High Shear Thrombus Formation under Pulsatile and Steady Flow

von Willebrand factor unfolding mediates platelet deposition in a model of high-shear thrombosis

Thrombosis under high-shear conditions is mediated by the mechanosensitive blood glycoprotein von Willebrand factor (vWF). vWF unfolds in response to strong flow gradients and facilitates rapid recruitment of platelets in flowing blood. While the thrombogenic effect of vWF is well recognized, its conformational response in complex flows has largely been omitted from numerical models of thrombosis. We recently presented a continuum model for the unfolding of vWF, where we represented vWF transport and its flow-induced conformational change using convection-diffusion-reaction equations. Here, we incorporate the vWF component into our multi-constituent model of thrombosis, where the local concentration of stretched vWF amplifies the deposition rate of free-flowing platelets and reduces the shear cleaning of deposited platelets. We validate the model using three benchmarks: in vitro model of atherothrombosis, a stagnation point flow, and the PFA-100, a clinical blood test commonly used for screening for von Willebrand disease (vWD). The simulations reproduced the key aspects of vWF-mediated thrombosis observed in these experiments, such as the thrombus location, thrombus growth dynamics, and the effect of blocking platelet-vWF interactions. The PFA-100 simulations closely matched the reported occlusion times for normal blood and several hemostatic deficiencies, namely, thrombocytopenia, vWD type 1, and vWD type 3. Overall, this multi-constituent model of thrombosis enables macro-scale 3D simulations of thrombus formation in complex geometries over a wide range of shear rates and accounts for qualitative and quantitative hemostatic deficiencies in patient blood.

Polymer Coating Integrity, Thrombogenicity and Computational Fluid Dynamics Analysis of Provisional Stenting Technique in the Left Main Bifurcation Setting: Insights from an In-Vitro Model

Currently, the provisional stenting technique is the gold standard in revascularization of lesions located in the left main (LM) bifurcation. The benefit of the routine kissing balloon technique (KBI) in bifurcation lesions is still debated, particularly following the single stent treatment. We compared the latest-generation drug-eluting stent (DES) with no side branch (SB) dilatation “keep it open” technique (KIO) vs. KBI technique vs. bifurcation dedicated drug-eluting stent (BD-DES) implantation. In vitro testing was performed under a static condition in bifurcation silicone vessel models. All the devices were implanted in accordance with the manufacturers’ recommendations. As a result, computational fluid dynamics (CFD) analysis demonstrated a statistically higher area of high shear rate in the KIO group when compared to KBI. Likewise, the maximal shear rate was higher in number in the KIO group. Floating strut count based on the OCT imaging was significantly higher in KIO than in KBI and BD-DES. Furthermore, according to OTC analysis, the thrombus area was numerically higher in both KIO and KBI than in the BD-DES. Scanning electron microscopy (SEM) analysis shows the highest degree of strut coating damage in the KBI group. This model demonstrated significant differences in CFD analysis at SB ostia with and without KBI optimization in the LM setting. The adoption of KBI was related to a meaningful reduction of flow disturbances in conventional DES and achieved results similar to BD-DES.

Computational investigation of blood cell transport in retinal microaneurysms

Microaneurysms (MAs) are one of the earliest clinically visible signs of diabetic retinopathy (DR). MA leakage or rupture may precipitate local pathology in the surrounding neural retina that impacts visual function. Thrombosis in MAs may affect their turnover time, an indicator associated with visual and anatomic outcomes in the diabetic eyes. In this work, we perform computational modeling of blood flow in microchannels containing various MAs to investigate the pathologies of MAs in DR. The particle-based model employed in this study can explicitly represent red blood cells (RBCs) and platelets as well as their interaction in the blood flow, a process that is very difficult to observe in vivo. Our simulations illustrate that while the main blood flow from the parent vessels can perfuse the entire lumen of MAs with small body-to-neck ratio (BNR), it can only perfuse part of the lumen in MAs with large BNR, particularly at a low hematocrit level, leading to possible hypoxic conditions inside MAs. We also quantify the impacts of the size of MAs, blood flow velocity, hematocrit and RBC stiffness and adhesion on the likelihood of platelets entering MAs as well as their residence time inside, two factors that are thought to be associated with thrombus formation in MAs. Our results show that enlarged MA size, increased blood velocity and hematocrit in the parent vessel of MAs as well as the RBC-RBC adhesion promote the migration of platelets into MAs and also prolong their residence time, thereby increasing the propensity of thrombosis within MAs. Overall, our work suggests that computational simulations using particle-based models can help to understand the microvascular pathology pertaining to MAs in DR and provide insights to stimulate and steer new experimental and computational studies in this area.

Comparison of overexpansion capabilities and thrombogenicity at the side branch ostia after implantation of four different drug eluting stents

Interventions in bifurcation lesions often requires aggressive overexpansion of stent diameter in the setting of long tapering vessel segment. Overhanging struts in front of the side branch (SB) ostium are thought to act as a focal point for thrombi formation and consequently possible stent thrombosis. This study aimed to evaluate the overexpansion capabilities and thrombogenicity at the SB ostia after implantation of four latest generation drug-eluting stents (DES) in an in-vitro bifurcation model. Four clinically available modern DES were utilized: one bifurcation dedicated DES (Bioss LIM C) and three conventional DES (Ultimaster, Xience Sierra, Biomime). All devices were implanted in bifurcation models with proximal optimization ensuring expansion before perfusing with porcine blood. Optical coherence tomography (OCT), immunofluorescence (IF) and scanning electron microscope analysis were done to determine thrombogenicity and polymer coating integrity at the over-expanded part of the stents. Computational fluid dynamics (CFD) was performed to study the flow disruption. OCT (p = 0.113) and IF analysis (p = 0.007) demonstrated lowest thrombus area at SB ostia in bifurcation dedicated DES with favorable biomechanical properties compared to conventional DES. The bifurcated DES also resulted in reduced area of high shear rate and maximum shear rate in the CFD analysis. This study demonstrated numerical differences in terms of mechanical properties and acute thrombogenicity at SB ostia between tested devices.

Biorheology of occlusive thrombi formation under high shear: in vitro growth and shrinkage

Occlusive thrombi formed under high flow shear rates develop very rapidly in arteries and may lead to myocardial infarction or stroke. Rapid platelet accumulation (RPA) and occlusion of platelet-rich thrombi and clot shrinkage have been studied after flow arrest. However, the influence of margination and shear rate on occlusive clot formation is not fully understood yet. In this study, the influence of flow on the growth and shrinkage of a clot is investigated. Whole blood (WB) and platelet-rich plasma (PRP) were perfused at high shear rates (> 3,000 s⁻¹) through two microfluidic systems with a stenotic section under constant pressure. The stenotic section of the two devices are different in stenotic length (1,000 vs 150 μm) and contraction angle of the stenosis (15° vs 80°). In all experiments, the flow chamber occluded in the stenotic section. Besides a significantly increased lag time and decreased RPA rate for PRP compared to WB (p < 0.01), the device with a shorter stenotic section and steeper contraction angle showed a shear-dependent occlusion and lag time for both PRP and WB. This shear-dependent behavior of the platelet aggregate formation might be caused by the stenotic geometry.

T and Small Protrusion (TAP) vs Double-Kissing Crush Technique: Insights From In Vitro Models

BACKGROUND

Percutaneous coronary interventions on complex bifurcation lesions may require implantation of two stents to appropriately treat diffuse side-branch (SB) disease. Comparisons among different bifurcation stenting techniques are continuously attempted by various study designs (bench tests, computer simulations, clinical studies). Among different techniques, double kissing crush (DKC) represents the last evolution for "crushing" while T and small Protrusion (TAP) represents the evolution of "T stenting". Both techniques are actually gaining popularity, but head-to-head comparisons are lacking.

METHODS AND RESULTS

Two last generation drug-eluting stents (Synergy™, Boston Scientific, MA, USA and Ultimaster™,Terumo Corp., Japan) were implanted in left main bifurcation bench models using TAP (n = 6 sets) and DKC (n = 6 sets) techniques. A peristaltic pump with fresh porcine blood was used to perfuse the blood through the silicone model at a flow rate of 200 ml/min for 4 min. Optical coherence tomography (OCT) was used to assess stent struts geometry and thrombus formation. SB cross sectional area as well as SB obstruction did not significantly differ between the two techniques. Numerical (but not statistically significant) differences were found in terms of malapposed struts (fewer with TAP) and floating struts (fewer with DKC). Thrombus formation after blood perfusion was similar between TAP and DKC technique (1.53 ± 1.12 vs. 1.20 ± 1.01 mm², p = 0.6).

CONCLUSION

The result of the present in-vitro study shows the absence of significant difference between TAP and DKC in terms of stent struts apposition and acute thrombus formation potential. Despite the completely different technical steps required, both techniques have similar performance according to such articulated pre-clinical evaluation. WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT?: Due to its limited complexity, T and Protrusion (TAP) technique is considered the to-go technique for bifurcation lesions with good long-term results. Recently, double kissing crush (DKC) technique has been gaining popularity and demonstrated promising results in randomized clinical trials. WHAT DOES THIS STUDY ADD?: This in-vitro bench test study provides a unique detailed OCT comparison and local hemodynamic environment analysis of the two techniques. HOW MIGHT THIS IMPACT ON CLINICAL PRACTICE?: New insights of acute thrombogenicity and computational flow model simulation may guide percutaneous therapeutic strategies of bifurcation lesions.

Platelet activation via dynamic conformational changes of von Willebrand factor under shear

Shear-induced conformational changes of von Willebrand factor (VWF) play an important role in platelet activation. A novel approach describing VWF unfolding on the platelet surface under dynamic shear stress is proposed. Cumulative effect of dynamic shear on platelet activation via conformational changes of VWF is analysed. The critical condition of shear-induced platelet activation is formulated. The explicit expression for the threshold value of cumulative shear stress as a function of VWF multimer size is derived. The results open novel prospects for pharmacological regulation of shear-induced platelet activation through control of VWF multimers size distribution.

Thrombus growth modelling and stenosis prediction in the cerebral microvasculature

Cerebral microvascular occlusions cause restriction of blood supply to the brain, thus potentially severely impacting cognitive abilities. Thus, accurate prediction of thrombus growth in realistic geometries is important. Thrombi growth in an existing 13-generation cerebral microvasculature network is simulated here to study the haemodynamic effects of single and multiple blockages on the occlusion of the network. Compared to a single vessel, in a network, the occlusion probability is found to be different. It is the downstream/smaller arterioles (i.e. the 3rd, 4th, 5th, 6th generation arterioles in this study) that tend to reach occlusion first in a network and thus are the critical vessels. Simulations of simultaneous growth of two independent thrombi in the network (referred to here as the two-block case) show a close coupling between the locations of the various blocks in the network, each influencing the other's growth. The presence of the lead block (LB) slows the growth of the trailing block (TB). In some cases, it stops the TB's growth thereby preventing it from occluding the vessel. Findings in this work thus indicate that, to prevent ischaemia, blocks in the smaller arterioles need to be identified and treated first, and that this is more critical if the number of simultaneous blocks is higher.

In Vitro models for thrombogenicity testing of blood-recirculating medical devices

INTRODUCTION

Blood-recirculating medical devices, such as mechanical circulatory support (MCS), extracorporeal membrane oxygenators (ECMO), and hemodialyzers, are commonly used to treat or improve quality of life in patients with cardiac, pulmonary, and renal failure, respectively. As part of their regulatory approval, guidelines for thrombosis evaluation in pre-clinical development have been established. In vitro testing evaluates a device's potential to produce thrombosis markers in static and dynamic flow loops.

AREAS COVERED

This review focuses on in vitro static and dynamic models to assess thrombosis in blood-recirculating medical devices. A summary of key devices is followed by a review of molecular markers of contact activation. Current thrombosis testing guidance documents, ISO 10993-4, ASTM F-2888, and F-2382 will be discussed, followed by analysis of their application to in vitro testing models.

EXPERT OPINION

In general, researchers have favored in vivo models to thoroughly evaluate thrombosis, limiting in vitro evaluation to hemolysis. In vitro studies are not standardized and it is often difficult to compare studies on similar devices. As blood-recirculating devices have advanced to include wearable and implantable artificial organs, expanded guidelines standardizing in vitro testing are needed to identify the thrombotic potential without excessive use of in vivo resources during pre-clinical development.

Investigation of the influence of fluid dynamics on thrombus growth at the interface between a connector and tube

Thrombus formation at the interface between connectors and tubes is a potential risk factor for complications. We investigated time-dependent relationships between formation of thrombus and hemodynamic factors at the interface between connectors and tubes using optical coherence tomography (OCT) under pulsatile flow. A swept-source OCT with the center wavelength of 1330 nm was employed. The sequential process of thrombus formation at the interface of connectors and tubes in the inlet and outlet was investigated. Connectors with and without tapers were tested using identical 50-ml air-contactless circuits. Fresh human blood from healthy volunteers was circulated under pulsatile flow. Thrombus initially formed at the interface between the connector tip and the tube. Geometries of thrombus growth were different between the 2 connectors, and between the inlet and the outlet. Growth of thrombus was observed at the interface between the connectors and tubes over time in 60 min circulation, except at the outlet part of connector without tapers. At the connector without tapers outlet, thrombus propagation length from the connector edge toward the flow downstream was comparable at 10 and 60 min (0.55 ± 0.35 vs. 0.51 ± 0.32 mm, p = 0.83). Analysis using particle image velocimetry showed the presence of a flow reattachment point 1.5 mm downstream from the connector edge. These results suggest that the flow reattachment point inhibits downstream thrombus growth. We quantitatively demonstrated sequential thrombus process at the interface between the connectors and tubes under pulsatile flow of human blood using OCT.

Prediction of Thrombus Growth: Effect of Stenosis and Reynolds Number

Shear stresses play a major role in platelet-substrate interactions and thrombus formation and growth in blood flow, where under both pathological and physiological conditions platelet adhesion and accumulation occur. In this study, a shear-dependent continuum model for platelet activation, adhesion and aggregation is presented. The model was first verified under three different shear conditions and at two heparin levels. Three-dimensional simulations were then carried out to evaluate the performance of the model for severely damaged (stripped) aortas with mild and severe stenosis degrees in laminar flow regime. For these cases, linear shear-dependent functions were developed for platelet-surface and platelet-platelet adhesion rates. It was confirmed that the platelet adhesion rate is not only a function of Reynolds number (or wall shear rate) but also the stenosis severity of the vessel. General correlations for adhesion rates of platelets as functions of stenosis and Reynolds number were obtained based on these cases. Finally using the new platelet adhesion rates, the model was applied to different experimental systems and shown to agree well with measured platelet deposition.

A General Shear-Dependent Model for Thrombus Formation

Modeling the transport, activation, and adhesion of platelets is crucial in predicting thrombus formation and growth following a thrombotic event in normal or pathological conditions. We propose a shear-dependent platelet adhesive model based on the Morse potential that is calibrated by existing invivo and invitro experimental data and can be used over a wide range of flow shear rates (100<γ˙<28,000s-1). We introduce an Eulerian-Lagrangian model where hemodynamics is solved on a fixed Eulerian grid, while platelets are tracked using a Lagrangian framework. A force coupling method is introduced for bidirectional coupling of platelet motion with blood flow. Further, we couple the calibrated platelet aggregation model with a tissue-factor/contact pathway coagulation cascade, representing the relevant biology of thrombin generation and the subsequent fibrin deposition. The range of shear rates covered by the proposed model encompass venous and arterial thrombosis, ranging from low-shear-rate conditions in abdominal aortic aneurysms and thoracic aortic dissections to thrombosis in stenotic arteries following plaque rupture, where local shear rates are extremely high.

Hemostasis (thrombus formation) is the normal physiological response that prevents significant blood loss after vascular injury. The resulting clots can form under different flow conditions in the veins as well as the arteries. The excessive and undesirable formation of clots (i.e., thrombosis) in our circulatory system may lead to significant morbidity and mortality. Some of these pathologies are deep vein thrombosis and pulmonary embolism and atherothrombosis (thrombosis triggered by plaque rupture) in coronary arteries, to name a few. The process of clot formation and growth at a site on a blood vessel wall involves a number of simultaneous processes including: multiple chemical reactions in the coagulation cascade, species transport and platelet adhesion all of which are strongly influenced by the hydrodynamic forces. Numerical models for blood clotting normally focus on one of the processes under a specific flow condition. Here, we propose a general numerical model that encompass a wide range of hemodynamic conditions in the veins and arteries, with individual platelets and their adhesive dynamics included explicitly in the models. Further, we include the biochemistry of coagulation cascade, which is essential to modeling thrombus formation, and couple that to our platelet aggregation model. The simulation results—tested against three different experiments—demonstrate that the proposed model is effective in capturing the invivo and invitro experimental observations.

Influence of catheter insertion on the hemodynamic environment in coronary arteries

Intravascular stenting is one of the most commonly used treatments to restore the vascular lumen and flow conditions, while perioperative complications such as thrombosis and restenosis are still nagging for patients. As the catheter with crimped stent and folded balloon is directly advanced through coronary artery during surgery, it is destined to cause interference as well as obstructive effect on blood flow. We wonder how the hemodynamic environment would be disturbed and weather these disturbances cause susceptible factors for those complications. Therefore, a realistic three-dimensional model of left coronary artery was reconstructed and blood flow patterns were numerically simulated at seven different stages in the catheter insertion process. The results revealed that the wall shear stress (WSS) and velocity in left anterior descending (LAD) were both significantly increased after catheter inserted into LAD. Besides, the WSS on the catheter, especially at the ending of the catheter, was also at high level. Compared with the condition before catheter inserted, the endothelial cells of LAD was exposed to high-WSS condition and the risk of platelet aggregation in blood flow was increased. These influences may make coronary arteries more vulnerable for perioperative complications.

Mathematical modeling of thrombus formation in idealized models of aortic dissection: initial findings and potential applications

Aortic dissection is a major aortic catastrophe with a high morbidity and mortality risk caused by the formation of a tear in the aortic wall. The development of a second blood filled region defined as the “false lumen” causes highly disturbed flow patterns and creates local hemodynamic conditions likely to promote the formation of thrombus in the false lumen. Previous research has shown that patient prognosis is influenced by the level of thrombosis in the false lumen, with false lumen patency and partial thrombosis being associated with late complications and complete thrombosis of the false lumen having beneficial effects on patient outcomes. In this paper, a new hemodynamics-based model is proposed to predict the formation of thrombus in Type B dissection. Shear rates, fluid residence time, and platelet distribution are employed to evaluate the likelihood for thrombosis and to simulate the growth of thrombus and its effects on blood flow over time. The model is applied to different idealized aortic dissections to investigate the effect of geometric features on thrombus formation. Our results are in qualitative agreement with in-vivo observations, and show the potential applicability of such a modeling approach to predict the progression of aortic dissection in anatomically realistic geometries.

Computational simulation of platelet interactions in the initiation of stent thrombosis due to stent malapposition

Coronary stenting is one of the most commonly used approaches to open coronary arteries blocked due to atherosclerosis. Stent malapposition can induce thrombosis but the microscopic process is poorly understood. The objective of this study was to determine the platelet-level process by which different extents of stent malapposition affect the initiation of stent thrombosis. We utilized a discrete element model to computationally simulate the transport, adhesion, and activation of thousands of individual platelets and red blood cells during thrombus initiation in stented coronary arteries. Simulated arteries contained a malapposed stent with a specified gap distance (0, 10, 25, 50, or 200 μm) between the struts and endothelium. Platelet-level details of thrombus formation near the proximal-most strut were measured during the simulations. The relationship between gap distance and amount of thrombus in the artery varied depending on different conditions (e.g., amount of dysfunctional endothelium, shear-induced activation of platelets, and thrombogenicity of the strut). Without considering shear-induced platelet activation, the largest gap distance (200 μm) produced no recirculation and less thrombus than the smallest two gap distances (0 and 10 μm) that created recirculation downstream of the strut. However, with the occurrence of shear-induced platelet activation, the largest gap distance produced more thrombus than the two smallest gap distances, but less thrombus than an intermediate gap distance (25 μm). A large gap distance was not necessarily the most thrombogenic, in contrast to implications of some computational fluid dynamics studies. The severity of stent malapposition affected initial stent thrombosis differently depending on various factors related to fluid recirculation, platelet trajectories, shear stress, and endothelial condition.

Simulation of the microscopic process during initiation of stent thrombosis

OBJECTIVE

Coronary stenting is one of the most commonly used approaches to open coronary arteries blocked due to atherosclerosis. However, stent struts can induce stent thrombosis due to altered hemodynamics and endothelial dysfunction, and the microscopic process is poorly understood. The objective of this study was to determine the microscale processes during the initiation of stent thrombosis.

METHODS

We utilized a discrete element computational model to simulate the transport, collision, adhesion, and activation of thousands of individual platelets and red blood cells in thrombus formation around struts and dysfunctional endothelium.

RESULTS

As strut height increased, the area of endothelium activated by low shear stress increased, which increased the number of platelets in mural thrombi. These thrombi were generally outside regions of recirculation for shorter struts. For the tallest strut, wall shear stress was sufficiently low to activate the entire endothelium. With the entire endothelium activated by injury or denudation, the number of platelets in mural thrombi was largest for the shortest strut. The type of platelet activation (by high shear stress or contact with activated endothelium) did not greatly affect results.

CONCLUSIONS

During the initiation of stent thrombosis, platelets do not necessarily enter recirculation regions or deposit on endothelium near struts, as suggested by previous computational fluid dynamics simulations. Rather, platelets are more likely to deposit on activated endothelium outside recirculation regions and deposit directly on struts. Our study elucidated the effects of different mechanical factors on the roles of platelets and endothelium in stent thrombosis.