Article

Estimation of wall shear stress in bypass grafts with computational fluid dynamics method.

Labor für Biofluidmechanik, Universitätsklinikum Charité, Humboldt Universität zu Berlin, Germany.
The International journal of artificial organs (Impact Factor: 1.45). 03/2001; 24(3):145-51.
Source: PubMed

ABSTRACT Coronary artery bypass graft (CABG) operation for coronary artery disease with different types of grafts has a large clinical application world wide. Immediately after this operation patients are usually relieved of their chest pain and have improved cardiac function. However, after a while, these bypass grafts may fail due to for example, neointimal hyperplasia or thrombosis. One of the causes for this bypass graft failure is assumed to be the blood flow with low wall shear stress. The aim of this research is to estimate the wall shear stress in a graft and thus to locate areas were wall shear stress is low. This was done with the help of a blood flow computer model. Post-operative biplane angiograms of the graft were recorded, and from these the three-dimensional geometry of the graft was reconstructed and imported into the computational fluid dynamics (CFD) program FLUENT. The stationary diastolic flow through the grafts was calculated, and the wall shear stress distribution was estimated. This procedure was carried out for one native vessel and two different types of bypass grafts. One bypass graft was a saphenous vein and the other one was a varicose saphenous vein encased in a fine, flexible metal mesh. The mesh was attached to give the graft a defined diameter. The computational results show that each graft has distinct areas of low wall shear stress. The graft with the metal mesh has an area of low wall shear stress (< 1 Pa, stationary flow), which is four times smaller than the respective areas in the other graft and in the native vessel. This is thought to be caused by the smaller and more uniform diameter of the metal mesh-reinforced graft.

1 Follower
 · 
140 Views
  • Source
    • "A complete description of hemodynamics within a particular vessel or lesion requires knowledge of the pattern of blood velocities within the flow [3], [4]. The latter depends on the geometry and mechanical properties of the vascular wall, an overall pressure difference, and the rheological characteristics of blood (viscosity, density). "
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Sterilization processes used on biologically-derived, intact arteries may change the vessel‟s structural integrity and biocompatibility from its native state. Remodeling processes that occur in biologically-derived vessels serve to restore homeostatic levels of shear and circumferential stresses, which arise from fluid flow and pressure loads. If the mechanical compliance of a decellularized artery changes from its native state, the vessel may be vulnerable to biological remodeling processes that occlude the vessel when used for small-diameter arterial bypass or replacement procedures. The effects of three distinct detergent-enzymatic decellularization protocols on porcine artery were characterized by mechanical and spectroscopic methods to evaluate cell removal efficacy, preservation of structural integrity, and altered mechanical behavior under physiological loading. Longitudinal porcine aortic strip samples were tested under uniaxial tension, and parameters optimized to a nonlinear rubber model were compared. Molecular-level changes between treated and untreated arterial tissue were assessed by observing shifts in Fourier Transform Infrared (FTIR) spectra processed by baseline-correction between amide I and II bands and optimized to Gaussian-Lorenztian curve distributions. Quasi-static inflation-extension tests were used to characterize effects of temperature and decellularization on the modeled distensibilities of native porcine carotid artery. Parameters for a nonlinear, orthotropic strain energy function were optimized to inflation-extension data to predict internal stresses at physiological loads. A four-step, 72-hour decellularizaton protocol that utilized TritonX-100, sodium-deoxycholate, RNase-A, DNase-I, and magnesium chloride as reagents was the most effective option for decellularizing porcine aorta. This protocol lysed all smooth muscle cells from porcine aorta and reduced the hyperelastic response of the vessel in the physiologic loading range. Analysis of FTIR spectra confirms that this decellularization method does not denature collagen protein structure in arterial tissue, but does alter intermolecular hydrogen bonding that supports the observed increase in bulk mechanical compliance. Three-dimensional mechanical characterization of porcine carotid artery suggests that room temperature testing conditions and the selected decellularization protocol increase arterial residual strains. Increased levels of residual strain increase the vessel‟s modeled distensibility and, consequently, its internal stresses. Use of a constitutive model that incorporates residual strains leads to better prediction of local circumferential stresses in comparison to mean circumferential stress approximations.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Many clinical studies suggest that local blood flow patterns are involved in the location and development of atherosclerosis. In coronary diseases, this assumption should be corroborated by quantitative information on local hemodynamic parameters such as pressure, velocity or wall shear stress. Nowadays, computational fluid dynamics (CFD) algorithms coupled to realistic 3-D reconstructions of such vessels make these data accessible. Nevertheless, they should be carefully analysed to avoid misinterpretations when the physiological parameters are not all considered. As an example, we propose here to compare the flow patterns calculated in a coronary vessel reconstructed by three different methods. In the three cases, the vessel trajectory respected the physiology. In the simplest reconstruction, the coronary was modelled by a tube of constant diameter while in the most complex one, the cross-sections corresponded to the reality. We showed that local pressures, wall shear rates and velocity profiles were severely affected by the geometrical modifications. In the constant cross-section vessel, the flow resembled to that of Poiseuille in a straight tube. On the contrary, velocity and shear rate exhibited sudden local variations in the more realistic vessels. As an example, velocity could be multiplied by 5 as compared to Poiseuille's flow and area of very low wall shear rates appeared. The results obtained with the most complex model clearly outlined that, in addition to a proper description of the vessel trajectory, the section area changes should be carefully taken into account, confirming assumptions already highlighted before the rise of commercially available and efficient CFD softwares.
    Journal of Biomechanics 11/2002; 35(10):1347-56. DOI:10.1016/S0021-9290(02)00179-3 · 2.50 Impact Factor
Show more