An S-type bypass can improve the hemodynamics in the bypassed arteries and suppress intimal hyperplasia along the host artery floor

A bypass flow model to study endothelial cell mechanotransduction across diverse flow environments

Disturbed flow is one of the pathological initiators of endothelial dysfunction in intimal hyperplasia (IH) which is commonly seen in vascular bypass grafts, and arteriovenous fistulas. Various in vitro disease models have been designed to simulate the hemodynamic conditions found in the vasculature. Nonetheless, prior investigations have encountered challenges in establishing a robust disturbed flow model, primarily attributed to the complex bifurcated geometries and distinctive flow dynamics. In the present study, we aim to address this gap by introducing an in vitro bypass flow model capable of inducing disturbed flow and other hemodynamics patterns through a pulsatile flow in the same model. To assess the model's validity, we employed computational fluid dynamics (CFD) to simulate hemodynamics and compared the morphology and functions of human umbilical venous endothelial cells (HUVECs) under disturbed flow conditions to those in physiological flow or stagnant conditions. CFD analysis revealed the generation of disturbed flow within the model, pinpointing the specific location in the channel where the effects of disturbed flow were observed. High-content screening, a single-cell morphological profile assessment, demonstrated that HUVECs in the disturbed flow area exhibited random orientation, and morphological features were significantly distinct compared to cells in the physiological flow or stagnant condition after a two days of flow exposure. Furthermore, HUVECs exposed to disturbed flow underwent extensive remodeling of the adherens junctions and expressed higher levels of endothelial cell activation markers compared to other hemodynamic conditions. In conclusion, our in vitro bypass flow model provides a robust platform for investigating the associations between disturbed flow pattern and vascular diseases.

A Comparative Study of the Etiology of Intracranial Vertebral Artery Dissection and Carotid Artery Dissection

BACKGROUND

Both extrinsic (trauma or violent movement) and intrinsic (structural abnormality, atherosclerosis, or hemodynamic instability) factors may result in arterial dissection. The role of these factors in the origin or progression of dissection remains unclear. This study aimed to characterize the clinical features, radiologic features, and outcomes of intracranial vertebral artery dissection compared with carotid artery dissection and to determine the major causative factors.

METHODS

Consecutive patients with craniocervical dissection (n=127) were retrospectively analyzed. Patients with intracranial vertebral artery dissection (n=77) and those with carotid artery dissection (n=35) were compared with respect to patient age, sex, cerebrovascular risk factors, laboratory indices, and radiologic features.

RESULTS

Intracranial vertebral artery dissection was the most common craniocervical arterial dissection in our cohort (n=77, 60.6%). Body mass index in the intracranial vertebral artery dissection group was significantly greater than that in carotid artery dissection group. Clinical manifestations of intracranial vertebral artery dissection included ischemic stroke (37.7%), dizziness or vertigo (39.0%), and headache or neck pain (44.2%). Two patients had a definite history of trauma. The frontal and lateral tortuosity ratios of the vertebral basilar artery were significantly greater while the vertex angle was smaller in the intracranial vertebral artery dissection group compared with carotid artery dissection group. A positive correlation between the tortuosity ratios and subarachnoid hemorrhage and a significant inverse correlation between the tortuosity ratios and lipid parameters (high-density lipoprotein; apolipoprotein A1) were identified.

CONCLUSIONS

Intrinsic causes may play a more important role in the development of intracranial vertebral artery dissection than carotid artery dissection.

'Heart-like' cross-sectional shape can better improve the hemodynamics in spiral laminar flow graft for small-caliber bypass application: a numerical study

Small-caliber grafts remain disappointed in the long-term bypass surgeries of coronary and peripheral arterial diseases. In order to improve the hemodynamics in small-caliber artery bypass grafts (ABGs), an improved spiral laminar flow (improved-SLF) graft with a 'heart-like' cross-sectional shape was proposed and verified by computational fluid dynamics simulation in this study. The results show that such graft can indeed induce a spiral flow and enhance the WSS distribution on the graft section. Furthermore, the helically distributed ribbon of unfavorable WSS observed in the original SLF graft was eliminated in the improved-SLF graft due to its smoothed and gentle helical ridge. On the other hand, improved-SLF ABG improved the WSS distribution in the distal anastomosis as well, because it maintained the strength of spiral flow when entering the anastomosis region. Finally, the improved-SLF ABG slightly increased the pressure drop along the bypass due to its small change of the general graft structure. As a proof-of-concept study, it can be concluded that improved-SLF graft can not only evenly enhance the WSS distribution in the graft section, but also improve the hemodynamic environment in the distal anastomosis without significantly increasing the pressure drop along the bypass, indicating such new helical-type graft may be more suitable to be used in the small-caliber graft bypass surgeries.

Mechanosensitive Piezo1 Channel Evoked-Mechanical Signals in Atherosclerosis

Recently, more and more works have focused and used extensive resources on atherosclerosis research, which is one of the major causes of death globally. Alongside traditional risk factors, such as hyperlipidemia, smoking, hypertension, obesity, and diabetes, mechanical forces, including shear stress, pressure and stretches exerted on endothelial cells by flow, is proved to be crucial in atherosclerosis development. Studies have recognized the mechanosensitive Piezo1 channel as a special sensor and transducer of various mechanical forces into biochemical signals, and recent studies report its role in atherosclerosis through different mechanical forces in pressure, stretching and turbulent shear stress. Based on our expertise in this field and considering the recent advancement of atherosclerosis research, we will be focusing on the function of Piezo1 and its involvement in various cellular mechanisms and consequent involvement in the development of atherosclerosis in this review. Also, we will discuss various functions of Piezo1 involvement in atherosclerosis and come up with new mechanistic insight for future research. Based on the recent findings, we suggest Piezo1 as a valid candidate for novel therapeutic innovations, in which deep exploration and translating its findings into the clinic will be a new therapeutic strategy for cardiovascular diseases, particularly atherosclerosis.

Study of helical flow inducers with different thread pitches and diameters in vena cava

Pulmonary embolism is a severe, potentially life-threatening condition. Inferior vena cava filters have been used to prevent recurrent pulmonary embolisms. However, the build-up of thrombosis in vena cava filters after deployment presents a severe problem to patients. Previous studies proposed that filters with helical flow are beneficial and capable of alleviating this problem. In this study, the hemodynamic performances of four typical helical flow inducers in the vena cava are determined using computational fluid dynamics simulations (steady-state and pulsatile flow) and compared. Pilot in vitro experiments were also conducted. The simulation results demonstrate that large-diameter inducers produce helical flow. Among inducers with identical diameter, those with a smaller thread pitch are more likely to induce increased helical flow. We also observed that the small thread pitch inducers can yield higher shear rates. Furthermore, a large diameter, small thread pitch helical flow inducer increases the time-averaged wall shear stress and reduces the oscillating shear index and relative residence time on the vessel wall in the vicinity of the helical flow inducer. In vitro experiments also verify that large diameter inducers generate a helical flow. A notable observation of this study is that the diameter is the key parameter that affects the induction of a helical flow. This study will likely provide important guidance for the design of interventional treatments and the deployment of filters associated with helical flow in the vena cava.

The Ratio of Diameters Between the Target Artery and the Bypass Modifies Hemodynamic Parameters Related to Intimal Hyperplasia in the Distal End-to-Side Anastomosis

Hemodynamics in the distal end-to-side anastomosis is related to early development of intimal hyperplasia and bypass failure. In this study we investigated the effect of diameter ratios between the target artery and the bypass at three different angles of the connection. The pulsatile flow field was visualized using particle image velocimetry in transparent models with three different angles of the connection (25°, 45°, 60°) and the diameter ratio between the bypass and the target artery was 4.6 mm : 6 mm, 6 mm : 6 mm, and 7.5 mm : 6 mm. Six parameters including location and oscillation of the stagnation point, local energy dissipation, wall shear stress (WSS), oscillatory shear index, spatial and temporal gradient of WSS and their distribution in the target artery were calculated from the flow field. In the wider bypass, the stagnation point oscillated in a greater range and was located more proximal to the anastomosis. Energy dissipation was minimal in a wider bypass with a more acute angle. The maximum WSS values were tree times greater in a narrow bypass and concentrated in a smaller circular region at the floor of the anastomosis. The oscillatory shear index increased with wider bypass and more acute angle. The maximum of spatial gradient of WSS concentrated around the floor and toe of the anastomosis and decreased with more acute angle and wider bypass, the temporal gradient of WSS was stretched more towards the side wall. Greater bypass to target vessel ratio and more acute anastomosis angle promote hemodynamics known to reduce formation of intimal hyperplasia.

Numerical simulation of compliant artery bypass grafts using fluid-structure interaction framework

Early researches on the artery bypass graft (ABG) generally took the assumption of rigid vessel wall that ignored the wall compliancy. To obtain more realistic and physiological hemodynamic parameters, a fluid structure interaction study on a complete ABG was carried out. It was concluded: (1) a compliant vessel is able to expand its vessel diameter and decrease its anastomosis angle to achieve a buffer for the blood, thereby helping to reduce endothelial cell injury. (2) The vessel walls experienced their maximum deformation at the time of peak pressure while the deformation could be ignored during diastole. However, the consideration of wall compliance did not quantitatively change the flow characters compared to those of rigid walls. (3) Generally, the hemodynamic priority of helical-type ABG over a conventional one was further strengthened by adopting compliant vessel wall. (4) The consideration of the wall deformation revealed a hidden fact by the rigid wall assumption: Helical ABG aggravated the risk of intimal hyperplasia at its toe region due to its geometry and flow asymmetry. The present study may be useful for surgeons and graft designers to optimize the current and future ABG configurations and selection of materials.

Novel modular anastomotic valve device for hemodialysis vascular access: preliminary computational hemodynamic assessment

PURPOSE

Arteriovenous graft patency is limited by terminal occlusion caused by intimal hyperplasia (IH). Motivated by evidence that flow disturbances promote IH progression, a modular anastomotic valve device (MAVD) was designed to isolate the graft from the circulation between dialysis periods (closed position) and enable vascular access during dialysis (open position). The objective of this study was to perform a preliminary computational assessment of the device ability to normalize venous flow between dialysis periods and potentially limit IH development and thrombogenesis.

METHODS

Computational fluid dynamics simulations were performed to compare flow and wall shear stress (WSS) in a native vein and MAVD prototypes featuring anastomotic angles of 90° and 30°. Low WSS (LWSS) regions prone to IH development were characterized in terms of temporal shear magnitude (TSM), oscillatory shear index (OSI), and relative residence time (RRT). Thrombogenic potential was assessed by investigating the loading history of fluid particles traveling through the device.

RESULTS

The closed MAVD exhibited the same flow characteristics as the native vein (0.3% difference in pressure drop, 3.5% difference in surface-averaged WSS). The open MAVD generated five LWSS regions (TSM <0.5 Pa) exhibiting different degrees of flow reversal (surface-averaged OSI: 0.03-0.36) and stagnation (max RRT: 2.50-37.16). Reduction in anastomotic angle resulted in the suppression of three LWSS regions and overall reductions in flow reversal (surface-averaged OSI <0.21) and stagnation (max RRT <18.05).

CONCLUSIONS

This study suggests the ability of the MAVD to normalize venous flow between dialysis periods while generating the typical hemodynamics of end-to-side vein-graft anastomoses during dialysis.

INTIMAL HYPERPLASIA AND WALL SHEAR IN ARTERIAL BYPASS Y-GRAFTING AND CONSEQUENCE GRAFTING: A NUMERICAL STUDY

The progression of intimal hyperplasia is considered to be the main cause of bypass failure and is directly related to the individual blood rheology, local arterial geometry and placement of the junctions, graft diameter and graft surface characteristics as well as the degree of compliance. In this paper we use commercial computational fluid dynamics (CFD) ANSYS to examine under the correct physiological flow conditions the hemodynamic forces of composite bypass with internal mammary artery in Y-grafting and consequence grafting which is known to achieve high patency rate and highly recommended by clinicians. Particular emphasis is given here on the parameters that could initiate the development of intimal hyperplasia within these bypass configurations. The hemodynamic flow patterns between the consequence grafting and the composite Y-grafting are observed here to be different. Moreover, on both end-to-side and side-to-side configurations, the circulating flows are detected in the vicinity of the junction area, while the Dean flow vortexes are only observed on the end-to-side configuration. Likewise, the hemodynamic flow on the end-to-side configuration on the LCX of both 45° and 90° Y-grafting is found to be smoother than that of the junction on the LCA, regardless of the changing of anastomosis angles. The high WSS gradients are observed at the vicinity of the toe and on the bed of the junction, while the low WSS are presented at the distal of the stenosis and at the stagnation point. The clinical relevance of the results are presented and discussed with particular focus on the factors and the flow patterns that trigger the development of intimal hyperplasia.

Numerical investigation of mass transport through patient-specific deformed aortae

Blood flow in human arteries has been investigated using computational fluid dynamics tools. This paper considers flow modeling through three aorta models reconstructed from cross-sectional magnetic resonance scans of female patients. One has the normal control configuration, the second has elongation of the transverse aorta, and the third has tortuosity of the aorta with stenosis. The objective of this study is to determine the impact of aortic abnormal geometries on the wall shear stress (WSS), luminal surface low-density lipoproteins (LDLs) concentration, and oxygen flux along the arterial wall. The results show that the curvature of the aortic arch and the stenosis have significant effects on the blood flow, and in turn, the mass transport. The location of hypoxia areas can be predicted well by ignoring the effect of hemoglobin on the oxygen transport. However, this simplification indeed alters the absolute value of Sherwood number on the wall.

The accelerated atherogenesis of venous grafts might be attributed to aggravated concentration polarization of low density lipoproteins: a numerical study

We hypothesize that after implantation the much elevated water filtration rate of venous grafts may cause aggravated concentration polarization of low density lipoproteins (LDLs), in turn lead to the accelerated atherogenesis of the grafts. To verify the hypothesis, we numerically simulated the transport of LDLs in various models of arterial bypasses with different grafts (veins or arteries) and geometrical configurations. The results showed that the venous grafts might endure abnormally high lipid infiltration/accumulation within the vessel wall due to severely elevated luminal surface LDL concentration. When compared to the conventional bypass models, the S-type bypass had the lowest luminal surface LDL concentration along its host artery floor, but the highest degree of risk to develop atherosclerotic lesions in its venous graft. Among the three conventional bypass models, the one with 30° anastomosis had the lowest risk to develop atherosclerosis in the venous graft. In conclusion, when compared with the bypass models with arterial grafts, the venous bypass models had rather high levels of LDL concentration polarization (cw) in the vein grafts, especially at the early stages of implantation. This might result in high infiltration/accumulation of LDLs within the walls of the venous grafts, leading to a fast genesis/development of atherosclerosis there.

Computational estimation of fluid mechanical benefits from a fluid deflector at the distal end of artificial vascular grafts

Intimal hyperplasia at the distal anastomosis is considered to be an important determinant for arterial and arteriovenous graft failure. The connection between unhealthy hemodynamics and intimal hyperplasia motivates the use of computational fluid dynamics modeling to search for improved graft design. However, studies on the fluid mechanical impact on intimal hyperplasia at the suture line intrusion have previously been scanty. In the present work, we focus on intimal hyperplasia at the suture line and illustrate potential benefits from the introduction of a fluid deflector to shield the suture line from unhealthily high wall shear stress.

Numerical analysis of coronary artery bypass grafts: an over view

Arterial bypass grafts tend to fail after some years due to the development of intimal thickening (restenosis). Non-uniform hemodynamics following a bypass operation contributes to restenosis and bypass failure can occur due to the focal development of anastomotic intimal hyperplasia. Additionally, surgical injury aggravated by compliance mismatch between the graft and artery has been suggested as an initiating factor for progress of wall thickening along the suture line Vascular grafts that are small in diameter tend to occlude rapidly. Computational fluid dynamics (CFD) methods have been effectively used to simulate the physical and geometrical parameters characterizing the hemodynamics of various arteries and bypass configurations. The effects of such changes on the pressure and flow characteristics as well as the wall shear stress during a cardiac cycle can be simulated. Recently, utilization of fluid and structure interactions have been used to determine fluid flow parameters and structure forces including stress and strains relationships under steady and transient conditions. In parallel to this, experimental diagnostics techniques such as Laser Doppler Anemometry, Particle Image Velocimetry, Doppler Guide wire and Magnetic Resonance Imaging have been used to provide essential information and to validate the numerical results. Moreover, clinical imaging techniques such as magnetic resonance or computed tomography have assisted considerably in gaining a detailed patient-specific picture of the blood flow and structure dynamics. This paper gives a review of recent numerical investigations of various configurations of coronary artery bypass grafts (CABG). In addition, the paper ends with a summary of the findings and the future directions.

NUMERICAL INVESTIGATION OF PULSATILE FLOW IN AN S-TYPE BYPASS GRAFT

Intimal hyperplasia developed at the end-to-side anastomosis of artery bypass is closely related to unphysiological hemodynamics. The helical flow as a normal physiological phenomenon in arteries is beneficial to endothelial damage repair. To deeply understand the physiological flow properties in a S-type bypass (StB) graft, four end-to-side bypass models including 30°, 45°, 60° conventional bypasses and a 45° StB were compared numerically under physiological pulsatile flow. The results showed that strong helical flow was observed at the distal anastomosis of StB. The distribution of hemodynamic parameters such as helicity, average wall shear stress and oscillating shear index, etc. were significantly improved at the S-type anastomosis as compared with those of three conventional models. The area-averaged normalized helicity in StB reached maxima at the moments of maximum flow rate and systolic deceleration. The hemodynamic performance in a 45° StB was improved as compared with a 30° conventional model. It is concluded that large StB anastomosis angle can be taken to achieve good hemodynamic performance while much smaller anastomosis angle has to be adopted for conventional bypass. As such, a S-type anastomosis should be a feasible choice of clinical artery bypass grafting due to its significant improvement in hemodynamic performance.

A novel way to reduce thrombus build-up in vena cava filters

OBJECTIVES AND BACKGROUND

The build-up of thromboses in vena cava filters after deployment presents serious problem to the patients. We proposed a novel way to overcome this problem in a belief that intentionally induced swirling flow can optimize blood flow patterns in vena cava filters, enhance the stirring motion of flow, in turn accelerate the dissolution of blood clots captured in the filter and facilitate blood to flow pass through the filters.

METHODS

In this study, we experimentally compared the work efficiency of a vena cava filter under swirling flow condition with that of the same filter under normal flow condition.

RESULTS

The results show that when compared to the normal flow, the swirling flow indeed has a significantly beneficial effect on a VCF which can decrease its flow-out time nearly 40% and reduce clot build-up in the filter more than 50%.

CONCLUSIONS

We therefore believe that the design of an ideal VCF should take how to create swirling flow in the filter into the consideration.

Effect of oxidized low-density lipoprotein concentration polarization on human smooth muscle cells' proliferation, cycle, apoptosis and oxidized low-density lipoprotein uptake

To clarify the effect of concentration polarization of oxidative modification of low-density lipoproteins (ox-LDLs) on human smooth muscle cells (SMCs), the proliferation, ox-LDL uptake and apoptosis with SMCs cultured on permeable (the permeable group) or non-permeable membranes (the non-permeable group) were analysed by 3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT) assay, spectrofluorometry and flow cytometry using a parallel-plate flow chamber technique. The concentration polarization of ox-LDLs at the surface of the cultured cell monolayer was assessed by confocal laser microscopy. The results showed that concentration polarization of ox-LDLs could indeed occur at the cultured cell monolayer surface of the permeable group, leading to an enhanced wall concentration of ox-LDLs that was over 15 per cent higher than the bulk concentration of the perfusion solution at a pressure of 100 mmHg. When concentration of ox-LDLs in the perfusion solution was less than or equal to 100 µg ml(-1), SMCs' proliferation was induced, while cell apoptosis was induced when its concentration was above 150 µg ml(-1). The uptake of ox-LDLs by the cultured cells was significantly higher for the permeable group than for the non-permeable group. In addition, the ox-LDL-induced cell death and apoptosis were much more severe in the permeable group than that in the non-permeable group. Therefore, the experimental study suggests that concentration polarization of ox-LDLs plays an adverse role in the vascular system owing to its toxicity to vascular cells, in turn enhance ox-LDL infiltration into the arterial wall and accelerate SMC apoptosis.

Disturbed flow in radial-cephalic arteriovenous fistulae for haemodialysis: low and oscillating shear stress locates the sites of stenosis

BACKGROUND

Despite recent clinical and technological advancements, the vascular access (VA) for haemodialysis still has significant early failure rates after arteriovenous fistula (AVF) creation. VA failure is mainly related to the haemodynamic conditions that trigger the phenomena of vascular wall disease such as intimal hyperplasia (IH) or atherosclerosis.

METHODS

We performed transient computational fluid dynamics simulations within idealized three-dimensional models of 'end-to-side' and 'end-to-end' radio-cephalic anastomosis, using non-Newtonian blood and previously measured flows and division ratio in subjects requiring primary access procedure as boundary conditions.

RESULTS

The numerical simulations allowed full characterization of blood flow inside the AVF and of patterns of haemodynamic shear stress, known to be the major determinant of vascular remodelling and disease. Wall shear stress was low and oscillating in zones where flow stagnation occurs on the artery floor and on the inner wall of the juxta-anastomotic vein.

CONCLUSIONS

Zones of low and oscillatory shear stress were located in the same sites where luminal reduction was documented in previous experimental studies on sites stenosis distribution in AVF. We conclude that even when exposed to high flow rates, there are spot regions along the AVF exposed to athero-prone shear stress that favour vessel stenosis by triggering IH.

3,3′-Dioctadecylindocarbocyanine-low-density lipoprotein uptake and flow patterns in the rabbit aorta–iliac bifurcation under three perfusion flow conditions

The aim of this study was to elucidate which of the following two factors plays a more important role in the localization of atherogenesis: the barrier function of the arterial endothelium modulated by wall shear stress or flow-dependent low-density lipoprotein (LDL) concentration at the blood/wall interface. To determine this, the rabbit aorto-iliac bifurcation was perfused with 3,3′-dioctadecylindocarbocyanine (DiI)-LDL solution under three different flow conditions: (i) forward flow (perfused in the in vivo flow direction); (ii) backward flow (perfused in a reversed flow direction); and (iii) static group (no flow). The results showed that there was a peak in the curve of DiI-LDL uptake distribution along the lateral wall of the bifurcation for all three groups, which was located in the branching areas where the endothelial cells were round and polygonal with no preferred orientation. Nevertheless, the peak of the forward flow group was much sharper than those of the other two groups. The overall DiI-LDL uptake was the highest for the static group. The present experimental study supports the concept that both the barrier function of the endothelium modulated by wall shear stress and the mass transport phenomenon of LDL concentration polarization are involved in the infiltration/accumulation of atherogenic lipids within the arterial wall. Nevertheless, the latter might play a larger role in the localization of atherogenesis.

Mechanistic insight into the physiological relevance of helical blood flow in the human aorta: an in vivo study

The hemodynamics within the aorta of five healthy humans were investigated to gain insight into the complex helical flow patterns that arise from the existence of asymmetries in the aortic region. The adopted approach is aimed at (1) overcoming the relative paucity of quantitative data regarding helical blood flow dynamics in the human aorta and (2) identifying common characteristics in physiological aortic flow topology, in terms of its helical content. Four-dimensional phase-contrast magnetic resonance imaging (4D PC MRI) was combined with algorithms for the calculation of advanced fluid dynamics in this study. These algorithms allowed us to obtain a 4D representation of intra-aortic flow fields and to quantify the aortic helical flow. For our purposes, helicity was used as a measure of the alignment of the velocity and the vorticity. There were two key findings of our study: (1) intra-individual analysis revealed a statistically significant difference in the helical content at different phases of systole and (2) group analysis suggested that aortic helical blood flow dynamics is an emerging behavior that is common to normal individuals. Our results also suggest that helical flow might be caused by natural optimization of fluid transport processes in the cardiovascular system, aimed at obtaining efficient perfusion. The approach here applied to assess in vivo helical blood flow could be the starting point to elucidate the role played by helicity in the generation and decay of rotating flows in the thoracic aorta.

Hemodynamic Performance of a Sutureless Anastomosis Device (the Graft Connector): A Numerical Study

Sutureless anastomosis devices have been developed to facilitate arterial bypass surgery on the beating heart. However, these devices can significantly alter the hemodynamics at the end-to-side anastomosis and in the host artery, leading to the formation of thrombus or/and intimal hyperplasia (IH). In this study, a numerical analysis was performed on the hemodynamic performance of the Graft Connector (GC), a sutureless anastomosis device, under pulsatile flow conditions. The results showed that blood flow was severely disturbed in the GC model with the formation of vortices and flow stagnation at the bed and the toe, and distal to each of the stent struts, which led to low wall shear stresses and high oscillating shear indices in these regions. This may cause severe IH in the host artery and compromise the performance of the device. Based on the numerical study, suggestions were proposed for the design of the GC to improve its performance.

A numerical study on the flow of blood and the transport of LDL in the human aorta: the physiological significance of the helical flow in the aortic arch

It has been proposed that a mass transfer phenomenon called concentration polarization of low-density lipoproteins (LDLs) may occur in the arterial system and is likely involved in the localization of atherogenesis. To test the hypothesis that concentration polarization of LDL may be suppressed by the helical flow pattern in the human aorta, hence sparing the ascending aorta from atherosclerosis, the effects of aortic torsion, branching, curvature, and taper on blood flow and LDL transport in the lumen were simulated numerically under steady-state flow conditions using four aorta models constructed based on in vivo MRI slices. The results showed that it was the aortic torsion that induced the helical flow in the aortic arch, stabilizing the flow of blood in the aorta, and compensated the adverse effects of the aortic curvature on blood flow and LDL transport. The helical flow reduced the luminal surface LDL concentration in the aortic arch and probably played a role in suppressing severe polarization of LDL at the entrances of the three branches on the arch, hence, protecting them from atherogenesis. The taper of the aorta was another important feature of the aorta that further stabilized the flow of blood and delayed the attenuation of the helical flow, making it move beyond the arch and into the beginning part of the descending aorta. The results therefore may account for why the ascending aorta and the arch are relatively free of atherosclerosis.