Ygael Grad

Hemodynamic Evaluation of Embolic Trajectory in an Arterial Bifurcation. An In-Vitro Experimental Model

Background and Purpose— Despite the importance of embolism as a major cause of brain infarction, little is known about the hemodynamic factors governing the path large emboli tend to follow. Our aim was to test in vitro, whether hemodynamic parameters other than flow ratios between bifurcation branches may affect the distribution of embolic particles in a Y-shaped bifurcation model, used as an analogue to an arterial bifurcation. Methods— In vitro experiments were conducted using suspensions of sphere-shaped particles (0.6, 1.6, and 3.2 mm) in water-glycerin mixture, using steady and pulsatile laminar flow regimes in a Y-shaped bifurcation model (identical branching angles [&thgr;1=&thgr;2=45o] with one daughter branch diameter wider than the other [D1=6 mm, D2=4 mm]; average Reynolds number 500). Results— Experiments using naturally buoyant particles under steady flow conditions and four outlet-flow ratios revealed that small (0.6 mm) and mid-sized (1.6 mm) particles entered into either the narrower or wider bifurcation daughter branch nonpreferentially, proportionally to the flow ratios. Large particles (3.2 mm), however, preferentially entered the wider daughter branch. Moreover, as the flow ratio increases this phenomenon was augmented. Further experiments revealed that the preference of the wider daughter branch for high particle-to-branch diameter-ratios further increases under pulsatile flow and by the density ratio between particles and fluid. Conclusion— Particles’ distribution in a bifurcation is affected, beyond its outlets-flow-ratios, by the particle-to-branch diameter-ratio. The tendency of large particles to preferentially enter the wider bifurcation branch, beyond the flow ratio, is augmented under pulsatile flow conditions and is affected by particle-to-fluid density-ratio. These findings may have important implications for understanding the hemodynamic mechanisms underlying the trajectory of large emboli.

A Novel Endovascular Device for Emboli Rerouting Part I: Evaluation in a Swine Model

Background and Purpose— The feasibility and safety of a novel endovascularly delivered tubular mesh designed to reroute emboli away from a critical artery as a means of ischemic stroke prevention was tested in vitro and in vivo. Methods— Emboli rerouting efficacy was assessed in vitro. Perfusion through the external femoral artery that was jailed by the device, cellular proliferation rate over the jailing mesh, and the resulting tissue coverage of the orifice were assessed in the swine iliofemoral bifurcation. Device-induced embolization was assessed in a swine kidney model. Results— In vitro experiments demonstrated that particles as small as 60% of the pore dimension can be rerouted by the device, although at low efficacy, and rerouting efficacy approached 100% as the particle size approached the pore dimension. Repeat assessment of flow preimplantation and at various follow-up times by Doppler ultrasound showed no significant changes in the perfusion ratio of the jailed branch to the parent artery or the jailed branch to the naive contralateral artery either as a result of device implantation or at the follow-up times. Tissue coverage over the jailed ostium was limited to approximately 12% after stabilization. Cellular proliferation rate gradually decreased to diminishing level approximately 22 weeks postimplantation. The devices implanted across the renal arteries did not demonstrate any device-induced embolization after 1 month. Conclusions— It is proposed that this device could be used to reroute emboli away from important intracranial vessels as a means of stroke prevention.

Similarity of the Swine Vasculature to the Human Carotid Bifurcation: Analysis of Arterial Diameters

PURPOSE We assessed the similarities of the iliac bifurcation in two breeds of swine to the known human carotid bifurcation, in order to assess its applicability as a preclinical model of the human carotid bifurcation. MATERIALS AND METHODS Mixed-Landrace crossbreed (domestic; n = 66) and Yucatan miniswine (Yucatan; n = 13) were studied. The diameters of the iliac bifurcation arteries were measured from angiographic pictures, the relations between the arterial diameters and the animals weight assessed, and diametrical ratios calculated. Findings were compared with the known human carotid bifurcation. RESULTS The external iliac diameter (approximately 6-7mm) in the lower weight swine was similar to the known human common carotid artery. The best similarity was found between the diametrical ratios of the human internal to common carotid artery (mean 0.63) to the swine profunda to external iliac (means 0.68 and 0.65 for the domestic and Yucatan, respectively). The arterial diameters of the domestic swine were highly correlated with their weight that increased considerably with time, while in the Yucatan group, arterial diameters did not change with increasing weight and the average weight increase rate was low. Thus, the estimated arterial diameter increase rate over time was high in the domestic while minor in the Yucatan group. CONCLUSIONS Similarities were found between the swine iliac bifurcation arteries to the human carotid bifurcation in terms of diameters, diametrical ratios and angle. The swine iliac bifurcation may be used for preclinical endovascular research of devices intended to the human carotid bifurcation, with miniswine strains a preferable model for long-term studies.

EFFICACY OF A NEW PERMANENT ARTERIAL FILTRATION DEVICE

The Diverter is a new permanent arterial diversion device designed to divert emboli away from the internal into the external carotid artery to prevent embolic stroke. The purpose of this study was to assess experimentally the hemodynamic performance and diverting capacity of the Diverter to be implanted at the carotid bifurcation.

In vivo assessment of a new long-term filtration device for embolic stroke prevention

Based on in vitro optimization, a new filtering implant (the Diverter) was designed to divert emboli away from the internal into the external carotid artery to prevent embolic stroke from proximal sources. Our aim was to test the patency of the permanent arterial filter crossing arterial bifurcations of swine. Biocompatible Diverters (29), composed of multiple fine wire meshes, were implanted in illio-femoral bifurcations of 45 kg swine for up to 18 weeks. The bifurcations were harvested and patency percentage of the filtered ostium was computed using image morphometry. Angiography and Duplex prior to animal sacrifice found the Diverter open to flow without occlusion or discernable stenosis up to 18 weeks post implantation in all devices except 3 mechanical failures. The Diverters were fully covered at the non-filtering portion area after 4 weeks. Morphometry showed that 90% (average) of filtered area of the 26 specimens remained patent, (93, 92, 72 and 93 [%] at 2, 4, 9, and 18 weeks respectively). Implantation of a permanent arterial filtration device is feasible and such a device remains patent. Our preliminary findings give hope to a new therapy for patients at high risk of embolic stroke.

Permanent arterial blood filtration for embolic stroke prevention - the hemodynamics of a new concept

A new permanent filtering device (the Diverter) was designed for prevention of embolic stroke from proximal sources such as atrial fibrillation, by implantation at the carotid bifurcation. It diverts emboli away from the internal into the external carotid artery (ICA, ECA). The aim was to study hemodynamic parameters governing blood filtration in-vitro, and then test the hemodynamically optimized arterial filtering device in-vivo. An in-vitro continuous digital particle image velocimetry (CDPIV) system was used to investigate the filters flow field in a compliant transparent model of the carotid bifurcation. The results were used to assess the hemodynamic parameters that govern the activation of the coagulation cascade and the design was optimized. An in vivo swine model was applied to assess the validity of the in-vitro results, by testing the patency of 30 harvested filtering devices implanted in arterial bifurcations. A thin filament based design was found to be hemodynamically optimal in-vitro, by invoking the minimally possible flow disturbances and values of activation parameter (AP). An arterial filter designed based on these considerations remained patent in a swine model and did not invoke occlusive thrombi. These findings serve as the basis for a novel vascular interventional approach for treatment of embolic disease.

Time-dependent aspects of the flow in the carotid bifurcation: continuous DPIV and computational results

Cerebrovascular disease (stroke) and cerebral ischemia are a leading cause of mortality and morbidity. Approximately 80% of all strokes are caused by ischemia. Occlusion or stenosis in the internal carotid artery (ICA), which starts at the bifurcation of the common carotid artery (CCA), is the result of disease of the arterial wall. Complication by thrombosis or embolism is the most frequent cause of cerebral ischemia and infarction. Biofluid factors such as abnormal shear stresses and turbulence have been cited to promote these disorders. Continuous digital particle image velocimetry (CDPIV) is used to map the flow field of vascular model of the ICA and obtain its time and spectral signatures. Computational fluid dynamics (CFD) is used to calculate and validate the hemodynamic factors governing the flow field. Continuous sequences of resulting images are given as animated presentation of distensible carotid bifurcation model. High shear stress peaks and vortices are observed at the bifurcation heel. Flow reversal or separation is Reynolds number dependent. The flow profile and spectra were dependent on geometry and elasticity. The combination of CDPIV and CFD seems to be a powerful tool enabling complementary time dependent assessments of the demanding field of cerebrovascular disease.