An experimental and computational study of the inferior vena cava hemodynamics under respiratory-induced collapse of the infrarenal IVC

Optimizing inferior vena cava filter design: A computational fluid dynamics study on strut configuration for enhanced hemodynamic performance and thrombosis reduction

Background and objective

Inferior vena cava filters have been shown to be effective in preventing deep vein thrombosis and its secondary complication, pulmonary embolism, thereby reducing the high mortality rate. Although inferior vena cava filters have evolved, specific complications like inferior vena cava thrombosis-induced deep vein thrombosis worsening and recurrent pulmonary embolism continue to pose challenges. This study analyzes the effects of geometric parameter variations of inferior vena cava filters, which have a significant impact on the thrombus formation inside the filter, the capture, dissolution, and hemodynamic flow of thrombus, as well as the shear stress on the filter and vascular wall.

Methods

This study used computational fluid dynamic simulations with the carreau model to investigate the impact of varying inferior vena cava filter design parameters (number of struts, strut arm length, and tilt angle) on hemodynamics.

Results

Recirculation and stagnation areas due to flow velocity and pressure, along with wall shear stress values, were identified as key factors. It is important to find a balance between wall shear stress high enough to aid thrombolysis and low enough to prevent platelet activation. The results of this paper show that the risk of platelet activation and thrombus filtration may be lowest when the wall shear stress of the filter ranges from 0 to 4 [Pa], minimizing stress concentration within the filter.

Conclusion

16 arm struts with a length of 20 mm and a tilt angle of 0° provide the best balance between thrombus capture and minimization of hemodynamic disturbance. This configuration minimizes the size of the stagnation and recirculation zones while maintaining sufficient wall shear stress for thrombus dissolution.

An ultrasound imaging and computational fluid dynamics protocol to assess hemodynamics in iliac vein compression syndrome

OBJECTIVE

Elevated shear rates are known to play a role in arterial thrombosis; however, shear rates have not been thoroughly investigated in patients with iliac vein compression syndrome (IVCS) owing to imaging limitations and assumptions on the low shear nature of venous flows. This study was undertaken to develop a standardized protocol that quantifies IVCS shear rates and can aid in the diagnosis and treatment of patients with moderate yet symptomatic compression.

METHODS

Study patients with and without IVCS had their iliac vein hemodynamics measured via duplex ultrasound (US) at two of the following three vessel locations: infrarenal inferior vena cava (IVC), right common iliac vein, and left common iliac vein, in addition to acquiring data at the right and left external iliac veins. US velocity spectra were multiplied by a weighted cross-sectional area calculated from US and computed tomography (CT) data to create flow waveforms. Flow waveforms were then scaled to enforce conservation of flow across the IVC and common iliac veins. A three-dimensional (3D), patient-specific model of the iliac vein anatomy was constructed from CT and US examination. Flow waveforms and the 3D model were used as a basis to run a computational fluid dynamics (CFD) simulation. Owing to collateral vessel flow and discrepancies between CT and US area measurements, flows in internal iliac veins and cross-sectional areas of the common iliac veins were calibrated iteratively against target common iliac flow. Simulation results on mean velocity were validated against US data at measurement locations. Simulation results were postprocessed to derive spatial and temporal values of quantities such as velocity and shear rate.

RESULTS

Using our modeling protocol, we were able to build CFD models of the iliac veins that matched common iliac flow splits within 2% and measured US velocities within 10%. Proof-of-concept analyses (1 subject, 1 control) have revealed that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein, more typical of the arterial rather than the venous circulation. These results encourage us to extend this protocol to a larger group of patients with IVCS and controls.

CONCLUSIONS

We developed a protocol that obtains hemodynamic measurements of the IVC and iliac veins from US, creates patient-specific 3D reconstructions of the venous anatomy using CT and US examinations, and computes shear rates using calibrated CFD methods. Proof-of-concept results have indicated that patients with IVCS may experience elevated shear rates in the compressed left common iliac vein. Larger cohorts are needed to assess the relationship between venous compression and shear rates in patients with IVCS as compared with controls with noncompressed iliac veins. Further studies using this protocol may also give promising insights into whether or not to treat patients with moderate, yet symptomatic compression.

Intraluminal Thrombus Characteristics in AAA Patients: Non-Invasive Diagnosis Using CFD

Abdominal aortic aneurysms (AAA) continue to pose a high mortality risk despite advances in medical imaging and surgery. Intraluminal thrombus (ILT) is detected in most AAAs and may critically impact their development. Therefore, understanding ILT deposition and growth is of practical importance. To assist in managing these patients, the scientific community has been researching the relationship between intraluminal thrombus (ILT) and hemodynamic parameters wall shear stress (WSS) derivatives. This study analyzed three patient-specific AAA models reconstructed from CT scans using computational fluid dynamics (CFD) simulations and a pulsatile non-Newtonian blood flow model. The co-localization and relationship between WSS-based hemodynamic parameters and ILT deposition were examined. The results show that ILT tends to occur in regions of low velocity and time-averaged WSS (TAWSS) and high oscillation shear index (OSI), endothelial cell activation potential (ECAP), and relative residence time (RRT) values. ILT deposition areas were found in regions of low TAWSS and high OSI independently of the nature of flow near the wall characterized by transversal WSS (TransWSS). A new approach is suggested which is based on the estimation of CFD-based WSS indices specifically in the thinnest and thickest ILT areas of AAA patients; this approach is promising and supports the effectiveness of CFD as a decision-making tool for clinicians. Further research with a larger patient cohort and follow-up data are needed to confirm these findings.

Clinical implications of hemodynamic analysis for the three-dimension iliac vein model with different stenosis

Background

The aim of this study was to perform hemodynamic simulations of a three-dimension ideal inferior vena cava-iliac vein model with artificial stenosis to determine the degree of stenosis that requires clinical intervention.

Methods

Four three-dimension stenosis models (30%, 50%, 70%, and 90% stenosis) were constructed using commercial software (Solidworks). The inlet flow rates were acquired from previous literatures to perform the hemodynamic simulations. Changes in the old blood volume fraction, as well conventional hemodynamic parameters including pressure, differential pressure, wall shear stress, and flow patterns, over time were recorded. The pressure at the telecentric region of the stenosis increased with increasing degree of stenosis.

Results

For the 70% stenosis model, the pressure at the telecentric region of the stenosis reached 341 Pa, and the differential pressure between the two ends of the stenosis was 363 Pa (approximately 2.7 mmHg). Moreover, in the 70% and 90% stenosis models, there was a marked change in wall shear stress in the stenosis and the proximal end region, and the flow patterns began to show the phenomenon of flow separation. Blood stasis analysis showed that the 70% stenosis model had the slowest decrease in old blood volume fraction, while the proximal end region had the largest blood residue (15%).

Conclusion

Iliac vein stenosis of approximately 70% is associated with clinically relevant hemodynamic changes, and is more closely related to DVT than other degrees of stenosis.

Evaluation of hemodynamic effects of different inferior vena cava filter heads using computational fluid dynamics

Inferior vena cava (IVC) filters are used to prevent pulmonary embolism in patients with deep vein thrombosis for whom anticoagulation is unresponsive. The head is a necessary structure for an Inferior vena cava filter (IVCF) in clinic use. At present, there are various head configurations for IVCFs. However, the effect of head pattern on the hemodynamics of IVCF is still a matter of unclear. In this study, computational fluid dynamics is used to simulate non-Newtonian blood flows around four IVCFs with different heads inside an IVC model, in which the Denali filter with a solid and hooked head is employed as a prototype, and three virtual variants are reconstructed either with a no-hook head or with a through-hole head for comparison. The simulation results show that the through-hole head can effectively avoid the recirculation region and weaken the blood flow stasis closely downstream the IVCF head. The shape change of the filter head has no significant effect on the blood flow acceleration inside the IVCF cone as well as little influence on the wall shear stress (WSS) distribution on the filter wire surface and IVC wall. The structure pattern of filter head greatly affects the flow resistance of its own. However, the flow drag of filter head only occupies a small proportion of the total resistance of IVCF. Therefore, to reduce the flow resistance of an IVCF should optimize its whole structure.

Influence of the Anatomical Structure On the Hemodynamics of Iliac Vein Stenosis

Few reports study the effects of the anatomical structure of the iliac vein on hemodynamics and the methods to reduce and delay in-stent thrombosis. The anatomical structure of iliac vein stenosis was used to establish vascular models with different stenosis rates, taper angle, and left branch tilt angle in the work. The influence of anatomical structure on hemodynamics was revealed through theoretical research and in vitro experimental verification. A real iliac vein model was built based on computed tomography angiography (CTA) images, and hemorheological parameters including time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI) and relative residence time (RRT) were analyzed by computational fluid dynamics (CFD). The results showed that iliac vein stenosis could significantly increase the wall shear stress (WSS) of the blood vessels at the stenosis site and outside the intersection area, which was easy to produce eddy currents in the distal blood vessels. With the increased taper angle, the proportion of low-wall shear stress areas and the risk of thrombosis increased. A small tilt angle could aggravate the influence of narrow blood vessels on the blood flow characteristics and vascular wall. The numerical simulation results were consistent with the theoretical research results, and the experimental study verified the correctness of the simulation. The work is helpful to further understand the hemodynamic characteristics of the iliac vein, providing a scientific reference for clinical treatment and diagnosis.

Patient-specific hemodynamic analysis of IVCS-induced DVT

The aim of this study is to perform patient-specific hemodynamic simulations of patients with iliac vein compression syndrome (IVCS) and evaluate the deep venous thrombosis (DVT) potential, with clinical observations as reference. 15 patient-specific IVCS models were reconstructed from computed tomography venography (CTV) data, and divided into three groups, i.e. two groups with thrombosis: Group A (complete obstruction) and Group B (incomplete obstruction), and a third group without DVT, Group C. Hemodynamic simulations were conducted with patient-specific inlet flow rates. The blood residue was predicted using the blood stasis model. Time histories of old blood volume fraction (OBVF) was obtained, in addition to conventional hemodynamic parameters such as wall shear stress (WSS). The mean area-averaged WSS of the stenosis region for Group A and Group B were 3.68 Pa and 1.78 Pa, respectively. For the telecentric end region, the WSS were 0.76 Pa and 0.58 Pa, respectively. For Group C, the WSS at these two regions were 4.61 Pa and 1.57 Pa, respectively. The OBVF was 74.0% at the stenosis region and 76.2% at the telecentric end region for Group A, much higher than 4.8% and 43.1% of Group B. For Group C, the OBVF at the two regions were close to 0. This corresponded well with clinical observations. The potential of DVT can be predicted through patient-specific hemodynamic simulations in combination of blood stasis model. The findings of this study are of great significance for the preoperative evaluation and treatment prognosis of IVCS patients with DVT.

Computational evaluation of inferior vena cava filters through computational fluid dynamics methods

Numerical simulation is growing in its importance toward the design, testing and evaluation of medical devices. Computational fluid dynamics and finite element analysis allow improved calculation of stress, heat transfer, and flow to better understand the medical device environment. Current research focuses not only on improving medical devices, but also on improving the computational tools themselves. As methods and computer technology allow for faster simulation times, iterations and trials can be performed faster to collect more data. Given the adverse events associated with long-term inferior vena cava (IVC) filter placement, IVC filter design and device evaluation are of paramount importance. This work reviews computational methods used to develop, test, and improve IVC filters to ultimately serve the needs of the patient.

In Vitro Clot Trapping Efficiency of the FDA Generic Inferior Vena Cava Filter in an Anatomical Model: An Experimental Fluid-Structure Interaction Benchmark

PURPOSE

Robust experimental data for performing validation of fluid-structure interaction (FSI) simulations of the transport of deformable solid bodies in internal flow are currently lacking. This in vitro experimental study characterizes the clot trapping efficiency of a new generic conical-type inferior vena cava (IVC) filter in a rigid anatomical model of the IVC with carefully characterized test conditions, fluid rheological properties, and clot mechanical properties.

METHODS

Various sizes of spherical and cylindrical clots made of synthetic materials (nylon and polyacrylamide gel) and bovine blood are serially injected into the anatomical IVC model under worst-case exercise flow conditions. Clot trapping efficiencies and their uncertainties are then quantified for each combination of clot shape, size, and material.

RESULTS

Experiments reveal the clot trapping efficiency increases with increasing clot diameter and length, with trapping efficiencies ranging from as low as approximately 42% for small 3.2 mm diameter spherical clots up to 100% for larger clot sizes. Because of the asymmetry of the anatomical IVC model, the data also reveal the iliac vein of clot origin influences the clot trapping efficiency, with the trapping efficiency for clots injected into the left iliac vein up to a factor of 7.5 times greater than that for clots injected into the right iliac (trapping efficiencies of approximately 10% versus 75%, respectively).

CONCLUSION

Overall, this data set provides a benchmark for validating simulations predicting IVC filter clot trapping efficiency and, more generally, low-Reynolds number FSI modeling.

Hemodynamic Analysis of VenaTech Convertible Vena Cava Filter Using Computational Fluid Dynamics

The VenaTech convertible filter (VTCF) has been widely used as an inferior vena cava (IVC) filter to prevent fatal pulmonary embolism in patients. However, its hemodynamics that greatly affect the filter efficacy and IVC patency are still unclear. This paper uses computational fluid dynamics with the Carreau model to simulate the non-Newtonian blood flows around the VTCF respectively deployed in the normal, reverse and three converted states in an IVC model. The results show that the prothrombotic stagnation zones are observed downstream from the normal, reverse and small open VTCFs, with the streamwise length is nearly eight times the IVC diameter. The no-slip boundary conditions of the thin-wire VTCF arms lead to the “viscous block” effect. The viscous block accelerates the blood flow by 5–15% inside the IVC and enhances the filter wall shear stress up to nearly 20 times that of the IVC only, which contributes to clot capture and thrombus lysis. The relative flow resistance is defined to evaluate the filter-induced resistance on the IVC blood flow that can be regarded as an index of IVC patency with the filter deployment. The flow resistance of the normal VTCF deployment increases dramatically by more than 60% compared with that of the IVC only and is a little higher (6%) than that of the reverse case. As the VTCF converts to a fully open configuration, the flow resistance gradually decreases to that of no filter. This work shows that even very thin VTCF arms can result in the viscous block effect and may cause significant hemodynamic impacts on clot capture, potential thrombosis and flow impedance inside the IVC. The present study also shows that CFD is a valuable and feasible in silico tool for analyzing the IVC filter hemodynamics to complement in vivo clinical and in vitro experimental studies.

The influence of inlet velocity profile on predicted flow in type B aortic dissection

In order for computational fluid dynamics to provide quantitative parameters to aid in the clinical assessment of type B aortic dissection, the results must accurately mimic the hemodynamic environment within the aorta. The choice of inlet velocity profile (IVP) therefore is crucial; however, idealised profiles are often adopted, and the effect of IVP on hemodynamics in a dissected aorta is unclear. This study examined two scenarios with respect to the influence of IVP—using (a) patient-specific data in the form of a three-directional (3D), through-plane (TP) or flat IVP; and (b) non-patient-specific flow waveform. The results obtained from nine simulations using patient-specific data showed that all forms of IVP were able to reproduce global flow patterns as observed with 4D flow magnetic resonance imaging. Differences in maximum velocity and time-averaged wall shear stress near the primary entry tear were up to 3% and 6%, respectively, while pressure differences across the true and false lumen differed by up to 6%. More notable variations were found in regions of low wall shear stress when the primary entry tear was close to the left subclavian artery. The results obtained with non-patient-specific waveforms were markedly different. Throughout the aorta, a 25% reduction in stroke volume resulted in up to 28% and 35% reduction in velocity and wall shear stress, respectively, while the shape of flow waveform had a profound influence on the predicted pressure. The results of this study suggest that 3D, TP and flat IVPs all yield reasonably similar velocity and time-averaged wall shear stress results, but TP IVPs should be used where possible for better prediction of pressure. In the absence of patient-specific velocity data, effort should be made to acquire patient’s stroke volume and adjust the applied IVP accordingly.

Inferior vena cava filter – comprehensive overview of current indications, techniques, complications and retrieval rates

Summary: Inferior vena cava (IVC) filter has been used to manage patients with pulmonary embolism and deep venous thrombosis. Its ease of use and the expansion of relative indications have led to a dramatic increase in IVC filter placement. However, IVC filters have been associated with a platitude of complications. Therefore, there exists a need to examine the current indications and identify the patient population at risk. In this paper, we comprehensively reviewed the current indications and techniques of IVC filter placement. Further, we examined the various complications associated with either permanent or retrievable IVC filters. Lastly, we examined the current data on filter retrieval.

Evaluation of 4D flow MRI-based non-invasive pressure assessment in aortic coarctations

Severity of aortic coarctation (CoA) is currently assessed by estimating trans-coarctation pressure drops through cardiac catheterization or echocardiography. In principle, more detailed information could be obtained non-invasively based on space- and time-resolved magnetic resonance imaging (4D flow) data. Yet the limitations of this imaging technique require testing the accuracy of 4D flow-derived hemodynamic quantities against other methodologies. With the objective of assessing the feasibility and accuracy of this non-invasive method to support the clinical diagnosis of CoA, we developed an algorithm (4DF-FEPPE) to obtain relative pressure distributions from 4D flow data by solving the Poisson pressure equation. 4DF-FEPPE was tested against results from a patient-specific fluid-structure interaction (FSI) simulation, whose patient-specific boundary conditions were prescribed based on 4D flow data. Since numerical simulations provide noise-free pressure fields on fine spatial and temporal scales, our analysis allowed to assess the uncertainties related to 4D flow noise and limited resolution. 4DF-FEPPE and FSI results were compared on a series of cross-sections along the aorta. Bland-Altman analysis revealed very good agreement between the two methodologies in terms of instantaneous data at peak systole, end-diastole and time-averaged values: biases (means of differences) were +0.4 mmHg, -1.1 mmHg and +0.6 mmHg, respectively. Limits of agreement (2 SD) were ±0.978 mmHg, ±1.06 mmHg and ±1.97 mmHg, respectively. Peak-to-peak and maximum trans-coarctation pressure drops obtained with 4DF-FEPPE differed from FSI results by 0.75 mmHg and -1.34 mmHg respectively. The present study considers important validation aspects of non-invasive pressure difference estimation based on 4D flow MRI, showing the potential of this technology to be more broadly applied to the clinical practice.

Steady Flow in a Patient-Averaged Inferior Vena Cava-Part I: Particle Image Velocimetry Measurements at Rest and Exercise Conditions

PURPOSE

Although many previous computational fluid dynamics (CFD) studies have investigated the hemodynamics in the inferior vena cava (IVC), few studies have compared computational predictions to experimental data, and only qualitative comparisons have been made. Herein, we provide particle image velocimetry (PIV) measurements of flow in a patient-averaged IVC geometry under idealized conditions typical of those used in the preclinical evaluation of IVC filters.

METHODS

Measurements are acquired under rest and exercise flow rate conditions in an optically transparent model fabricated using 3D printing. To ensure that boundary conditions are well-defined and to make follow-on CFD validation studies more convenient, fully-developed flow is provided at the inlets (i.e., the iliac veins) by extending them with straight rigid tubing longer than the estimated entrance lengths. Velocity measurements are then obtained at the downstream end of the tubing to confirm Poiseuille inflow boundary conditions.

RESULTS

Measurements in the infrarenal IVC reveal that flow profiles are blunter in the sagittal plane (minor axis) than in the coronal plane (major axis). Peak in-plane velocity magnitudes are 4.9 cm/s and 27 cm/s under the rest and exercise conditions, respectively. Flow profiles are less parabolic and exhibit more inflection points at the higher flow rate. Bimodal velocity peaks are also observed in the sagittal plane at the elevated flow condition.

CONCLUSIONS

The IVC geometry, boundary conditions, and infrarenal velocity measurements are provided for download on a free and publicly accessible repository at https://doi.org/10.6084/m9.figshare.7198703 . These data will facilitate future CFD validation studies of idealized, in vitro IVC hemodynamics and of similar laminar flows in vascular geometries.