Mechanisms of Platelet Capture Under Very High Shear

Material strengths of shear-induced platelet aggregation clots and coagulation clots

Arterial occlusion by thrombosis is the immediate cause of some strokes, heart attacks, and peripheral artery disease. Most prior studies assume that coagulation creates the thrombus. However, a contradiction arises as whole blood (WB) clots from coagulation are too weak to stop arterial blood pressures (> 150 mmHg). We measure the material mechanical properties of elasticity and ultimate strength for Shear-Induced Platelet Aggregation (SIPA) type clots, that form under stenotic arterial hemodynamics in comparison with coagulation clots. The ultimate strength of SIPA clots averaged 4.6 ± 1.3 kPa, while WB coagulation clots had a strength of 0.63 ± 0.3 kPa (p < 0.05). The elastic modulus of SIPA clots was 3.8 ± 1.5 kPa at 1 Hz and 0.5 mm displacement, or 2.8 times higher than WB coagulation clots (1.3 ± 1.2 kPa, p < 0.0001). This study shows that the SIPA thrombi, formed quickly under high shear hemodynamics, is seven-fold stronger and three-fold stiffer compared to WB coagulation clots. A force balance calculation shows a SIPA clot has the strength to resist arterial pressure with a short length of less than 2 mm, consistent with coronary pathology.

Platelet Biorheology and Mechanobiology in Thrombosis and Hemostasis: Perspectives from Multiscale Computation

Thrombosis is the pathological clot formation under abnormal hemodynamic conditions, which can result in vascular obstruction, causing ischemic strokes and myocardial infarction. Thrombus growth under moderate to low shear (<1000 s-1) relies on platelet activation and coagulation. Thrombosis at elevated high shear rates (>10,000 s-1) is predominantly driven by unactivated platelet binding and aggregating mediated by von Willebrand factor (VWF), while platelet activation and coagulation are secondary in supporting and reinforcing the thrombus. Given the molecular and cellular level information it can access, multiscale computational modeling informed by biology can provide new pathophysiological mechanisms that are otherwise not accessible experimentally, holding promise for novel first-principle-based therapeutics. In this review, we summarize the key aspects of platelet biorheology and mechanobiology, focusing on the molecular and cellular scale events and how they build up to thrombosis through platelet adhesion and aggregation in the presence or absence of platelet activation. In particular, we highlight recent advancements in multiscale modeling of platelet biorheology and mechanobiology and how they can lead to the better prediction and quantification of thrombus formation, exemplifying the exciting paradigm of digital medicine.

A computational investigation of occlusive arterial thrombosis

The generation of occlusive thrombi in stenotic arteries involves the rapid deposition of millions of circulating platelets under high shear flow. The process is mediated by the formation of molecular bonds of several distinct types between platelets; the bonds capture the moving platelets and stabilize the growing thrombi under flow. We investigated the mechanisms behind occlusive thrombosis in arteries with a two-phase continuum model. The model explicitly tracks the formation and rupture of the two types of interplatelet bonds, the rates of which are coupled with the local flow conditions. The motion of platelets in the thrombi results from competition between the viscoelastic forces generated by the interplatelet bonds and the fluid drag. Our simulation results indicate that stable occlusive thrombi form only under specific combinations for the ranges of model parameters such as rates of bond formation and rupture, platelet activation time, and number of bonds required for platelet attachment.

Dose-Dependent Von Willebrand Factor Inhibition by Aptamer BB-031 Correlates with Thrombolysis in a Microfluidic Model of Arterial Occlusion

Von Willebrand Factor (VWF) plays a critical role in thrombus formation, stabilization, and propagation. Previous studies have demonstrated that targeted inhibition of VWF induces thrombolysis when administered in vivo in animal models of ischemic stroke. The study objective was to quantify dose-dependent inhibition of VWF-platelet function and its relationship with thrombolysis using BB-031, an aptamer that binds VWF and inhibits its function. VWF:Ac, VWF:RCo, T-TAS, and ristocetin-induced impedance aggregometry were used to assess BB-031-mediated inhibition of VWF. Reductions in original thrombus surface area and new deposition during administration of treatment were measured in a microfluidic model of arterial thrombolysis. Rotational thromboelastometry was used to assess changes in hemostasis. BB-031 induced maximal inhibition at the highest dose (3384 nM) in VWF:Ac, and demonstrated dose-dependent responses in all other assays. BB-031, but not vehicle, induced recanalization in the microfluidic model. Maximal lytic efficacy in the microfluidic model was seen at 1692 nM and not 3384 nM BB-031 when assessed by surface area. Minor changes in ROTEM parameters were seen at 3384 nM BB-031. Targeted VWF inhibition by BB-031 results in clinically measurable impairment of VWF function, and specifically VWF-GPIb function as measured by VWF:Ac. BB-031 also induced thrombolysis as measured in a microfluidic model of occlusion and reperfusion. Moderate correlation between inhibition and lysis was observed. Additional studies are required to further examine off-target effects of BB-031 at high doses, however, these are expected to be above the range of clinical targeted dosing.

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.

SIPA in 10 milliseconds: VWF tentacles agglomerate and capture platelets under high shear

Agglomeration and capture of agglomerates after travelling a lag distance of >100 µm creates SIPA as fast as 10 milliseconds.

Phase diagrams of SIPA controlled by VWF length and concentration provide mechanistic insights for various thrombotic and hemostatic events.

Shear-induced platelet aggregation (SIPA) occurs under elevated shear rates (10 000 s⁻¹) found in stenotic coronary and carotid arteries. The pathologically high shear environment can lead to occlusive thrombosis by SIPA from the interaction of nonactivated platelets and von Willebrand factor (VWF) via glycoprotein Ib–A1 binding. This process under high shear rates is difficult to visualize experimentally with concurrent molecular- and cellular-resolutions. To understand this fast bonding, we employ a validated multiscale in silico model incorporating measured molecular kinetics and a thrombosis-on-a-chip device to delineate the flow-mediated biophysics of VWF and platelets assembly into mural microthrombi. We show that SIPA begins with VWF elongation, followed by agglomeration of platelets in the flow by soluble VWF entanglement before mural capture of the agglomerate by immobilized VWF. The entire SIPA process occurs on the order of 10 milliseconds with the agglomerate traveling a lag distance of a few hundred microns before capture, matching in vitro results. Increasing soluble VWF concentration by ∼20 times in silico leads to a ∼2 to 3 times increase in SIPA rates, matching the increase in occlusion rates found in vitro. The morphology of mural aggregates is primarily controlled by VWF molecular weight (length), where normal-length VWF leads to cluster or elongated aggregates and ultra-long VWF leads to loose aggregates seen by others’ experiments. Finally, we present phase diagrams of SIPA, which provides biomechanistic rationales for a variety of thrombotic and hemostatic events in terms of platelet agglomeration and capture.

In Vitro Measurements of Shear-Mediated Platelet Adhesion Kinematics as Analyzed through Machine Learning

Platelet adhesion to blood vessel walls in shear flow is essential to initiating the blood coagulation cascade and prompting clot formation in vascular disease processes and prosthetic cardiovascular devices. Validation of predictive adhesion kinematics models at the single platelet level is difficult due to gaps in high resolution, dynamic morphological data or a mismatch between simulation and experimental parameters. Gel-filtered platelets were perfused at 30 dyne/cm² in von Willebrand Factor (vWF)-coated microchannels, with flipping platelets imaged at high spatial and temporal resolution. A semi-unsupervised learning system (SULS), consisting of a series of convolutional neural networks, was used to segment platelet geometry, which was compared with expert-analyzed images. Resulting time-dependent rotational angles were smoothed with wavelet-denoising and shifting techniques to characterize the rotational period and quantify flipping kinematics. We observed that flipping platelets do not follow the previously-modeled modified Jefferey orbit, but are characterized by a longer lift-off and shorter reattachment period. At the juncture of the two periods, rotational velocity approached 257.48 ± 13.31 rad/s. Our SULS approach accurately segmented large numbers of moving platelet images to identify distinct adhesive kinematic characteristics which may further validate the physical accuracy of individual platelet motion in multiscale models of shear-mediated thrombosis.

Computational investigation of platelet thrombus mechanics and stability in stenotic channels

The stability of a platelet thrombus under flow is believed to depend strongly on the local hemodynamics and on the thrombus' porosity, permeability, and elasticity. A two-phase continuum model is used to investigate the biomechanics of thrombus stability in stenotic channels. It treats the thrombus as a porous, viscoelastic material moving differently than the background fluid. The dynamic clot-flow interaction is modeled through a frictional drag term. The model explicitly tracks the formation and breaking of interplatelet molecular bonds, which directly determine the viscoelastic property of the thrombus and govern its ability to resist fluid drag. We characterize the stability/fragility of thrombi for various flow speeds, porosities, bond concentrations, and bond types.

Mechanobiology of shear-induced platelet aggregation leading to occlusive arterial thrombosis: A multiscale in silico analysis

Occlusive thrombosis in arteries causes heart attacks and strokes. The rapid growth of thrombus at elevated shear rates (~10,000 1/s) relies on shear-induced platelet aggregation (SIPA) thought to come about from the entanglement of von Willebrand factor (VWF) molecules. The mechanism for SIPA is not yet understood in terms of cell- and molecule-level dynamics in fast flowing bloodstreams. Towards this end, we develop a multiscale computational model to recreate SIPA in silico, where the suspension dynamics and interactions of individual platelets and VWF multimers are resolved directly. The platelet-VWF interaction via GP1b-A1 bonds is prescribed with intrinsic binding rates theoretically derived and informed by single-molecule measurements. The model is validated against existing microfluidic SIPA experiments, showing good agreement with the in vitro observations in terms of the morphology, traveling distance and capture time of the platelet aggregates. Particularly, the capture of aggregates can occur in a few milliseconds, comparable to the platelet transit time through pathologic arterial stenotic sections and much shorter than the time for shear-induced platelet activation. The multiscale SIPA simulator provides a cross-scale tool for exploring the biophysical mechanisms of SIPA in silico that are difficult to access with single-molecule measurements or micro-/macro-fluidic assays only.

Advances in extracorporeal membrane oxygenator design for artificial placenta technology

Extreme prematurity, defined as a gestational age of fewer than 28 weeks, is a significant health problem worldwide. It carries a high burden of mortality and morbidity, in large part due to the immaturity of the lungs at this stage of development. The standard of care for these patients includes support with mechanical ventilation, which exacerbates lung pathology. Extracorporeal life support (ECLS), also called artificial placenta technology when applied to extremely preterm (EPT) infants, offers an intriguing solution. ECLS involves providing gas exchange via an extracorporeal device, thereby doing the work of the lungs and allowing them to develop without being subjected to injurious mechanical ventilation. While ECLS has been successfully used in respiratory failure in full-term neonates, children, and adults, it has not been applied effectively to the EPT patient population. In this review, we discuss the unique aspects of EPT infants and the challenges of applying ECLS to these patients. In addition, we review recent progress in artificial placenta technology development. We then offer analysis on design considerations for successful engineering of a membrane oxygenator for an artificial placenta circuit. Finally, we examine next-generation oxygenators that might advance the development of artificial placenta devices.

Lysis of arterial thrombi by perfusion of N,N'-Diacetyl-L-cystine (DiNAC)

The search persists for a safe and effective agent to lyse arterial thrombi in the event of acute heart attacks or strokes due to thrombotic occlusion. The culpable thrombi are composed either primarily of platelets and von Willebrand Factor (VWF), or polymerized fibrin, depending on the mechanism of formation. Current thrombolytics were designed to target red fibrin-rich clots, but may be not be efficacious on white VWF-platelet-rich arterial thrombi. We have developed an in vitro system to study the efficacy of known and proposed thrombolytic agents on white clots formed from whole blood in a stenosis with arterial conditions. The agents and adjuncts tested were tPA, ADAMTS-13, abciximab, N-acetyl cysteine, and N,N'-Diacetyl-L-cystine (DiNAC). Most of the agents, including tPA, had little thrombolytic effect on the white clots. In contrast, perfusion of DiNAC lysed thrombi as quickly as 1.5 min, which ranged up to 30 min at lower concentrations, and resulted in an average reduction in surface area of 71 ± 20%. The clot burden was significantly reduced compared to both tPA and a saline control (p<0.0001). We also tested the efficacy of all agents on red fibrinous clots formed in stagnant conditions. DiNAC did not lyse red clots, whereas tPA significantly lysed red clot over 48 h (p<0.01). These results lead to a novel use for DiNAC as a possible thrombolytic agent against acute arterial occlusions that could mitigate the risk of hyper-fibrinolytic bleeding.

Haemodynamic flow conditions at the initiation of high-shear platelet aggregation: a combined in vitro and cellular in silico study

The influence of the flow environment on platelet aggregation is not fully understood in high-shear thrombosis. The objective of this study is to investigate the role of a high shear rate in initial platelet aggregation. The haemodynamic conditions in a microfluidic device are studied using cell-based blood flow simulations. The results are compared with in vitro platelet aggregation experiments performed with porcine whole blood (WB) and platelet-rich-plasma (PRP). We studied whether the cell-depleted layer in combination with high shear and high platelet flux can account for the distribution of platelet aggregates. High platelet fluxes at the wall were found in silico. In WB, the platelet flux was about twice as high as in PRP. Additionally, initial platelet aggregation and occlusion were observed in vitro in the stenotic region. In PRP, the position of the occlusive thrombus was located more downstream than in WB. Furthermore, the shear rates and stresses in cell-based and continuum simulations were studied. We found that a continuum simulation is a good approximation for PRP. For WB, it cannot predict the correct values near the wall.

Clot Permeability, Agonist Transport, and Platelet Binding Kinetics in Arterial Thrombosis

The formation of wall-adherent platelet aggregates is a critical process in arterial thrombosis. A growing aggregate experiences frictional drag forces exerted on it by fluid moving over or through the aggregate. The magnitude of these forces is strongly influenced by the permeability of the developing aggregate; the permeability depends on the aggregate's porosity. Aggregation is mediated by formation of ensembles of molecular bonds; each bond involves a plasma protein bridging the gap between specific receptors on the surfaces of two different platelets. The ability of the bonds existing at any time to sustain the drag forces on the aggregate determines whether it remains intact or sheds individual platelets or larger fragments (emboli). We investigate platelet aggregation in coronary-sized arteries using both computational simulations and in vitro experiments. The computational model tracks the formation and breaking of bonds between platelets and treats the thrombus as an evolving porous, viscoelastic material, which moves differently from the background fluid. This relative motion generates drag forces which the fluid and thrombus exert on one another. These forces are computed from a permeability-porosity relation parameterized from experimental measurements. Basing this relation on measurements from occlusive thrombi formed in our flow chamber experiments, along with other physiological parameter values, the model produced stable dense thrombi on a similar timescale to the experiments. When we parameterized the permeability-porosity relation using lower permeabilities reported by others, bond formation was insufficient to balance drag forces on an early thrombus and keep it intact. Under high shear flow, soluble agonist released by platelets was limited to the thrombus and a boundary layer downstream, thus restricting thrombus growth into the vessel lumen. Adding to the model binding and activation of unactivated platelets through von Willebrand-factor-mediated processes allowed greater growth and made agonist-induced activation more effective.

Platelet α-granules are required for occlusive high-shear-rate thrombosis

von Willebrand factor (VWF) is essential for the induction of arterial thrombosis. In this study, we investigated the critical role of platelet VWF in occlusive thrombosis formation at high shear in mice that do not express platelet VWF (Nbeal2-/-). Using in silico modeling, in vitro high-shear microfluidics, and an in vivo Folts model of arterial thrombosis we reproduced the platelet dynamics that occur under pathological flow in a stenosed vessel. Computational fluid dynamics (CFDs) simulated local hemodynamics in a stenosis based on arterial geometries. The model predicted shear rates, time course of platelet adhesion, and time to occlusion. These predictions were validated in vitro and in vivo. Occlusive thrombosis developed in wild-type control mice that had normal levels of plasma VWF and platelet VWF in vitro and in vivo. Occlusive thrombosis did not form in the Nbeal2-/- mice that had normal plasma VWF and an absence of platelet VWF. Occlusive thrombosis was corrected in Nbeal2-/- microfluidic assays by the addition of exogenous normal platelets with VWF. Combining model and experimental data, we demonstrated the necessary requirement of platelet VWF in α-granules in forming an occlusive thrombus under high shear. These results could inspire new pharmacological targets specific to pathological conditions and prevent arterial thrombosis.

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.

Occlusive thrombosis in arteries

Thrombus formation in major arteries is life threatening. In this review article, we discuss how an arterial thrombus can form under pathologically high shear stresses, with bonding rates estimated to be the fastest Kon values in biochemistry. During occlusive thrombosis in arteries, the growth rate of the thrombus explodes to capture a billion platelets in about 10 min. Close to 100% of all platelets passing the thrombus are captured by long von Willebrand factor (vWF) strands that quickly form tethered nets. The nets grow in patches where shear stress is high, and the local concentration of vWF is elevated due to α-granule release by previously captured platelets. This rapidly formed thrombus has few red blood cells and so has a white appearance and is much stronger and more porous than clots formed through coagulation. Understanding and modeling the biophysics of this event can predict totally new approaches to prevent and treat heart attacks and strokes.

Identifying the start of a platelet aggregate by the shear rate and the cell-depleted layer

Computer simulations were performed to study the transport of red blood cells and platelets in high shear flows, mimicking earlier published in vitro experiments in microfluidic devices with high affinity for platelet aggregate formation. The goal is to understand and predict where thrombus formation starts. Additionally, the need of cell-based modelling in these microfluidic devices is demonstrated by comparing our results with macroscopic models, wherein blood is modelled as a continuous fluid. Hemocell, a cell-based blood flow simulation framework is used to investigate the transport physics in the microfluidic devices. The simulations show an enlarged cell-depleted layer at the site where a platelet aggregate forms in the experiments. In this enlarged cell-depleted layer, the probability to find a platelet is higher than in the rest of the microfluidic device. In addition, the shear rates are sufficiently high to allow for the von Willebrand factor to elongate in this region. We hypothesize that the enlarged cell-depleted layer combined with a sufficiently large platelet flux and sufficiently high shear rates result in an haemodynamic environment that is a preferred location for initial platelet aggregation.

Accumulations of von Willebrand factor within ECMO oxygenators: Potential indicator of coagulation abnormalities in critically ill patients?

Clot formation within membrane oxygenators (MOs) remains a critical problem during extracorporeal membrane oxygenation (ECMO). The composition of the clots-in particular, the presence of von Willebrand factor (vWF)-may be an indicator for prevalent nonphysiological flow conditions, foreign body reactions, or coagulation abnormalities in critically ill patients. Mats of interwoven gas exchange fibers from randomly collected MOs (PLS, Maquet, Rastatt, Germany) of 21 patients were stained with antibodies (anti-vWF and anti-P-selectin) and counterstained with 4',6-diamidino-2-phenylindole. The extent of vWF-loading was correlated with patient and technical data. While 12 MOs showed low vWF-loadings, 9 MOs showed high vWF-loading with highest accumulations close to crossing points of adjacent gas fibers. The presence and the extent of vWF-fibers/"cobwebs," leukocytes, platelet-leukocyte aggregates (PLAs), and P-selectin-positive platelet aggregates were independent of the extent of vWF-loading. However, the highly loaded MOs were obtained from patients with a significantly elevated SOFA score, severe thrombocytopenia, and persistent liver dysfunction. The coagulation abnormalities of these critically ill patients may cause an accumulation of the highly thrombogenic and elongated high-molecular-weight vWF multimers in the plasma which will be trapped in the MOs during the ECMO therapy.

Inhibition of high shear arterial thrombosis by charged nanoparticles

Platelet accumulation under high shear rates at the site of atherosclerotic plaque rupture leads to myocardial infarction and stroke. Current antiplatelet therapies remain ineffective within a large percentage of the population, while presenting significant risks for bleeding. We explore a novel way to inhibit arterial thrombus formation by biophysical means without the use of platelet inactivating drugs. Our computational multi-scale dynamics model has predicted that charged particles of a specific size may entangle von Willebrand Factor (vWF) polymers and reduce the amount of elongation at high shear rates. We tested this hypothesis experimentally for negatively charged nanoparticles (CNP) to inhibit arterial thrombus formation. CNP of a particular size and charge inhibited thrombus formation, with a 10-fold peak inhibition over control conditions of thrombotic occlusion. Particles of differing material composition, size, and charge had little effect as predicted by computational studies. Surprisingly, the dose response curve was not sigmoidal, but exhibited a peak at 1.5 CNP:vWF proteins, which was not predicted by the model. This study describes a new antithrombotic agent that may have a different mechanism of action than current pharmaceutical therapies.

Choice of a hemodynamic model for occlusive thrombosis in arteries

Intravascular thrombosis can lead to heart attacks and strokes that together are the leading causes of death in the US (Kochanek, K.D., Murphy, S.L., Xu, J.Q., 2014). The ability to identify the offending biofluid mechanical conditions and predict the timescale of thrombotic occlusion in vessels and devices may improve patient outcomes. A computational model was developed to describe the growth of thrombus based on the local hemodynamic shear rate. The model predicts thrombus deposition based on initial geometric and fluid mechanical conditions, which are updated throughout the simulation to reflect the changing lumen dimensions. Thrombus growth and occlusion from whole blood was measured in in vitro experiments using stenotic glass capillary tubes, a PDMS microfluidic channel, and a PTFE stenotic aorto-iliac graft. Comparison of the predicted occlusion times to experimental results shows excellent agreement. The results indicate that local shear rate plays a critical role in acute thrombosis, and that hemodynamic characterization may have clinical utility.

The function of ultra-large von Willebrand factor multimers in high shear flow controlled by ADAMTS13

The paradigm that platelet aggregation, which contributes to bleeding arrest and also to thrombovascular disorders, initiates after signaling-induced platelet activation has been refuted in past recent years. Platelets can form aggregates independently of activation when soluble von Willebrand factor (VWF) is present and the shear rate exceeds a certain threshold where active A1 domains become exposed in soluble VWF multimers and can bind to platelet glycoprotein Ib. Subsequently - fostering each other - VWF can self-assemble into large nets combining with platelets into large conglomerates, which are entirely reversible when they enter a flow region with shear rates below the threshold. In addition the threshold changes from approximately 20 000 s⁻¹ in wall parallel flow to approximately 10 000 s⁻¹ in stagnation point flow. VWF containing ultra-large multimers - as when just released from endothelial storage sites - has been shown to have the highest binding potential to platelets and to each other, thus facilitating rapid platelet accrual to sites of vessel injury and exposed subendothelial structures, i.e. collagen. The VWF nets as well as the platelet-VWF conglomerates are controlled by the cleaving protease ADAMTS13 within minutes under high shear flow. Therewith the hemostatic potential is delivered where needed and the thrombogenic potential is highly controlled twofold: by flow and enzymatic proteolytic cleavage.

Platelet transport rates and binding kinetics at high shear over a thrombus

Thrombus formation over a ruptured atherosclerotic plaque cap can occlude an artery with fatal consequences. We describe a computational model of platelet transport and binding to interpret rate-limiting steps seen in experimental thrombus formation over a collagen-coated stenosis. The model is used to compute shear rates in stenoses with growing boundaries. In the model, moving erythrocytes influence platelet transport based on shear-dependent enhanced diffusivity and a nonuniform platelet distribution. Adhesion is modeled as platelet-platelet binding kinetics. The results indicate that observed thrombus growth rates are limited by platelet transport to the wall for shear rates up to 6000 s(-1). Above 7000 s(-1), the thrombus growth rate is likely limited by binding kinetics (10(-4) m/s). Thrombus growth computed from these rate-limiting steps match the thrombus location and occlusion times for experimental conditions if a lag time for platelet activation is included. Using fitted parameters, the model is then used to predict thrombus size and shape at a higher Reynolds number flow consistent with coronary artery disease.

A low-volume, single pass in-vitro system of high shear thrombosis in a stenosis

INTRODUCTION

Arterial thrombosis leading to heart attack and stroke requires the rapid accumulation of millions of platelets under pathologically high shear. Previous in vitro systems studying platelets typically use endpoints that emphasize platelet-surface effects rather than large-scale platelet-platelet accumulation that precedes occlusion. Further, most platelet tests do not recreate shear rates present during arterial occlusion. We present an alternative flow system to study large thrombus formation under pathologic shear conditions in an anatomic stenosis with reasonable volumes of human blood.

MATERIALS AND METHODS

An in-vitro system using a syringe pump was created to subject low volume (<30 mLs), whole blood samples to very high shear rates (>3,500 s(-1)) through a stenosis. Thrombus was quantified using an optical microscope from initial deposition to large scale accumulation. Images were taken using a high definition camera in real time.

RESULTS AND CONCLUSIONS

Occlusive thrombus blocks the collagen-coated lumen with millions of platelets using human whole, heparinized blood. Rapid Platelet Accumulation rates in human blood are 4.5±2.4 μm(3)/μm(2)/min (n=21). There is an initial lag time of 7.4±3.8 min (n=21) before the onset of large scale thrombosis. The rates of platelet accumulation in vitro are consistent with the clinical timescale of coronary or carotid artery occlusion. Porcine blood has a faster accumulation rate of 9.6±6.1 μm(3)/μm(2)/min (n=7, p<0.05) and a shorter lag time of 2.7±0.5 min (n=7, p<0.05). The long lag time for large thrombus formation suggests that some in-vitro assays will miss the main mechanism creating thrombotic occlusion.