Analysis of hemodynamic fluid phase mass transport in a separated flow region

Convection-Enhanced Transport into Open Cavities : Effect of Cavity Aspect Ratio

Recirculating fluid regions occur in the human body both naturally and pathologically. Diffusion is commonly considered the predominant mechanism for mass transport into a recirculating flow region. While this may be true for steady flows, one must also consider the possibility of convective fluid exchange when the outer (free stream) flow is transient. In the case of an open cavity, convective exchange occurs via the formation of lobes at the downstream attachment point of the separating streamline. Previous studies revealed the effect of forcing amplitude and frequency on material transport rates into a square cavity (Horner in J Fluid Mech 452:199-229, 2002). This paper summarizes the effect of cavity aspect ratio on exchange rates. The transport process is characterized using both computational fluid dynamics modeling and dye-advection experiments. Lagrangian analysis of the computed flow field reveals the existence of turnstile lobe transport for this class of flows. Experiments show that material exchange rates do not vary linearly as a function of the cavity aspect ratio (A = W/H). Rather, optima are predicted for A ≈ 2 and A ≈ 2.73, with a minimum occurring at A ≈ 2.5. The minimum occurs at the point where the cavity flow structure bifurcates from a single recirculating flow cell into two corner eddies. These results have significant implications for mass transport environments where the geometry of the flow domain evolves with time, such as coronary stents and growing aneurysms. Indeed, device designers may be able to take advantage of the turnstile-lobe transport mechanism to tailor deposition rates near newly implanted medical devices.

EGF as a New Therapeutic Target for Medulloblastoma Metastasis

Medulloblastoma (MB) is a malignant pediatric brain tumor known for its aggressive metastatic potential. Despite the well-documented migration of MB cells to other parts of the brain and spinal column, MB chemotaxis is poorly understood. Herein, we examined the in vitro migratory and cellular responses of MB-derived cells to external signaling of Epidermal Growth Factor (EGF), hepatocyte growth factor (HGF), platelet-derived growth factor (PDGF-BB), and the stromal cell-derived factors 1-alpha (SDF-1). Experiments utilized transwell assays and immunocytochemistry to identify receptor activation in MB migration, and used a microfluidic platform to examine directionality, trajectory, and gradient-dependence of motile cells. Data illustrates that MB-derived cells respond strongly to EGF in a dosage and gradient-dependent manner with increased EGF-R activation, and show that high EGF gradient fields cause an increased number of cells to migrate longer directed distances. Our results provide evidence that EGF and its receptor play an important role than previously documented in MB chemotactic migration than previously documented and should be considered for developing migration-target therapies against MB metastasis.

Experimental validation of convection-diffusion discretisation scheme employed for computational modelling of biological mass transport

BACKGROUND

The finite volume solver Fluent (Lebanon, NH, USA) is a computational fluid dynamics software employed to analyse biological mass-transport in the vasculature. A principal consideration for computational modelling of blood-side mass-transport is convection-diffusion discretisation scheme selection. Due to numerous discretisation schemes available when developing a mass-transport numerical model, the results obtained should either be validated against benchmark theoretical solutions or experimentally obtained results.

METHODS

An idealised aneurysm model was selected for the experimental and computational mass-transport analysis of species concentration due to its well-defined recirculation region within the aneurysmal sac, allowing species concentration to vary slowly with time. The experimental results were obtained from fluid samples extracted from a glass aneurysm model, using the direct spectrophometric concentration measurement technique. The computational analysis was conducted using the four convection-diffusion discretisation schemes available to the Fluent user, including the First-Order Upwind, the Power Law, the Second-Order Upwind and the Quadratic Upstream Interpolation for Convective Kinetics (QUICK) schemes. The fluid has a diffusivity of 3.125 x 10-10 m2/s in water, resulting in a Peclet number of 2,560,000, indicating strongly convection-dominated flow.

RESULTS

The discretisation scheme applied to the solution of the convection-diffusion equation, for blood-side mass-transport within the vasculature, has a significant influence on the resultant species concentration field. The First-Order Upwind and the Power Law schemes produce similar results. The Second-Order Upwind and QUICK schemes also correlate well but differ considerably from the concentration contour plots of the First-Order Upwind and Power Law schemes. The computational results were then compared to the experimental findings. An average error of 140% and 116% was demonstrated between the experimental results and those obtained from the First-Order Upwind and Power Law schemes, respectively. However, both the Second-Order upwind and QUICK schemes accurately predict species concentration under high Peclet number, convection-dominated flow conditions.

CONCLUSION

Convection-diffusion discretisation scheme selection has a strong influence on resultant species concentration fields, as determined by CFD. Furthermore, either the Second-Order or QUICK discretisation schemes should be implemented when numerically modelling convection-dominated mass-transport conditions. Finally, care should be taken not to utilize computationally inexpensive discretisation schemes at the cost of accuracy in resultant species concentration.

Blood flow and macromolecular transport in complex blood vessels

Numerical simulations of pulsatile flows and macromolecular (such as LDL) transport in complex blood vessels, including the cerebral artery, are carried out using the FLUENT software. The hemodynamic factors such as axial velocity, secondary flow as well as LDL concentration distribution in the complex vessel are obtained. It is found that in the case of pulsatile flow, the LDL concentration is higher in the central region of the flow than on the wall. Under the precondition of impermeability, the numerical results indicate that the blood flow is quite complicated in complex blood vessel. The complex flow can reduce the LDL concentration on the vessel wall, which is helpful to prevent the concentration polarization.

Numerical analysis of the cooling effect of blood over inflamed atherosclerotic plaque

Atherosclerotic plaques with high likelihood of rupture often show local temperature increase with respect to the surrounding arterial wall temperature. In this work, atherosclerotic plaque temperature was numerically determined during the different levels of blood flow reduction produced by the introduction of catheters at the vessel lumen. The temperature was calculated by solving the energy equation and the Navier-Stokes equations in 2D idealized arterial models. Arterial wall temperature depends on three basic factors: metabolic activity of the inflammatory cells embedded in the plaque, heat convection due to luminal blood flow, and heat conduction through the arterial wall and plaque. The calculations performed serve to simulate transient blood flow reduction produced by the presence of thermography catheters used to measure arterial wall temperature. The calculations estimate the spatial and temporal alterations in the cooling effect of blood flow and plaque temperature during the measurement process. The mathematical model developed provides a tool for analyzing the contribution of factors known to affect heat transfer at the plaque surface. Blood flow reduction leads to a nonuniform temperature increase ranging from 0.1 to 0.25 degrees Celsius in the plaque/lumen interface of the arterial geometries considered in this study. The temperature variation as well as the Nusselt number calculated along the plaque surface strongly depended on the arterial geometry and distribution of inflammatory cells. The calculations indicate that the minimum required time to obtain a steady temperature profile after arterial occlusion is 6 s. It was seen that in arteries with geometries involving bends, the temperature profiles appear asymmetrical and lean toward the downstream edge of the plaque.

Calculation of arterial wall temperature in atherosclerotic arteries: effect of pulsatile flow, arterial geometry, and plaque structure

Background

This paper presents calculations of the temperature distribution in an atherosclerotic plaque experiencing an inflammatory process; it analyzes the presence of hot spots in the plaque region and their relationship to blood flow, arterial geometry, and inflammatory cell distribution. Determination of the plaque temperature has become an important topic because plaques showing a temperature inhomogeneity have a higher likelihood of rupture. As a result, monitoring plaque temperature and knowing the factors affecting it can help in the prevention of sudden rupture.

Methods

The transient temperature profile in inflamed atherosclerotic plaques is calculated by solving an energy equation and the Navier-Stokes equations in 2D idealized arterial models of a bending artery and an arterial bifurcation. For obtaining the numerical solution, the commercial package COMSOL 3.2 was used. The calculations correspond to a parametric study where arterial type and size, as well as plaque geometry and composition, are varied. These calculations are used to analyze the contribution of different factors affecting arterial wall temperature measurements. The main factors considered are the metabolic heat production of inflammatory cells, atherosclerotic plaque length l_p, inflammatory cell layer length l_mp, and inflammatory cell layer thickness d_mp.

Results

The calculations indicate that the best location to perform the temperature measurement is at the back region of the plaque (0.5 ≤ l/l_p ≤ 0.7). The location of the maximum temperature, or hot spot, at the plaque surface can move during the cardiac cycle depending on the arterial geometry and is a direct result of the blood flow pattern. For the bending artery, the hot spot moves 0.6 millimeters along the longitudinal direction; for the arterial bifurcation, the hot spot is concentrated at a single location due to the flow recirculation observed at both ends of the plaque. Focusing on the thermal history of different points selected at the plaque surface, it is seen that during the cardiac cycle the temperature at a point located at l/l_p = 0.7 can change between 0.5 and 0.1 degrees Celsius for the bending artery, while no significant variation is observed in the arterial bifurcation. Calculations performed for different values of inflammatory cell layer thickness d_mp indicate the same behavior reported experimentally; that corresponds to an increase in the maximum temperature observed, which for the bending artery ranges from 0.6 to 2.0 degrees Celsius, for d_mp = 25 and 100 micrometers, respectively.

Conclusion

The results indicate that direct temperature measurements should be taken (1) as close as possible to the plaque/lumen surface, as the calculations show a significant drop in temperature within 120 micrometers from the plaque surface; (2) in the presence of blood flow, temperature measurement should be performed in the downstream edge of the plaque, as it shows higher temperature independently of the arterial geometry; and (3) it is necessary to perform measurements at a sampling rate that is higher than the cardiac cycle; the measurement should be extended through several cardiac cycles, as variations of up to 0.7 degrees Celsius were observed at l/l_p = 0.7 for the bending artery.

Lipoprotein transport in the blood stream to the arterial wall in a carotid artery bifurcation

The carotid arteries are a common site of atherosclerotic plaque formation, which has been linked to the blood flow patterns and the mass transport phenomenon. The purpose of this research was to study the lipid transport in a human carotid artery model, focusing on the effects of local geometric and hemodynamic factors on mass transfer from blood flow to vessel wall and its concentration at the luminal surface of the artery. The Reynolds number, 250, and the Schmidt number, 6.66x10⁵, were selected to model the mass transfer of LDL macro molecules, and in order to see the effect of Reynolds and Schmidt numbers to mass transport, the model was analyzed with different conditions. The steady state flow was used for two dimensional carotid geometry. At the inlet, the blood flow was assumed a steady fully developed laminar velocity profile with a uniform LDL concentration. The vessel wall was assumed permeable to water and semi-permeable to LDL macro molecules. The problem was analyzed with the finite element method. The results show 26% increase of LDL concentration from inlet value at the luminal surface of the artery located in the separated flow region. The maximum value of LDL concentration occurred at the separation point.

Mass Transport Disturbances in the Distal Graft/Artery Junction of a Peripheral Bypass Graft

Intimal hyperplasia (IH) development is a primary cause of failure of reconstructive bypass surgery. While the exact mechanism by which IH initiates and proliferates has yet to be fully elucidated, it is clear that the abnormal haemodynamics present in the downstream graft/artery junction are intrinsic in its development. Mass transport disturbances owing to abnormal haemodynamics have been associated with atherogenesis and it is for this reason that an investigation into transport of platelet-derived growth factor (PDGF), a known promoter of the intimal hyperplastic response, at the downstream graft/artery junction was carried out. A steady flow analysis in a three-dimensional, idealized, downstream graft/artery junction was carried out using commercial computational fluid dynamics software. It was found that there is a two-and-half fold increase in the transport of PDGF to the artery wall at the bed of the junction when compared with an idealized, healthy artery. The presence of secondary flows in the downstream arterial section also leads to large disturbances in mass transport. It was concluded that PDGF transport in the downstream graft/artery junction tends to be highly disturbed and that there may be a role of this disturbance in the initiation and subsequent development of distal anastomotic intimal hyperplasia.

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.

Computational modeling of vascular anastomoses

Recent development of computational technology allows a level of knowledge of biomechanical factors in the healthy or pathological cardiovascular system that was unthinkable a few years ago. In particular, computational fluid dynamics (CFD) and computational structural (CS) analyses have been used to evaluate specific quantities, such as fluid and wall stresses and strains, which are very difficult to measure in vivo. Indeed, CFD and CS offer much more variability and resolution than in vitro and in vivo methods, yet computations must be validated by careful comparison with experimental and clinical data. The enormous parallel development of clinical imaging such as magnetic resonance or computed tomography opens a new way toward a detailed patient-specific description of the actual hemodynamics and structural behavior of living tissues. Coupling of CFD/CS and clinical images is becoming a standard evaluation that is expected to become part of the clinical practice in the diagnosis and in the surgical planning in advanced medical centers. This review focuses on computational studies of fluid and structural dynamics of a number of vascular anastomoses: the coronary bypass graft anastomoses, the arterial peripheral anastomoses, the arterio-venous graft anastomoses and the vascular anastomoses performed in the correction of congenital heart diseases.