In vitro model for the evaluation of inferior vena cava filters: effect of experimental parameters on thrombus-capturing efficacy of the Vena Tech-LGM filter

Design and performance analysis of a new inferior vena cava filter

This study proposes a novel inferior vena cava filter (IVCF) design, "Lotus," aiming to enhance release stability and endothelialization. A catheter-filter-vessel model was established for IVCF property analysis, validated by comparing numerical simulations and in vitro tests. Lotus's mechanical properties were analyzed, and optimization suggestions are provided. Compared to existing clinical filters, Lotus demonstrates improved release stability and thrombus capture ability. This work suggests Lotus as a potential technical reference for improved IVCF treatment.

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.

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.

A Computational Method for Predicting Inferior Vena Cava Filter Performance on a Patient-Specific Basis

A computational methodology for simulating virtual inferior vena cava (IVC) filter placement and IVC hemodynamics was developed and demonstrated in two patient-specific IVC geometries: a left-sided IVC and an IVC with a retroaortic left renal vein. An inverse analysis was performed to obtain the approximate in vivo stress state for each patient vein using nonlinear finite element analysis (FEA). Contact modeling was then used to simulate IVC filter placement. Contact area, contact normal force, and maximum vein displacements were higher in the retroaortic IVC than in the left-sided IVC (144 mm2, 0.47 N, and 1.49 mm versus 68 mm2, 0.22 N, and 1.01 mm, respectively). Hemodynamics were simulated using computational fluid dynamics (CFD), with four cases for each patient-specific vein: (1) IVC only, (2) IVC with a placed filter, (3) IVC with a placed filter and model embolus, all at resting flow conditions, and (4) IVC with a placed filter and model embolus at exercise flow conditions. Significant hemodynamic differences were observed between the two patient IVCs, with the development of a right-sided jet, larger flow recirculation regions, and lower maximum flow velocities in the left-sided IVC. These results support further investigation of IVC filter placement and hemodynamics on a patient-specific basis.

Limitations of using synthetic blood clots for measuring in vitro clot capture efficiency of inferior vena cava filters

The purpose of this study was first to evaluate the clot capture efficiency and capture location of six currently-marketed vena cava filters in a physiological venous flow loop, using synthetic polyacrylamide hydrogel clots, which were intended to simulate actual blood clots. After observing a measured anomaly for one of the test filters, we redirected the focus of the study to identify the cause of poor clot capture performance for large synthetic hydrogel clots. We hypothesized that the uncharacteristic low clot capture efficiency observed when testing the outlying filter can be attributed to the inadvertent use of dense, stiff synthetic hydrogel clots, and not as a result of the filter design or filter orientation. To study this issue, sheep blood clots and polyacrylamide (PA) synthetic clots were injected into a mock venous flow loop containing a clinical inferior vena cava (IVC) filter, and their captures were observed. Testing was performed with clots of various diameters (3.2, 4.8, and 6.4 mm), length-to-diameter ratios (1:1, 3:1, 10:1), and stiffness. By adjusting the chemical formulation, PA clots were fabricated to be soft, moderately stiff, or stiff with elastic moduli of 805 ± 2, 1696 ± 10 and 3295 ± 37 Pa, respectively. In comparison, the elastic moduli for freshly prepared sheep blood clots were 1690 ± 360 Pa. The outlying filter had a design that was characterized by peripheral gaps (up to 14 mm) between its wire struts. While a low clot capture rate was observed using large, stiff synthetic clots, the filter effectively captured similarly sized sheep blood clots and soft PA clots. Because the stiffer synthetic clots remained straight when approaching the filter in the IVC model flow loop, they were more likely to pass between the peripheral filter struts, while the softer, physiological clots tended to fold and were captured by the filter. These experiments demonstrated that if synthetic clots are used as a surrogate for animal or human blood clots for in vitro evaluation of vena cava filters, the material properties (eg, elastic modulus) and dynamic behavior of the surrogate should first be assessed to ensure that they accurately mimic an actual blood clot within the body.

Avaliação in vitro de um novo filtro de veia cava

OBJETIVO: O objetivo do estudo foi avaliar a eficácia de um novo filtro de veia cava, de baixo perfil, na retenção de coágulos em modelo in vitro. MÉTODO: O filtro consiste em dois cones opostos pelo ápice. O cone distal é formado por oito hastes de aço inoxidável, que têm a função de retenção dos êmbolos. O cone proximal é constituído de quatro hastes, cuja função é ancorar e centralizar. Os filtros foram introduzidos e fixados no interior de um tubo de PVC transparente de 25, 30 e 35 mm de diâmetro interno, em posição vertical, e conectados com um sistema pulsátil de fluxo (bomba peristáltica). Foi utilizado, para veículo, um reservatório com solução salina (0,9%) com 40% de glicerina, mantido em temperatura ambiente. Confeccionaram-se trombos com sangue bovino em tubos plásticos de 3, 4,5 e 6 mm de diâmetro e, posteriormente, foram segmentados nas medidas de 10, 15, 20 e 30 mm de comprimento, totalizando 12 diferentes tamanhos. Realizaram-se 100 liberações para cada tipo de êmbolo e tamanho das cânulas, totalizando 3.600 eventos. Foram feitos lançamentos seqüenciais com cinco êmbolos, sendo 10 para cada tamanho de êmbolo e cânulas, totalizando 360 eventos. Fez-se avaliação da capacidade de retenção dinâmica utilizando os três diferentes tamanhos de cânulas com 100 eventos cada, totalizando 300 eventos. RESULTADOS: Detectou-se que o diâmetro e comprimento dos êmbolos, assim como diâmetros da cânula, podem comprometer a eficácia do filtro. A média de captura de êmbolos pelos filtros foi de 80,5% nas cânulas de 35 mm, 88,7% para cânulas de 30 mm e 86,6% para cânulas de 25 mm. CONCLUSÃO: Conclui-se que a eficácia desse filtro sofre interferência relacionada ao tamanho dos êmbolos e diâmetro da cânula.

Evaluation of the LGM Vena-Tech infrarenal vena cava filter in an ovine venous thromboembolism model

PURPOSE

To validate a new percutaneous model of venous thrombosis in sheep and evaluate the use of the LGM Vena-Tech vena cava filter with use of this model.

MATERIALS AND METHODS

After implantation of a LGM Vena-Tech filter in the infrarenal vena cava (IVC), thrombus was obtained by blocking the iliac vein with an inflated balloon (Wedge catheter) for 15 minutes and simultaneously injecting 20 mL of fresh thrombus into the femoral vein. Clot migration of the thrombus was induced by balloon deflation and injection of contrast medium. Migration and capture of the thrombus by the filter were filmed under fluoroscopy at 1 frame/sec. Euthanasia followed by pathologic examination of the IVC, heart, and lungs was performed immediately after the procedure in five sheep (group 1). Sheep in groups 2, 3, and 4, (five in each group), were killed at 2, 4, and 8 weeks, respectively, after vena cavography. Histologic examination was performed to analyze the evolution of the thrombus captured, the incorporation of the filter in the caval wall, and the physical and mechanical effects of captured thrombi on the filter.

RESULTS

The Vena-Tech filter captured a large amount of thrombus in all cases except one, in which the filter captured a small strand of thrombus, related to incomplete occlusion of the iliac vein during the clot formation procedure. In the 15 animals in which follow-up was performed, fibrous evolution of the thrombus was observed at gross examination in 14, leading to the formation of fibrous webs between the filter and the IVC wall in eight. These observations were not related to the incidence of filter retraction (n = 4), caudal migration (n = 2), tilt (n = 2), or caval perforation (n = 1). Heart and lung thrombi were present in six animals. Histologic results confirmed the fibrous evolution of the thrombus and its organization during follow-up. Neointima increases significantly (P <.02) during follow-up, from 135.7 microm +/- 13.4 at 2 weeks to 192.2 microm +/- 125.7 at 4 weeks and 334.2 microm +/- 144.1 at 8 weeks.

CONCLUSION

The model used is suitable for the formation of a large amount of venous thrombus and analysis of its migration and capture by the LGM Vena-Tech filter. Fibrous evolution of the thrombus, including development of webs and changes in filter shape and position, were the main outcomes observed.