used in 15 coronary lesions to obtain per-beat averages of pressure drop and velocity after an intracoronary bolus of adenosine. DeltaP-v relations from resting to maximal hyperemic velocity were constructed before and after stepwise executed percutaneous coronary intervention (PCI). Before PCI, half of the DeltaP-v relations revealed the presence of a compliant stenosis, which was stabilized by angioplasty. Fractional flow reserve (FFR), coronary flow reserve (CFVR), and velocity-based indices of stenosis resistance (h-SRv) and microvascular resistance (h-MRv) at maximal hyperemia were compared. Stepwise PCI significantly lowered h-SRv, with an initial marked reduction in hyperemic pressure drop followed by further gains in velocity. A concomitant significant reduction of h-MRv accounted for half of the gain in velocity after PCI. The average magnitude of absolute incremental hemodynamic changes was highest for h-SRv (56.8+/-39.2%) compared with CFVR (35.3+/-34.5%, P<0.005) or FFR (19.5+/-25.2%, P<0.0001).

CONCLUSIONS

DeltaP-v relations comprehensively visualize improvements in coronary hemodynamics after PCI. h-SRv is a powerful and sensitive descriptor of the functional gain achieved by PCI, combining information about both pressure gradient and velocity, which are oppositely affected by PCI. Simultaneous assessment of stenosis and microvascular resistance may provide a valuable tool for guidance of PCI.

Hemodynamics of Carotid Artery Atherosclerotic Occlusive Disease

Hemodynamic mechanisms for the initiation and progression of carotid bifurcation atherosclerotic occlusive disease have been extensively researched during the past few decades. Attention has focused on the carotid bulb, or sinus, where most atherosclerotic plaques are found. Herein, the authors review the seminal works that have led to an understanding of not only complex local hemodynamics but also the elicited specific biologic response. In addition, new analysis of the age-dependent morphologic maturation of the human carotid bifurcation is unveiled. Understanding the role of hemodynamics in atherogenesis may lead to the improvement of minimally invasive endovascular therapy and noninvasive strategies.

Computational modeling of arterial biomechanics: insights into pathogenesis and treatment of vascular disease

We review how advances in computational techniques are improving our understanding of the biomechanical behavior of the healthy and diseased cardiovascular system. Numerical modeling of biomechanics is being used in a wide variety of ways, including assessment of effects of mural and hemodynamically induced stresses on atherogenesis, development of risk measures for aneurysm rupture, improvement in interpretation of medical images, and quantification of oxygen transport in diseased and healthy arteries. Although not amenable to routine clinical use, numerical modeling of cardiovascular biomechanics is a powerful research tool.

A nonlinear axisymmetric model with fluid-wall interactions for steady viscous flow in stenotic elastic tubes

Arteries with high-grade stenoses may compress under physiologic conditions due to negative transmural pressure caused by high-velocity flow passing through the stenoses. To quantify the compressive conditions near the stenosis, a nonlinear axisymmetric model with fluid-wall interactions is introduced to simulate the viscous flow in a compliant stenotic tube. The nonlinear elastic properties of the tube (tube law) are measured experimentally and used in the model. The model is solved using ADINA (Automatic Dynamic Incremental Nonlinear Analysis), which is a finite element package capable of solving problems with fluid-structure interactions. Our results indicate that severe stenoses cause critical flow conditions such as negative pressure and high and low shear stresses, which may be related to artery compression, plaque cap rupture, platelet activation, and thrombus formation. The pressure filed near a stenosis has a complex pattern not seen in one-dimensional models. Negative transmural pressure as low as -24 mmHg for a 78 percent stenosis by diameter is observed at the throat of the stenosis for a downstream pressure of 30 mmHg. Maximum shear stress as a high as 1860 dyn/cm2 occurs at the throat of the stenoses, while low shear stress with reversed direction is observed right distal to the stenosis. Compressive stresses are observed inside the tube wall. The maximal principal stress and hoop stress in the 78 percent stenosis are 80 percent higher than that from the 50 percent stenosis used in our simulation. Flow rates under different pressure drop conditions are calculated and compared with experimental measurements and reasonable agreement is found for the prebuckling stage.

Wall stress as a possible mechanism for the development of transverse intimal tears in aortic dissections

In aortic dissection intimal tear develops in a transverse direction. Since dissection is associated with the aneurysm of the aorta, its mechanism was investigated by analysing the pressure induced wall stress as a function of 'growth' of the aneurysm. The stresses were determined using a finite element analysis where the aorta was modelled as an isotropic, nonlinear, hyperelastic material. Growth of the aneurysm was simulated by dilating an aortic segment in increments of 10% of the initial diameter. At each dilation luminal pressure of 120 mm Hg was applied and stress determined. In the aneurysm bulb, longitudinal stress increased significantly as the bulb became larger, while circumferential stress changed only a little. In the undilated segment both the longitudinal and circumferential stresses remained relatively unchanged. The increase in the longitudinal stress in the bulb occurred primarily due to change in shape of the aorta from cylindrical to ellipsoidal to spherical. Hence, as the aneurysm 'grows', the longitudinal stress in the bulb increases rapidly and could be responsible for the transverse tear in the aortic dissection.

Compliance mismatch may promote graft-artery intimal hyperplasia by altering suture-line stresses

The role of graft-artery compliance mismatch in the development of distal anastomotic intimal hyperplasia (DAIH) is not yet resolved. Although DAIH develops at all surgically created anastomoses, increased compliance mismatch does not lead to greater hyperplasia formation in end-to-end anastomoses, but in end-to-side anastomoses, it leads to a profound increase in hyperplasia. The current study was undertaken to determine whether suture-induced anastomotic stresses could explain these findings. A large strain finite element analysis of vascular wall mechanics was performed to compare the influence of compliance mismatch on intramural stresses in end-to-end versus end-to-side anastomoses. A novel modelling approach was implemented which includes suture-induced stress concentrations. End-to-end and end-to-side graft-artery simulations were executed using (1) artery (compliance = C = 0.44% kPa(-1)), (2) vein (C = 0.33% kPa(-1)), and (3) Dacron (C = 0.14% kPa(-1)) grafts. Residual stresses due to axial tension were included and the anastomoses were statically inflated to 13.3 kPa (100 mmHg). Elevated intramural stresses were found to exist at both the end-to-end and end-to-side graft-artery junctions; however, in the end-to-end anastomosis, the maximum anastomotic stress was not a function of the graft compliance, whereas in the end-to-side anastomosis, the maximum stress was a strong function of graft compliance. For the 45 degree end-to-side geometry considered in this study, the maximum anastomotic stress concentration obtained using a stiff Dacron graft was more than 40% greater than that obtained using a compliant artery graft. In the end-to-end anastomosis, the Dacron graft led to a less than 5% increase in maximum stress over the artery graft. Therefore, increased compliance mismatch increases stresses and promotes DAIH in end-to-side junctions, but, it has little influence on either stresses or DAIH in end-to-end junctions. Thus, the proliferative influence of increased compliance mismatch on suture-line hyperplasia in end-to-side anastomoses can be explained by the resulting increase in intramural stresses. In addition, since high stresses were found in both geometries, elevated suture-line intramural stresses may be an important proliferative stimulus for intimal hyperplasia formation in all vascular reconstructions.

Effects of frictional losses and pulsatile flow on the collapse of stenotic arteries

High-grade stenosis can produce conditions in which the artery may collapse. A one-dimensional numerical model of a compliant stenosis was developed from the collapsible tube theory of Shapiro. The model extends an earlier model by including the effects of frictional losses and unsteadiness. The model was used to investigate the relative importance of several physical parameters present in the in vivo environment. The results indicated that collapse can occur within the stenosis. Frictional loss was influential in reducing the magnitude of collapse. Large separation losses could prevent collapse outright even with low downstream resistances. However, the degree of stenosis was still the primary parameter governing the onset of collapse. Pulsatile solutions demonstrated conditions that produce cyclic collapse within the stenosis. This study predicts certain physiologic conditions in which collapse of arteries may occur for high-grade stenoses.

Fluid dynamics of a partially collapsible stenosis in a flow model of the coronary circulation

The influence of passive vasomotion on the pressure drop-flow (delta P-Q) characteristics of a partially compliant stenosis was studied in an in vitro model of the coronary circulation. Twelve stenosis models of different severities (50 to 90 percent area reduction) and degrees of flexible wall (0 to 1/2 of the wall circumference) were inserted into thin-walled latex tubing and pressure and flow data were collected during simulated cardiac cycles. In general, the pressure drop increased with increasing fraction of flexible wall for a given flow rate and stenosis severity. The magnitude of this effect was directly dependent upon the underlying stenosis severity. The diastolic delta P-Q relationship of severe, compliant models exhibited features of partial collapse with an increase in pressure drop at a decreasing flow rate. It is concluded that passive vasomotion of a normal wall segment at an eccentric stenosis in response to periodic changes in intraluminal pressure causes dimensional changes in the residual lumen area which can strongly affect the hemodynamic characteristics of the stenosis during the cardiac cycle. This mechanism may have important implications for the onset of plaque fracture and the prediction of the functional significance of a coronary stenosis based on quantitative angiogram analysis.

Viscoelastic property detection by elastic displacement NMR measurements

An attempt was undertaken to analyze the phase changes in the transverse magnetization component, M[perp], determined in NMR response by an elastic wave in the presence of two pulses with opposite magnetic field gradient directions. The obtained theoretical results indicate that such changes are significant in biologic tissues. Their measurement by NMR opens up new possibilities for detecting the displacement and some viscoelastic properties such as the adiabatic compressibility coefficient, amplitude wave damping coefficient, frequency and dispersion strength of elastic relaxation processes, etc. The method may be useful in basic applied research and in medical diagnosis of some diseases resulting in the variation of elastic properties of biological tissues, eg, atherosclerosis.