Elucidating creating an entirely new flow-field causing abnormal

Elucidating blood flow and transport of macromolecules
in the human arterial system is crucial for the in depth understanding of
genesis and progression of atherosclerotic plaques, 1. The Computational Fluid Dynamics (CFD)
has proven to be an excellent tool for obtaining qualitative and detailed
quantitative information on stent flow patterns, 2. Oscillating flow develops complex flow field
and from computational point of view poses a difficult task.

Nowadays stent implantations are widely used to unblock
arteries with stenosis. Stents are usually deployed in arteries with significant
amount of stenosis (plaque). However, stent implantation drastically alters the
blood flow characteristics creating an entirely new flow-field causing abnormal
endothelium response. Furthermore, stent designs
greatly influence the reaction of the arterial wall. Questions arise as to how
increased heart rate affect Low Density Lipoprotein (LDL) distribution in stented
arteries. Of equal importance is the answer to the question: does diastole in relation to systole favors
elevated LDL distribution at increased heart rates? Stent
implantation affects the Wall Shear Stress (WSS) in the surrounding upstream
and downstream strut flow regions 3. Considering oscillatory flow, low time-Averaged Wall Shear
Stress (AWSS) values give rise to atherosclerotic lesions at specific coronary
arterial regions as it is now widely accepted. The developed highly complex
laminar flow yields low and oscillatory WSS, increase LDL uptake and
permeability, 4.
Thus, the progressing of atherosclerosis is present. Oscillating WSS, lower
than <1.0-1.2 N/m2, is considered to be responsible for thrombus formation via endothelium injury, 5. Complex configuration of the vessel, after stent implantation, promotes disturbances with low AWSS and high shear stress temporal oscillations during accelerating, peak and decelerating flows 6. The strut spacing, strut diameter, longitudinal strut connectors and strut embedment may be responsible for increased LDL concentration, 22. Struts oriented to flow direction reduce the flow recirculation, 23. Instantaneous hemodynamic conditions have important influence on LDL transmural transport in arteries with disturbed and complicated flow patterns 7. There have been very few studies investigating the fluid-wall coupled mass transport under oscillating flow condition 7, 8. Regional variations in the permeability of arterial endothelium may contribute to the localization of atherosclerosis 9, 10. Luminal surface level of LDL transport across the artery wall is considered to be important in atherogenesis 11, 12. Elevated LDL nearly co-localize with known sites of atherosclerotic plaque development, 13. Results indicate that lipids accumulate at the luminal surface in areas where blood flow velocity and WSS are low and where the permeability of the endothelial layer is enhanced, 14. Few 3D oscillating CFD studies have been carried out on stented arteries incorporating flow and mass transfer elucidating at the same time the LDL concentration differentiation between systole and diastole at resting and/or at exercise condition (pulse wave). Our objective was to calculate the flow disturbance induced by the stent via the AWSS, Relative Residence Time (RRT) and the LDL transport distributions under resting and exercise condition. Computational analysis is performed for the: half stent (strut) exposure into lumen, while the other half is embedded into endothelium. In this research work the semi-permeable arterial wall treatment is incorporated. No transmural flow or mass transfer analysis within the stented arterial segment wall is introduced. The infiltration velocity and the endothelial permeability are treated as wall shear stress independent. Calculated results provide insight into how the fluid biomechanical factors, induced by a certain stent design, affect LDL concentration distribution during systole as it is compared to diastole at resting and exercise heart condition.  

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