Flow Effects on Coagulation and Thrombosis

Four-dimensional characterization of thrombosis in a live-cell, shear-flow assay: development and application to xenotransplantation

Background

Porcine xenografts are a promising source of scarce transplantable organs, but stimulate intense thrombosis of human blood despite targeted genetic and pharmacologic interventions. Current experimental models do not enable study of the blood/endothelial interface to investigate adhesive interactions and thrombosis at the cellular level under physiologic conditions. The purpose of this study was to develop and validate a live-cell, shear-flow based thrombosis assay relevant to general thrombosis research, and demonstrate its potential in xenotransplantation applications.

Methodology/Principal Findings

Confluent wild-type (WT, n = 48) and Gal transferase knock-out (GalTKO, which resist hyperacute rejection; n = 11) porcine endothelia were cultured in microfluidic channels. To mimic microcirculatory flow, channels were perfused at 5 dynes/cm² and 37°C with human blood stained to fluorescently label platelets. Serial fluorescent imaging visualized percent surface area coverage (SA, for adhesion of labeled cells) and total fluorescence (a metric of clot volume). Aggregation was calculated by the fluorescence/SA ratio (FR). WT endothelia stimulated diffuse platelet adhesion (SA 65 ± 2%) and aggregation (FR 120 ± 1 a.u.), indicating high-grade thrombosis consistent with the rapid platelet activation and consumption seen in whole-organ lung xenotransplantation models. Experiments with antibody blockade of platelet aggregation, and perfusion of syngeneic and allo-incompatible endothelium was used to verify the biologic specificity and validity of the assay. Finally, with GalTKO endothelia thrombus volume decreased by 60%, due primarily to a 58% reduction in adhesion (P < 0.0001 each); importantly, aggregation was only marginally affected (11% reduction, P < 0.0001).

Conclusions/Significance

This novel, high-throughput assay enabled dynamic modeling of whole-blood thrombosis on intact endothelium under physiologic conditions, and allowed mechanistic characterization of endothelial and platelet interactions. Applied to xenogeneic thrombosis, it enables future studies regarding the effect of modifying the porcine genotype on sheer-stress-dependent events that characterize xenograft injury. This in-vitro platform is likely to prove broadly useful to study thrombosis and endothelial interactions under dynamic physiologic conditions.

Quantitative Assessment of Turbulence and Flow Eccentricity in an Aortic Coarctation: Impact of Virtual Interventions

Turbulence and flow eccentricity can be measured by magnetic resonance imaging (MRI) and may play an important role in the pathogenesis of numerous cardiovascular diseases. In the present study, we propose quantitative techniques to assess turbulent kinetic energy (TKE) and flow eccentricity that could assist in the evaluation and treatment of stenotic severities. These hemodynamic parameters were studied in a pre-treated aortic coarctation (CoA) and after several virtual interventions using computational fluid dynamics (CFD), to demonstrate the effect of different dilatation options on the flow field. Patient-specific geometry and flow conditions were derived from MRI data. The unsteady pulsatile flow was resolved by large eddy simulation including non-Newtonian blood rheology. Results showed an inverse asymptotic relationship between the total amount of TKE and degree of dilatation of the stenosis, where turbulent flow proximal the constriction limits the possible improvement by treating the CoA alone. Spatiotemporal maps of TKE and flow eccentricity could be linked to the characteristics of the jet, where improved flow conditions were favored by an eccentric dilatation of the CoA. By including these flow markers into a combined MRI-CFD intervention framework, CoA therapy has not only the possibility to produce predictions via simulation, but can also be validated pre- and immediate post treatment, as well as during follow-up studies.

Computational fluid dynamics characterization of blood flow in central aorta to pulmonary artery connections: importance of shunt angulation as a determinant of shear stress-induced thrombosis

The central aortic shunt, consisting of a Gore-Tex (polytetrafluoroethylene) tube (graft) connecting the ascending aorta to the pulmonary artery, is a palliative operation for neonates with cyanotic congenital heart disease. These tubes often have an extended length, and therefore must be angulated to complete the connection to the posterior pulmonary arteries. Thrombosis of the graft is not uncommon and can be life-threatening. We have shown that a viscous fluid (such as blood) traversing a curve or bend in a small-caliber vessel or conduit can give rise to marked increases in wall shear stress, which is the major mechanical factor responsible for vascular thrombosis. Thus, the objective of this study was to use computational fluid dynamics to investigate whether wall shear stress (and shear rate) generated in angulated central aorta-to-pulmonary artery connections, in vivo, can be of magnitude and distribution to initiate platelet activation/aggregation, ultimately leading to thrombus formation. Anatomical features required to construct the computer-simulated blood flow pathways were verified from angiograms of central aortic shunts in patients. For the modeled central aortic shunts, we found wall shear stresses of (80-200 N/m(2)), with shear rates of (16,000-40,000/s), at sites of even modest curvature, to be high enough to cause platelet-mediated shunt thrombosis. The corresponding energy losses for the fluid transitions through the aorta-to-pulmonary connections constituted (70 %) of the incoming flow's mechanical energy. The associated velocity fields within these shunts exhibited vortices, eddies, and flow stagnation/recirculation, which are thrombogenic in nature and conducive to energy dissipation. Angulation-induced, shear stress-mediated shunt thrombosis is insensitive to aspirin therapy alone. Thus, for patients with central aortic shunts of longer length and with angulation, aspirin alone will provide insufficient protection against clotting. These patients are at risk for shunt thrombosis and significant morbidity and mortality, unless their anticoagulation regimen includes additional antiplatelet medications.

Comparison of fibrin-based clot elasticity parameters measured by free oscillation rheometry (ReoRox ®) versus thromboelastometry (ROTEM ®)

Background. Whole blood viscoelastic tests such as the fibrin-based thromboelastometry (ROTEM^®) test FIBTEM are increasingly used in the perioperative setting to quickly identify deficits in fibrin quality, and to guide hemostatic therapy. The recently developed FibScreen2 test of the ReoRox^® method, based on free oscillation rheometry, also provides an evaluation of fibrin clot quality. To date, little information is available on the performance of this test in hemodiluted blood, by comparison to FIBTEM. Methods. Whole blood samples from eight healthy volunteers were analyzed using FIBTEM and Fibscreen2. Native and diluted (to 33% and 50% using saline, gelatin or hydroxyethyl starch [HES]) samples were analyzed. Clot strength parameters, including FIBTEM maximum clot firmness (MCF), FIBTEM maximum clot elasticity (MCE) and Fibscreen2 maximum elasticity (G'max), were measured. Results. In repeatedly measured samples from two volunteers, FIBTEM MCF and Fibscreen2 G'max revealed a coefficient of variation (CV) of 5.3 vs. 16.3% and 5.6 vs. 31.7% for each volunteer, respectively. Hemodilution decreased clot strength. Both Fibscreen2 G'max and FIBTEM parameters decreased proportionally to the dilution ratio when saline was used. The observed reductions in FIBTEM and Fibscreen2 parameters were more severe in samples diluted with gelatin and HES, compared to saline. Finally, a regression analysis between FIBTEM MCE and Fibscreen2 G'max revealed a poor goodness of fit (r² = 0.37, p < 0.0001). Conclusions. ReoRox^® Fibscreen2 test has a high coefficient of variation, and its application in various hemodilution conditions showed limited comparability with the ROTEM^® FIBTEM test.

Using a Systems Pharmacology Approach to Study the Effect of Statins on the Early Stage of Atherosclerosis in Humans

More than 100,000 people have participated in controlled trials of statins (lowering cholesterol drugs) since the introduction of lovastatin in the 1980s. Meta-analyses of this data have shown that statins have a beneficial effect on treated groups compared to control groups, reducing cardiovascular risk. Inhibiting the HMG-CoA reductase in the liver, statins can reduce cholesterol levels, thus reducing LDL levels in circulation. Published data from intravascular ultrasound studies (IVUS) was used in this work to develop and validate a unique integrative system model; this consisted of analyzing control groups from two randomized controlled statins trials (24/97 subjects respectively), one treated group (40 subjects, simvastatin trial), and 27 male subjects (simvastatin, pharmacokinetic study). The model allows to simulate the pharmacokinetics of statins and its effect on the dynamics of lipoproteins (e.g., LDL) and the inflammatory pathway while simultaneously exploring the effect of flow-related variables (e.g., wall shear stress) on atherosclerosis progression.

Physiologic and hematologic concerns of rotary blood pumps: what needs to be improved?

Over the past few decades, advances in ventricular assist device (VAD) technology have provided a promising therapeutic strategy to treat heart failure patients. Despite the improved performance and encouraging clinical outcomes of the new generation of VADs based on rotary blood pumps (RBPs), their physiologic and hematologic effects are controversial. Currently, clinically available RBPs run at constant speed, which results in limited control over cardiac workload and introduces blood flow with reduced pulsatility into the circulation. In this review, we first provide an update on the new challenges of mechanical circulatory support using rotary pumps including blood trauma, increased non-surgical bleeding rate, limited cardiac unloading, vascular malformations, end-organ function, and aortic valve insufficiency. Since the non-physiologic flow characteristic of these devices is one of the main subjects of scientific debate in the literature, we next emphasize the latest research regarding the development of a pulsatile RBP. Finally, we offer an outlook for future research in the field.

Effects of viscosity on cerebral blood flow after cardiac arrest

OBJECTIVES

To determine blood viscosity in adult comatose patients treated with mild therapeutic hypothermia after cardiac arrest and to assess the relation between blood viscosity, cerebral blood flow, and cerebral oxygen extraction.

DESIGN

Observational study.

SETTING

Tertiary care university hospital.

PATIENTS

Ten comatose patients with return of spontaneous circulation after out-of-hospital cardiac arrest.

INTERVENTION

Treatment with mild therapeutic hypothermia for 24 hours followed by passive rewarming to normothermia.

MEASUREMENTS AND MAIN RESULTS

Median viscosity at shear rate 50/s was 5.27 mPa · s (4.29-5.91 mPa · s) at admission; it remained relatively stable during the first 12 hours and decreased significantly to 3.00 mPa · s (2.72-3.58 mPa · s) at 72 hours (p < 0.001). Median mean flow velocity in the middle cerebral artery was low (27.0 cm/s [23.8-30.5 cm/s]) at admission and significantly increased to 63.0 cm/s (51.0-80.0 cm/s) at 72 hours. Median jugular bulb saturation at the start of the study was 61.5% (55.5-75.3%) and significantly increased to 73.0% (69.0-81.0%) at 72 hours. Median hematocrit was 0.41 L/L (0.36-0.44 L/L) at admission and subsequently decreased significantly to 0.32 L/L (0.27-0.35 L/L) at 72 hours. Median C-reactive protein concentration was low at admission (2.5 mg/L [2.5-6.5 mg/L]) and increased to 101 mg/L (65-113.3 mg/L) in the following hours. Median fibrinogen concentration was increased at admission 2,795 mg/L (2,503-3,565 mg/L) and subsequently further increased to 6,195 mg/L (5,843-7,368 mg/L) at 72 hours. There was a significant negative association between blood viscosity and the mean flow velocity in the middle cerebral artery (p = 0.0008).

CONCLUSIONS

Changes in blood viscosity in vivo are associated with changes in flow velocity in the middle cerebral artery. High viscosity early after cardiac arrest may reduce cerebral blood flow and may contribute to secondary brain injury. Further studies are needed to determine the optimal viscosity during the different stages of the postcardiac arrest syndrome.

Quantification of local hemodynamic alterations caused by virtual implantation of three commercially available stents for the treatment of aortic coarctation

Patients with coarctation of the aorta (CoA) are prone to morbidity including atherosclerotic plaque that has been shown to correlate with altered wall shear stress (WSS) in the descending thoracic aorta (dAo). We created the first patient-specific computational fluid dynamics (CFD) model of a CoA patient treated by Palmaz stenting to date, and compared resulting WSS distributions to those from virtual implantation of Genesis XD and modified NuMED CP stents, also commonly used for CoA. CFD models were created from magnetic resonance imaging, fluoroscopy and blood pressure data. Simulations incorporated vessel deformation, downstream vascular resistance and compliance to match measured data and generate blood flow velocity and time-averaged WSS (TAWSS) results. TAWSS was quantified longitudinally and circumferentially in the stented region and dAo. While modest differences were seen in the distal portion of the stented region, marked differences were observed downstream along the posterior dAo and depended on stent type. The Genesis XD model had the least area of TAWSS values exceeding the threshold for platelet aggregation in vitro, followed by the Palmaz and NuMED CP stents. Alterations in local blood flow patterns and WSS imparted on the dAo appear to depend on the type of stent implanted for CoA. Following confirmation in larger studies, these findings may aid pediatric interventional cardiologists in selecting the most appropriate stent for each patient, and ultimately reduce long-term morbidity following treatment for CoA by stenting.

Macro- and microscale variables regulate stent haemodynamics, fibrin deposition and thrombomodulin expression

Drug eluting stents are associated with late stent thrombosis (LST), delayed healing and prolonged exposure of stent struts to blood flow. Using macroscale disturbed and undisturbed fluid flow waveforms, we numerically and experimentally determined the effects of microscale model strut geometries upon the generation of prothrombotic conditions that are mediated by flow perturbations. Rectangular cross-sectional stent strut geometries of varying heights and corresponding streamlined versions were studied in the presence of disturbed and undisturbed bulk fluid flow. Numerical simulations and particle flow visualization experiments demonstrated that the interaction of bulk fluid flow and stent struts regulated the generation, size and dynamics of the peristrut flow recirculation zones. In the absence of endothelial cells, deposition of thrombin-generated fibrin occurred primarily in the recirculation zones. When endothelium was present, peristrut expression of anticoagulant thrombomodulin (TM) was dependent on strut height and geometry. Thinner and streamlined strut geometries reduced peristrut flow recirculation zones decreasing prothrombotic fibrin deposition and increasing endothelial anticoagulant TM expression. The studies define physical and functional consequences of macro- and microscale variables that relate to thrombogenicity associated with the most current stent designs, and particularly to LST.

Effect of exercise on patient specific abdominal aortic aneurysm flow topology and mixing

Computational fluid dynamics modeling was used to investigate changes in blood transport topology between rest and exercise conditions in five patient-specific abdominal aortic aneurysm models. MRI was used to provide the vascular anatomy and necessary boundary conditions for simulating blood velocity and pressure fields inside each model. Finite-time Lyapunov exponent fields and associated Lagrangian coherent structures were computed from blood velocity data and were used to compare features of the transport topology between rest and exercise both mechanistically and qualitatively. A mix-norm and mix-variance measure based on fresh blood distribution throughout the aneurysm over time were implemented to quantitatively compare mixing between rest and exercise. Exercise conditions resulted in higher and more uniform mixing and reduced the overall residence time in all aneurysms. Separated regions of recirculating flow were commonly observed in rest, and these regions were either reduced or removed by attached and unidirectional flow during exercise, or replaced with regional chaotic and transiently turbulent mixing, or persisted and even extended during exercise. The main factor that dictated the change in flow topology from rest to exercise was the behavior of the jet of blood penetrating into the aneurysm during systole.

An investigation on platelet transport during thrombus formation at micro-scale stenosis

This paper reports on an investigation of mass transport of blood cells at micro-scale stenosis where local strain-rate micro-gradients trigger platelet aggregation. Using a microfluidic flow focusing platform we investigate the blood flow streams that principally contribute to platelet aggregation under shear micro-gradient conditions. We demonstrate that relatively thin surface streams located at the channel wall are the primary contributor of platelets to the developing aggregate under shear gradient conditions. Furthermore we delineate a role for red blood cell hydrodynamic lift forces in driving enhanced advection of platelets to the stenosis wall and surface of developing aggregates. We show that this novel microfluidic platform can be effectively used to study the role of mass transport phenomena driving platelet recruitment and aggregate formation and believe that this approach will lead to a greater understanding of the mechanisms underlying shear-gradient dependent discoid platelet aggregation in the context of cardiovascular diseases such as acute coronary syndromes and ischemic stroke.

Comparison of stress-hemoconcentration correction techniques for stress-induced coagulation

When examining stress effects on coagulation, arithmetic correction is typically used to adjust for concomitant hemoconcentration but may be inappropriate for coagulation activity assays. We examined a new physiologically relevant method of correcting for stress-hemoconcentration. Blood was drawn from healthy men (N = 40) during baseline, mental stress, and recovery, and factor VII activity (FVII:C), factor VIII activity (FVIII:C), activated partial thromboplastin time (APTT), prothrombin time (PT%), fibrinogen, D-dimer, and plasma volume were determined. Three hemoconcentration correction techniques were assessed: arithmetic correction and two reconstitution techniques using baseline plasma or physiological saline. Area-under-the-curve (AUC) was computed for each technique. For FVII:C, uncorrected AUC was significantly greater than AUC corrected arithmetically. For PT%, uncorrected AUC was significantly greater than AUC corrected with saline or arithmetically. For APTT, uncorrected AUC was significantly less than AUC corrected with saline and greater than AUC corrected arithmetically. For fibrinogen, uncorrected AUC was significantly greater than AUC corrected with saline or arithmetically. For D-dimer, uncorrected AUC was significantly greater than AUC corrected arithmetically. No differences in AUC were observed for FVIII:C. Saline reconstitution seems most appropriate when adjusting for hemoconcentration effects on clotting time and activity. Stress-hemoconcentration accounted for the majority of coagulation changes.

Platelet-mediated mesenchymal stem cells homing to the lung reduces monocrotaline-induced rat pulmonary hypertension

Bone marrow mesenchymal stem cell (BM-MSC) transplantation has been suggested to be a promising method for the treatment of pulmonary arterial hypertension (PAH), a fatal disease currently without effective preventive/therapeutic strategies. However, the detailed mechanisms underlying BM-MSC therapy are largely unknown. We designed the present study to test the hypothesis that circulating platelets facilitate BM-MSC homing to the lung vasculature in a rat model of PAH induced by monocrotalin (MCT). A single subcutaneous administration of MCT induced a marked rise in right ventricular systolic pressure (RVSP) and the weight ratio of right to left ventricle plus septum (RV/LV+S) 3 weeks after injection. The injection of MSCs via tail vein 3 days after MCT significantly reduced the increase of RVSP and RV/LV+S. The fluorescence-labeled MSCs injected into the PAH rat circulation were found mostly distributed in the lungs, particularly on the pulmonary vascular wall, whereas cell homing was abolished by an anti-P-selectin antibody and the GPIIb/IIIa inhibitor tirofiban. Furthermore, using an in vitro flow chamber, we demonstrated that MSC adhesion to the major extracellular matrix collagen was facilitated by platelets and their P-selectin and GPIIb/IIIa. Therefore, the current study suggested that platelet-mediated MSC homing prevented the aggravation of MCT-induced rat PAH, via P-selectin and GPIIb/IIIa-mediated mechanisms.

Biocompatibility assessment of a long-term wearable artificial pump-lung in sheep

The purpose of this study was to assess the biocompatibility of a newly developed long-term wearable artificial pump-lung (APL) in a clinically relevant ovine animal model. The wearable APL device was implanted in five sheep through left thoracotomy. The device was connected between the right atrium and pulmonary artery and evaluated for 30 days. Three sheep were used as the sham control. Platelet activation was assessed by measuring platelet surface P-selectin (CD62P) expression with flow cytometry and plasma soluble P-selectin with an enzyme-linked immunosorbent assay. Thrombotic deposition on the device components and hollow fiber membranes were analyzed with digital imaging and scanning electron microscopy. Surface P-selectin of the APL and sham groups changed significantly over the study period, but without significant differences between the two groups. Soluble P-selectin for the two groups peaked in the first 24 h after the surgery. Soluble P-selectin of the APL group remained slightly elevated over the study period compared to the presurgical baseline value and was slightly higher compared to that of the sham group. Plasma free hemoglobin remained in the normal ranges in all the animals. In spite of the surgery-related alteration in laboratory tests and elevation of platelet activation status, the APL devices in all the animals functioned normally (oxygen transfer and blood pumping) during the 30-day study period. The device flow path and membrane surface were free of gross thrombus. Electron microscopy images showed only scattered thrombi on the fibers (membrane surface and weft). In summary, the APL exhibited excellent biocompatibility. Two forms of platelet activation, surgery-related and device-induced, in the animals implanted with the wearable APL were observed. The limited device-induced platelet activation did not cause gross thrombosis and impair the long-term device performance.

In vitro perfused human capillary networks

Replicating in vitro the complex in vivo tissue microenvironment has the potential to transform our approach to medicine and also our understanding of biology. In order to accurately model the 3D arrangement and interaction of cells and extracellular matrix, new microphysiological systems must include a vascular supply. The vasculature not only provides the necessary convective transport of oxygen, nutrients, and waste in 3D culture, but also couples and integrates the responses of organ systems. Here we combine tissue engineering and microfluidic technology to create an in vitro 3D metabolically active stroma (∼1 mm(3)) that, for the first time, contains a perfused, living, dynamic, interconnected human capillary network. The range of flow rate (μm/s) and shear rate (s(-1)) within the network was 0-4000 and 0-1000, respectively, and thus included the normal physiological range. Infusion of FITC dextran demonstrated microvessels (15-50 μm) to be largely impermeable to 70 kDa. Our high-throughput biology-directed platform has the potential to impact a broad range of fields that intersect with the microcirculation, including tumor metastasis, drug discovery, vascular disease, and environmental chemical toxicity.

Tanshinone IIA protects rabbits against LPS-induced disseminated intravascular coagulation (DIC)

AIM

To evaluate the effects of tanshinone IIA (Tan IIA), a lipophilic diterpene from the Chinese herb Salvia miltiorrhiza, on lipopolysaccharide (LPS)-induced disseminated intravascular coagulation (DIC) in rabbits.

METHODS

LPS-induced DIC model was made in adult male New Zealand rabbits by continuous intravenous infusion of LPS (0.5 mg/kg) via marginal ear vein for 6 h. The animals were simultaneously administered with Tan IIA (1, 3 and 10 mg/kg) or heparin (500 000 IU/kg) through continuous infusion via the contralateral marginal ear vein for 6 h. Before and 2 and 6 h after the start of LPS infusion, blood samples were taken for biochemical analyses.

RESULTS

Continuous infusion of LPS into the rabbits gradually impaired the hemostatic parameters, damaged renal and liver functions, increased the plasma TNF-α level, and led to a high mortality rate (80%). Treatment of the rabbits with Tan IIA dose-dependently attenuated the increase in activated partial thromboplastin time (APTT), prothrombin time (PT) and fibrin-fibrinogen degradation products (FDP); ameliorated the decrease in plasma levels of fibrinogen and platelets; and reversed the decline in activity of protein C and antithrombin III. Meanwhile, the treatment significantly suppressed the increase in the plasma levels of aminotransferase, creatinine and TNF-α, and led to much lower mortality (46.7% and 26.7% for the medium- and high-dose groups). Treatment of the rabbits with the high dose of heparin also effectively improved the hemostatic parameters, ameliorated liver and renal injuries, and reduced the plasma level of TNF-α, and significantly reduced the mortality (33.3%).

CONCLUSION

Tan IIA exerts a protective effect against DIC in rabbits.

Rivaroxaban delivery and reversal at a venous flow rate

OBJECTIVE

Rivaroxaban is an oral anticoagulant that directly targets both free factor Xa and factor Xa in complex with its protein cofactor, factor Va, in the prothrombinase complex. It is approved in the United States for the prophylaxis of deep vein thrombosis and stroke in patients with atrial fibrillation; however, it also carries a black box warning regarding the risk of thrombosis after discontinuation of treatment. The purpose of this study was to determine the degree to which rivaroxaban, over a range of physiologically relevant free plasma concentrations, inhibits preassembled prothrombinase at a typical venous shear rate (100 s(-1)) and to determine the dynamics of rivaroxaban washout.

METHODS AND RESULTS

Prothrombinase was assembled on phospholipid-coated glass capillaries. Its activity was characterized with respect to the activation of prothrombin (mean plasma concentration, 1.4 μmol/L) in the absence and presence of rivaroxaban (2, 5, and 10 nmol/L). The degree of inactivation of preassembled prothrombinase is sensitive to the solution-phase rivaroxaban concentration; however, prothrombinase unmasking upon removal of rivaroxaban is concentration independent.

CONCLUSIONS

The model system presented suggests that when rivaroxaban plasma concentrations decrease after cessation of therapy, there will be an unmasking of thrombus-associated prothrombinase that may be related to the reported rebound phenomena.

Prothrombin activation by platelet-associated prothrombinase proceeds through the prethrombin-2 pathway via a concerted mechanism

The protease α-thrombin is a key enzyme of the coagulation process as it is at the cross-roads of both the pro- and anti-coagulant pathways. The main source of α-thrombin in vivo is the activation of prothrombin by the prothrombinase complex assembled on either an activated cell membrane or cell fragment, the most relevant of which is the activated platelet surface. When prothrombinase is assembled on synthetic phospholipid vesicles, prothrombin activation proceeds with an initial cleavage at Arg-320 yielding the catalytically active, yet effectively anticoagulant intermediate meizothrombin, which is released from the enzyme complex ∼30-40% of the time. Prothrombinase assembled on the surface of activated platelets has been shown to proceed through the inactive intermediate prethrombin-2 via an initial cleavage at Arg-271 followed by cleavage at Arg-320. The current work tests whether or not platelet-associated prothrombinase proceeds via a concerted mechanism through a study of prothrombinase assembly and function on collagen-adhered, thrombin-activated, washed human platelets in a flow chamber. Prothrombinase assembly was demonstrated through visualization of bound factor Xa by confocal microscopy using a fluorophore-labeled anti-factor Xa antibody, which demonstrated the presence of distinct platelet subpopulations capable of binding factor Xa. When prothrombin activation was monitored at a typical venous shear rate over preassembled platelet-associated prothrombinase neither potential intermediate, meizothrombin or prethrombin-2, was observed in the effluent. Collectively, these findings suggest that platelet-associated prothrombinase activates prothrombin via an efficient concerted mechanism in which neither intermediate is released.

High platelet reactivity on clopidogrel therapy correlates with increased coronary atherosclerosis and calcification: a volumetric intravascular ultrasound study

OBJECTIVES

This study sought to evaluate the relationship between platelet reactivity and atherosclerotic burden in patients undergoing percutaneous coronary intervention (PCI) with pre-intervention volumetric intravascular ultrasound (IVUS) imaging.

BACKGROUND

Atherosclerosis progresses by the pathologic sequence of subclinical plaque rupture, thrombosis, and healing. In this setting, increased platelet reactivity may lead to more extensive arterial thrombosis at the time of plaque rupture, leading to a more rapid progression of the disease. Alternatively, abnormal vessel wall biology with advanced atherosclerosis is known to enhance platelet reactivity. Therefore, it is possible that by either mechanism, increased platelet reactivity may be associated with greater atherosclerotic burden.

METHODS

This study included patients who underwent PCI with pre-intervention IVUS imaging and platelet reactivity functional assay (P2Y(12) reaction units) performed >16 h after PCI, after the stabilization of clopidogrel therapy (administered before PCI). Platelet reactivity >230 P2Y(12) reaction units defined high on-treatment platelet reactivity (HPR).

RESULTS

Among 335 patients (mean age 65.0 years, 71% men), there were 109 patients with HPR (32.5%) and 226 without HPR (67.5%), with HPR being associated with diabetes and chronic renal insufficiency. By IVUS analysis, patients with HPR had significantly greater target lesion calcium lengths, calcium arcs, and calcium indexes. Furthermore, patients with HPR tended to have longer lesions and greater volumetric dimensions, indicating higher plaque volume, larger total vessel volume, and also greater luminal volume, despite similar plaque burden. By multivariate analysis controlling for baseline clinical variables, HPR was the single consistent predictor of all IVUS parameters examined, including plaque volume, calcium length, and calcium arc.

CONCLUSIONS

Increased platelet reactivity on clopidogrel treatment, defined as >230 P2Y(12) reaction units, is associated with greater coronary artery atherosclerotic disease burden and plaque calcification.

The effect of shear on in vitro platelet and leukocyte material-induced activation

The failure to understand the mechanisms of biomaterial-associated thrombosis prevents us from improving the blood compatibility of stents and mechanical heart valves. Blood-material interactions trigger a complex series of events and anticoagulant and anti-platelet therapies are needed to reduce the risks of thrombotic complications with most cardiovascular materials. While material interaction with platelets has been widely studied, little is currently known on material-induced leukocyte activation in the presence of shear. In vitro experiments were performed to assess the effect of flow on blood cell activation induced by medical grade metals, ST316L and TiAl6V4. Blood was circulated in flow chambers preloaded with or without metal wires at shear rates of 100, 500, and 1500 s−1. Platelet and leukocyte activation, leukocyte-platelet aggregation, and tissue factor expression on monocytes were measured by flow cytometry. Metal surfaces were characterized by scanning electron microscopy. Under physiological shear rates, no significant platelet microparticle formation was observed. However, significant CD11b up-regulation, leukocyte-platelet aggregates, and tissue factor expression were observed at 100 s−1. As shear rate increased to 1500 s−1, leukocyte activation reduced to control values. TiAl6V4-induced leukocyte activation was generally lower than that of ST316L. Adhesion significantly decreased with increasing shear rate to 1500 s−1.

In blood, increase within physiological shear rates led to a significant reduction in in vitro material-induced leukocyte activation, suggesting that difference between material biocompatibility may be better identified at low shear rates or under pathological shear conditions.

Computational fluid dynamics of carotid arteries after carotid endarterectomy or carotid artery stenting based on postoperative patient-specific computed tomography angiography and ultrasound flow data

BACKGROUND

There are significant differences in the postoperative morphological and hemodynamic conditions of the carotid arteries between carotid artery stenting (CAS) and endarterectomy (CEA).

OBJECTIVE

To compare the postoperative rheological conditions after CAS with those after CEA with patch angioplasty (patch CEA) through the use of computational fluid dynamics (CFD) based on patient-specific data.

METHODS

The rheological conditions in the carotid arteries were simulated in 2 patients after CAS and in 2 patients after patch CEA by CFD calculations. Three-dimensional reconstruction of the carotid arteries was performed with the images obtained with computed tomography angiography. The streamlines and wall shear stress (WSS) were calculated by a supercomputer. Adequate boundary conditions were determined by comparing the simulation results with ultrasound flow data.

RESULTS

CFD was successfully calculated for all patients. The differences between the flow velocities of ultrasound data and those of the simulation results were limited. In the streamline analysis, the maximum flow velocities in the internal carotid artery after patch CEA were around two-thirds of those after CAS. Rotational slow flow was observed in the internal carotid artery bulb after patch CEA. WSS analysis found regional low WSS near the outer wall of the bulb. High WSS was observed at the distal end of the arteriotomy after patch CEA and at the residual stenosis after CAS.

CONCLUSION

CFD of postoperative carotid arteries disclosed the differences in streamlines and WSS between CAS and patch CEA. CFD may allow us to obtain adequate rheological conditions conducive to achieving the best clinical results.

Stress-induced alterations in coagulation: assessment of a new hemoconcentration correction technique

OBJECTIVE

For the examination of psychological stress effects on coagulation, the Dill and Costill correction (DCC) for hemoconcentration effects has been used to adjust for stress-induced plasma volume changes. Although the correction is appropriate for adjusting concentrations of various large blood constituents, it may be inappropriate for time-dependent or functional coagulation assays. Two new plasma reconstitution techniques for correcting hemoconcentration effects on stress-induced changes in coagulation were compared with the DCC.

METHODS

Blood was collected from 31 men during baseline, the Trier Social Stress Test (TSST), and after 20-minute recovery. For the reconstitution techniques, TSST plasma samples were reconstituted with either baseline plasma or physiological saline equal to the amount of plasma lost during stress.

RESULTS

Uncorrected activated partial thromboplastin time (APTT) decreased, whereas fibrinogen, factor VIII clotting activity (FVIII:C), D-dimer and prothrombin time (PT%) increased significantly during the TSST. The DCC produced a significantly greater decrease in APTT during stress compared to uncorrected APTT, a significant decrease in PT% compared to uncorrected PT%, and stress D-dimer and fibrinogen and FVIII:C being no different than baseline. APTT, fibrinogen, D-dimer and PT% after saline reconstitution were not different from baseline, whereas FVIII:C after saline reconstitution remained elevated. APTT, PT%, fibrinogen and D-dimer after plasma reconstitution were no different from uncorrected values, whereas FVIII:C remained significantly elevated.

CONCLUSIONS

The observed changes in coagulation are likely in part a consequence of stress and hemoconcentration, but the DCC seems to be an inappropriate hemoconcentration correction technique of time-dependent assays. The saline reconstitution technique may be more biologically relevant when examining stress-hemoconcentration effects on coagulation.

Hemoconcentration and hemostasis during acute stress: interacting and independent effects

BACKGROUND

Acute psychological stress can produce significant hemoconcentration as well as prothrombotic changes in blood, both of which may have potentially harmful effects on the cardiovascular system. It is unclear whether these effects are independent or have influence on each other.

PURPOSE

This review discusses research investigating the effects of acute psychological stress on hemoconcentration and hemostasis and explores future directions for psychohematology research. Physiology, associations with cardiovascular disease, and relationships between acute psychological stress are discussed independently for hemoconcentration and hemostasis, followed by an examination of the effects of stress-hemoconcentration on hemostasis.

CONCLUSIONS

Traditional methods of adjusting for stress-hemoconcentration effects (e.g., calculated plasma volume or hematocrit level corrections) may not be appropriate when examining stress-induced changes in hemostasis. The effects of acute stress on hemostasis should be examined in conjunction with hemoconcentration.

Membrane binding events in the initiation and propagation phases of tissue factor-initiated zymogen activation under flow

This study investigates the dynamics of zymogen activation when both extrinsic tenase and prothrombinase are assembled on an appropriate membrane. Although the activation of prothrombin by surface-localized prothrombinase is clearly mediated by flow-induced dilutional effects, we find that when factor X is activated in isolation by surface-localized extrinsic tenase, it exhibits characteristics of diffusion-mediated activation in which diffusion of substrate to the catalytically active region is rate-limiting. When prothrombin and factor X are activated coincident with each other, competition for available membrane binding sites masks the diffusion-limiting effects of factor X activation. To verify the role of membrane binding in the activation of factor X by extrinsic tenase under flow conditions, we demonstrate that bovine lactadherin competes for both factor X and Xa binding sites, limiting factor X activation and forcing the release of bound factor Xa from the membrane at a venous shear rate (100 s(-1)). Finally, we present steady-state models of prothrombin and factor X activation under flow showing that zymogen and enzyme membrane binding events further regulate the coagulation process in an open system representative of the vasculature geometry.

Procoagulant activity in hemostasis and thrombosis: Virchow's triad revisited

Virchow's triad is traditionally invoked to explain pathophysiologic mechanisms leading to thrombosis, alleging concerted roles for abnormalities in blood composition, vessel wall components, and blood flow in the development of arterial and venous thrombosis. Given the tissue-specific bleeding observed in hemophilia patients, it may be instructive to consider the principles of Virchow's triad when investigating mechanisms operant in hemostatic disorders as well. Blood composition (the function of circulating blood cells and plasma proteins) is the most well studied component of the triad. For example, increased levels of plasma procoagulant proteins such as prothrombin and fibrinogen are established risk factors for thrombosis, whereas deficiencies in plasma factors VIII and IX result in bleeding (hemophilia A and B, respectively). Vessel wall (cellular) components contribute adhesion molecules that recruit circulating leukocytes and platelets to sites of vascular damage, tissue factor, which provides a procoagulant signal of vascular breach, and a surface upon which coagulation complexes are assembled. Blood flow is often characterized by 2 key variables: shear rate and shear stress. Shear rate affects several aspects of coagulation, including transport rates of platelets and plasma proteins to and from the injury site, platelet activation, and the kinetics of fibrin monomer formation and polymerization. Shear stress modulates adhesion rates of platelets and expression of adhesion molecules and procoagulant activity on endothelial cells lining the blood vessels. That no one abnormality in any component of Virchow's triad fully predicts coagulopathy a priori suggests coagulopathies are complex, multifactorial, and interactive. In this review, we focus on contributions of blood composition, vascular cells, and blood flow to hemostasis and thrombosis, and suggest that cross-talk among the 3 components of Virchow's triad is necessary for hemostasis and determines propensity for thrombosis or bleeding. Investigative models that permit interplay among these components are necessary to understand the operant pathophysiology, and effectively treat and prevent thrombotic and bleeding disorders.

Impact of altered venous hemodynamic conditions on the formation of platelet layers in thromboemboli

Although it is generally believed that the structure of venous thromboemboli is a homogeneous red blood cell-fibrin clot, their structure may be heterogeneous, with non-uniformly distributed platelet layers, known as the lines of Zahn. We tested (a) whether venous thromboemboli ex vivo contained platelet layers, i.e. the lines of Zahn, and (b) whether, according to mathematical modeling, eddies can arise in the venous system, possibly contributing to platelet aggregation. The structure of venous thromboemboli ex vivo was determined by high-resolution magnetic resonance imaging (MRI) and by immunohistochemistry (IHC). High-resolution ultrasound (US) imaging was employed to determine the popliteal vein geometry and hemodynamics in healthy subjects and in subjects with previous venous thrombosis. The US data were then used as input for numerical simulations of venous hemodynamics. MRI and IHC confirmed that 42 of 49 ex vivo venous thromboemboli were structurally heterogeneous with platelet layers. The peak venous flow velocity was higher in patients with partly recanalized deep vein thrombosis than in healthy subjects in the prone position (46±4cm/s vs. 16±3cm/s). Our numerical simulation showed that partial venous obstruction with stenosis or malfunctioning venous valves creates the conditions for eddy blood flow. Our experimental results and computer simulation confirmed that the heterogeneous structure of venous thromboemboli with twisted platelet layers may be associated with eddy flow at the sites of their formation.

Survival analysis of gastric cancer patients with tumor thrombus in the portal vein

BACKGROUND

A tumor thrombus in the portal vein originating from gastric cancer is a rare condition. Little is known about the development of portal vein tumor thrombus (PVTT) and its effect on the survival of gastric cancer patients.

METHODS

PVTT originating from gastric cancer was identified retrospectively with computed tomography in 51 patients, from January 2002 to June 2007. PVTT was classified by metastatic routes as follows: lymph node (LN), hepatic mass, and hematogenous type. The median survival from PVTT, and the factors affecting the survival of gastric cancer patients with PVTT were analyzed.

RESULTS

The median survival of gastric cancer patients with PVTT (N = 51) was 5.4 months. The median survival was compared according to clinical characteristics, and multivariate analysis proved that female and hepatic mass type were independent risk factors for poor prognosis. The characteristics of the hepatic mass type were old age, higher proportion of tumors located in the upper-third of the stomach, and liver metastasis, compared to the LN or the hematogeneous type.

CONCLUSIONS

PVTT originating from gastric cancer was a poor prognostic factor and the median survival from PVTT was different according to several clinical factors.

Dilutional control of prothrombin activation at physiologically relevant shear rates

The generation of proteolyzed prothrombin species by preassembled prothrombinase in phospholipid-coated glass capillaries was studied at physiologic shear rates (100-1000 s(-1)). The concentration of active thrombin species (α-thrombin and meizothrombin) reaches a steady state, which varies inversely with shear rate. When corrected for shear rate, steady-state levels of active thrombin species exhibit no variation and a Michaelis-Menten analysis reveals that chemistry of this reaction is invariant between open and closed systems; collectively, these data imply that variations with shear rate arise from dilutional effects. Significantly, the major products observed include nonreactive species arising from the loss of prothrombin's phospholipid binding domain (des F1 species). A numerical model developed to investigate the spatial and temporal distribution of active thrombin species within the capillary reasonably approximates the observed output of total thrombin species at different shears; it also predicts concentrations of active thrombin species in the wall region sufficient to account for observed levels of des FI species. The predominant feedback formation of nonreactive species and high levels of the primarily anticoagulant intermediate meizothrombin (∼40% of total active thrombin species) may provide a mechanism to prevent thrombus propagation downstream of a site of thrombosis or hemorrhage.

A local increase in red blood cell aggregation can trigger deep vein thrombosis: evidence based on quantitative cellular ultrasound imaging

BACKGROUND

Recurrent deep vein thrombosis (DVT) risk factors include a first idiopathic DVT, strongly suggesting the existence of undiagnosed and/or unidentified prothrombotic abnormalities.

OBJECTIVES

To evaluate the impact of locally increased red blood cell (RBC) aggregation on DVT pathogenesis in a rabbit model.

METHODS

DVT presence, flow and aggregation were measured in situ with ultrasound. Greatly enhanced aggregation was achieved by covalent linkage of Pluronic F98 to the RBC surface; coating with Pluronic F68, which very mildly enhances aggregation, was used as a coating control. On day 1, endothelial damage and a partial stenosis were surgically created on the left femoral vein whereas the right femoral vein was not manipulated.

RESULTS

A thrombus was formed within 30 min in six out of seven left femoral veins of animals receiving a 30% volume blood exchange with F98-coated RBC, whereas a thrombus occurred in only one out of seven veins in F68-transfused controls. In vivo imaging using quantitative ultrasound confirmed increased aggregation in the thrombosed veins of the F98 group compared with the F68 group and the contralateral vessel. For each group, five animals were followed for 2 weeks before being killed. In F98-transfused animals, lysis of clots occurred and the presence of chronic thrombi totally occluding the vein in three out of five animals was confirmed by histology. Conversely, in the F68 group, a single disorganized blood clot was observed in one out of five animals.

CONCLUSIONS

A marked increase in RBC aggregation promotes thrombosis in rabbit femoral veins, confirming a pathophysiological role of locally altered hemorheology in the onset of DVT.

Blood compatibility of photografted hydrogel coatings

In this work, we have evaluated the haemocompatibility of different surface modifications, intended for biomaterials and bioanalytical applications. Polystyrene slides were coated with thin hydrogel films by self-initiated photografting and photopolymerization (SIPGP) of four different monomers. The hydrogel surface modifications were thoroughly characterized and tested for their protein resistance and ability to resist platelet adhesion and activation of the coagulation system. There was very little protein adsorption from human plasma on the hydrogels prepared from poly(ethylene glycol) methacrylate and 2-hydroxyethyl methacrylate. Platelet adhesion tests performed under both static and flow conditions showed that these coatings also demonstrated very high resistance towards platelet adhesion. A small amount of platelets were found to adhere to hydrogels formed from ethylene glycol methyl ether methacrylate and 2-carboxyethyl methacrylate. The polystyrene substrates themselves facilitated large amounts of platelet adhesion under both static and flow conditions. Utilizing a novel setup for imaging of coagulation, it was confirmed that none of the hydrogel surfaces activated the coagulation system to any great extent. We suggest that this simple fabrication method can be used to produce hydrogel coatings with unusually high blood compatibility, suitable for demanding biomaterials applications.

The pathophysiology of peri-operative myocardial infarction

SUMMARY

It is generally believed that plaque rupture and myocardial oxygen supply-demand imbalance contribute approximately equally to the burden of peri-operative myocardial infarction. This review critically analyses data of post-mortem, pre-operative coronary angiography, troponin surveillance, other pre-operative non-invasive investigations, and peri-operative haemodynamic predictors of myocardial ischaemia and/or myocardial infarction. The current evidence suggests that myocardial oxygen supply-demand imbalance predominates in the early postoperative period. It is likely that flow stagnation and thrombus formation is an important pathway in the development of a peri-operative myocardial infarction, in addition to the more commonly recognised role of peri-operative tachycardia. Research and therapeutic interventions should be focused on the prediction and therapy of flow stagnation and thrombus formation. Plaque rupture appears to be a more random event, distributed over the entire peri-operative admission.

Reduction of Procoagulant Potential of b-Datum Leakage Jet Flow in Bileaflet Mechanical Heart Valves via Application of Vortex Generator Arrays

Current designs of bileaflet mechanical heart valves put patients at an increased risk of thromboembolism. In particular, regurgitant flow through the b-datum line is associated with nonphysiologic flow characteristics such as elevated shear stresses, regions of recirculation, and increased mixing, all of which may promote thrombus formation. We have previously shown that passive flow control in the form of vortex generators mounted on the downstream leaflet surfaces can effectively diminish turbulent stresses. The objective of the current work is thus to determine the effect of vortex generators on the thromboembolic potential of the b-datum line leakage jet and to correlate that effect with the vortex generator-induced changes to the flow structure. Flow experiments were performed using a steady model of the transient b-datum line jet. These experiments encompassed flow visualization to gain an overall picture of the flow system, particle image velocimetry to quantify the flow field in detail, and in vitro experiments with human blood to quantify thrombus formation in response to the applied passive flow control. Thrombus formation was quantified over time by an assay for thrombin-antithrombin III (TAT III). In comparing results with and without vortex generators, significantly lower mean TAT III levels were observed at one time point for the case with vortex generators. Also, the TAT III growth rate of the case with vortex generators was significantly lower. While no differences in jet spreading were found with and without vortex generators, lower peak turbulent stresses were observed for the case with vortex generators. The results thus demonstrate the potential of applying passive flow control to cardiovascular hardware in order to mitigate the hemodynamic factors leading to thrombus formation.

Disturbed blood flow induces erosive injury to smooth muscle cell-rich neointima and promotes thrombus formation in rabbit femoral arteries

BACKGROUND

Plaque erosion is a cause of atherothrombosis that preferentially occurs on smooth muscle cell (SMC)- and proteoglycan-rich rather than lipid-rich plaques. However, its underlying mechanisms remain unknown.

OBJECTIVE

To determine whether disturbed blood flow induces erosive injury and thrombus formation on SMC-rich neointima.

METHODS

Three weeks after balloon injury, SMC-rich neointima with increased tissue factor (TF) activity developed in rabbit femoral arteries that were narrowed with a vascular occluder to disturb blood flow after stenosis. Neointimal injury and thrombus formation were assessed at 15, 30, and 180 min after the vascular narrowing.

RESULTS

Endothelial detachment, platelet adhesion and neointimal cell apoptosis became evident at the post-stenotic regions of all femoral arteries (n = 5) within 15 min of narrowing. Mural thrombi composed of platelet and fibrin developed after 30 min, and then occlusive thrombi were generated in three out of five vessels after 180 min. The identical vascular narrowing of normal femoral arteries also induced endothelial detachment with small platelet thrombi at post-stenotic regions, but fibrin and occlusive thrombi did not develop. Computational simulation analysis indicated that oscillatory shear stress contributes to the development of erosive damage to the neointima.

CONCLUSIONS

These results suggest that disturbed post-stenotic blood flow can induce erosive injury in SMC-rich plaques and promote thrombus formation that results in vascular events.

Permanent occlusion of feeding arteries and draining veins in solid mouse tumors by vascular targeted photodynamic therapy (VTP) with Tookad

Background

Antiangiogenic and anti-vascular therapies present intriguing alternatives to cancer therapy. However, despite promising preclinical results and significant delays in tumor progression, none have demonstrated long-term curative features to date. Here, we show that a single treatment session of Tookad-based vascular targeted photodynamic therapy (VTP) promotes permanent arrest of tumor blood supply by rapid occlusion of the tumor feeding arteries (FA) and draining veins (DV), leading to tumor necrosis and eradication within 24–48 h.

Methodology/Principal Findings

A mouse earlobe MADB106 tumor model was subjected to Tookad-VTP and monitored by three complementary, non-invasive online imaging techniques: Fluorescent intravital microscopy, Dynamic Light Scattering Imaging and photosensitized MRI. Tookad-VTP led to prompt tumor FA vasodilatation (a mean volume increase of 70%) with a transient increase (60%) in blood-flow rate. Rapid vasoconstriction, simultaneous blood clotting, vessel permeabilization and a sharp decline in the flow rates then followed, culminating in FA occlusion at 63.2 sec±1.5SEM. This blockage was deemed irreversible after 10 minutes of VTP treatment. A decrease in DV blood flow was demonstrated, with a slight lag from FA response, accompanied by frequent changes in flow direction before reaching a complete standstill. In contrast, neighboring, healthy tissue vessels of similar sizes remained intact and functional after Tookad-VTP.

Conclusion/Significance

Tookad-VTP selectively targets the tumor feeding and draining vessels. To the best of our knowledge, this is the first mono-therapeutic modality that primarily aims at the larger tumor vessels and leads to high cure rates, both in the preclinical and clinical arenas.