In vitro assessment of the hemodynamic effects of a partial occlusion in a vena cava filter

Purpose: Flow fields surrounding a vena cava filter were studied with a noninvasive method of flow visualization to examine the underlying hemodynamic factors that contribute to its function and patency. Methods: The photochromic technique was used to measure axial velocity profiles and wall shear stress distributions in a 12F titanium Greenfield filter partially occluded with a simulated volume of entrapped thrombi. These results were compared with similar measurements obtained for an unoccluded filter and with a theoretic prediction of the velocity profile and wall shear stress without a vena cava filter. Shear stress distributions were obtained along the vessel wall and for the partially occluded filter along the surface of the simulated clot. Results: The unoccluded filter was observed to have little effect on the flow field. In the case of the partially occluded filter, the results of these measurements show that caval blood flow is preserved by the creation of an annular region of increased flow around the periphery of the clot. Within this region high shear stresses that develop as a result of the increased flow are observed along the vessel wall and along the surface of the simulated clot. No vortices or turbulence were observed with either the unoccluded or the partially occluded filter. Conclusions: The elevated levels of shear stress may be a factor in the lysis of trapped clots observed in vivo. Although increased shear stress is reported to cause thrombogenesis in an in vivo study in canines, particularly under turbulent flow conditions, the levels of shear measured in this study around the simulated clot were well below such values.