Flow-dependent Parameters Research Articles

Computational Hemodynamic Analysis of a Patient Specific Abdominal Aortic Aneurysm

Abdominal aortic aneurysm (AAA) is a cardiovascular disease caused by the enlargement of the aorta in the abdomen over time. Unless treated, the growth of AAA continues, resulting in 80% death in the case of rupture. Today, the width of the aneurysm diameter is taken into account in clinical practice to examine the status of AAA. Although there are aneurysms that do not rupture despite reaching a diameter of 9 cm, it is reported that aneurysms with a diameter of 3 cm are ruptured in several cases. Therefore, analyzing only the AAA diameter is not a reliable method, and a deeper investigation is necessary for the rupture risk assessment. In this study, a patient's situation is analyzed using computational fluid dynamics (CFD) simulations, which allows to elucidate the flow dependent parameters such as velocity, vorticity, pressure, and wall shear stress (WSS). First, the patient-specific geometry was obtained and boundary conditions were defined at the inlet and the outlet of the flow domain. The effects of intraluminal thrombus (ILT) formation and patient’s effort conditions were also included in the analysis. According to the results, WSS and vorticity increase with the increasing blood flow velocity. In terms of the rupture risk, it has been found that the effect of patient’s effort level is more critical than the amount of ILT in the AAA.

In vitro study of red blood cell and VWF damage in mechanical circulatory support devices based on blood-shearing platform.

Blood damage induced by mechanical circulatory support devices (MCSDs) remains a significant challenge to optimal clinical care. Although researchers have been conducting in vitro studies, the major determinant of blood damage is still unclear. An optimized capillary tube blood-shearing platform with custom designed parts was constructed to investigate the influence of two flow-dependent parameters (shear stress and exposure time) on the shear-induced damage of red blood cells and von Willebrand factor (VWF). Blood samples under different high shear stress and instantaneous exposure time were obtained by changing the flow rate and the length of capillary tube. Plasma free hemoglobin assay and immunoblotting of VWF were then performed on the sheared blood samples. The quantitative correlation between the hemolysis index and the two flow-dependent parameters was found following the power law mathematical model under the flow condition with high shear stress and instantaneous exposure time. The degradation of high molecular weight VWF was not obvious under high shear stress factor. However, the degradation of high molecular weight VWF was found as the result of the accumulation over exposure time under non-physiological shear stress, which was consistent with the different mechanism of VWF damage comparing to red blood cell damage. Compared to peak shear stress, exposure time has a greater effect on both red blood cell and VWF damage. To improve the hemocompatibility of MCSDs, it is more important to avoid regions of slow blood flow with non-physiological shear stress under laminar flow conditions.

Mathematical models for shear-induced blood damage based on vortex platform.

Non-physiological shear stress in Ventricular Assist Device (VAD) is considered to be an important trigger of blood damage, which has become the biggest shackle for clinical application. The researches on blood damage in literature were limited to qualitative but did not make much quantitative analysis. The purpose of this study was to investigate the quantitative influence of two flow-dependent parameters: shear stress (rotational speed) and exposure time on the shear-induced damage of red blood cells and von Willebrand Factor (vWF). A vortex blood-shearing platform was constructed to conduct in vitro experiments. Free hemoglobin assay and vWF molecular weight analysis were then performed on the sheared blood samples. MATLAB was used for regression fitting of original experimental data. The quantitative correlations between the hemolysis index, the degradation of high molecular weight vWF and the two flow-dependent parameters were found both following the power law model. The mathematic models indicated that the sensitivity of blood damage on red blood cells and vWF to exposure time was both greater than that of shear stress. Besides, the damage of vWF was more serious than that of red blood cells at the same flow condition. The models could be used to predict blood damage in blood-contacting medical devices, especially for the slow even stagnant blood flow regions in VAD, thus may provide useful guidance for VAD development and improvement. It also indicated that the vortex platform can be used to study the law of blood damage for the simple structure and easy operation.

Sutureless valves and the quality of perfusion: towards a goal directed aortic valve replacement.

Aim of this study was to assess the potential differences in respecting the principles of goal-directed perfusion between aortic valve replacement with sutureless vs. conventional stented bioprostheses. Data from 94 consecutive patients undergoing aortic valve replacement with Perceval sutureless valve, with or without concomitant coronary artery bypass grafting, were compared to a contemporary cohort of 116 patients receiving conventional stented bioprostheses. Propensity score matching was used to reduce selection bias. Flow-dependent estimate of oxygen delivery and carbon dioxide production were compared in accordance to the principles of goal directed perfusion. Hospital outcomes were assessed as well. Propensity score matching resulted in 39 pairs with similar characteristics and operative risk. Cross-clamping time (41.6±20.6 vs. 89.6±48.2; P<0.001) and cardiopulmonary bypass time (66.9±29.4 vs. 121.2±56.6, P<0.001) were shorter in Perceval patients. Flow-dependent parameters were significantly better in Perceval patients (peak VCO2i P=0.010; nadir VO2i P=0.035; nadir DO2i/VCO2i P=0.015; peak RQ P=0.020) and therefore goal-directed perfusion principles were met more frequently (all P<0.001). Peak lactates were higher in controls (P=0.001), as well as number of patients receiving transfusions during CPB (P=0.044). QualyP Score was also higher in controls (P<0.001). Perceval patients experienced reduced postoperative bleeding, had a lower incidence of surgical revisions and transfusions. Duration of ventilation was longer in controls. The use of sutureless valves significantly reduces cardiopulmonary bypass and cross-clamping times and contributes to achieve a goal-directed perfusion. This results in better perfusion quality, with a positive clinical impact.

Estimation of Models for the Upper Part of Murray River with Flow Dependent Parameters

Data-based modelling is an important tool for efficient planning and management of rivers. However, linear data-based models often only perform well in a specific operating range. The larger the operating range is, the better the model is considered for prediction and control. An increase in the operating range can be achieved by incorporating non-linearities in the model. In this paper we used Data-Based Mechanistic (DBM) approach (Young (2011)) to derive models of the upper part of Murray River in Australia. The approach can identify static non-linearities in the system, and we used it to see how significant the non-linearities in the river model are. We analysed models obtained from historical data spanning a decade. In the data considered we could not find any strong indication that significant non-linearities were present.

Vascular and pharmacokinetic effects of EndoTAG-1 in patients with advanced cancer and liver metastasis

EndoTAG-1 (ET), a novel formulation of cationic liposomes carrying embedded paclitaxel (Taxol), shows antitumoral activity, targeting tumor endothelial cells in solid tumors. Patients with advanced metastatic cancer were evaluated investigating effects on pharmacokinetics and tumor vasculature using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and contrast-enhanced ultrasound (CEUS). The pharmacokinetic (PK) profile of ET (22 mg/m(2) i.v.) was evaluated after single and repeated doses. DCE-MRI and CEUS explored hepatic metastases before, during and after the 4-week treatment cycle. Angiogenic biomarkers were assessed. Tumor response was evaluated by modified RECIST. The PK profile demonstrated slight accumulation of paclitaxel after repeated doses. DCE-MRI parameters K(trans) and/or iAUC(60) showed a trend to decrease. Changes of blood flow-dependent parameters of DCE-MRI and CEUS were well correlated. Angiogenic biomarkers revealed no clear trend. ET was generally well tolerated; common toxic effects were fatigue and hypersensitivity reactions. Nine (9 of 18) patients had stable disease after the first treatment cycle. Four patients without disease progression continued treatment. This study including multiple pretreated patients with different metastatic cancer revealed individually distinctive hemodynamic alterations by DCE-MRI. The PK profiles of ET were similar as observed previously.

Study of Flow-Induced Hemolysis Using Novel Couette-Type Blood-Shearing Devices

To assist the development and application of blood-contacting medical devices, two novel flow-through Couette-type blood-shearing devices have been developed to study the quantitative relationship between blood damage indexes and flow-dependent parameters. One device is an axial flow-through Couette-type device supported by a pair of pin bearings adapted from the adult Jarvik 2000 blood pump. The other is a centrifugal flow-through Couette-type device supported with magnetic bearings adapted from the CentriMag blood pump. In both devices, a rotor spindle was used to replace the original impeller blades so that a small gap was created between the housing and the rotating spindle surface. Computational fluid dynamics simulations have shown that a uniform, high shear stress region can be generated inside the small gap while the shear stresses elsewhere are relatively low. The possibility of secondary blood damage caused by mechanical seals was eliminated due to the use of a magnetic rotor system. Blood flow through the gap was driven by an externally pressurized reservoir. By adjusting the rotational speed and blood flow rate, shear-induced hemolysis was quantified at a matrix of exposure time (0.039 to 1.48 s) and shear stress (50 to 320 Pa). All of the experiments were conducted at room temperature using heparinized ovine blood with a hematocrit value of 30%. The measured hemolysis levels were much lower than those published in the literature, and the overestimation of those earlier studies may be attributable to device-related secondary blood-damaging effects. A new set of coefficients for the power law model was derived from the regression of the experimental data.

Multi-class percentile user equilibrium with flow-dependent stochasticity

Travelers often reserve a buffer time for trips sensitive to arrival time in order to hedge against the uncertainties in a transportation system. To model the effects of such behavior, travelers are assumed to choose routes to minimize the percentile travel time, i.e. the travel time budget that ensures their preferred probability of on-time arrival; in doing so, they drive the system to a percentile user equilibrium (UE), which can be viewed as an extension of the classic Wardrop equilibrium. The stochasticity in the supply of transportation are incorporated by modeling the service flow rate of each road segment as a random variable. Such stochasticity is flow-dependent in the sense that the probability density functions of these random variables, from which the distribution of link travel time are constructed, are specified endogenously with flow-dependent parameters. The percentile route travel time, obtained by directly convolving the link travel time distributions in this paper, is not available in closed form in general and has to be numerically evaluated. To reveal their structural properties, percentile UE solutions are examined in special cases and verified with numerical results. For the general multi-class percentile UE traffic assignment problem, a variational inequality formulation is given and solved using a route-based algorithm. The algorithm makes use of the diagonal elements in the Jacobian of percentile route travel time, which is approximated through recursive convolution. Preliminary numerical experiments indicate that the algorithm is able to achieve highly precise equilibrium solutions.

A dynamic subgrid-scale tensorial Eddy viscosity model

In the Navier-Stokes equations the removal of the turbulent fluctuating velocities with a frequency above a certain fixed threshold, employed in the Large Eddy Simulation (LES), causes the appearance of a turbulent stress tensor that requires a number of closure assumptions. In this paper insufficiencies are demonstrated for those closure models which are based on a scalar eddy viscosity coefficient. A new model, based on a tensorial eddy viscosity, is therefore proposed; it employs the Germano identity [1] and allows dynamical evaluation of the single required input coefficient. The tensorial expression for the eddy viscosity is deduced by removing the widely used scalar assumption of the high-frequency viscous dissipation and replacing it by its tensorial counterpart arising in the balance of the Reynolds stress tensor. The numerical simulations performed for a lid driven cavity flow show that the proposed model allows to overcome the drawbacks encountered by the scalar eddy viscosity models. In numerical simulations of turbulent hydrodynamic field quantities different levels of the representation of the frequency spectrum of hydrodynamic quantities can be pursued. A full representation of this spectrum, achieved by Direct Numerical Simulation (DNS), needs a numerical discretization of the continuum hydrodynamic field that is able to resolve all turbulent high-frequency microscales; in this scheme the viscous stresses dissipate the turbulent kinetic energy into heat. The representation of the high frequency turbulent eddies, thus, imposes massive computational efforts and restricts DNS to simple flows and low Reynolds numbers. At the opposite extreme is the statistical approach, based on an averaging process of the Navier-Stokes equations (Reynolds Averaged Navier-Stokes, RANS); it defines the filtered hydrodynamic fields and removes the turbulence length-scales except those of the macroscales whose dimension is comparable with that of the domain in which the equations are solved. As a consequence of the averaging process, the non-linear convective terms of the Navier-Stokes equations cause the appearance of a turbulent stress tensor related to the removed turbulent fluctuating velocities. Since this tensor is an additional unknown, suitable closure assumptions must be introduced. The low-frequency fluctuating velocities are predominantly dependent on the averaged hydrodynamic fields and simple closure conditions may be introduced. However, as the intended resolution of the frequency spectrum is increased to higher frequencies, more complex closure models are required to accomodate for the increased resolution of the subgrid fluctuations. They, in general, also require the determination of a large number of flow dependent parameters. It is important that these equations satisfy the necessary form invariance under Euclidean transformations [2]. Large Eddy Simulation (LES) [3] is a compromise approach between DNS and RANS since it defines a turbulence mesoscale that allows to distinguish the large energy-carrying scales that are numerically resolved

HEADCUT MIGRATION ANALYSIS OF A COMPACTED SOIL

Predicting headcut migration is a complex problem. The rate of headcut migration is of specific interest forengineers designing earthen spillways and embankments. Headcut migration tests of a compacted soil were conducted ina 1.8-m wide and 29-m long flume with 2.4-m high sidewalls. Three simple physically based headcut migration equationsthat separately group material-dependent parameters and flow-dependent parameters are used in this study to analyzeresults of the headcut migration study. The purpose of this article is (1) to relate the material-dependent factors of theseequations to material property measurements; and (2) to determine the appropriate adjustment algorithm for thesematerial-dependent factors for multiple material layers.