RESUMEN
BACKGROUND AND PURPOSE: The reconstruction of aneurysm geometry is a main factor affecting the accuracy of hemodynamics simulations in patient-specific aneurysms. We analyzed the effects of the inlet artery length on intra-aneurysmal flow estimations by using 10 ophthalmic aneurysm models. MATERIALS AND METHODS: We successively truncated the inlet artery of each model, first at the cavernous segment and second at the clinoid segment. For each aneurysm, we obtained 3 models with different artery lengths: the originally segmented geometry with the longest available inlet from scans and 2 others with successively shorter artery lengths. We analyzed the velocity, wall shear stress, and the oscillatory shear index inside the aneurysm and compared the 2 truncations with the original model. RESULTS: We found that eliminating 1 arterial turn resulted in root mean square errors of <18% with no visual differences for the contours of the flow parameters in 8 of the 10 models. In contrast, truncating at the second turn led to root mean square errors between 18% and 32%, with consistently large errors for wall shear stress and the oscillatory shear index in 5 of the 10 models and visual differences for the contours of the flow parameters. For 3 other models, the largest errors were between 43% and 55%, with large visual differences in the contour plots. CONCLUSIONS: Excluding 2 arterial turns from the inlet artery length of the ophthalmic aneurysm resulted in large quantitative differences in the calculated velocity, wall shear stress, and oscillatory shear index distributions, which could lead to erroneous conclusions if used clinically.
Asunto(s)
Arterias/patología , Simulación por Computador , Aneurisma Intracraneal/fisiopatología , Modelos Cardiovasculares , Hemodinámica/fisiología , Humanos , Modelos Teóricos , Estrés MecánicoRESUMEN
BACKGROUND AND PURPOSE: Predicting the outcome of flow diversion treatment of cerebral aneurysms remains challenging. Our aim was to investigate the relationship between hemodynamic conditions created immediately after flow diversion and subsequent occlusion of experimental aneurysms in rabbits. MATERIALS AND METHODS: The hemodynamic environment before and after flow-diversion treatment of elastase-induced aneurysms in 20 rabbits was modeled by using image-based computational fluid dynamics. Local aneurysm occlusion was quantified by using a voxelization technique on 3D images acquired 8 weeks after treatment. Global and local voxel-by-voxel hemodynamic variables were used to statistically compare aneurysm regions that later thrombosed to regions that remained patent. RESULTS: Six aneurysms remained patent at 8 weeks, while 14 were completely or nearly completely occluded. Patent aneurysms had statistically larger neck sizes (P = .0015) and smaller mean transit times (P = .02). The velocity, vorticity, and shear rate were approximately 2.8 times (P < .0001) larger in patent regions-that is, they had larger "flow activity" than regions that progressed to occlusion. Statistical models based on local hemodynamic variables were capable of predicting local occlusion with good precision (84% accuracy), especially away from the neck (92%-94%). Predictions near the neck were poorer (73% accuracy). CONCLUSIONS: These results suggests that the dominant healing mechanism of occlusion within the aneurysm dome is related to slow-flow-induced thrombosis, while near the neck, other processes could be at play simultaneously.
Asunto(s)
Hemodinámica/fisiología , Aneurisma Intracraneal/terapia , Animales , Modelos Animales de Enfermedad , Hidrodinámica , Imagenología Tridimensional , Conejos , StentsRESUMEN
Under the dynamic conditions of pulsatile flow, the forces exerted by the fluid on the vessel wall create considerable displacements and stresses within the thickness of the vessel wall. We review a series of analytical options for exploring the dynamics of the vessel wall, specifically displacements and stresses within the depth of the vessel wall, under a range of conditions including the degree of external tethering and the mechanical consistency of the wall material. It is shown that one of the most important effects of tethering is that of drastically restricting radial displacements of and within the vessel wall. This restriction in turn places limits on the length and speed of the propagating wave. Specifically, the wave speed is significantly reduced as a result of tethering. This has important consequences because the wave speed, or pulse wave velocity as it is referred to in the clinical setting, is used as an index of vascular stiffening in relation to aging or age related hypertension. It is found further that the extent of displacements and shear stresses within the vessel wall depend critically on the relative proportions of viscous and elastic content within the wall. In particular, loss of viscous consistency leads to higher shear stresses within the wall, thus putting higher loading on elastin and may ultimately lead to elastin fatigue. As elastin gradually fails, its load bearing function is presumably taken over by collagen which renders the vessel wall less elastic and more rigid as is observed in the aging process.
Asunto(s)
Arterias/fisiología , Fenómenos Mecánicos , Flujo Pulsátil , Fenómenos Biomecánicos , Humanos , Modelos Biológicos , Estrés MecánicoRESUMEN
An analytical solution is presented of the governing equations for the coupled radial and longitudinal displacements and stresses within the finite thickness of the arterial wall in pulsatile flow. The results are used to examine the extent of coupling between the radial and longitudinal dynamics within the vessel wall, particularly when the wall is fully tethered. In the case of a free wall, it is found that the dynamics in the two directions are fairly decoupled from each other when the wavelength is at least of the order of 100 times the vessel radius. At 10 times the vessel radius, however, there is strong coupling between the two. These findings are consistent with expectations in the case of a free wall where the long-wave approximation has been applied in the past. In the case of a tethered wall, however, the results indicate that in general the long-wave approximation is strictly valid only when the combination of wall material and tethering allow the wave to be long.
Asunto(s)
Arterias/fisiología , Modelos Biológicos , Flujo Pulsátil , Estrés Mecánico , Fenómenos BiomecánicosRESUMEN
Pulse wave velocity (PWV) is often used as a clinical index of aging, vascular disease, or age related hypertension. This practice is based on the assumption that a higher wave speed indicates vascular stiffening. This assumption is well grounded in the physics of pulsatile flow of an incompressible fluid where it is fully established that a pulse wave travels faster in a tube of stiffer wall, the wave speed becoming infinite in the mathematical limit of a rigid wall. However, in this paper we point out that the physical principal of higher pulse wave velocity in a stiffer tube is strictly valid only when the wall is free from outside constraints, which in the physiological setting is present in the form of tethering of the vessel wall. The use of PWV as an index of arterial stiffening may thus lose its validity if tethering is involved. A solution of the problem of vessel wall mechanics as they arise from the physiological pulsatile flow problem is presented for the purpose of resolving this issue. The vessel wall is considered to have finite thickness with or without tethering and with a range of mechanical properties ranging from viscoelastic to stiff. The results show that, indeed, while the wave speed becomes infinite in the mathematical limit of a rigid free wall, the opposite actually happens if the vessel wall is tethered. Here the wave speed actually diminishes as the degree of tethering increases. This dichotomy in the effects of tethering versus stiffening of the arterial wall may clearly lead to error in the interpretation of PWV as an index of vessel wall stiffness. In particular, a normal value of PWV may lead to the conclusion that vessel wall stiffening is absent while this value may in fact have been lowered by tethering. In other words, the diagnostic test may lead to a false negative diagnosis. Our results indicate that the reason for which PWV is lower in a tethered wall compared with that in a free wall of the same stiffness is that the radial movements of the wall are greatly reduced by tethering. More precisely, the results show that PWV depends strongly on the ratio of radial to axial displacements and that this ratio is much lower in a tethered wall than it is in a free wall of the same stiffness.
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Aorta/fisiopatología , Arterias/fisiopatología , Frecuencia Cardíaca , Adulto , Anciano de 80 o más Años , Fenómenos Biomecánicos , Biofisica/métodos , Velocidad del Flujo Sanguíneo , Presión Sanguínea/fisiología , Elasticidad , Endotelio Vascular/fisiopatología , Humanos , Hipertensión/fisiopatología , Modelos Estadísticos , Flujo Pulsátil , Pulso Arterial , Resistencia Vascular/fisiología , ViscosidadRESUMEN
Change in arterial stiffness is generally considered a risk factor for cardiovascular disease and, in various ways, has been associated with hypertension, diabetes, hyperlipidemia, atherosclerosis, and heart failure, likely because of altered dynamics of the wall and of the fluid-wall interplay in pulsatile flow. We present a comprehensive analytical study of longitudinal displacements and stresses within the thickness of the vessel wall induced by pulsatile flow at different times within the cardiac cycle, using the fractional derivative model which has been found to provide a good descriptor of the rheological material's response to frequency. The results indicate that the extent of displacement and shear stress within the depth of the vessel wall depend critically on the degree to which the wall is tethered to surrounding tissue and on the mechanical consistency of the wall material, particularly on the relative proportions of viscous and elastic content within the wall. In particular, loss of viscous consistency leads to higher shear stresses within the wall thus putting higher loading on elastin and may ultimately lead to elastin fatigue and, as elastin gradually fails, its load bearing function is presumably taken over by collagen which renders the vessel wall less elastic and more rigid as is indeed observed in the aging process. It is thus concluded that loss of viscous content within the vessel wall, whether by disease or aging, may be a prelude to elastin fatigue and elastin failure within the vessel wall.
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Arterias/fisiología , Elastina/fisiología , Animales , Fenómenos Biomecánicos , Velocidad del Flujo Sanguíneo , Elasticidad , Modelos Cardiovasculares , Flujo Pulsátil/fisiología , Resistencia al Corte/fisiología , Estrés Mecánico , ViscosidadRESUMEN
A standing difficulty in the problem of blood vessel tethering has been that only one of the two required boundary conditions can be fully specified, namely, that at the inner (endothelial) wall surface. The other, at the outer layer of the vessel wall, is not known except in the limiting case where the wall is fully tethered such that its outer layer is prevented from any displacement. In all other cases, where the wall is either free or partially tethered, a direct boundary condition is not available. We present a method of determining this missing boundary condition by considering the limiting case of a semi-infinite wall. The result makes it possible to define the degree of tethering imposed by surrounding tissue more accurately in terms of the displacement of the outer layer of the vessel wall, rather than in terms of equivalent added mass which has been done in the past. This new approach makes it possible for the first time to describe the effect of partial tethering in its full range, from zero to full tethering. The results indicate that high tethering leads to high stresses and low displacements within the vessel wall, while low tethering leads to low stresses and high displacements. Since both extremes would be damaging to wall tissue, particularly elastin, this suggest that moderate tethering would be optimum in the physiological setting.
Asunto(s)
Arterias/fisiología , Tejido Conectivo/fisiología , Modelos Cardiovasculares , Animales , Simulación por Computador , HumanosRESUMEN
Mechanical events within the thickness of the vessel wall caused by pulsatile blood flow are considered, with focus on axial dynamics of the wall, driven by the oscillatory drag force exerted by the fluid on the endothelial layer of the wall. It is shown that the focus on the axial direction makes it possible to derive simplified equations of motion which, combined with a viscoelastic model of the wall material, makes it possible in turn to obtain solutions in closed form for the displacement and stress of material elements within the wall. The viscoelastic model allows a study of the dynamics of the wall with different ratios of viscosity to elasticity of the wall material, to mimic changes in the properties of the arterial wall caused by disease or aging. It is found that when the wall is highly viscous the displacements and stresses caused by the flow are confined to a thin layer close to the inner boundary of the wall, while as the wall material becomes less viscous and more rigid the displacements and stresses spread deeper into the thickness of the wall to affect most of its elements.