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

Abstract

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.

In vitro evaluation of clot capture efficiency of an absorbable vena cava filter

“…The slightly lower elastic modulus of the synthetic clots was by design, as it represents a balance between matching elasticity and foldability – both important parameters in simulating thrombus flow. Similar to Robinson et al 21 , the PAM clots in this study were slightly less foldable than blood clots even with the lower elastic modulus.…”

“…Glycerin solution (52% by weight in water, 1.12 g/ml) was used to simulate the viscosity and density of blood. 21 Aheating coil wrapped around stainless steel pipe with a thermostat controller (Love Controls, Michigan City, IN) maintained the temperature at 37°C +/− 2°C.…”

“…21 The PAM was cast in tubing with inner diameters of 2.4, 4.8, 5.7, 9.5, and 25.4 mm for 3 hours. These were cut to lengths of approximately 3.6mm (2.4 mm ID), 7.1 mm (2.4, 4.8, and 5.7 mm IDs), 14.3 mm (2.4, 4.8, and 5.7 mm ID’s), 17.1 mm (9.5 mm ID), and 10 mm (25.4 mm ID).…”

In vitro comparison of Günther Tulip and Celect filters. Testing filtering efficiency and pressure drop

“…As blood flow velocity is highest in the center of the IVC, emboli are believed to be carried along the center of the blood stream (Qian et al, 1994). As shown by Lorch et al (2002), Katsamouris et al (1988), and Robinson et al (2013), clot capture rates were significantly higher with the device in the vertical position. In this report, tests were only performed in a stable horizontal position similar to that in supine patients.…”

“…The blood clots were modeled as spheres for these experiments. Although the shape of the clots varies widely and has been approximated by different shapes in various works (Couch et al, 1997;Stewart et al, 2008;Robinson et al, 2013), no single shape can be rigorously assigned to a clot due to the inherently complex and Both of them have a half-basket shape with a length of 45 mm and a maximum diameter of 30 mm (Cook, 2014) and have a retrieval hook at the appex to allow percutaneous retrieval or repositioning (de Gregorio et al, 2003). random nature of its formation.…”