Platelet activation in a circulating flow loop: combined effects of shear stress and exposure time

Research approaches for studying flow-induced thromboembolic complications in blood recirculating devices

The advent of implantable blood recirculating devices has provided life-saving solutions to patients with severe cardiovascular diseases. Recently it has been reported that ventricular assist devices are superior to drug therapy. The implantable total artificial heart is showing promise as a potential solution to the chronic shortage of available heart transplants. Prosthetic heart valves are routinely used for replacing diseased heart valves. However, all of these devices share a common problem--significant complications such as hemolysis and thromboembolism often arise after their implantation. Elevated flow stresses that are present in the nonphysiologic geometries of blood recirculating devices, enhance their propensity to initiate thromboembolism by chronically activating the blood platelets. This, rather than hemolysis, appears to be the salient aspect of blood trauma in devices. Limitations in characterizing and controlling relevant aspects of the flow-induced mechanical stimuli and the platelet response, hampers our ability to achieve design optimization for these devices. The main objective of this article is to describe state-of-the-art numerical, experimental, and in vivo tools, that facilitate elucidation of flow-induced mechanisms leading to thromboembolism in prosthetic devices. Such techniques are giving rise to an accountable model for flow-induced thrombogenicity, and to a methodology that has the potential to transform current device design and testing practices. It might lead to substantial time and cost savings during the research and development phase, and has the potential to reduce the risks that patients implanted with these devices face, lower the ensuing healthcare costs, and offer viable long-term solutions for these patients.

A simplified mathematical model for thrombin generation

A new phenomenological mathematical model based directly on laboratory data for thrombin generation and having a patient-specific character is described. A set of the solved equations for cell-based models of blood coagulation that can reproduce the temporal evolution of thrombin generation is proposed; such equations are appropriate for use in computational fluid dynamic (CFD) simulations. The initial values for the reaction rates are either taken from already existing model or experimental data, or they can obtained from simple reasoning under certain assumptions; it is shown that coefficients can be adjusted in order to fit a range of different thrombin generation curves as derived from thrombin generation assays. The behaviour of the model for different platelet concentration seems to be in good agreement with reported experimental data. It is shown that the reduced set of equations used represents to a good approximation a low-order model of the detailed mechanism and thus it can represent a cost-effective and-case specific mathematical model of coagulation reactions up to thrombin generation.

Platelet size and density affect shear-induced thrombus formation in tortuous arterioles

Thrombosis accounts for 80% of deaths in patients with diabetes mellitus. Diabetic patients demonstrate tortuous microvessels and larger than normal platelets. Large platelets are associated with increased platelet activation and thrombosis, but the physical effects of large platelets in the microscale processes of thrombus formation are not clear. Therefore, the objective of this study was to determine the physical effects of mean platelet volume (MPV), mean platelet density (MPD) and vessel tortuosity on platelet activation and thrombus formation in tortuous arterioles. A computational model of the transport, shear-induced activation, collision, adhesion and aggregation of individual platelets was used to simulate platelet interactions and thrombus formation in tortuous arterioles. Our results showed that an increase in MPV resulted in a larger number of activated platelets, though MPD and level of tortuosity made little difference on platelet activation. Platelets with normal MPD yielded the lowest amount of mural thrombus. With platelets of normal MPD, the amount of mural thrombus decreased with increasing level of tortuosity but did not have a simple monotonic relationship with MPV. The physical mechanisms associated with MPV, MPD and arteriole tortuosity play important roles in platelet activation and thrombus formation.

A novel mathematical model of activation and sensitization of platelets subjected to dynamic stress histories

Blood recirculating devices, such as ventricular assist devices and prosthetic heart valves, are burdened by thromboembolic complications requiring complex and lifelong anticoagulant therapy with its inherent hemorrhagic risks. Pathologic flow patterns occurring in such devices chronically activate platelets, and the optimization of their thrombogenic performance requires the development of flow-induced platelet activation models. However, existing models are based on empirical correlations using the well-established power law paradigm of constant levels of shear stress during certain exposure times as factors for mechanical platelet activation. These models are limited by their range of application and do not account for other relevant phenomena, such as loading rate dependence and platelet sensitization to high stress conditions, which characterize the dynamic flow conditions in devices. These limitations were addressed by developing a new class of phenomenological stress-induced platelet activation models that specifies the rate of platelet activation as a function of the entire stress history and results in a differential equation that can be directly integrated to calculate the cumulative levels of activation. The proposed model reverts to the power law under constant shear stress conditions and is able to describe experimental results in response to a diverse range of highly dynamic stress conditions found in blood recirculating devices. The model was tested in vitro under emulated device flow conditions and correlates well with experimental results. This new model provides a reliable and robust mathematical tool that can be incorporated into computational fluid dynamic studies in order to optimize design, with the goal of improving the thrombogenic performance of blood recirculating devices.

High wall shear stress and spatial gradients in vascular pathology: a review

Cardiovascular pathologies such as intracranial aneurysms (IAs) and atherosclerosis preferentially localize to bifurcations and curvatures where hemodynamics are complex. While extensive knowledge about low wall shear stress (WSS) has been generated in the past, due to its strong relevance to atherogenesis, high WSS (typically >3 Pa) has emerged as a key regulator of vascular biology and pathology as well, receiving renewed interests. As reviewed here, chronic high WSS not only stimulates adaptive outward remodeling, but also contributes to saccular IA formation (at bifurcation apices or outer curves) and atherosclerotic plaque destabilization (in stenosed vessels). Recent advances in understanding IA pathogenesis have shed new light on the role of high WSS in pathological vascular remodeling. In complex geometries, high WSS can couple with significant spatial WSS gradient (WSSG). A combination of high WSS and positive WSSG has been shown to trigger aneurysm initiation. Since endothelial cells (ECs) are sensors of WSS, we have begun to elucidate EC responses to high WSS alone and in combination with WSSG. Understanding such responses will provide insight into not only aneurysm formation, but also plaque destabilization and other vascular pathologies and potentially lead to improved strategies for disease management and novel targets for pharmacological intervention.

Device thrombogenicity emulation: a novel methodology for optimizing the thromboresistance of cardiovascular devices

Thrombotic complications with mechanical circulatory support (MCS) devices remain a critical limitation to their long-term use. Device-induced shear forces may enhance the thrombotic potential of MCS devices through chronic activation of platelets, with a known dose-time response of the platelets to the accumulated stress experienced while flowing through the device-mandating complex, lifelong anticoagulation therapy. To enhance the thromboresistance of these devices for facilitating their long-term use, a universal predictive methodology entitled device thrombogenicity emulation (DTE) was developed. DTE is aimed at optimizing the thromboresistance of any MCS device. It is designed to test device-mediated thrombogenicity, coupled with virtual design modifications, in an iterative approach. This disruptive technology combines in silico numerical simulations with in vitro measurements, by correlating device hemodynamics with platelet activity coagulation markers-before and after iterative design modifications aimed at achieving optimized thrombogenic performance. The design changes are first tested in the numerical domain, and the resultant device conditions are then emulated in a hemodynamic shearing device (HSD) in which platelet activity is measured under device emulated conditions. As such, DTE can be easily incorporated during the device research and development phase-achieving minimization of the device thrombogenicity before prototypes are built and tested thereby reducing the ultimate cost of preclinical and clinical trials. The robust capability of this predictive technology is demonstrated here in various MCS devices. The presented examples indicate the potential of DTE for reducing device thrombogenicity to a level that may obviate or significantly reduce the extent of anticoagulation currently mandated for patients implanted with MCS devices for safe long-term clinical use.

Evaluation of shear stress accumulation on blood components in normal and dysfunctional bileaflet mechanical heart valves using smoothed particle hydrodynamics

Evaluating shear induced hemodynamic complications is one of the major concerns in design of the mechanical heart valves (MHVs). The monitoring of these events relies on both numerical simulations and experimental measurements. Currently, numerical approaches are mainly based on a combined Eulerian-Lagrangian approach. A more straightforward evaluation can be based on the Lagrangian analysis of the whole blood. As a consequence, Lagrangian meshfree methods are more adapted to such evaluation. In this study, smoothed particle hydrodynamics (SPH), a fully meshfree particle method originated to simulate compressible astrophysical flows, is applied to study the flow through a normal and a dysfunctional bileaflet mechanical heart valves (BMHVs). The SPH results are compared with the reference data. The accumulation of shear stress patterns on blood components illustrates the important role played by non-physiological flow patterns and mainly vortical structures in this issue. The statistical distribution of particles with respect to shear stress loading history provides important information regarding the relative number of blood components that can be damaged. This can be used as a measure of the response of blood components to the presence of the valve implant or any implantable medical device. This work presents the first attempt to simulate pulsatile flow through BMHVs using SPH method.

FLOW DYNAMIC COMPARISON BETWEEN RECESSED HINGE AND OPEN PIVOT BI-LEAFLET HEART VALVE DESIGNS

The flow dynamics through the peripheral and hinge regions of a bi-leaflet mechanical heart valve are complex and result in abnormally high shear stresses particularly during the closing phase of the valve function. It has been observed that, the late stages of closure is more significant in the dynamics of platelet activation; therefore, the later stages of closure is simulated by solving the two-dimensional Navier-Stokes equations using an Eulerian Levelset based sharp interface Cartesian grid method. Using a fixed Cartesian mesh incorporating local mesh refinement for solution accuracy and efficiency, the flow through and within a recessed hinge design and an open pivot hinge design is compared. Platelets are modelled as point particles by Lagrangian particle tracking algorithm with one way coupling. A dilute particle flow is assumed and particle-particle interactions are neglected. It was observed that the hinge region of the open pivot valve indicated a lower potential for activation of platelets compared to that in valves with a recessed hinge design.

Design optimization of a mechanical heart valve for reducing valve thrombogenicity-A case study with ATS valve

Patients implanted with mechanical heart valves (MHV) or with ventricular assist devices that use MHV require mandatory lifelong anticoagulation for secondary stroke prevention. We recently developed a novel Device Thrombogenicity Emulator (DTE) methodology that interfaces numerical and experimental approaches to optimize the thrombogenic performance of the device and reduce the bleeding risk associated with anticoagulation therapy. Device Thrombogenicity Emulator uses stress-loading waveforms in pertinent platelet flow trajectories that are extracted from highly resolved numerical simulations and emulates these flow conditions in a programmable hemodynamic shearing device (HSD) by which platelet activity is measured. We have previously compared two MHV, ATS and the St. Jude Medical, and demonstrated that owing to its nonrecessed hinge design, the ATS valve offers improved thrombogenic performance. In this study, we further optimize the ATS valve thrombogenic performance, by modifying various design features of the valve, intended to achieve reduced thrombogenicity: 1) optimizing the leaflet-housing gap clearance; 2) increasing the effective maximum opening angle of the valve; and 3) introducing a streamlined channel between the leaflet stops of the valve that increases the effective flow area. We have demonstrated that the DTE optimization methodology can be used as test bed for developing devices with significantly improved thombogenic performance.

Quantification of particle residence time in abdominal aortic aneurysms using magnetic resonance imaging and computational fluid dynamics

Hemodynamic conditions are hypothesized to affect the initiation, growth, and rupture of abdominal aortic aneurysms (AAAs), a vascular disease characterized by progressive wall degradation and enlargement of the abdominal aorta. This study aims to use magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) to quantify flow stagnation and recirculation in eight AAAs by computing particle residence time (PRT). Specifically, we used gadolinium-enhanced MR angiography to obtain images of the vessel lumens, which were used to generate subject-specific models. We also used phase-contrast MRI to measure blood flow at supraceliac and infrarenal locations to prescribe physiologic boundary conditions. CFD was used to simulate pulsatile flow, and PRT, particle residence index, and particle half-life of PRT in the aneurysms were computed. We observed significant regional differences of PRT in the aneurysms with localized patterns that differed depending on aneurysm geometry and infrarenal flow. A bulbous aneurysm with the lowest mean infrarenal flow demonstrated the slowest particle clearance. In addition, improvements in particle clearance were observed with increase of mean infrarenal flow. We postulate that augmentation of mean infrarenal flow during exercise may reduce chronic flow stasis that may influence mural thrombus burden, degradation of the vessel wall, and aneurysm growth.

On the Representation of Turbulent Stresses for Computing Blood Damage

Computational prediction of blood damage has become a crucial tool for evaluating blood-wetted medical devices and pathological hemodynamics. A difficulty arises in predicting blood damage under turbulent flow conditions because the total stress is indeterminate. Common practice uses the Reynolds stress as an estimation of the total stress causing damage to the blood cells. This study investigates the error introduced by making this substitution, and further shows that energy dissipation is a more appropriate metric of blood trauma.

Ensembles of uncertain mathematical models can identify network response to therapeutic interventions

The role of mechanistic modeling and systems biology in molecular medicine remains unclear. In this study, we explored whether uncertain models could be used to understand how a network responds to a therapeutic intervention. As a proof of concept, we modeled and analyzed the response of the human coagulation cascade to recombinant factor VIIa (rFVIIa) and prothrombin (fII) addition in normal and hemophilic plasma. An ensemble of parametrically uncertain human coagulation models was developed (N = 437). Each model described the time evolution of 193 proteins and protein complexes interconnected by 301 interactions under quiescent flow. The 467 unknown model parameters were estimated, using multiobjective optimization, from published in vitro coagulation studies. The model ensemble was validated using published in vitro thrombin measurements and thrombin measurements taken from coronary artery disease patients. Sensitivity analysis was then used to rank-order the importance of model parameters as a function of experimental or physiological conditions. A novel strategy for the systematic comparison of ranks identified a family of fX/FXa and fII/FIIa interactions that became more sensitive with decreasing fVIII/fIX. The fragility of these interactions was preserved following the addition of exogenous rFVIIa and fII. This suggested that exogenous rFVIIa did not alter the qualitative operation of the cascade. Rather, exogenous rFVIIa and fII took advantage of existing fluid and interfacial fX/FXa and fII/FIIa sensitivity to restore normal coagulation in low fVIII/fIX conditions. The proposed rFVIIa mechanism of action was consistent with experimental literature not used in model training. Thus, we demonstrated that an ensemble of uncertain models could unravel key facets of the mechanism of action of a focused intervention. Whereas the current study was limited to coagulation, perhaps the general strategy used could be extended to other molecular networks relevant to human health.

Principles of biomimetic vascular network design applied to a tissue-engineered liver scaffold

Branched vascular networks are a central component of scaffold architecture for solid organ tissue engineering. In this work, seven biomimetic principles were established as the major guiding technical design considerations of a branched vascular network for a tissue-engineered scaffold. These biomimetic design principles were applied to a branched radial architecture to develop a liver-specific vascular network. Iterative design changes and computational fluid dynamic analysis were used to optimize the network before mold manufacturing. The vascular network mold was created using a new mold technique that achieves a 1:1 aspect ratio for all channels. In vitro blood flow testing confirmed the physiologic hemodynamics of the network as predicted by computational fluid dynamic analysis. These results indicate that this biomimetic liver vascular network design will provide a foundation for developing complex vascular networks for solid organ tissue engineering that achieve physiologic blood flow.

Flow residence time and regions of intraluminal thrombus deposition in intracranial aneurysms

Thrombus formation in intracranial aneurysms, while sometimes stabilizing lesion growth, can present additional risk of thrombo-embolism. The role of hemodynamics in the progression of aneurysmal disease can be elucidated by patient-specific computational modeling. In our previous work, patient-specific computational fluid dynamics (CFD) models were constructed from MRI data for three patients who had fusiform basilar aneurysms that were thrombus-free and then proceeded to develop intraluminal thrombus. In this study, we investigated the effect of increased flow residence time (RT) by modeling passive scalar advection in the same aneurysmal geometries. Non-Newtonian pulsatile flow simulations were carried out in base-line geometries and a new postprocessing technique, referred to as "virtual ink" and based on the passive scalar distribution maps, was used to visualize the flow and estimate the flow RT. The virtual ink technique clearly depicted regions of flow separation. The flow RT at different locations adjacent to aneurysmal walls was calculated as the time the virtual ink scalar remained above a threshold value. The RT values obtained in different areas were then correlated with the location of intra-aneurysmal thrombus observed at a follow-up MR study. For each patient, the wall shear stress (WSS) distribution was also obtained from CFD simulations and correlated with thrombus location. The correlation analysis determined a significant relationship between regions where CFD predicted either an increased RT or low WSS and the regions where thrombus deposition was observed to occur in vivo. A model including both low WSS and increased RT predicted thrombus-prone regions significantly better than the models with RT or WSS alone.

Towards non-thrombogenic performance of blood recirculating devices

Implantable blood recirculating devices have provided life saving solutions to patients with severe cardiovascular diseases. However, common problems of hemolysis and thromboembolism remain an impediment to these devices. In this article, we present a brief review of the work by several groups in the field that has led to the development of new methodologies that may facilitate achieving the daunting goal of optimizing the thrombogenic performance of blood recirculating devices. The aim is to describe work which pertains to the interaction between flow-induced stresses and the blood constituents, and that supports the hypothesis that thromboembolism in prosthetic blood recirculating devices is initiated and maintained primarily by the non-physiological flow patterns and stresses that activate and enhance the aggregation of blood platelets, increasing the risk of thromboembolism and cardioembolic stroke. Such work includes state-of-the-art numerical and experimental tools used to elucidate flow-induced mechanisms leading to thromboembolism in prosthetic devices. Following the review, the paper describes several efforts conducted by some of the groups active in the field, and points to several directions that should be pursued in the future in order to achieve the goal for blood recirculating prosthetic devices becoming more effective as destination therapy in the future.

High-shear stress sensitizes platelets to subsequent low-shear conditions

Individuals with mechanical heart valve implants are plagued by flow-induced thromboembolic complications, which are undoubtedly caused by platelet activation. Flow fields in or around the affected regions involve brief exposure to pathologically high-shear stresses on the order of 100 to 1000 dyne/cm(2). Although high shear is known to activate platelets directly, their subsequent behavior is not known. We hypothesize that the post-high-shear activation behavior of platelets is particularly relevant in understanding the increased thrombotic risk associated with blood-recirculating prosthetic cardiovascular devices. Purified platelets were exposed to brief (5-40 s) periods of high-shear stress, and then exposed to longer periods (15-60 min) of low shear. Their activation state was measured using a prothrombinase-based assay. Platelets briefly exposed to an initial high-shear stress (e.g., 60 dyne/cm(2) for 40 s) activate a little, but this study shows that they are now sensitized, and when exposed to subsequent low shear stress, they activate at least 20-fold faster than platelets not initially exposed to high shear. The results show that platelets in vitro exposed beyond a threshold of high-shear stress are primed for subsequent activation under normal cardiovascular circulation conditions, and they do not recover from the initial high-shear insult.

Glycated albumin modulates platelet susceptibility to flow induced activation and aggregation

A diabetic vasculature promotes cardiovascular diseases via endothelial cell activation induced by advanced glycation end products. It has recently become clear that activated platelets are a hallmark of cardiovascular disease and diabetes progression, by initiating and/or perpetuating the endothelial cell response. However, the role that platelets play in diabetic cardiovascular diseases remains elusive. Our objective was to evaluate the effects of glycated serum albumin on flow induced platelet activation and platelet aggregation. Albumin was glycated for up to 8 weeks. Timed samples of glycated or non-glycated albumin were removed to determine the effects of the extent of glycation on platelet functional changes. Thrombin receptor agonist peptide 6 (TRAP(6), residues 42-47 of the thrombin receptor) and collagen I induced platelet aggregation was measured as a time course of glycated albumin incubation. The thrombogenicity of platelets incubated with glycated albumin was also measured under static and dynamic flow conditions using the modified prothrombinase assay. CD41 and CD62P expression was examined using flow cytometry to validate aggregation and activation studies. Platelets subjected to glycated albumin were more susceptible to TRAP(6)- and collagen-induced aggregation and flow induced activation. The extent of albumin glycation modulates these changes. As the albumin glycation time increased, this enhancement in platelet function was more pronounced. These results indicate that under diabetic conditions activated platelets may act to promote cardiovascular disease progression.

Two-dimensional simulation of flow and platelet dynamics in the hinge region of a mechanical heart valve

The hinge region of a mechanical bileaflet valve is implicated in blood damage and initiation of thrombus formation. Detailed fluid dynamic analysis in the complex geometry of the hinge region during the closing phase of the bileaflet valve is the focus of this study to understand the effect of fluid-induced stresses on the activation of platelets. A fixed-grid Cartesian mesh flow solver is used to simulate the blood flow through a two-dimensional geometry of the hinge region of a bileaflet mechanical valve. Use of local mesh refinement algorithm provides mesh adaptation based on the gradients of flow in the constricted geometry of the hinge. Leaflet motion is specified from the fluid-structure interaction analysis of the leaflet dynamics during the closing phase from a previous study, which focused on the fluid mechanics at the gap between the leaflet edges and the valve housing. A Lagrangian particle tracking method is used to model and track the platelets and to compute the magnitude of the shear stress on the platelets as they pass through the hinge region. Results show that there is a boundary layer separation in the gaps between the leaflet ear and the constricted hinge geometry. Separated shear layers roll up into vortical structures that lead to high residence times combined with exposure to high-shear stresses for particles in the hinge region. Particles are preferentially entrained into this recirculation zone, presenting the possibility of platelet activation, aggregation, and initiation of thrombi.

Platelet activation due to hemodynamic shear stresses: damage accumulation model and comparison to in vitro measurements

The need to optimize the thrombogenic performance of blood recirculating cardiovascular devices, e.g., prosthetic heart valves (PHV) and ventricular assist devices (VAD), is accentuated by the fact that most of them require lifelong anticoagulation therapy that does not eliminate the risk of thromboembolic complications. The formation of thromboemboli in the flow field of these devices is potentiated by contact with foreign surfaces and regional flow phenomena that stimulate blood clotting, especially platelets. With the lack of appropriate methodology, device manufacturers do not specifically optimize for thrombogenic performance. Such optimization can be facilitated by formulating a robust numerical methodology with predictive capabilities of flow-induced platelet activation. In this study, a phenomenological model for platelet cumulative damage, identified by means of genetic algorithms (GAs), was correlated with in vitro experiments conducted in a Hemodynamic Shearing Device (HSD). Platelets were uniformly exposed to flow shear representing the lower end of the stress levels encountered in devices, and platelet activity state (PAS) was measured in response to six dynamic shear stress waveforms representing repeated passages through a device, and correlated to the predictions of the damage accumulation model. Experimental results demonstrated an increase in PAS with a decrease in "relaxation" time between pulses. The model predictions were in very good agreement with the experimental results.

In vitro model of platelet-endothelial activation due to cigarette smoke under cardiovascular circulation conditions

Cigarette smoke has been shown to increase platelet activation and endothelial cell (EC) adhesion molecule expression. In the present study, we utilized a hemodynamic shearing device (HSD) to investigate the above effects in vitro in a combined system of platelets and cultured HUVECs (Human Umblical Vein ECs) under physiological shear stress. We investigated the alteration of E-selectin expression on ECs upon exposure to: (1) platelets and nicotine-free smoke extract (NFE), (2) platelets alone, (3) NFE alone, under physiological shear stress. We additionally confirmed the protective effect of nicotine on platelet activation. We found that: (i) surface expression of E-selectin on ECs was significantly increased upon simultaneous exposure of ECs and platelets to NFE relative to exposure of ECs to either platelets or NFE alone (p < 0.05). (ii) Platelet activation was significantly increased in the presence of NFE (p < 0.05). (iii) Nicotine (200 nM) when added to NFE, significantly reduced platelet activation due to NFE (p < 0.05), an effect additionally confirmed by conventional cigarette extracts which contain nicotine (p < 0.05). We therefore conclude that: (a) NFE and platelets additively increase EC E-selectin surface expression, and (b) nicotine modulates platelet activation regardless of ECs.

The effects of coronary artery disease severity on time-dependent changes in platelet activation indices in stored whole blood

BACKGROUND

Platelets play a pivotal role in the pathogenesis of the thrombotic complications in cardiovascular disease (CVD). Abnormal platelet activation indices are evolving as potentially useful markers in CVD risk stratification. Whilst there has been some investigation into the effects of storage time on several of these indices, the effects of underlying disease severity on these temporal changes have not been previously studied.

METHODS

Using the ADVIA 120 haematology analyser, we assessed the effects of time-dependent storage of whole blood in EDTA, on a number of platelet activation indices: mean platelet volume (MPV), mean platelet component (MPC, measure of platelet density) and platelet component distribution width (PCDW, a marker of platelet shape change. We studied three age- and sex-matched patient groups: (i) healthy controls (n = 10), (ii) stable patients with coronary artery disease (CAD, n = 9); and (iii) patients with acute myocardial infarction (n = 8). Whole blood samples were processed at exactly 5 min following venesection and at 15, 30, 60 and 120 min later in storage in EDTA tubes at room temperature.

RESULTS

There was a significant and stepwise increase in MPV (P = 0.01) and decrease in PCDW (P = 0.03), with a non-significant trend to increasing MPM and decreasing MPC with increasing underlying disease (that is healthy, 'stable' and 'acute' artery disease). There was a significant time-dependent increase in MPV and decrease in MPC and PCDW (all P < 0.05), which were all significant on 'post-hoc' analyses by 30 min. There were no significant changes in platelet count or MPM with time. There was no interaction of underlying disease with whole-blood storage time for any of the platelet indices reported (P = NS).

CONCLUSION

There is a temporal increase in MPV and decrease in MPC and PCDW in venous blood stored over 2 h in EDTA. These changes are not influenced by the underlying CVD disease severity.

DPIV prediction of flow induced platelet activation-comparison to numerical predictions

Flow induced platelet activation (PA) can lead to platelet aggregation, deposition onto the blood vessel wall, and thrombus formation. PA was thoroughly studied under unidirectional flow conditions. However, in regions of complex flow, where the platelet is exposed to varying levels of shear stress for varying durations, the relationship between flow and PA is not well understood. Numerical models were developed for studying flow induced PA resulting from stress histories along Lagrangian trajectories in the flow field. However, experimental validation techniques such as Digital Particle Image Velocimetry (DPIV) were not extended to include such models. In this study, a general experimental tool for PA analysis by means of continuous DPIV was utilized and compared to numerical simulation in a model of coronary stenosis. A scaled up (5:1) 84% eccentric and axisymetric coronary stenosis model was used for analysis of shear stress and exposure time along particle trajectories. Flow induced PA was measured using the PA State (PAS) assay. An algorithm for computing the PA level in pertinent trajectories was developed as a tool for extracting information from DPIV measurements for predicting the flow induced thrombogenic potential. CFD, DPIV and PAS assay results agreed well in predicting the level of PA. In addition, the same trend predicted by the DPIV was measured in vitro using the Platelet Activity State (PAS) assay, namely, that the symmetric stenosis activated the platelets more as compared to the eccentric stenosis.

Flow Effects on Coagulation and Thrombosis

Thrombosis occurs in a dynamic rheological field that constantly changes as the thrombus grows to occlusive dimensions. In the initiation of thrombosis, flow conditions near the vessel wall regulate how quickly reactive components are delivered to the injured site and how rapidly the reaction products are disseminated. Whereas the delivery and removal of soluble coagulation factors to the vessel is thought to occur via classic convection-diffusion phenomena, the movement of cells and platelets to the injured wall is strongly augmented by flow-dependent cell-cell collisions that enhance their ability to interact with the wall. In addition, increased shear conditions have been shown to activate platelets, alter the cellular localization of proteins such as tissue factor (TF) and TF pathway inhibitor, and regulate gene production. In the absence of high shearing forces, red cells, leukocytes, and platelets can form stable aggregates with each other or cells lining the vessel wall, which, in addition to altering the biochemical makeup of the aggregate or vessel wall, effectively increases the local blood viscosity. Thus, hemodynamic forces not only regulate the predilection of specific anatomic sites to thrombosis, but they strongly influence the biochemical makeup of thrombi and the reaction pathways involved in thrombus formation.

The extent of platelet activation under shear depends on platelet count: differential expression of anionic phospholipid and factor Va

It is widely accepted that shear stress activates platelets. However, this may have two linked but separate causes: a direct effect of shear stress on individual platelets, and secondary inter-platelet activation dependent on the release of agonists caused by shear. Gel-filtered platelets were exposed to intermittent low shear at 20,000 and 200,000 platelets/microl and their activation was measured with a prothrombinase-based assay. At the lower count, activation was slow and essentially linear, but at the higher count, it rose exponentially with time, leading to 3-fold more prothrombinase activity. Inclusion of apyrase and/or prostaglandin I(2) slightly reduced activation at high platelet counts, but did not abolish the nonlinear kinetics, and antibodies against von Willebrand factor had no significant effect. The contributions of anionic phospholipid and factor Va to the prothrombinase activity were assessed by measurements in the presence of exogenous factor Va. The results strongly suggest that anionic phospholipid appearance is caused directly by shear exposure, but that factor Va release from the alpha-granules is a secondary event and largely the result of platelet-platelet signaling.

The hemodynamics of the Berlin pulsatile VAD and the role of its MHV configuration

The 3D flow in a model of the Berlin ventricular assist device (VAD) chamber with monoleaflet valves placed in S-shape conduits was simulated numerically. The blood flow dynamics were described in terms of flow patterns, velocity, pressure, and shear stress. The hemodynamic properties and the VAD's potential risk for thrombosis were evaluated in terms of mixing and washout properties, and global estimations of platelet level of activation (LOA). In order to evaluate the role of valves on the flow in the chamber, the flow in a model with bileaflet valves in straight conduits was simulated and compared with the original case. The results showed that in both models a large rotating flow was developed in the chamber during filling. This vortex filled the entire chamber and moved constantly up to the peak ejection phase, resulting in relatively low shear stress (up to 0.4 Pa) and no lasting stagnation regions. Significant shear stresses were found near the valves with higher values near the outlet valve in both models. The configuration of valves and conduits had a large effect on VAD washout and mixing properties, with advantage to the bileaflet model. However, since the bileaflet valves exhibited higher shear stresses, higher LOA were found for the bileaflet model.

Thrombogenic performance of a st. Jude bileaflet mechanical heart valve in a sheep model

The sheep model is preferred for chronic evaluation of prosthetic heart valves, surgical techniques, and endocardiographic studies. A bileaflet mechanical heart valve (MHV) was implanted into a sheep model to study its in vivo performance and to evaluate the thrombogenic potential of the valve. Transesophageal echocardiography and transcranial Doppler ultrasonography measurements were conducted before and after the valve implantation. Platelet activity state (PAS) assay measurements were also conducted before and after the implantation surgery. After sheep euthanasia, the MHV was explanted and scanning electron microscopy (SEM) was performed on the explanted valve to examine changes to the MHV surface. Tissue blocks were taken from the sheep brain, left ventricle, aorta, spleen, and lung lobes for histological examination. Our results indicated that after the MHV implantation, more embolic signals were detected in the sheep carotid artery, increasing monotonously as a function of implantation time. Echocardiographic parameters including blood aortic velocity, transvalvular pressure gradient, and velocity time integral increased. PAS increased significantly after valve implantation. SEM pictures demonstrated calcium and phosphate deposition on the valve surfaces. Histological examination demonstrated hemorrhage in the lung tissue, pulmonary thrombosis, and osteogenesis in heart tissue.

Flow-induced platelet activation in a St. Jude mechanical heart valve, a trileaflet polymeric heart valve, and a St. Jude tissue valve

Polymer heart valves have been under investigation since the 1960s, but their success has been hampered by an overall lack of durability mainly due to calcification of the leaflets and a relatively high rate of thromboembolic complications. A new polymer (Quatromer) trileaflet design was tested for its thrombogenic potential and was compared to that of existing prosthetic heart valves routinely implanted in patients: a St. Jude Medical bileaflet mechanical heart valve (MHV) and a St. Jude porcine bioprosthetic tissue valve. The valves were mounted in a left ventricular assist device and the procoagulant activity of the platelets was measured using a platelet activation state (PAS) assay. The PAS measurements indicated that the platelet activation level induced by the polymeric valve was very similar to that induced by the St. Jude Medical MHV and the St. Jude tissue valve. No significant difference was observed between the three valves, indicating that they have a comparable thrombogenic potential.

Th2 shift in juvenile muscular atrophy of distal upper extremity: a combined allergological and flow cytometric analysis

Juvenile muscular atrophy of the distal upper extremity (JMADUE) is considered to be a type of flexion myelopathy; however, we recently reported cases of JMADUE associated with airway allergy successfully treated by plasma exchange. To further characterize the allergo-immunological features of JMADUE, 11 consecutive JMADUE patients in the neurology clinic at Kyushu University Hospital were studied. Past and present together with family histories of common allergic disorders were investigated. Total serum IgE was measured by an enzyme linked immunosorbent assay (ELISA) and allergen-specific IgE by a liquid phase enzyme immunoassay. Intracellular interferon (IFN) gamma-, interleukin (IL)-4-, IL-5- and IL-13-producing T cells in peripheral blood were analyzed by flow cytometry. Data from 42 healthy subjects were used as controls for allergological studies. Flow cytometric data from 21 healthy subjects were also used for comparison. The patients exhibited significantly higher frequencies of coexisting airway allergies such as allergic rhinitis (p=0.0057) and pollinosis (p=0.0064), family histories of allergic disorders (p=0.0075), and mite antigen specific IgE (p=0.0361) compared with the healthy subjects. Patients with JMADUE had a significantly higher percentage of IFNgamma-IL-4+CD4+T cells (p=0.0017), but not IL-5- or IL-13-producing CD4+T cells, and a reduced intracellular IFNgamma/IL-4 ratio in CD4+T cells (p=0.002) compared to the controls. These findings suggest that JMADUE has a significant T helper 2 (Th2) shift, which may in part contribute to the development of spinal cord damage.

Dynamics of Blood Flow and Platelet Transport in Pathological Vessels

Arterial disease, characterized by arterial occlusion (stenosis), is a leading cause of cardiovascular diseases and a major healthcare problem in the Western world. One of the main mechanisms leading to vessel occlusion is thrombus formation, which may be initiated by platelet activation. Shear rates and flow patterns (fluid dynamics factors) and concentration of coagulation factors and platelet agonists (biological factors) modulate platelet function and may lead to platelet activation and aggregation. Here, we examine the flow-induced mechanisms leading to platelet activation in models of stenosed coronary vessels. Experimental and numerical methods were used to investigate and characterize the influence of the flow field on platelet activation. As it passes through pathological geometries characteristic of arterial stenosis, a platelet is exposed to varying levels of shear stress. The cumulative effect of the shear stress level and the duration of the platelet's exposure to it determine whether the platelet is brought beyond its activation threshold. Stress histories of individual platelets can be tracked within the flow field to locate the regions where activated platelets might be found and subsequently aggregate and/or adhere to the wall.

A shear-restricted pathway of platelet procoagulant activity is regulated by IQGAP1

Circulating blood platelets regulate the initial phase of the hemostatic response through adhesive and aggregatory events and by providing the necessary procoagulant surface for prothrombinase complex assembly and thrombin generation. The signaling pathway(s) that regulate platelet procoagulant activity are largely unknown, although they are distinct from platelet aggregatory signals linked to fibrinogen ligation to the conformationally active alpha(IIB)beta(3) integrin. We describe a novel intracellular signaling mechanism involving platelet IQGAP1 that specifically regulates the development of platelet procoagulant activity under conditions of mechanical shear stress. Murine platelets that are deficient in IQGAP1 demonstrate increased prothrombinase activity compared with wild-type littermate controls when activated by a physiological shear stress of 16 dynes/cm(2) (shear rates of 1600 s(-1)) (p < 0.0001), corresponding to approximately 2.5 times the normal shear stress, or approximately 40% degree of stenosis in coronary arteries. The exaggerated prothrombinase activity is not associated with enhanced platelet microvesiculation (cytoskeletal proteolysis) and occurs independently of the intracellular calcium release, [Ca(2+)](i), but it is specifically coupled to the alpha-granule exocytic pathway without concomitant effects on aminophospholipid exposure. These observations identify platelet IQGAP1 as an important modulator of normal hemostasis and as an appropriate pharmacological target for control of platelet procoagulant function.

Differences between mainstream and sidestream cigarette smoke extracts and nicotine in the activation of platelets under static and flow conditions

BACKGROUND

Cigarette smoke is a primary risk factor for cardiovascular diseases. Enhanced function of the hemostatic system, in which platelets play a major role, is a significant underlying mechanism in cardiovascular disease and its progression. Epidemiological studies, complemented by physiological and biochemical data, show that cigarette smoke adversely affects platelet function, both in smokers and in nonsmokers exposed to sidestream smoke.

METHODS AND RESULTS

The thrombogenic potential of platelets subjected to mainstream smoke extracts, sidestream extracts, and nicotine was measured in vitro under static and dynamic flow conditions. Platelet activation state was measured with a modified prothrombinase-based method. Mainstream and sidestream smoke extracts caused increased platelet activation. Although low-tar mainstream extracts activated platelets less than high-tar extracts, the sidestream extracts were almost equally potent. Modification of the filters of low-tar cigarettes, by blocking the air-bypass holes, raised activation rates by mainstream extracts to the level of high-tar extracts. Nicotine (50 nmol/L and 5 micromol/L) inhibited platelet activation under both flow and static conditions.

CONCLUSIONS

Cigarette smoke extracts directly cause platelet activation but also markedly increase the susceptibility of platelets to activation by shear stress. In contrast, nicotine, although also a constituent of cigarette smoke, significantly reduces platelet susceptibility to shear stress.

Physical and occupational therapy for children with rheumatic diseases

Total management of rheumatic disorders of children includes antiinflammatory drugs, active therapy, maintenance of ADLs, and attention to the psychosocial development of the child. This article focuses on the role that physical and occupational therapists play in the management of children with arthritis. The complexity of the problems of these children necessitates a multidisciplinary team approach, with professionals who are committed to helping the child lead as normal a life as possible. This objective can be accomplished only by teaching families and school personnel how to manage the child's daily therapeutic needs.