High Frequency Components of Hemodynamic Shear Stress Profiles are a Major Determinant of Shear-Mediated Platelet Activation in Therapeutic Blood Recirculating Devices

Linking Computational Fluid Dynamics Modeling to Device-Induced Platelet Defects in Mechanically Assisted Circulation

Thrombotic and bleeding events are the most common hematologic complications in patients with mechanically assisted circulation and are closely related to device-induced platelet dysfunction. In this study, we sought to link computational fluid dynamics (CFD) modeling of blood pumps with device-induced platelet defects. Fresh human blood was circulated in circulatory loops with four pumps (CentriMag, HVAD, HeartMate II, and CH-VAD) operated under a total of six clinically representative conditions. Blood samples were collected and analyzed for glycoprotein (GP) IIb/IIIa activation and receptor shedding of GPIbα and GPVI. In parallel, CFD modeling was performed to characterize the blood flow in these pumps. Numerical indices of platelet defects were derived from CFD modeling incorporating previously derived power-law models under constant shear conditions. Numerical results were correlated with experimental results by regression analysis. The results suggested that a scalar shear stress of less than 75 Pa may have limited contribution to platelet damage. The platelet defect indices predicted by the CFD power-law models after excluding shear stress <75 Pa correlated excellently with experimentally measured indices. Although numerical prediction based on the power-law model cannot directly reproduce the experimental data. The power-law model has proven its effectiveness, especially for quantitative comparisons.

Application of 1,000 fps High-Speed Angiography to In-Vitro Hemodynamic Evaluation of Left Ventricular Assist Device Outflow Graft Configurations

Left ventricular assist device (LVAD)-induced hemodynamics are characterized by fast-moving flow with large variations in velocity, making quantitative assessments difficult with existing imaging methods. This study demonstrates the ability of 1,000 fps high-speed angiography (HSA) to quantify the effect of the surgical implantation angle of a LVAD outflow graft on the hemodynamics within the ascending aorta in vitro . High-speed angiography was performed on patient-derived, three-dimensional-printed optically opaque aortic models using a nonsoluble contrast media, ethiodol, as a flow tracer. Outflow graft configuration angles of 45° and 90° with respect to the central aortic axis were considered. Projected velocity distributions were calculated from the high-speed experimental sequences using two methods: a physics-based optical flow algorithm and tracking of radio-opaque particles. Particle trajectories were also used to evaluate accumulated shear stress. Results were then compared with computational fluid dynamics (CFD) simulations to confirm the results of the high-speed imaging method. Flow patterns derived from HSA coincided with the impingement regions and recirculation zones formed in the aortic root as seen in the CFD for both graft configurations. Compared with the 45° graft, the 90° configuration resulted in 81% higher two-dimensional-projected velocities (over 100 cm/s) along the contralateral wall of the aorta. Both graft configurations suggest elevated accumulated shear stresses along individual trajectories. Compared with CFD simulations, HSA successfully characterized the fast-moving flow and hemodynamics in each LVAD graft configuration in vitro , demonstrating the potential utility of this technology as a quantitative imaging modality.

A Clinical-Scale Microfluidic Respiratory Assist Device with 3D Branching Vascular Networks

Recent global events such as COVID-19 pandemic amid rising rates of chronic lung diseases highlight the need for safer, simpler, and more available treatments for respiratory failure, with increasing interest in extracorporeal membrane oxygenation (ECMO). A key factor limiting use of this technology is the complexity of the blood circuit, resulting in clotting and bleeding and necessitating treatment in specialized care centers. Microfluidic oxygenators represent a promising potential solution, but have not reached the scale or performance required for comparison with conventional hollow fiber membrane oxygenators (HFMOs). Here the development and demonstration of the first microfluidic respiratory assist device at a clinical scale is reported, demonstrating efficient oxygen transfer at blood flow rates of 750 mL min⁻1 , the highest ever reported for a microfluidic device. The central innovation of this technology is a fully 3D branching network of blood channels mimicking key features of the physiological microcirculation by avoiding anomalous blood flows that lead to thrombus formation and blood damage in conventional oxygenators. Low, stable blood pressure drop, low hemolysis, and consistent oxygen transfer, in 24-hour pilot large animal experiments are demonstrated - a key step toward translation of this technology to the clinic for treatment of a range of lung diseases.

Prosthesis Geometrical Predictors of Leaflet Thrombosis Following Transcatheter Aortic Valve Replacement With Intra-Annular Prostheses

OBJECTIVE

To determine the association between prosthesis geometry with leaflet thrombosis (LT).

BACKGROUND

Leaflet thrombosis following transcatheter aortic valve replacement (TAVR) is a recognised entity. The association between prosthesis geometry with LT is unclear but maybe a potential modifiable factor in its prevention.

METHODS

Patients who received an intra-annular TAVR prosthesis and were prospectively planned to undergo post-procedural computed tomography (CT) imaging were included. Leaflet thrombosis was defined as at least 50% restricted leaflet motion on CT. Prosthesis expansion and eccentricity was measured at prosthesis inflow, annulus and outflow levels. Prosthesis misalignment was defined as the average angle deviation between native and prosthesis leaflet commissure, greater than 30°.

RESULTS

Prevalence of LT was 13.7% in 117 patients. None of the patients with LT were on anticoagulation therapy. Patients with LT had reduced prosthesis annular expansion (89.4±5.2% vs 97.0±4.4%, p<0.01), greater prosthesis misalignment (81.3% vs 48.5%, p=0.02) and deeper implants (6.3±1.7 mm vs 4.3±1.5 mm, p<0.01). Threshold for the presence of LT on ROC analysis was an implant depth of 5.7 mm (AUC [area under curve]=0.81). Independent predictors of LT were annular under-expansion (Odds ratio [OR] 1.4, 95% confidence interval [CI] 1.2-1.7, p=0.03) prosthesis misalignment (OR 6.8, 95%CI 1.1-45.5, p=0.04) and implant depth (OR 1.9, 95%CI 1.1-3.2, p=0.03). Anticoagulation therapy was a protective factor (OR 0.2; 95%CI 0.1-0.4, p<0.01).

CONCLUSION

Geometrical predictors of LT post intra-annular TAVR were reduced prosthesis expansion at the annular level, lower implant depth and greater prosthesis misalignment. These factors may be important considerations during procedural planning for TAVR.

Investigation of the influence of blade configuration on the hemodynamic performance and blood damage of the centrifugal blood pump

PURPOSE

The design and optimization of centrifugal blood pumps is crucial for improved extracorporeal membrane oxygenation system performances. Secondary flow passages are common in centrifugal blood pumps, allowing for a high volume of unstable flow. Traditional design theory offers minimal guidance on the design and optimization of centrifugal blood pumps, so it's critical to understand how design parameter variables affect hydraulic performances and hemocompatibility.

METHODS

Computational fluid dynamics (CFD) was employed to investigate the effects of blade number, blade wrap angle, blade thickness, and splitters on pressure head, hemolysis, and platelet activation state. Eulerian and Lagrangian features were used to analyze the flow fields and hemocompatibility metrics such as scalar shear stress, velocity distribution, and their correlation.

RESULTS

The equalization of frictional and flow losses allow impellers with more blades and smaller wrap angles to have higher pressure heads, whereas the trade-off between the volume of high scalar shear stress and exposure time allows impellers with fewer blades and larger blade wrap angles to have a lower HI; there are configurations that increase the possibility of platelet activation for both number of blades and wrap angles. The hydraulic performance and hemocompatibility of centrifugal blood pumps are not affected by blade thickness. Compared to the main blades, a splitters can improve the blood compatibility of a centrifugal blood pump with a small reduction in pressure head, but there is a trade-off between the length and location of the splitter that suppresses flow losses while reducing the velocity gradient. According to correlation analysis, pressure head, HI, and the volume of high shear stress were all substantially connected, and exposure time had a significant impact on HI. The platelet activation state was influenced by the average scalar shear stress and the volume of low velocity.

CONCLUSION

The findings reveal the impact of design variables on the performance of centrifugal blood pumps with secondary flow passages, as well as the relationship between hemocompatibility, hydraulic performance, and flow characteristics, and are useful for the design and optimization of this type of blood pump, as well as the prediction of clinical complications.

Models of Shear-Induced Platelet Activation and Numerical Implementation With Computational Fluid Dynamics Approaches

Shear-induced platelet activation is one of the critical outcomes when blood is exposed to elevated shear stress. Excessively activated platelets in the circulation can lead to thrombus formation and platelet consumption, resulting in serious adverse events such as thromboembolism and bleeding. While experimental observations reveal that it is related to the shear stress level and exposure time, the underlying mechanism of shear-induced platelet activation is not fully understood. Various models have been proposed to relate shear stress levels to platelet activation, yet most are modified from the empirically calibrated power-law model. Newly developed multiscale platelet models are tested as a promising approach to capture a single platelet's dynamic shape during activation, but it would be computationally expensive to employ it for a large-scale analysis. This paper summarizes the current numerical models used to study the shear-induced platelet activation and their computational applications in the risk assessment of a particular flow pattern and clot formation prediction.

Insights Into the Low Rate of In-Pump Thrombosis With the HeartMate 3: Does the Artificial Pulse Improve Washout?

While earlier studies reported no relevant effect of the HeartMate 3 (HM3) artificial pulse (AP) on bulk pump washout, its effect on regions with prolonged residence times remains unexplored. Using numerical simulations, we compared pump washout in the HM3 with and without AP with a focus on the clearance of the last 5% of the pump volume. Results were examined in terms of flush-volume (V_f, number of times the pump was flushed with new blood) to probe the effect of the AP independent of changing flow rate. Irrespective of the flow condition, the HM3 washout scaled linearly with flush volume up to 70% washout and slowed down for the last 30%. Flush volumes needed to washout 95% of the pump were comparable with and without the AP (1.3–1.4 V_f), while 99% washout required 2.1–2.2 V_f with the AP vs. 2.5 V_f without the AP. The AP enhanced washout of the bend relief and near-wall regions. It also transiently shifted or eliminated stagnation regions and led to rapid wall shear stress fluctuations below the rotor and in the secondary flow path. Our results suggest potential benefits of the AP for clearance of fluid regions that might elicit in-pump thrombosis and provide possible mechanistic rationale behind clinical data showing very low rate of in-pump thrombosis with the HM3. Further optimization of the AP sequence is warranted to balance washout efficacy while limiting blood damage.

Discrete responses of erythrocytes, platelets, and von Willebrand factor to shear

Despite decades of technological advancements in blood-contacting medical devices, complications related to shear flow-induced blood trauma are still frequently observed in clinic. Blood trauma includes haemolysis, platelet activation, and degradation of High Molecular Weight von Willebrand Factor (HMW vWF) multimers, all of which are dependent on the exposure time and magnitude of shear stress. Specifically, accumulating evidence supports that when blood is exposed to shear stresses above a certain threshold, blood trauma ensues; however, it remains unclear how various constituents of blood are affected by discrete shears experimentally. The aim of this study was to expose blood to discrete shear stresses and evaluate blood trauma indices that reflect red cell, platelet, and vWF structure. Citrated human whole blood (n = 6) was collected and its haematocrit was adjusted to 30 ± 2% by adding either phosphate buffered saline (PBS) or polyvinylpyrrolidone (PVP). Viscosity of whole blood was adjusted to 3.0, 12.5, 22.5 and 37.5 mPa·s to yield stresses of 3, 6, 9, 12, 50, 90 and 150 Pa in a custom-developed shearing system. Blood samples were exposed to shear for 0, 300, 600 and 900 s. Haemolysis was measured using spectrophotometry, platelet activation using flow cytometry, and HMW vWF multimer degradation was quantified with gel electrophoresis and immunoblotting. For tolerance to 300, 600 and 900 s of exposure time, the critical threshold of haemolysis was reached after blood was exposed to 90 Pa for 600 s (P < 0.05), platelet activation and HMW vWF multimer degradation were 50 Pa for 600 s and 12 Pa for 300 s respectively (P < 0.05). Our experimental results provide simultaneous comparison of blood trauma indices and thus also the relation between shear duration and magnitude required to induce damage to red cells, platelets, and vWF. Our results also demonstrate that near-physiological shear stress (<12 Pa) is needed in order to completely avoid any form of blood trauma. Therefore, there is an urgent need to design low shear-flow medical devices in order to avoid blood trauma in this blood-contacting medical device field.

The effect of turbulence modelling on the assessment of platelet activation

Pathological platelet activation by abnormal shear stresses is regarded as a main clinical complication in recipients of cardiovascular mechanical devices. In order to improve their performance computational fluid dynamics (CFD) are used to evaluate flow fields and related shear stresses. CFD models are coupled with mathematical models that describe the relation between fluid dynamics variables, and in particular shear stresses, and the platelet activation state (PAS). These models typically use a Lagrangian approach to compute the shear stresses along possible platelet trajectories. However, in the case of turbulent flow, the choice of the proper turbulence closure is still debated for both concerning its effect on shear stress calculation and Lagrangian statistics. In this study different numerical simulations of the flow through a mechanical heart valve were performed and then compared in terms of Eulerian and Lagrangian quantities: a direct numerical simulation (DNS), a large eddy simulation (LES), two Reynolds-averaged Navier-Stokes (RANS) simulations (SST k-ω and RSM) and a "laminar" (no turbulence modelling) simulation. Results exhibit a large variability in the PAS assessment depending on the turbulence model adopted. "Laminar" and RSM estimates of platelet activation are about 60% below DNS, while LES is 16% less. Surprisingly, PAS estimated from the SST k- ω velocity field is only 8% less than from DNS data. This appears more artificial than physical as can be inferred after comparing frequency distributions of PAS and of the different Lagrangian variables of the mechano-biological model of platelet activation. Our study indicates how much turbulence closures may affect platelet activation estimates, in comparison to an accurate DNS, when assessing blood damage in blood contacting devices.

Shear-mediated platelet activation in the free flow II: Evolving mechanobiological mechanisms reveal an identifiable signature of activation and a bi-directional platelet dyscrasia with thrombotic and bleeding features

Shear-mediated platelet activation (SMPA) in the "free flow" is the net result of a range of cell mechanobiological mechanisms. Previously, we outlined three main groups of mechanisms including: 1) mechano-destruction - i.e. additive platelet (membrane) damage; 2) mechano-activation - i.e. activation of shear-sensitive ion channels and pores; and 3) mechano-transduction - i.e. "outside-in" signaling via a range of transducers. Here, we report on recent advances since our original report which describes additional features of SMPA. A clear "signature" of SMPA has been defined, allowing differentiation from biochemically-mediated activation. Notably, SMPA is characterized by mitochondrial dysfunction, platelet membrane eversion, externalization of anionic phospholipids, and increased thrombin generation on the platelet surface. However, SMPA does not lead to integrin αIIbβ3 activation or P-selectin exposure due to platelet degranulation, as is commonly observed in biochemical activation. Rather, downregulation of GPIb, αIIbβ3, and P-selectin surface expression is evident. Furthermore, SMPA is accompanied by a decrease in overall platelet size coupled with a concomitant, progressive increase in microparticle generation. Shear-ejected microparticles are highly enriched in GPIb and αIIbβ3. These observations indicate the enhanced diffusion, migration, or otherwise dispersion of platelet adhesion receptors to membrane zones, which are ultimately shed as receptor-rich PDMPs. The pathophysiological consequence of this progressive shear accumulation phenomenon is an associated dyscrasia of remaining platelets - being both reduced in size and less activatable via biochemical means - a tendency to favor bleeding, while concomitantly shed microparticles are highly prothrombotic and increase the tendency for thrombosis in both local and systemic milieu. These mechanisms and observations offer direct clinical utility in allowing measurement and guidance of the net balance of platelet driven events in patients with implanted cardiovascular therapeutic devices.

Characterization of the competing role of surface-contact and shear stress on platelet activation in the setting of blood contacting devices

Supraphysiological shear stress and surface-contact are recognized as driving mechanisms of platelet activation (PA) in blood contacting devices (BCDs). However, the competing role of these mechanisms in triggering thrombogenic events is poorly understood. Here, we characterized the dynamics of PA in response to the combined effect of shear stress and material exposure. Human platelets were stimulated with different levels of shear stress (500, 750, 1000 dynes/cm²) over a range of exposure times (10, 20, and 30 min) within capillary tubes made of various polymeric materials. Polyethylene (PE), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), and polyether ether ketone (PEEK), used for BCDs fabrication, were investigated as compared to glass and thromboresistant Sigma™-coated glass. PA was quantified using the Platelet Activity State assay. Our results indicate that mechanical stimulation and polymer surface-contact both significantly contribute to PA. Notably, the contribution of the mechanical stimulus ranges between +36% and +43%, while that associated with polymer surface-contact ranges from +48% to +59%, depending on the exposure time. In more detail, our results indicate that: (i) PA increases with increasing shear stress magnitude; (ii) PA has a non-linear, time-dependent relationship to exposure time; (iii) PA is largely influenced by biomaterials, with PE and PEEK having respectively the lowest and highest prothrombotic potential; (iv) the effects of polymer surface-contact and shear stress are not correlated and can be studied separately. Our results suggest the importance of incorporating the evaluation of platelet activation driven by the combined effect of shear stress and polymer surface-contact for the comprehensive assessment, and eventually minimization, of BCDs thrombogenic potential.

Lagrangian Trajectory Simulation of Platelets and Synchrotron Microtomography Augment Hemodynamic Analysis of Intracranial Aneurysms Treated With Embolic Coils

As frequency of endovascular treatments for intracranial aneurysms increases, there is a growing need to understand the mechanisms for coil embolization failure. Computational fluid dynamics (CFD) modeling often simplifies modeling the endovascular coils as a homogeneous porous medium (PM), and focuses on the vascular wall endothelium, not considering the biomechanical environment of platelets. These assumptions limit the accuracy of computations for treatment predictions. We present a rigorous analysis using X-ray microtomographic imaging of the coils and a combination of Lagrangian (platelet) and Eulerian (endothelium) metrics. Four patient-specific, anatomically accurate in vitro flow phantoms of aneurysms are treated with the same patient-specific endovascular coils. Synchrotron tomography scans of the coil mass morphology are obtained. Aneurysmal hemodynamics are computationally simulated before and after coiling, using patient-specific velocity/pressure measurements. For each patient, we analyze the trajectories of thousands of platelets during several cardiac cycles, and calculate residence times (RTs) and shear exposure, relevant to thrombus formation. We quantify the inconsistencies of the PM approach, comparing them with coil-resolved (CR) simulations, showing the under- or overestimation of key hemodynamic metrics used to predict treatment outcomes. We fully characterize aneurysmal hemodynamics with converged statistics of platelet RT and shear stress history (SH), to augment the traditional wall shear stress (WSS) on the vascular endothelium. Incorporating microtomographic scans of coil morphology into hemodynamic analysis of coiled intracranial aneurysms, and augmenting traditional analysis with Lagrangian platelet metrics improves CFD predictions, and raises the potential for understanding and clinical translation of computational hemodynamics for intracranial aneurysm treatment outcomes.

Influence of shear rate and surface chemistry on thrombus formation in micro-crevice

Thromboembolic complications remain a central issue in management of patients on mechanical circulatory support. Despite the best practices employed in design and manufacturing of modern ventricular assist devices, complexity and modular nature of these systems often introduces internal steps and crevices in the flow path which can serve as nidus for thrombus formation. Thrombotic potential is influenced by multiple factors including the characteristics of the flow and surface chemistry of the biomaterial. This study explored these elements in the setting of blood flow over a micro-crevice using a multi-constituent numerical model of thrombosis. The simulations reproduced the platelet deposition patterns observed experimentally and elucidated the role of flow, shear rate, and surface chemistry in shaping the deposition. The results offer insights for design and operation of blood-contacting devices.

Clinical predictors and sequelae of computed tomography defined leaflet thrombosis following transcatheter aortic valve replacement at medium-term follow-up

BACKGROUND

The clinical predictors and sequelae of leaflet thrombosis (LT) following transcatheter aortic valve replacement (TAVR) is still unclear. Therefore, our aim was to determine the clinical predictors and sequelae at mid-term follow-up of computed tomography (CT)-defined LT following TAVR.

METHODS AND RESULTS

We performed a prospective evaluation with a 320-multislice CT following TAVR for the presence of LT, defined as hypo-attenuated leaflet thickening (HALT). Four-dimensional CT image-rendering was performed to determine the presence of reduced leaflet motion (RELM). 172 patients [89 (51.7%) male, mean age 82.8 ± 5.7 years] treated with commercially available TAVR device (Lotus 54%, CoreValve 32% and Sapien 3 14%) were included, with median CT-scan at 6.0 weeks post-TAVR. Prevalence of HALT was 14.0% (24 cases) and RELM was 9.8% (17 cases). On multivariate analysis, patients with HALT were less prescribed oral anticoagulation (OAC) (OR 9.9), received larger TAVR prostheses (OR 5.7) and higher rates of moderate-severe para-valvular regurgitation (PVR) (OR 16.3). There was no difference in clinical outcomes at a median follow-up of 2.3 years. Patients with RELM had significantly higher transvalvular gradients after discharge when compared to those without RELM.

CONCLUSIONS

Absence of OAC, large TAVR prostheses and moderate-severe PVR were predictors for LT. Transvalvular gradients were higher in patients that developed RELM but not HALT. Further studies are warranted to determine the long-term impact of LT on TAVR durability. Prevalence of different sub-types of CT-defined LT (HALT and RELM) and the clinical predictors of developing LT following TAVR. CT computed tomography, HALT hypo-attenuated leaflet thickening, LT leaflet thrombosis, RELM reduced leaflet motion, TAVR transcatheter aortic valve replacement.

Influence of Different Antithrombotic Regimens on Platelet-Mediated Thrombin Generation in Patients with Left Ventricular Assist Devices

We characterized the biologic background of prothrombotic platelet function in the setting of durable left ventricular assist devices (LVADs) evaluating the role of different antithrombotic regimens. Platelet-mediated thrombin generation was quantified using the Platelet Activity State (PAS) Assay and the Thrombin Generation Test (TGT) in 78 patients implanted with the HeartMate II (n = 10, 13%), the HeartMate 3 (HM3) (n = 30, 38%), or the HVAD (n = 38, 49%) and managed with oral anticoagulation plus aspirin (n = 46, 59%) or anticoagulation alone (n = 32, 41%). Coagulation parameters (platelet count, International Normalized Ratio (INR), activated Partial Thromboplastin Time, Fibrinogen and D-Dimer levels) and hemolysis (lactate dehydrogenase levels [LDH]) were also recorded to comprehensively characterize the hemostatic profile in the two groups. In patients without aspirin, the PAS assay revealed low-intensity increase in platelet prothrombinase activity (1.11-fold, p = 0.03). Similarly the TGT revealed moderate higher platelet reactivity when compared with patients receiving aspirin, consistent with reduction in lag time (0.87-fold, p < 0.001), increase in peak of thrombin generation (1.5-fold, p = 0.002) and thrombin generation rate (2-fold, p = 0.02), but comparable endogenous thrombin potential (p = 0.50). Coagulation parameters and LDH were comparable in the two groups (p > 0.05). Moreover, no differences were noted in platelet prothrombinase activity of patients implanted with the HM3 or HVAD. Our results suggest that, in the setting of durable LVADs, aspirin minimally modulates the biochemical pathway of platelet-mediated thrombin generation. Accordingly, re-evaluation of current antithrombotic management criteria in patients stratified according to bleeding/thromboembolic risk might be safe and beneficial to prevent adverse events.

Thrombotic Risk of Rotor Speed Modulation Regimes of Contemporary Centrifugal Continuous-flow Left Ventricular Assist Devices

Contemporary centrifugal continuous-flow left ventricular assist devices (LVADs) incorporate dynamic speed modulation algorithms. Hemocompatibility of these periodic unsteady pump operating conditions has been only partially explored. We evaluated whether speed modulation induces flow alterations associated with detrimental prothrombotic effects. For this aim, we evaluated the thrombogenic profile of the HeartWare ventricular assist device (HVAD) Lavare Cycle (LC) and HeartMate3 (HM3) artificial pulse (AP) via comprehensive numerical evaluation of (i) pump washout, (ii) stagnation zones, (iii) shear stress regimens, and (iv) modeling of platelet activation status via the platelet activity state (PAS) model. Data were compared between different simulated operating scenarios, including: (i) constant rotational speed and pump pressure head, used as reference; (ii) unsteady pump pressure head as induced by cardiac pulsatility; and (iii) unsteady rotor speed modulation of the LC (HVAD) and AP (HM3). Our results show that pump washout did not improve across the different simulated scenarios in neither the HVAD nor the HM3. The LC reduced but did not eliminate flow stagnation (-57%) and did not impact metrics of HVAD platelet activation (median PAS: +0.4%). The AP reduced HM3 flow stagnation by up to 91% but increased prothrombotic shear stress and simulated platelet activation (median PAS: +124%). Our study advances understanding of the pathogenesis of LVAD thrombosis, suggesting mechanistic implications of rotor speed modulation. Our data provide rationale criteria for the future design optimization of next generation LVADs to further reduce hemocompatibility-related adverse events.

Fluid dynamics characterization and thrombogenicity assessment of a levitating centrifugal pump with different impeller designs

Technical guidelines nowadays recommend and regulate the use Computational Fluid Dynamics (CFD) to assess the performance of medical devices. CFD coupled to blood damage models has emerged as a powerful tool to evaluate the hemocompatibility of blood recirculating devices. The present study is aimed at evaluating the hydrodynamic performance and the thrombogenic potential of two prototypes of magnetically levitating centrifugal pumps. The two devices differ in the impeller configuration - 6-blades vs. 12-blades - and have been designed to be used in Cardiopulmonary Bypass (CPB) circuits during open heart surgery and in Extracorporeal Membrane Oxygenation (ECMO) to support patients with severe cardiac or respiratory failure. The pumps have been modelled using Direct Numerical Simulation coupled to Lagrangian analysis to predict platelet activation due to abnormal shear stress histories. Numerical results have been compared with experimental data in terms of head generation for different working points. Results show that the 6-blades pump has i) smaller stagnation areas, ii) lower stress levels and iii) higher strain rate, resulting in a lower thrombogenic potential, whereas the 12-blade impeller guarantees a more stable performance at high flow rates, suggesting its preferential use for more demanding applications, such as CPB.

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

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

Modeling sensitivity and uncertainties in platelet activation models applied on centrifugal pumps for extracorporeal life support

Two platelet activation models were studied with respect to uncertainties of model parameters and variables. The sensitivity was assessed using two direct/deterministic approaches as well as the statistical Monte Carlo method. The first two, are linear in character whereas the latter is non-linear. The platelet activation models were applied on platelets moving within an extracorporeal centrifugal blood pump. The phenomenological, Lagrangian stress- and time-based power law-based models under consideration, have experimentally calibrated parameters and the stress expressed in a scalar form. The sensitivity of the model with respect to model parameters and the expression of the scalar stress was examined focusing on a smaller group of platelets associated with an elevated risk of activation. The results showed a high disparity between the models in terms of platelet activation state, found to depend on the platelets’ trajectory in the pump and the expression used for the scalar stress. Monte Carlo statistics was applied to the platelets at risk for activation and not to the entire platelet population. The method reveals the non-linear sensitivity of the activation models. The results imply that power-law based models have a restricted range of validity. The conclusions of this study apply to both platelet activation and hemolysis models.

Prothrombotic activity of cytokine-activated endothelial cells and shear-activated platelets in the setting of ventricular assist device support

BACKGROUND

We systematically analyzed the synergistic effect of: (i) cytokine-mediated inflammatory activation of endothelial cells (ECs) with and (ii) shear-mediated platelet activation (SMPA) as a potential contributory mechanism to intraventricular thrombus formation in the setting of left ventricular assist device (LVAD) support.

METHODS

Intact and shear-activated human platelets were exposed to non-activated and cytokine-activated ECs. To modulate the level of LVAD-related shear activation, platelets were exposed to shear stress patterns of varying magnitude (30, 50, and 70 dynes/cm², 10 minutes) via a hemodynamic shearing device. ECs were activated via exposure to inflammatory tumor necrosis factor-α (TNF-α 10 and 100 ng/ml, 24 hours), consistent with inflammatory activation recorded in patients on LVAD circulatory support.

RESULTS

Adhesivity of shear-activated platelets to ECs was significantly higher than that of intact/unactivated platelets, regardless of the initial activation level (70 dynes/cm² shear-activated platelets vs intact platelets: +80%, p < 0.001). Importantly, inflammatory activation of ECs amplified platelet prothrombinase activity progressively with increasing shear stress magnitude and TNF-α concentration: thrombin generation of 70 dynes/cm² shear-activated platelets was 2.6-fold higher after exposure and adhesion to 100 ng/ml TNF-α‒activated ECs (p < 0.0001).

CONCLUSIONS

We demonstrated synergistic effect of SMPA and cytokine-mediated EC inflammatory activation to enhance EC‒platelet adhesion and platelet prothrombotic function. These mechanisms may contribute to intraventricular thrombosis in the setting of mechanical circulatory support.

Fluid Dynamics in the HeartMate 3: Influence of the Artificial Pulse Feature and Residual Cardiac Pulsation

Ventricular assist devices (VADs), among which the HeartMate 3 (HM3) is the latest clinically approved representative, are often the therapy of choice for patients with end-stage heart failure. Despite advances in the prevention of pump thrombosis, rates of stroke and bleeding remain high. These complications are attributed to the flow field within the VAD, among other factors. One of the HM3's characteristic features is an artificial pulse that changes the rotor speed periodically by 4000 rpm, which is meant to reduce zones of recirculation and stasis. In this study, we investigated the effect of this speed modulation on the flow fields and stresses using high-resolution computational fluid dynamics. To this end, we compared Eulerian and Lagrangian features of the flow fields during constant pump operation, during operation with the artificial pulse feature, and with the effect of the residual native cardiac cycle. We observed good washout in all investigated situations, which may explain the low incidence rates of pump thrombosis. The artificial pulse had no additional benefit on scalar washout performance, but it induced rapid variations in the flow velocity and its gradients. This may be relevant for the removal of deposits in the pump. Overall, we found that viscous stresses in the HM3 were lower than in other current VADs. However, the artificial pulse substantially increased turbulence, and thereby also total stresses, which may contribute to clinically observed issues related to hemocompatibility.

Blood damage in Left Ventricular Assist Devices: Pump thrombosis or system thrombosis?

Introduction:

Despite significant technical advancements in the design and manufacture of Left Ventricular Assist Devices, post-implant thrombotic and thromboembolic complications continue to affect long-term outcomes. Previous efforts, aimed at optimizing pump design as a means of reducing supraphysiologic shear stresses generated within the pump and associated prothrombotic shear-mediated platelet injury, have only partially altered the device hemocompatibility.

Methods:

We examined hemodynamic mechanisms that synergize with hypershear within the pump to contribute to the thrombogenic potential of the overall Left Ventricular Assist Device system.

Results:

Numerical simulations of blood flow in differing regions of the Left Ventricular Assist Device system, that is the diseased native left ventricle, the pump inflow cannula, the impeller, the outflow graft and the anastomosed downstream aorta, reveal that prothrombotic hemodynamic conditions might occur at these specific sites. Furthermore, we show that beyond hypershear, additional hemodynamic abnormalities exist within the pump, which may elicit platelet activation, such as recirculation zones and stagnant platelet trajectories. We also provide evidences that particular Left Ventricular Assist Device implantation configurations and specific post-implant patient management strategies, such as those allowing aortic valve opening, are more hemodynamically favorable and reduce the thrombotic risk.

Conclusion:

We extend the perspective of pump thrombosis secondary to the supraphysiologic shear stress environment of the pump to one of Left Ventricular Assist Device system thrombosis, raising the importance of comprehensive characterization of the different prothrombotic risk factors of the total system as the target to achieve enhanced hemocompatibility and improved clinical outcomes.

Couette shearing device for the investigation of shear-induced damage of the primary hemostasis by left ventricular assist devices

INTRODUCTION:

Continuous-flow left ventricular assist devices have evolved from short-time therapy into permanent or so-called destination therapy. One complication in long-term usage is bleeding, which is presumably attributed to shear-induced interference of left ventricular assist devices with the coagulation system.

METHODS:

The influence of dynamic shear stresses on primary hemostasis by single or multiple passes through left ventricular assist devices was investigated. A novel Couette-type shearing device, especially fitted to simulate left ventricular assist devices with highly dynamic and repetitive stresses, was developed. To evaluate the clotting ability of the blood and thus the bleeding tendency, the closure time of the platelet function analyzer (PFA-100^®, Dade Behring, Marburg, Germany) was used. The relationship of the PFA-100 closure time was fitted to measurement points with shear stress and exposure time as parameters.

RESULTS:

76 samples of human blood collected from four different healthy donors in sodium-citrate anticoagulant solution were tested, including 20 control samples. A damage model according to the power law approach could be developed. A linear correlation of shear stress and exposure time to the PFA-100 closure time could be determined. In addition, a model was developed to calculate the increase in the PFA closure time on the basis of shear stress over time curves.

DISCUSSION:

With the shearing device, half-sine-wave-shaped shear stress patterns relevant to rotary blood pumps can be achieved with very good repeatability. The proposed damage model could be used to compare and optimize left ventricular assist devices under development. The tests showed a significant decrease in coagulability after only a few repetitions.

Microfluidic flow-based platforms for induction and analysis of dynamic shear-mediated platelet activation-Initial validation versus the standardized hemodynamic shearing device

A microfluidic flow-based platform (μFP), able to stimulate platelets via exposure of shear stress patterns pertinent to cardiovascular devices and prostheses, was compared to the Hemodynamic Shearing Device (HSD)-a state-of-the-art bench-top system for exposure of platelets to defined levels and patterns of shear. Platelets were exposed to time-varying shear stress patterns in the two systems; in detail, platelets were recirculated in the μFP or stimulated in the HSD to replicate comparable exposure time. Shear-mediated platelet activation was evaluated via (i) the platelet activity state assay, allowing the measurement of platelet-mediated thrombin generation and associated prothrombotic tendencies, (ii) scanning electron microscopy to evaluate morphological changes of sheared platelets, and (iii) flow cytometry for the determination of platelet phosphatidylserine exposure as a marker of shear activation. The results revealed good matching and comparability between the two systems, with similar trends of platelet activation, formation of microaggregates, and analogous trends of activation marker exposure for both the HSD and microfluidic-stimulated samples. These findings support future translation of the microfluidic platform as a Point-of-Care facsimile system for the diagnosis of thrombotic risk in patients implanted with cardiovascular devices.

Routine clinical anti-platelet agents have limited efficacy in modulating hypershear-mediated platelet activation associated with mechanical circulatory support

INTRODUCTION

Continuous flow ventricular assist devices (cfVADs) continue to be limited by thrombotic complications associated with disruptive flow patterns and supraphysiologic shear stresses. Patients are prescribed complex antiplatelet therapies, which do not fully prevent recurrent thromboembolic events. This is partially due to limited data on antiplatelet efficacy under cfVAD-associated shear conditions.

MATERIALS AND METHODS

We investigated the efficacy of antiplatelet drugs directly acting on three pathways: (1) cyclooxygenase (aspirin), (2) phosphodiesterase (dipyridamole, pentoxifylline, cilostazol), and (3) glycoprotein IIb-IIIa (eptifibatide). Gel-filtered platelets treated with these drugs were exposed for 10min to either constant shear stresses (30dyne/cm² and 70dyne/cm²) or dynamic shear stress profiles extracted from simulated platelet trajectories through a cfVAD (Micromed DeBakey). Platelet activation state (PAS) was measured using a modified prothrombinase-based assay, with drug efficacy quantified based on PAS reduction compared to untreated controls.

RESULTS AND CONCLUSIONS

Significant PAS reduction was observed for all drugs after exposure to 30dyne/cm² constant shear stress, and all drugs but dipyridamole after exposure to the 30th percentile shear stress waveform of the cfVAD. However, only cilostazol was significantly effective after 70dyne/cm² constant shear stress exposure, though no significant reduction was observed upon exposure to median shear stress conditions in the cfVAD. These results, coupled with the persistence of reported clinical thrombotic complication, suggest the need for the development of new classes of drugs that are especially designed to mitigate thrombosis in cfVAD patients, while reducing or eliminating the risk of bleeding.

Microfludic platforms for the evaluation of anti-platelet agent efficacy under hyper-shear conditions associated with ventricular assist devices

Thrombus formation is a major adverse event affecting patients implanted with ventricular assist devices (VADs). Despite anti-thrombotic drug administration, thrombotic events remain frequent within the first year post-implantation. Platelet activation (PA) is an essential process underling thrombotic adverse events in VAD systems. Indeed, abnormal shear forces, correlating with specific flow trajectories of VADs, are strong agonists mediating PA. To date, the ability to determine efficacy of anti-platelet (AP) agents under shear stress conditions is limited. Here, we present a novel microfluidic platform designed to replicate shear stress patterns of a clinical VAD, and use it to compare the efficacy of two AP agents in vitro. Gel-filtered platelets were incubated with i) acetylsalicylic acid (ASA) and ii) ticagrelor, at two different concentrations (ASA: 125 and 250 µM; ticagrelor: 250 and 500 nM) and were circulated in the VAD-emulating microfluidic platform using a peristaltic pump. GFP was collected after 4 and 52 repetitions of exposure to the VAD shear pattern and tested for shear-mediated PA. ASA significantly inhibited PA only at 2-fold higher concentration (250 µM) than therapeutic dose (125 µM). The effect of ticagrelor was not dependent on drug concentration, and did not show significant inhibition with respect to untreated control. This study demonstrates the potential use of microfluidic platforms as means of testing platelet responsiveness and AP drug efficacy under complex and realistic VAD-like shear stress conditions.