Stress and Exposure Time on von Willebrand Factor Degradation

Effect of veno-arterial extracorporeal membrane oxygenation lower-extremity cannulation on intra-arterial flow characteristics, oxygen content, and thrombosis risk

PURPOSE

This study aimed to investigate the effects of lower-extremity cannulation on the intra-arterial hemodynamic environment, oxygen content, blood damage, and thrombosis risk under different levels of veno-arterial (V-A) ECMO support.

METHODS

Computational fluid dynamics methods were used to investigate the effects of different levels of ECMO support (ECMO flow ratios supplying oxygen-rich blood 100-40 %). Flow rates and oxygen content in each arterial branch were used to determine organ perfusion. A new thrombosis model considering platelet activation and deposition was proposed to determine the platelet activation and thrombosis risk at different levels of ECMO support. A red blood cell damage model was used to explore the risk of hemolysis.

RESULTS

Our study found that partial recovery of cardiac function improved the intra-arterial hemodynamic environment, with reduced impingement of the intra-arterial flow field by high-velocity blood flow from the cannula, a flow rate per unit time into each arterial branch closer to physiological levels, and improved perfusion in the lower extremities. Partial recovery of cardiac function helps reduce intra-arterial high shear stress and residence time, thereby reducing blood damage. The overall level of hemolysis and platelet activation in the aorta decreased with the gradual recovery of cardiac contraction function. The areas at high risk of thrombosis under V-A ECMO femoral cannulation support were the aortic root and the area distal to the cannula, which moved to the descending aorta when cardiac function recovered to 40-60 %. However, with the recovery of cardiac contraction function, hypoxic blood pumped by the heart is insufficient in supplying oxygen to the front of the aortic arch, which may result in upper extremity hypoxia.

CONCLUSION

We developed a thrombosis risk prediction model applicable to ECMO cannulation and validated the model accuracy using clinical data. Partial recovery of cardiac function contributed to an improvement in the aortic hemodynamic environment and a reduction in the risk of blood damage; however, there is a potential risk of insufficient perfusion of oxygen-rich blood to organs.

Multi-Method Investigation of Blood Damage Induced By Blood Pumps in Different Clinical Support Modes

To investigate the effects of blood pumps operated in different modes on nonphysiologic flow patterns, cell and protein function, and the risk of bleeding, thrombosis, and hemolysis, an extracorporeal blood pump (CentriMag) was operated in three clinical modalities including heart failure (HF), venous-venous (V-V) extracorporeal membrane oxygenation (ECMO), and venous-arterial (V-A) ECMO. Computational fluid dynamics (CFD) methods and coupled hemolysis models as well as recently developed bleeding and thrombosis models associated with changes in platelet and von Willebrand factor (vWF) function were used to predict hydraulic performance and hemocompatibility. The V-A ECMO mode had the highest flow losses and shear stress levels, the V-V ECMO mode was intermediate, and the HF mode was the lowest. Different nonphysiologic flow patterns altered cell/protein morphology and function. The V-A ECMO mode resulted in the highest levels of platelet activation, receptor shedding, vWF unfolding, and high molecular weight multimers vWF (HMWM-vWF) degradation, leading to the lowest platelet adhesion and the highest vWF binding capacity, intermediate in the V-V ECMO mode, and opposite in the HF mode. The V-A ECMO mode resulted in the highest risk of bleeding, thrombosis, and hemolysis, with the V-V ECMO mode intermediate and the HF mode lowest. These findings are supported by published experimental or clinical statistics. Further studies found that secondary blood flow passages resulted in the highest risk of blood damage. Nonphysiologic blood flow patterns were strongly associated with cell and protein function changing, blood damage, and complications.

Development of the PSU Child Pump

The Pennsylvania State University (PSU) Child Pump, a centrifugal continuous-flow ventricular assist device (cf-VAD), is being developed as a suitable long-term implantable device for pediatric heart failure patients between 10 and 35 kg, body surface area (BSA) of 0.5-1.2 m2, 1-11 years of age, and requiring a mean cardiac output of 1.0-3.5 L/min. In-vitro hydraulic and hemodynamic performances were evaluated on a custom mock circulatory loop with ovine blood. Normalized index of hemolysis (NIH) was evaluated under four conditions: 1) 8,300 rpm, 3.5 L/min, ΔP = 60 mm Hg, 2) 8,150 rpm, 5.1 L/min, ΔP = 20 mm Hg, 3) 8,400 rpm, 3.2 L/min, ΔP = 70 mm Hg, and 4) 9,850 rpm, 5.0 L/min, ΔP = 80 mm Hg, resulting in normalized index of hemolysis = 0.027 ± 0.013, 0.015 ± 0.006, 0.016 ± 0.008, and 0.026 ± 0.011 mg/dl, respectively. A mock fit study was conducted using a three-dimensional printed model of a 19 kg patient's thoracic cavity to compare the size of the PSU Child Pump to the HeartMate3 and the HVAD. Results indicate the PSU Child Pump will be a safer, appropriately sized device capable of providing the given patient cohort proper support while minimizing the risks of blood trauma as they wait for a transplant.

Development and validation of a mathematical model for evaluating shear-induced damage of von Willebrand factor

PURPOSE

To develop a mathematical model for predicting shear-induced von Willebrand factor (vWF) function modification which can be used to guide ventricular assist devices (VADs) design, and evaluate the damage of high molecular weight multimers (HMWM)-vWF in VAD patients for reducing clinical complications.

METHODS

Mathematical models were constructed based on three morphological variations (globular vWF, unfolded vWF and degraded vWF) of vWF under shear stress conditions, in which parameters were obtained from previous studies or fitted by experimental data. Different clinical support modes (pediatric vs. adult mode), different VAD operating states (pulsation vs. constant mode) and different clinical VADs (HeartMate II, HeartWare and CentriMag) were utilized to analyze shear-induced damage of HMWM-vWF based on our vWF model. The accuracy and feasibility of the models were evaluated using various experimental and clinical cases, and the biomechanical mechanisms of HMWM-vWF degradation induced by VADs were further explained.

RESULTS

The mathematical model developed in this study predicted VAD-induced HMWM-vWF degradation with high accuracy (correlation with experimental data r2 > 0.99). The numerical results showed that VAD in the pediatric mode resulted in more HMWM-vWF degradation per unit time and per unit flow rate than in the adult mode. However, the total degradation of HMWM-vWF is less in the pediatric mode than in the adult mode because the pediatric mode has fewer times of blood circulation than the adult mode in the same amount of time. The ratio of HMWM-vWF degradation was lower in the pulsation mode than in the constant mode. This is due to the increased flushing of VADs in the pulsation mode, which avoids prolonged stagnation of blood in high shear regions. This study also found that the design feature, rotor size and volume of the VADs, and the superimposed regions of high shear stress and long residence time inside VADs affect the degradation of HMWM-vWF. The axial flow VADs (HeartMate II) showed higher degradation of HMWM-vWF compared to centrifugal VADs (HeartWare and CentriMag). Compared to fully magnetically suspended VADs (CentriMag), hydrodynamic suspended VADs (HeartWare) produced extremely high degradation of HWMW-vWF in its narrow hydrodynamic clearance. Finally, the study used a mathematical model of HMWM-vWF degradation to interpret the clinical statistics from a biomechanical perspective and found that minimizing the rotating speed of VADs within reasonable limits helps to reduce HWMW-vWF degradation. All predicted conclusions are supported by the experimental and clinical data.

CONCLUSION

This study provides a validated mathematical model to assess the shear-induced degradation of HMWM-vWF, which can help to evaluate the damage of HMWM-vWF in patients implanted with VADs for reducing clinical complications, and to guide the optimization of VADs for improving hemocompatibility.

Turbulence in surgical suction heads as detected by MRI

BACKGROUND

Blood loss is common during surgical procedures, especially in open cardiac surgery. Allogenic blood transfusion is associated with increased morbidity and mortality. Blood conservation programs in cardiac surgery recommend re-transfusion of shed blood directly or after processing, as this decreases transfusion rates of allogenic blood. But aspiration of blood from the wound area is often associated with increased hemolysis, due to flow induced forces, mainly through development of turbulence.

METHODS

We evaluated magnetic resonance imaging (MRI) as a qualitative tool for detection of turbulence. MRI is sensitive to flow; this study uses velocity-compensated T1-weighted 3D MRI for turbulence detection in four geometrically different cardiotomy suction heads under comparable flow conditions (0-1250 mL/min).

RESULTS

Our standard control suction head Model A showed pronounced signs of turbulence at all flow rates measured, while turbulence was only detectable in our modified Models 1-3 at higher flow rates (Models 1 and 3) or not at all (Model 2).

CONCLUSIONS

The comparison of flow performance of surgical suction heads with different geometries via acceleration-sensitized 3D MRI revealed significant differences in turbulence development between our standard control Model A and the modified alternatives (Models 1-3). As flow conditions during measurement have been comparable, the specific geometry of the respective suction heads must have been the main factor responsible. The underlying mechanisms and causative factors can only be speculated about, but as other investigations have shown, hemolytic activity is positively associated with degree of turbulence. The turbulence data measured in this study correlate with data from other investigations about hemolysis induced by surgical suction heads. The experimental MRI technique used showed added value for further elucidating the underlying physical phenomena causing blood damage due to non-physiological flow.

Kinetic and Dynamic Effects on Degradation of von Willebrand Factor

The loss of high molecular weight multimers (HMWM) of von Willebrand factor (vWF) in aortic stenosis (AS) and continuous-flow left ventricular assist devices (cf-LVADs) is believed to be associated with high turbulent blood shear. The objective of this study is to understand the degradation mechanism of HMWM in terms of exposure time (kinetic) and flow regime (dynamics) within clinically relevant pathophysiologic conditions. A custom high-shear rotary device capable of creating fully controlled exposure times and flows was used. The system was set so that human platelet-poor plasma flowed through at 1.75 ml/sec, 0.76 ml/sec, or 0.38 ml/sec resulting in the exposure time ( texp ) of 22, 50, or 100 ms, respectively. The flow was characterized by the Reynolds number (Re). The device was run under laminar (Re = 1,500), transitional (Re = 3,000; Re = 3,500), and turbulent (Re = 4,500) conditions at a given texp followed by multimer analysis. No degradation was observed at laminar flow at all given texp . Degradation of HMWM at a given texp increases with the Re. Re ( p < 0.0001) and texp ( p = 0.0034) are significant factors in the degradation of HMWM. Interaction between Re and texp , however, is not always significant ( p = 0.73).

A mathematical model for assessing shear induced bleeding risk

PURPOSE

The objective of this study is to develop a bleeding risk model for assessing device-induced bleeding risk in patients supported with blood contact medical devices (BCMDs).

METHODS

The mathematical model for evaluating bleeding risk considers the effects of shear stress on von Willebrand factor (vWF) unfolding, high molecular weight multimers-vWF (HMWM-vWF) degradation, platelet activation and receptor shedding and platelet-vWF binding ability. Functions of the effect of shear stress on the above factors are fitted/employed and solved by the Eulerian transport equation. An axial flow-through Couette device and two clinical VADs which are HeartWare Ventricular Assist Device (HVAD) and HeartMate II (HM II) blood pump were employed to perform the simulation to evaluate platelet receptor shedding (GPIbα and GPIIb/IIIa), loss of HWMW-vWF, platelet-vWF binding ability and bleeding risk for validating the accuracy of our model.

RESULTS

The platelet-vWF binding ability after being subjected to high shear region in the axial flow-through Couette device predicted by our bleeding model was highly consistent with reported experimental data. As indicated by our CFD simulation results in the axial flow-through Couette device, it can find that an increase in shear stress led to a decrease in the adhesion ability of platelets on vWF, while the binding ability of vWF with platelets first increase and then decrease as shear stress elevates gradually beyond a threshold. The factor of exposure time can enhance the effect of shear stress. Additionally, the shear-induced bleeding risk predicted by our model increases with increasing shear stress and exposure time in an axial flow-through Couette device. As indicated by our numerical model, the bleeding risk in HVAD was higher than HMII, which is highly consistent with the meta-analysis based on clinical statistics. Our simulation investigations in these two clinical VADs also found that HVAD caused a higher rate of platelet receptor shedding and lower damage to HWMW-vWF than HeartMate II. The high shear stress generated in the narrow and turbulent regions of both VADs was the underlying cause of device-induced bleeding.

CONCLUSION

In this study, the shear-induced bleeding risk predicted by our bleeding model in axial flow-through Couette device and two clinical VADs is consistent or highly correlated with experimental and clinical findings, which proves the accuracy of our bleeding model. Our bleeding model can be used to aid the development of new BCMDs with improved functional characteristics and biocompatibility, and help to reduce risk of device-induced adverse events in patients.

Performance of a Hydrogel Coated Nitinol with Oligonucleotide-Modified Nanoparticles Within Turbulent Conditions of Blood-Contacting Devices

INTRODUCTION

Hydrogels offer a wide range of applications in the antithrombotic modification of biomedical devices. The functionalization of these hydrogels with potentially drug-laden nanoparticles in the context of deviceassociated turbulence is critically under-studied. Thus, the purpose of this study was to use a hydrogel-coating nitinol surface as a model to understand the functions of hydrogels and the capture of nanoparticles under clinically relevant flow conditions.

METHODS

Nitinol was coated by an oligonucleotide (ON) functionalized hydrogel. Nanoparticles were functionalized with complementary oligonucleotides (CONs). The capture of CONfunctionalized nanoparticles by the ON-functionalized hydrogel surfaces was studied under both static and dynamic attachment conditions. Fluorescent-labelling of nanoparticles was utilized to assess capture efficacy and resistance to removal by device-relevant flow conditions.

RESULTS

The specificity of the ON-CON bond was verified, exhibiting a dose-dependent attachment response. The hydrogel coating was resistant to stripping by flow, retaining >95% after exposure to one hour of turbulent flow. Attachment of nanoparticles to the hydrogel was higher in the static condition than under laminar flow (p < 0.01), but comparable to that of attachment under turbulent flow. Modified nitinol samples underwent one hour of flow treatment under both laminar and turbulent regimes and demonstrated decreased nanoparticle loss following static conjugation rather than turbulent conjugation (36.1% vs 53.8%, p < 0.05). There was no significant difference in nanoparticle functionalization by upstream injection between laminar and turbulent flow.

CONCLUSION

The results demonstrate promising potential of hydrogelfunctionalized nitinol for capturing nanoparticles using nucleic acid hybridization. The hydrogel structure and ONCON bond integrity both demonstrated a resistance to mechanical damage and loss of biomolecular functionalization by exposure to turbulence. Further investigation is warranted to highlight drug delivery and antithrombogenic modification applications of nanoparticle-functionalized hydrogels.

In vitro study of red blood cell and VWF damage in mechanical circulatory support devices based on blood-shearing platform

Blood damage induced by mechanical circulatory support devices (MCSDs) remains a significant challenge to optimal clinical care. Although researchers have been conducting in vitro studies, the major determinant of blood damage is still unclear. An optimized capillary tube blood-shearing platform with custom designed parts was constructed to investigate the influence of two flow-dependent parameters (shear stress and exposure time) on the shear-induced damage of red blood cells and von Willebrand factor (VWF). Blood samples under different high shear stress and instantaneous exposure time were obtained by changing the flow rate and the length of capillary tube. Plasma free hemoglobin assay and immunoblotting of VWF were then performed on the sheared blood samples. The quantitative correlation between the hemolysis index and the two flow-dependent parameters was found following the power law mathematical model under the flow condition with high shear stress and instantaneous exposure time. The degradation of high molecular weight VWF was not obvious under high shear stress factor. However, the degradation of high molecular weight VWF was found as the result of the accumulation over exposure time under non-physiological shear stress, which was consistent with the different mechanism of VWF damage comparing to red blood cell damage. Compared to peak shear stress, exposure time has a greater effect on both red blood cell and VWF damage. To improve the hemocompatibility of MCSDs, it is more important to avoid regions of slow blood flow with non-physiological shear stress under laminar flow conditions.

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.

Distribution and history of extensional stresses on vWF surrogate molecules in turbulent flow

The configuration of proteins is critical for their biochemical behavior. Mechanical stresses that act on them can affect their behavior leading to the development of decease. The von Willebrand factor (vWF) protein circulating with the blood loses its efficacy when it undergoes non-physiological hemodynamic stresses. While often overlooked, extensional stresses can affect the structure of vWF at much lower stress levels than shear stresses. The statistical distribution of extensional stress as it applies on models of the vWF molecule within turbulent flow was examined here. The stress on the molecules of the protein was calculated with computations that utilized a Lagrangian approach for the determination of the molecule trajectories in the flow filed. The history of the stresses on the proteins was also calculated. Two different flow fields were considered as models of typical flows in cardiovascular mechanical devises, one was a Poiseuille flow and the other was a Poiseuille–Couette flow field. The data showed that the distribution of stresses is important for the design of blood flow devices because the average stress can be below the critical value for protein damage, but tails of the distribution can be outside the critical stress regime.

Dynamics of Blood Flows in Aortic Stenosis: Mild, Moderate, and Severe

Supraphysiologic high shear stresses created in calcific aortic stenosis (AS) are known to cause hemostatic abnormalities, however, the relationship between the complex blood flows over the severity of AS and hemostatic abnormalities still remains unclear. This study systematically characterized the blood flow in mild, moderate, and severe AS. A series of large eddy simulations (LES) validated by particle image velocimetry were performed on physiologically representative AS models with a peak physiologic flow condition of 18 liter per minute. Time-accurate velocity fields, transvalvular pressure gradient, and laminar viscous-and turbulent (or Reynolds) shear stresses (RSSmax) were evaluated for each degree of severity. The peak velocities of mild, moderate, and severe AS were on the order of 2.0, 4.0, and 8.0 m/s, respectively. Jet velocity in severe AS was highly skewed with extremely high velocity (as high as 8 m/s) and mainly traveled through the posterior aortic wall up to the aortic arch while still carrying a relatively high velocity, that is, >4 m/s. The mean laminar viscous wall shear stresses (WSS) for mild, moderate, and severe AS were on the order of 40, 100, and 180 Pa, respectively. The RSSmax were on the order of 260, 490, and 2,500 Pa for mild, moderate, and severe AS, respectively. This study may provide a link between altered flows in AS and hemostatic abnormalities such as acquired von Willebrand syndrome and hemolysis, thus, help diagnosing and timing of the treatment.

Mathematical models for shear-induced blood damage based on vortex platform

Non-physiological shear stress in Ventricular Assist Device (VAD) is considered to be an important trigger of blood damage, which has become the biggest shackle for clinical application. The researches on blood damage in literature were limited to qualitative but did not make much quantitative analysis. The purpose of this study was to investigate the quantitative influence of two flow-dependent parameters: shear stress (rotational speed) and exposure time on the shear-induced damage of red blood cells and von Willebrand Factor (vWF). A vortex blood-shearing platform was constructed to conduct in vitro experiments. Free hemoglobin assay and vWF molecular weight analysis were then performed on the sheared blood samples. MATLAB was used for regression fitting of original experimental data. The quantitative correlations between the hemolysis index, the degradation of high molecular weight vWF and the two flow-dependent parameters were found both following the power law model. The mathematic models indicated that the sensitivity of blood damage on red blood cells and vWF to exposure time was both greater than that of shear stress. Besides, the damage of vWF was more serious than that of red blood cells at the same flow condition. The models could be used to predict blood damage in blood-contacting medical devices, especially for the slow even stagnant blood flow regions in VAD, thus may provide useful guidance for VAD development and improvement. It also indicated that the vortex platform can be used to study the law of blood damage for the simple structure and easy operation.

Antiplatelet and anticoagulation strategies for left ventricular assist devices

Left ventricular assist devices (LVAD) have revolutionized the management of advanced heart failure. However, complications rates remain high, among which hemorrhagic and thrombotic complications are the most important. Antiplatelet and anticoagulation strategies form a cornerstone of LVAD management and may directly affect LVAD complications. Concurrently, LVAD complications influence anticoagulation and anticoagulation management. A thorough understanding of device, patient, and management, including anticoagulation and antiplatelet therapies, are important in optimizing LVAD outcomes. This article provides a comprehensive state of the art review of issues related to antiplatelet and anticoagulation management in LVADs. We start with a historical overview, the epidemiology and pathophysiology of bleeding and thrombotic complications in LVADs. We then discuss platelet and anticoagulation biology followed by considerations prior to, during, and after LVAD implantation. This is followed by discussion of anticoagulation and the management of thrombotic and hemorrhagic complications. Specific problems, including management of heparin-induced thrombocytopenia, anticoagulant reversal, novel oral anticoagulants, artificial heart valves, and noncardiac surgeries are covered in detail.

Shear-dependent platelet aggregation size

Nonsurgical bleeding is the most frequent complication of left ventricular assist device (LVAD) support. Supraphysiologic shear rates generated in LVAD causes impaired platelet aggregation, which increases the risk of bleeding. The effect of shear rate on the formation size of platelet aggregates has never been reported experimentally, although platelet aggregation size can be considered to be directly relevant to bleeding complications. Therefore, this study investigated the impact of shear rate and exposure time on the formation size of platelet aggregates, which is vital in predicting bleeding in patients with an LVAD. Human platelet‐poor plasma (containing von Willebrand factor, vWF) and fluorochrome‐labeled platelets were subjected to a range of shear rates (0‐10 000 s⁻¹) for 0, 5, 10, and 15 minutes using a custom‐built blood‐shearing device. Formed sizes of platelet aggregates under a range of shear‐controlled environment were visualized and measured using microscopy. The loss of high molecular weight (HMW) vWF multimers was quantified using gel electrophoresis and immunoblotting. An inhibition study was also performed to investigate the reduction in platelet aggregation size and HMW vWF multimers caused by either mechanical shear or enzymatic (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13—ADAMTS13, the von Willebrand factor protease) mechanism under low and high shear conditions (360 and 10 000 s⁻¹). We found that the average size of platelet aggregates formed under physiological shear rates of 360‐3000 s⁻¹ (200‐300 μm²) was significantly larger compared to those sheared at >6000 s⁻¹ (50‐100 μm²). Furthermore, HMW vWF multimers were reduced with increased shear rates. The inhibition study revealed that the reduction in platelet aggregation size and HWM vWF multimers were mainly associated with ADAMTS13. In conclusion, the threshold of shear rate must not exceed >6000 s⁻¹ in order to maintain the optimal size of platelet aggregates to “plug off” the injury site and stop bleeding.

Pathologic Shear and Elongation Rates Do Not Cause Cleavage of Von Willebrand Factor by ADAMTS13 in a Purified System

INTRODUCTION

Pathological flows in patients with severe aortic stenosis are associated with acquired von Willebrand syndrome. This syndrome is characterized by excessive cleavage of von Willebrand factor by its main protease, A Disintegrin and Metalloproteinase with a Thrombospondin Type 1 Motif, Member 13 (ADAMTS13) leading to decreased VWF function and mucocutaneous bleeding. Aortic valve replacement and correction of the flow behavior to physiological levels reverses the syndrome, supporting the association between pathological flow and acquired von Willebrand syndrome. We investigated the effects of shear and elongational rates on von Willebrand factor cleavage in the presence of ADAMTS13.

METHODS

We identified acquired von Willebrand syndrome in five patients with severe aortic stenosis. Doppler echography values from these patients were used to develop three computational fluid dynamic (CFD) aortic valve models (normal, mild and severe stenosis). Shear, elongational rates and exposure times identified in the CFD simulations were used as parameters for the design of microfluidic devices to test the effects of pathologic shear and elongational rates on the structure and function of von Willebrand factor.

RESULTS

The shear rates (0-10,000s-1), elongational rates (0-1000 s-1) and exposure times (1-180 ms) tested in our microfluidic designs mimicked the flow features identified in patients with aortic stenosis. The shear and elongational rates tested in vitro did not lead to excessive cleavage or decreased function of von Willebrand factor in the presence of the protease.

CONCLUSIONS

High shear and elongational rates in the presence of ADAMTS13 are not sufficient for excessive cleavage of von Willebrand Factor.