von Willebrand factor unfolding mediates platelet deposition in a model of high-shear thrombosis

Numerical study of ultra-large von Willebrand factor multimers in coagulopathy

An excessive von Willebrand factor (VWF) secretion, coupled with a moderate to severe deficiency of ADAMTS13 activity, serves as a linking mechanism between inflammation to thrombosis. The former facilitates platelet adhesion to the vessel wall and the latter is required to cleave VWF multimers. As a result, the ultra-large VWF (UL-VWF) multimers released by Weibel-Palade bodies remain uncleaved. In this study, using a computational model based on first principles, we quantitatively show how the uncleaved UL-VWF multimers interact with the blood cells to initiate microthrombosis. We observed that platelets first adhere to unfolded and stretched uncleaved UL-VWF multimers anchored to the microvessel wall. By the end of this initial adhesion phase, the UL-VWF multimers and platelets make a mesh-like trap in which the red blood cells increasingly accumulate to initiate a gradually growing microthrombosis. Although high-shear rate and blood flow velocity are required to activate platelets and unfold the UL-VWFs, during the initial adhesion phase, the blood velocity drastically drops after thrombosis, and as a result, the wall shear stress is elevated near UL-VWF roots, and the pressure drops up to 6 times of the healthy condition. As the time passes, these trends progressively continue until the microthrombosis fully develops and the effective size of the microthrombosis and these flow quantities remain almost constant. Our findings quantitatively demonstrate the potential role of UL-VWF in coagulopathy.

In Vitro and In Silico Characterization of the Aggregation of Thrombi on Textured Ventricular Cannula

The unacceptably high stroke rate associated with HeartMate 3 ventricular assist device (VAD) without signs of adherent pump thrombosis is hypothesized to be the result of the emboli produced by the inflow cannula, that are ingested and ejected from the pump. This in vitro and numerical study aimed to emulate the surface features and supraphysiological shear of a ventricular cannula to provide insight into their effect on thrombogenesis. Human whole blood was perfused at calibrated flow rates in a microfluidic channel to achieve shear rates 1000-7500 s-1, comparable to that experienced on the cannula. The channel contained periodic teeth representative of the rough sintered surface of the HeartMate 3 cannula. The deposition of fluorescently labeled platelets was visualized in real time and analyzed with a custom entity tracking algorithm. Numerical simulations of a multi-constituent thrombosis model were performed to simulate laminar blood flow in the channel. The sustained growth of adherent platelets was observed in all shear conditions ( p <  0.05). However, the greatest deposition was observed at the lower shear rates. The location of deposition with respect to the microfluidic teeth was also found to vary with shear rate. This was confirmed by CFD simulation. The entity tracking algorithm revealed the spatial variation of instances of embolic events. This result suggests that the sintered surface of the ventricular cannula may engender unstable thrombi with a greater likelihood of embolization at supraphysiological shear rates.

A continuum model for the elongation and orientation of Von Willebrand factor with applications in arterial flow

The blood protein Von Willebrand factor (VWF) is critical in facilitating arterial thrombosis. At pathologically high shear rates, the protein unfolds and binds to the arterial wall, enabling the rapid deposition of platelets from the blood. We present a novel continuum model for VWF dynamics in flow based on a modified viscoelastic fluid model that incorporates a single constitutive relation to describe the propensity of VWF to unfold as a function of the scalar shear rate. Using experimental data of VWF unfolding in pure shear flow, we fix the parameters for VWF's unfolding propensity and the maximum VWF length, so that the protein is half unfolded at a shear rate of approximately 5000 s - 1 . We then use the theoretical model to predict VWF's behaviour in two complex flows where experimental data are challenging to obtain: pure elongational flow and stenotic arterial flow. In pure elongational flow, our model predicts that VWF is 50% unfolded at approximately 2000 s - 1 , matching the established hypothesis that VWF unfolds at lower shear rates in elongational flow than in shear flow. We demonstrate the sensitivity of this elongational flow prediction to the value of maximum VWF length used in the model, which varies significantly across experimental studies, predicting that VWF can unfold between 2000 and 3200 s - 1 depending on the selected value. Finally, we examine VWF dynamics in a range of idealised arterial stenoses, predicting the relative extension of VWF in elongational flow structures in the centre of the artery compared to high shear regions near the arterial walls.

Influence of Hematocrit Level and Integrin αIIbβIII Function on vWF-Mediated Platelet Adhesion and Shear-Induced Platelet Aggregation in a Sudden Expansion

Purpose

Shear-mediated thrombosis is a clinically relevant phenomenon that underlies excessive arterial thrombosis and device-induced thrombosis. Red blood cells are known to mechanically contribute to physiological hemostasis through margination of platelets and vWF, facilitating the unfurling of vWF multimers, and increasing the fraction of thrombus-contacting platelets. Shear also plays a role in this phenomenon, increasing both the degree of margination and the near-wall forces experienced by vWF and platelets leading to unfurling and activation. Despite this, the contribution of red blood cells in shear-induced platelet aggregation has not been fully investigated-specifically the effect of elevated hematocrit has not yet been demonstrated.

Methods

Here, a microfluidic model of a sudden expansion is presented as a platform for investigating platelet adhesion at hematocrits ranging from 0 to 60% and shear rates ranging from 1000 to 10,000 s-1. The sudden expansion geometry models nonphysiological flow separation characteristic to mechanical circulatory support devices, and the validatory framework of the FDA benchmark nozzle. PDMS microchannels were fabricated and coated with human collagen. Platelets were fluorescently tagged, and blood was reconstituted at variable hematocrit prior to perfusion experiments. Integrin function of selected blood samples was inhibited by a blocking antibody, and platelet adhesion and aggregation over the course of perfusion was monitored.

Results

Increasing shear rates at physiological and elevated hematocrit levels facilitate robust platelet adhesion and formation of large aggregates. Shear-induced platelet aggregation is demonstrated to be dependent on both αIIbβIII function and the presence of red blood cells. Inhibition of αIIbβIII results in an 86.4% reduction in overall platelet adhesion and an 85.7% reduction in thrombus size at 20-60% hematocrit. Hematocrit levels of 20% are inadequate for effective platelet margination and subsequent vWF tethering, resulting in notable decreases in platelet adhesion at 5000 and 10,000 s-1 compared to 40% and 60%. Inhibition of αIIbβIII triggered dramatic reductions in overall thrombus coverage and large aggregate formation. Stability of platelets tethered by vWF are demonstrated to be αIIbβIII-dependent, as adhesion of single platelets treated with A2A9, an anti-αIIbβIII blocking antibody, is transient and did not lead to sustained thrombus formation.

Conclusions

This study highlights driving factors in vWF-mediated platelet adhesion that are relevant to clinical suppression of shear-induced thrombosis and in vitro assays of platelet adhesion. Primarily, increasing hematocrit promotes platelet margination, permitting shear-induced platelet aggregation through αIIbβIII-mediated adhesion at supraphysiological shear rates.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00796-0.

A computational investigation of occlusive arterial thrombosis

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

Exploratory Simulation of Thrombosis in a Temporary LVAD Catheter Pump within a Virtual In-vivo Left Heart Environment

Percutaneous catheter pumps are intraventricular temporary mechanical circulatory support (MCS) devices that are positioned across the aortic valve into the left ventricle (LV) and provide continuous antegrade blood flow from the LV into the ascending aorta (AA). MCS devices are most often computationally evaluated as isolated devices subject to idealized steady-state blood flow conditions. In clinical practice, MCS devices operate connected to or within diseased pulsatile native hearts and are often complicated by hemocompatibility related adverse events such as stroke, bleeding, and thrombosis. Whereas aspects of the human circulation are increasingly being simulated via computational methods, the precise interplay of pulsatile LV hemodynamics with MCS pump hemocompatibility remains mostly unknown and not well characterized. Technologies are rapidly converging such that next-generation MCS devices will soon be evaluated in virtual physiological environments that increasingly mimic clinical settings. The purpose of this brief communication is to report results and lessons learned from an exploratory CFD simulation of hemodynamics and thrombosis for a catheter pump situated within a virtual in-vivo left heart environment.

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.

A Computational Investigation of Occlusive Arterial Thrombosis

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