BACKGROUND:Despite the significant advances made in the field of computational fluid dynamics (CFD) to simulate the left atrium (LA) in atrial fibrillation (AF) conditions, the connection between atrial structure, flow dynamics, and blood stagnation in the left atrial appendage (LAA) remains unclear. Deepening our understanding of this relationship would have important clinical implications, as the thrombi formed within the LAA are one of the main causes of stroke. AIM:To highlight and better understand the fundamental role of the PV orientation in forming atrial flow patterns and systematically quantifying its effect on blood stasis within the LAA. METHODS:Two patients with opposite atrial flow patterns were selected for the study. The atria were segmented and subsequently morphed to modify the pulmonary vein (PV) orientations in a highly controlled manner. CFD analysis were performed using a kinematic model able to reproduce AF conditions. Results were projected into the universal left atrial appendage coordinate (ULAAC) system to enhance data visualization and comparison. RESULTS:The position of the main atrial vortex can be modified by controlled changes in the PV orientations, which to the best of our knowledge was not demonstrated before. This finding may have important clinical implications, as the behavior and position of the main atrial vortex is crucial to define the LA flow patterns and thus the LAA washing, making possible to assess the stroke risk for a particular patient.

BACKGROUND:Despite the recent advances in computational fluid dynamics (CFD) techniques applied to blood flow within the left atrium (LA), the relationship between atrial geometry, flow patterns, and blood stasis within the left atrial appendage (LAA) remains unclear. A better understanding of this relationship would have important clinical implications, as thrombi originating in the LAA are a common cause of stroke in patients with atrial fibrillation (AF). AIM:To identify the most representative atrial flow patterns on a patient-specific basis and study their influence on LAA blood stasis by varying the flow split ratio and some common atrial modeling assumptions. METHODS:Three recent techniques were applied to nine patient-specific computational fluid dynamics (CFD) models of patients with AF: a kinematic atrial model to isolate the influence of wall motion because of AF, projection on a universal LAA coordinate system, and quantification of stagnant blood volume (SBV). RESULTS:We identified three different atrial flow patterns based on the position of the center of the main circulatory flow. The results also illustrate how atrial flow patterns are highly affected by the flow split ratio, increasing the SBV within the LAA. As the flow split ratio is determined by the patient's lying position, the results suggest that the most frequent position adopted while sleeping may have implications for the medium- and long-term risks of stroke.

We examined the effect of minimal lumen segmentation uncertainty on Fractional Flow Reserve obtained from Coronary Computed Tomography Angiography FFR CT . A total of 14 patient-specific coronary models with different stenosis locations and degrees of severity were enrolled in this study. The optimal segmented coronary lumens were disturbed using intra ± 6 % and inter-operator ± 15 % variations on the segmentation threshold. FFR CT was evaluated in each case by 3D-OD CFD simulations. The findings suggest that the sensitivity of FFR CT to this type of uncertainty increases distally and with the stenosis severity. Cases with moderate or severe distal coronary lesions should undergo either exact and thorough segmentation operations or invasive FFR measurements, particularly if the FFR CT is close to the cutoff (0.80). Therefore, we conclude that it is crucial to consider the lesion's location and degree of severity when evaluating FFR CT results.

BACKGROUND:Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, affecting over 1% of the population. It is usually triggered by irregular electrical impulses that cause the atria to contract irregularly and ineffectively. It increases blood stasis and the risk of thrombus formation within the left atrial appendage (LAA) and aggravates adverse atrial remodeling. Despite recent efforts, LAA flow patterns representative of AF conditions and their association with LAA stasis remain poorly characterized.AIM:To develop reduced-order data-driven models of LAA flow patterns during atrial remodeling in order to uncover flow disturbances concurrent with LAA stasis that could add granularity to clinical decision criteria.METHODS:We combined a geometric data augmentation process with projection of results from 180 CFD atrial simulations on a universal LAA coordinate (ULAAC) system. The projection approach enhances data visualization and facilitates direct comparison between different anatomical and functional states. ULAAC projections were used as input for a proper orthogonal decomposition (POD) algorithm to build reduced-order models of hemodynamic metrics, extracting flow characteristics associated with AF and non-AF anatomies.RESULTS:We verified that the ULAAC system provides an adequate representation to visualize data distributions on the LAA surface and to build POD-based reduced-order models. These models revealed significant differences in LAA flow patterns for atrial geometries that underwent adverse atrial remodeling and experienced elevated blood stasis. Together with anatomical morphing-based patient-specific data augmentation, this approach could facilitate data-driven analyses to identify flow features associated with thrombosis risk due to atrial remodeling.

Polydimethylsiloxane (PDMS) has become a popular material in microfluidic and macroscale in vitro models due to its elastomeric properties and versatility. PDMS-based biomodels are widely used in blood flow studies, offering a platform for improving flow models and validating numerical simulations. This review highlights recent advances in bioflow studies conducted using both PDMS microfluidic devices and macroscale biomodels, particularly in replicating physiological environments. PDMS microchannels are used in studies of blood cell deformation under confined conditions, demonstrating the potential to distinguish between healthy and diseased cells. PDMS also plays a critical role in fabricating arterial models from real medical images, including pathological conditions such as aneurysms. Cutting-edge applications, such as nanofluid hemodynamic studies and nanoparticle drug delivery in organ-on-a-chip platforms, represent the latest developments in PDMS research. In addition to these applications, this review critically discusses PDMS properties, fabrication methods, and its expanding role in micro- and nanoscale flow studies.

This study investigates the impact of hemodynamics on real intracranial aneurysms (IAs) using experiments and computational fluid dynamics (CFD) simulations. A particle tracking velocimetry (PTV) approach was used to study the vortical structures inside a real aneurysm and validate numerical simulations performed at a steady regime for different flow rates. Moreover, this and two additional patient-specific cases have been numerically analyzed, focusing on flow patterns, wall shear stress (WSS), relative residence time (RRT), and oscillatory shear index (OSI) for transient studies. For the transient simulations, vorticity profiles indicated significant rotation of fluid particles in the neck and outlet arteries. TAWSS analysis revealed high WSS values in the bifurcation zone, neck, and middle cerebral artery (MCA), with variations among the patients. OSI and RRT plots provided insights into disturbed flow patterns, low or oscillatory WSS areas, and regions with prolonged residence time. This study shows great potential for combining PTV and CFD to obtain detailed insights into flow structures in aneurysms, which are crucial to developing effective treatments and interventions for IA management.

This study focuses on the performance of various turbulence models in predicting hemodynamic variables within a patient-specific geometry of the Brachiocephalic trunk exhibiting a severe stenosis. Numerical simulations employing Reynolds-averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) models were conducted, comparing four RANS with two LES models. Results indicated significant differences in turbulent structures between RANS and LES models, with the k-ω RANS model closely approximating LES throughout the cardiac cycle. The velocity and Turbulent Kinetic Energy (TKE) results were consistent in the stenosis region due to the proximity to the inlet and plug flow conditions. However, notable disparities were observed in bifurcation and outlet regions. Time-Averaged Wall Shear Stress (TAWSS) comparisons revealed that the k-ω-based models provided closer agreement with LES, particularly in atherosclerosis-prone areas. The study highlights the limitations and strengths of each turbulence model, emphasizing the importance of model selection in simulating complex cardiovascular conditions. While RANS models demonstrated computational efficiency, their accuracy varied across regions. The effectiveness of the k-ω model in capturing the intricacies of such a complex flow field suggests its potential as a reliable and accurate tool for simulating physiological conditions with strong curvature and advanced stenosis.

: The hemodynamics of Intracranial Aneurysm (IA) involves complex phenomena that influence its growth and rupture. The progress of additive manufacturing techniques has allowed the development of biomodels suitable to perform in vitro flow experiments. Hence, this work presents the manufacturing process to fabricate flow biomodels by using the additive manufacturing technique known as Fused Deposition Modeling (FDM). The biomodels obtained through the proposed technique has proved to be suitable for in vitro flow experiments using imaging techniques and for validation of numerical studies

Coronary stenosis is mainly responsible for myocardial ischemia as the blood supply to a portion of the heart stops or is severely reduced. The Fractional Flow Reserve is the benchmark for the hemodynamic significance assessment of coronary stenoses. Its value is employed as a gatekeeper/planning tool for revascularization in clinical practice. Non-invasive alternatives have been successfully proposed to guide cardiologists. However, simulation values are not accurate enough in the 0.75–0.85 range, so invasive Fractional Flow Reserve should be used. Several authors argue about where distal pressure should be measured. Therefore, our aim is to use simulation to assess how this value changes and to detect the correct measurement region. First, we have adjusted the simulation method to the segmentations of two patients whose invasive Fractional Flow Reserve is known. We then extended our analysis to four patients and obtained the simulated value at multiple points distal to the stenosis. This is an advantage over invasive measurements, whose locations are restricted. The results are also essential for locating the best region for invasive distal pressure measurements. We propose a hybrid invasive and in-silico procedure that would avoid false results and prevent cardiologists from making erroneous clinical decisions.

Background and objective: This paper presents a comprehensive (qualitative and quantitative) comparison of four classic Reynolds Average Navier-Stokes models (RANS) (k-ε, k-ω, Shear Stress Transport k-ω, Transitional Shear Stress Transport), and two Large Eddy Simulation (LES), (Smagorinsky-Lilly, Dynamic Kinetic Energy Subgrid-Scale Model). The purpose of this paper is to assess whether the RANS turbulence models compare with LES, in the prediction of hemodynamic parameters of a clinical case. Methods: The turbulence models are compared on the simulation of blood flow on real stenosed geometry of the brachiocephalic trunk, derived from medical images of a Computerized Tomography scan. Doppler measurements were used to obtain patient-specific boundary conditions, both major developments over the published studies within this field. Velocity inlet, and outflow outlet boundary conditions are applied in the Right Subclavian and Right Common Carotid Arteries, with a distribution of 70/30 of the flow rate, respectively. Results: The results of this research revealed that the RANS models required 13.2 times less computational time relatively to LES. The comparison between the two LES models showed that both produced similar results for the hemodynamic variables analyzed. Of the RANS models, in general, the k-ε displayed the worst agreement with the LES results, with exception of the turbulent kinetic energy. The k-ω model provided the best agreement with results of the LES models, particularly for the near-the-wall variables, such as the pressure, Wall Shear Stress and its derivatives. The results of independent samples Kolmogorov-Smirnov test revealed a statistical equality between the pressure and force on the wall calculations of the k-ω and LES models. Conclusion: Considering the less computational requirements and favorable comparative results of the k-ω model with the more complex LES models, the k-ω model is faster and cheaper alternative that can be applied in blood flow studies with transitional and low Reynolds turbulent flows.

BACKGROUND:Aortic Regurgitation (AR) produces the entrance of an abnormal amount of blood in the left ventricle. This disease is responsible for high morbidity and mortality worldwide and may be caused by an aortic valve dysfunction. Surgical and transcatheter aortic valve replacement (TAVR) are the current options for treating AR. They have replaced older procedures such as Hufnagel's one. However, some physicians have reconsidered this procedure as a less aggressive alternative for patients not eligible for surgical or TAVR. Although Hufnagel suggested a 75% regurgitation reduction when a valve is placed in the descending aorta, a quantification of this value has not been reported.METHODS:In this paper, CFD/FSI numerical simulation is conducted on an idealized geometry. We quantify the effect of placing a bileaflet mechanical heart valve in the descending aorta on a moderate-severe AR case. A three-element Windkessel model is employed to prescribe pressure outlet boundary conditions. We calculate the resulting flow rates and pressures at the aorta and first-generation vessels. Moreover, we evaluate several indices to assess the improvement due to the valve introduction.RESULTS AND CONCLUSIONS:Regurgitation fraction (RF) is reduced from 37.5% (without valve) to 18.0% (with valve) in a single cardiac cycle. This reduction clearly shows the remarkable efficacy of the rescued technique. It will further ameliorate the left ventricle function in the long-term. Moreover, the calculations show that the implantation in that location introduces fewer incompatibilities' risks than a conventional one. The proposed methodology can be extended to any particular conditions (pressure waveforms/geometry) and is designed to assess usual clinical parameters employed by physicians.

Cerebral aneurysm is an abnormal dilatation of the blood vessel which affects a high percentage of the worldwide population. One way to investigate this pathology is using in vivo techniques, but these types of experiments have a high cost and low reproducibility. Thus, to understand the local hemodynamics of brain aneurysms, it is imperative to manufacture in vitro models that simulate real brain aneurysms. These biomodels are suitable for experimental testing, as well as for evaluating and validating computational models. In this work, was manufactured a biomodel of a cerebral aneurysm made by polydimethylsiloxane (PDMS), combining rapid prototyping technology with a PDMS gravity casting process. Experimental flow visualizations were performed at different flow rates. The flow visualizations results have shown that there is a transition from laminar to turbulent flow for a flow rate near 6 ml/min. The proposed PDMS biomodels have shown the ability to perform flow visualizations and have the potential to help the development and validation of computational models.

This paper presents a qualitative and quantitative comparison between the finite element and the finite volume methods for the fluid-structure interaction simulation of blood flow through a real stenosed artery. The artery geometry corresponds to a severely stenosed (around 75% lumen reduction) portion of the brachiocephalic trunk, located immediately upstream of the bifurcation of this vessel into the right subclavian and right common carotid arteries. The patient-specific geometry was segmented from medical images of a computerized tomography scanner from an individual with the subclavian steal syndrome. Doppler ultrasound velocity measurements were used to determine and impose patient-specific boundary conditions. The numerical simulations were performed in commercial software, Ansys and COMSOL, with a comparative second order discretization for the pressure, velocity and displacement variables. The results of this research disclosed a reasonable overall agreement between the predicted hemodynamics for both approaches. The finite volume method software (Ansys) proved to be more efficient in computational time and memory requirements.

BACKGROUND:Recently, advances in medical imaging, segmentation techniques, and high-performance computing have supported the use of patient-specific computational fluid dynamics (CFD) simulations. At present, CFD-compatible atrium geometries can be easily reconstructed from atrium images, providing important insight into the atrial fibrillation (AF) phenomenon, and assistance during therapy selection and surgical procedures. However, the hypothesis assumed for such CFD models should be adequately validated.AIM:This work aims to perform an extensive study of the different hypotheses that are commonly assumed when performing atrial simulations for AF patients, as well as to evaluate and compare the range of indices that are usually applied to assess thrombus formation within the left atrium appendage (LAA).METHODS:The atrial geometries of two AF patients have been segmented. The resulting geometries have been registered and interpolated to construct a dynamic mesh, which has been employed to compare the rigid and flexible models. Two families of hemodynamic indices have been calculated and compared: wall shear-based and blood age distribution-based.RESULTS:The findings of this study illustrate the importance of validating the rigid atrium hypothesis when utilizing an AF CFD model. In particular, the absence of the A-wave contraction does not avoid a certain degree of passive atrial contraction, making the rigid model a poor approximation in some cases. Moreover, a new thrombosis predicting index has been proposed, i.e., M4, which has been shown to predict stasis more effectively than other indicators.

We propose a method to measure the interfacial tension characterizing the interface between two immiscible liquids of practically the same density. In this method, a cylindrical liquid bridge made of one the liquids is vibrated laterally inside a tank filled with the other. The first resonance frequency is determined and equated to the first eigenfrequency of the m=1 linear mode to infer the interfacial tension value. The method does not involve the density jump across the interface. Therefore, its accuracy is affected neither by the smallness of the Bond number nor by errors of the density difference. The experimental setup is relatively simple, and the procedure does not use image processing techniques. The results satisfactorily agree with those measured by TIFA-AI (Theoretical Fitting Image Analysis-Axisymmetric Interfaces) for the same liquid bridges when the density difference is sufficiently large for TIFA-AI to be valid. We conduct numerical simulations of the Navier-Stokes equations to determine the best parameter conditions for the proposed method. The transfer function characterizing the frequency response of the fluid configuration is measured in some experiments to quantify non-linear effects and to study the role played by the outer bath vibration.

Background and ObjectiveHealth care costs represent a substantial an increasing percentage of global expenditures. One key component is treatment of respiratory diseases, which account for one in twelve deaths in Europe. Computational simulations of lung airflow have potential to provide considerable cost reduction and improved outcomes. Such simulations require accurate in silico modeling of the lung airway. The geometry of the lung is extremely complex and for this reason very simple morphologies have primarily been used to date. The objective of this work is to develop an effective methodology for the creation of hybrid pulmonary geometries combining patient-specific models obtained from CT images and idealized pulmonary models, for the purpose of carrying out experimental and numerical studies on aerosol/particle transport and deposition in inhaled drug delivery.MethodsFor the construction of the hybrid numerical model, lung images obtained from computed tomography were exported to the DICOM format to be treated with a commercial software to build the patient-specific part of the model. At the distal terminus of each airway of this portion of the model, an idealization of a single airway path is connected, extending to the sixteenth generation. Because these two parts have different endings, it is necessary to create an intermediate solid to link them together. Physically realistic treatment of truncated airway boundaries in the model was accomplished by mapping of the flow velocity distribution from corresponding conducting airway segments.ResultsThe model was verified using two sets of simulations, steady inspiration/expiration and transient simulation of forced spirometry. The results showed that the hybrid model is capable of providing a realistic description of air flow dynamics in the lung while substantially reducing computational costs relative to models of the full airway tree.ConclusionsThe model development outlined here represents an important step toward computational simulation of lung dynamics for patient-specific applications. Further research work may consist of investigating specific diseases, such as chronic bronchitis and pulmonary emphysema, as well as the study of the deposition of pollutants or drugs in the airways.

There is a continuous search for better and more complete in vitro models with mechanical properties closer to in vivo conditions. In this work a manufacturing process, based on a lost core casting technique, is herein reported to produce aneurysm biomodels to perform experimental hemodynamic studies. By using real artery images combined with a lost core casting technique, three materials were tested: paraffin, beeswax and glycerin-based soap. All in vitro biomodels were compared according to their transparency and final structure. Additionally, comparisons between experimental and numerical flow studies were also performed. The results have shown that the biomodels produced with beeswax and glycerine-based soap were the most suitable in vitro models to perform direct flow visualizations of particulate blood analogue fluids. The biomodels proposed in this works, have the potential to provide further insights into the complex blood flow phenomena happening at different kinds of pathologies and answer to important hemodynamics questions that otherwise cannot be tackled with the existing in vitro models.

In this paper, we present a numerical study of the pulsatile transitional flow crossing a severe real stenosis located right in front of the bifurcation between the right subclavian and right common carotid arteries. The simulation allows one to determine relevant features of this subject-specific flow, such as the pressure waves in the right subclavian and right common carotid arteries. We explain the subclavian steal syndrome suffered by the patient in terms of the drastic pressure drop in the right subclavian artery. This pressure drop is caused by both the diverging part of the analyzed stenosis and the reverse flow in the bifurcation induced by another stenosis in the right internal carotid artery.

A method is proposed to improve the quality of the three-dimensional airway geometric models using a commercial software, checking the number of elements, meshing time, and aspect ratio and skewness parameters. The use of real and virtual topologies combined with patch-conforming and patch-independent meshing algorithms results in four different models being the best solution the combination of virtual topology and patch-independent algorithm, due to an excellent aspect ratio and skewness of the elements, and minimum meshing time. The result is a reduction in the computational time required for both meshing and simulation due to a smaller number of cells. The use of virtual topologies combined with patch-independent meshing algorithms could be extended in bioengineering because the geometries handling is similar to this case. The method is applied to a healthy person using their computed tomography images. The resulting numerical models are able to simulate correctly a forced spirometry.

The air flow that crosses the cylinderhead of a low-capacity engine is studied both theoretically and experimentally in the steady regime. We analyze the dependence of both the discharge coefficient and swirl number on the Reynolds number and valve lift. The formation of the turbulent vortex in the cylinder is described by measuring the 2D velocity distribution over several cylinder cross sections. The integration of the Reynolds-averaged Navier–Stokes (RANS) equations reproduces satisfactorily the experimental data, especially the swirl number values.