A two-phase flow approach for modeling blood stasis and estimating the thrombosis potential of a ventricular assist device

A multi-constituent model for assessing the effect of impeller shroud on the thrombosis potential of a centrifugal blood pump

Centrifugal blood pumps can be used for treating heart failure patients. However, pump thrombosis has remained one of the complications that trouble clinical treatment. This study analyzed the effect of impeller shroud on the thrombosis risk of the blood pump, and predicted areas prone to thrombosis. Multi-constituent transport equations were presented, considering mechanical activation and biochemical activation. It was found that activated platelets concentration can increase with shear stress and adenosine diphosphate(ADP) concentration increasing, and the highest risk of thrombosis inside the blood pump was under extracorporeal membrane oxygenation (ECMO) mode. Under the same condition, ADP concentration and thrombosis index of semi-shroud impeller can increase by 7.3% and 7.2% compared to the closed-shroud impeller. The main reason for the increase in thrombosis risk was owing to elevated scalar shear stress and more coagulation promoting factor-ADP released. The regions with higher thrombosis potential were in the center hole, top and bottom clearance. As a novelty, the findings revealed that impeller shroud can influence mechanical and biochemical activation factors. It is useful for identifying potential risk regions of thrombus formation based on relative comparisons.

Clot embolization studies and computational framework for embolization in a canonical tube model

Despite recent advances in the development of computational methods of modeling thrombosis, relatively little effort has been made in developing methods of modeling blood clot embolization. Such a model would provide substantially greater understanding of the mechanics of embolization, as in-vitro and in-vivo characterization of embolization is difficult. Here, a method of computationally simulating embolization is developed. Experiments are performed of blood clots formed in a polycarbonate tube, where phosphate-buffered saline is run through the tube at increasing flow rates until the clot embolizes. The experiments revealed embolization can be initiated by leading edge and trailing edge detachment or by non-uniform detachment. Stress-relaxation experiments are also performed to establish values of constitutive parameters for subsequent simulations. The embolization in the tube is reproduced in silico using a multiphase volume-of-fluid approach, where the clot is modeled as viscoelastic. By varying the constitutive parameters at the wall, embolization can be reproduced in-silico at varying flow rates, and a range of constitutive parameters fitting the experiments is reported. Here, the leading edge embolization is simulated at flow rates consistent with the experiments demonstrating excellent agreement in this specific behavior.

Computational modeling of hemodynamics and risk of thrombosis in the left atrial appendage using patient-specific blood viscosity and boundary conditions at the mitral valve

Hemodynamics play a vital role for the risk of thrombosis in the left atrial appendage (LAA) and left atrium (LA) for patients with atrial fibrillation. Accurate prediction of hemodynamics in the LA can provide important guidance for assessing the risk of thrombosis in the LAA. Patient specificity is a crucial factor in representing the true hemodynamic fields. In this study, we investigated the effects of blood rheology (as a function of hematocrit and shear rate), as well as patient-specific mitral valve (MV) boundary conditions (MV area and velocity profiles measured by ultrasound) on the hemodynamics and thrombosis potential of the LAA. Four scenarios were setup with different degrees of patient specificity. Though using a constant blood viscosity can classify the thrombus and non-thrombus patients for all the hemodynamic indicators, the risk of thrombosis was underestimated for all patients compared with patient-specific viscosities. The results with least patient specificities showed that patients prone to thrombosis predicted by three hemodynamic indicators were inconsistent with clinical observations. Moreover, though patients had the same MV inlet flow rate, different MV models lead to different trends in the risk of thrombosis in different patients. We also found that endothelial cell activation potential and relative residence time can effectively distinguish thrombus and non-thrombus patients for all the scenarios, relatively insensitive to patient specificities. Overall, the findings of this study provide useful insights on patients-specific hemodynamic simulations of the LA.

Effect of stent treatment on hemodynamics in iliac vein compression syndrome with collateral vein

BACKGROUND

Iliac vein compression syndrome (IVCS) leads to blood flow obstruction in the lower extremities and is usually treated with stents, but stenting may worsen the hemodynamics and increase the risk of thrombosis in the iliac vein. The present work evaluates the advantages and disadvantages of the stent on IVCS with a collateral vein.

METHODS

The computational fluid dynamics method is adopted to analyze the preoperative and postoperative flow fields in a typical IVCS. The geometric models of the iliac vein are constructed from medical imaging data. The porous model is used to simulate the flow obstruction in IVCS.

RESULTS

The preoperative and postoperative hemodynamic characteristics in the iliac vein are obtained, e.g., the pressure gradient at two ends of the compressive region and the wall shear stress. It is found that the stenting restores the blood flow in the left iliac vein.

CONCLUSION

Impacts of the stent are classified into short-term and long-term effects. The short-term effects are beneficial in relieving IVCS, i.e., shortening the blood stasis and reducing the pressure gradient. The long-term effects increase the risk of thrombosis in the stent, i.e., enlarging wall shear stress due to a large corner and a diameter constriction in the distal vessel, and suggests the need to develop a venous stent for IVCS.

Influence of Inlet Boundary Conditions on the Prediction of Flow Field and Hemolysis in Blood Pumps Using Large-Eddy Simulation

Inlet boundary conditions (BC) are one of the uncertainties which may influence the prediction of flow field and hemolysis in blood pumps. This study investigated the influence of inlet BC, including the length of inlet pipe, type of inlet BC (mass flow rate or experimental velocity profile) and turbulent intensity (no perturbation, 5%, 10%, 20%) on the prediction of flow field and hemolysis of a benchmark centrifugal blood pump (the FDA blood pump) and a commercial axial blood pump (Heartmate II), using large-eddy simulation. The results show that the influence of boundary conditions on integral pump performance metrics, including pressure head and hemolysis, is negligible. The influence on local flow structures, such as velocity distributions, mainly existed in the inlet. For the centrifugal FDA blood pump, the influence of type of inlet BC and inlet position on velocity distributions can also be observed at the diffuser. Overall, the effects of position of inlet and type of inlet BC need to be considered if local flow structures are the focus, while the influence of turbulent intensity is negligible and need not be accounted for during numerical simulations of blood pumps.

Clinical implications of hemodynamic analysis for the three-dimension iliac vein model with different stenosis

Background

The aim of this study was to perform hemodynamic simulations of a three-dimension ideal inferior vena cava-iliac vein model with artificial stenosis to determine the degree of stenosis that requires clinical intervention.

Methods

Four three-dimension stenosis models (30%, 50%, 70%, and 90% stenosis) were constructed using commercial software (Solidworks). The inlet flow rates were acquired from previous literatures to perform the hemodynamic simulations. Changes in the old blood volume fraction, as well conventional hemodynamic parameters including pressure, differential pressure, wall shear stress, and flow patterns, over time were recorded. The pressure at the telecentric region of the stenosis increased with increasing degree of stenosis.

Results

For the 70% stenosis model, the pressure at the telecentric region of the stenosis reached 341 Pa, and the differential pressure between the two ends of the stenosis was 363 Pa (approximately 2.7 mmHg). Moreover, in the 70% and 90% stenosis models, there was a marked change in wall shear stress in the stenosis and the proximal end region, and the flow patterns began to show the phenomenon of flow separation. Blood stasis analysis showed that the 70% stenosis model had the slowest decrease in old blood volume fraction, while the proximal end region had the largest blood residue (15%).

Conclusion

Iliac vein stenosis of approximately 70% is associated with clinically relevant hemodynamic changes, and is more closely related to DVT than other degrees of stenosis.

The impact of rotor configurations on hemodynamic features, hemocompatibility and dynamic balance of the centrifugal blood pump: A numerical study

To investigate the effect of rotor design configuration on hemodynamic features, hemocompatibility and dynamic balance of blood pumps. Computational fluid dynamics was employed to investigate the effects of rotor type (closed impeller, semi-open impeller), clearance height and back vanes on blood pump performance. In particular, the Eulerian hemolysis model based on a power-law function and the Lagrangian thrombus model with integrated stress accumulation and residence time were applied to evaluate the hemocompatibility of the blood pump. This study shows that compared to the closed impeller, the semi-open impeller can improve hemolysis at a slight sacrifice in head pressure, but increase the risk of thrombogenic potential and disrupt rotor dynamic balance. For the semi-open impeller, the pressure head, hemolysis, and axial thrust of the blood pump decrease with increasing front clearance, and the risk of thrombosis increases first and then decreases with increasing front clearance. Variations in back clearance have little effect on pressure head, but larger on back clearance, worsens hemolysis, thrombogenic potential and rotor dynamic balance. The employment of back vanes has little effect on the pressure head. All back vanes configurations have an increased risk of hemolysis in the blood pump but are beneficial for the improvement of the rotor dynamic balance of the blood pump. Reasonable back vanes configuration (higher height, wider width, longer length and more number) decreases the flow separation, increases the velocity of blood in the back clearance, and reduces the risk of blood pooling and thrombosis. It was also found that hemolysis index (HI) was highly negatively correlated with pressure difference between the top and back clearances (r = -.87), and thrombogenic potential was positively correlated with pressure difference between the top and back clearances (r = .71). This study found that rotor type, clearance height, and back vanes significantly affect the hydraulic performance, hemocompatibility and rotor dynamic balance of centrifugal blood pumps through secondary flow. These parameters should be carefully selected when designing and optimizing centrifugal blood pumps for improving the blood pump clinical outcomes.

Impact of volute design features on hemodynamic performance and hemocompatibility of centrifugal blood pumps used in ECMO

BACKGROUND

The centrifugal blood pump volute has a significant impact on its hemodynamic performance hemocompatibility. Previous studies about the effect of volute design features on the performance of blood pumps are relatively few.

METHODS

In the present study, the computational fluid dynamics (CFD) method was utilized to evaluate the impact of volute design factors, including spiral start position, volute tongue radius, inlet height, size, shape and diffuser pipe angle on the hemolysis index and thrombogenic potential of the centrifugal blood pump.

RESULTS

Correlation analysis shows that flow losses affect the hemocompatibility of the blood pump by influencing shear stress and residence time. The closer the spiral start position of the volute, the better the hydraulic performance and hemocompatibility of the blood pump. Too large or too small volute inlet heights can worsen hydraulic performance and hemolysis, and higher volute inlet height can increase the thrombogenic potential. Small volute sizes exacerbate hemolysis and large volute sizes increase the thrombogenic risk, but volute size does not affect hydraulic performance. When the diffuser pipe is tangent to the base circle of the volute, the best hydraulic performance and hemolysis performance of the blood pump is achieved, but the thrombogenic potential is increased. The trapezoid volute has poor hydraulic performance and hemocompatibility. The round volute has the best hydraulic and hemolysis performance, but the thrombogenic potential is higher than that of the rectangle volute.

CONCLUSION

This study found that the hemolysis index shows a significant correlation with spiral start position, volute size, and diffuser pipe angle. Thrombogenic potential exhibits a good correlation with all the studied volute design features. The flow losses affect the hemocompatibility of the blood pump by influencing shear stress and residence time. The finding of this study can be used to guide the optimization of blood pump for improving the hemodynamic performance and hemocompatibility.

The hemodynamics and blood trauma in axial blood pump under different operating model

Speed modulation of blood pump has been proved to help restore vascular pulsatility and implemented clinically during treatment for cardiac failure. However, its effect on blood trauma has not been studied thoroughly. In this paper, we study the flow field of an axial pump FW-X under the modes of co-pulse, counter pulse and constant speed to evaluate the blood trauma. Based on the coupling model of cardiovascular system and axial blood pump, aortic pressure and the pump flow were obtained and applied as the boundary conditions at the pump outlet and inlet. The level of shear stress and hemolysis index were derived from computational fluid dynamics (CFD) simulation. Results showed the constant speed mode had the lowest shear stress level and hemolytic index at the expense of diminished pulsatility. Compared with the constant speed mode, the hemolysis index of co-pulse and counter pulse mode was higher, but it was helpful to restore vascular pulsatility. This method can be easily incorporated in the in vitro testing phase to analyze and decrease a pump's trauma before animal experimentation, thereby reducing the cost of blood pump development.

A New Mathematical Numerical Model to Evaluate the Risk of Thrombosis in Three Clinical Ventricular Assist Devices

(1) Background: Thrombosis is the main complication in patients supported with ventricular assist devices (VAD). Models that accurately predict the risk of thrombus formation in VADs are still lacking. When VADs are clinically assisted, their complex geometric configuration and high rotating speed inevitably generate complex flow fields and high shear stress. These non-physiological factors can damage blood cells and proteins, release coagulant factors and trigger thrombosis. In this study, a more accurate model for thrombus assessment was constructed by integrating parameters such as shear stress, residence time and coagulant factors, so as to accurately assess the probability of thrombosis in three clinical VADs. (2) Methods: A mathematical model was constructed to assess platelet activation and thrombosis within VADs. By solving the transport equation, the influence of various factors such as shear stress, residence time and coagulation factors on platelet activation was considered. The diffusion equation was applied to determine the role of activated platelets and substance deposition on thrombus formation. The momentum equation was introduced to describe the obstruction to blood flow when thrombus is formed, and finally a more comprehensive and accurate model for thrombus assessment in patients with VAD was obtained. Numerical simulations of three clinically VADs (CH-VAD, HVAD and HMII) were performed using this model. The simulation results were compared with experimental data on platelet activation caused by the three VADs. The simulated thrombogenic potential in different regions of MHII was compared with the frequency of thrombosis occurring in the regions in clinic. The regions of high thrombotic risk for HVAD and HMII observed in experiments were compared with the regions predicted by simulation. (3) Results: It was found that the percentage of activated platelets within the VAD obtained by solving the thrombosis model developed in this study was in high agreement with the experimental data (r² = 0.984), the likelihood of thrombosis in the regions of the simulation showed excellent correlation with the clinical statistics (r² = 0.994), and the regions of high thrombotic risk predicted by the simulation were consistent with the experimental results. Further study revealed that the three clinical VADs (CH-VAD, HVAD and HMII) were prone to thrombus formation in the inner side of the secondary flow passage, the clearance between cone and impeller, and the corner region of the inlet pipe, respectively. The risk of platelet activation and thrombus formation for the three VADs was low to high for CH-VAD, HVAD, and HM II, respectively. (4) Conclusions: In this study, a more comprehensive and accurate thrombosis model was constructed by combining parameters such as shear stress, residence time, and coagulation factors. Simulation results of thrombotic risk received with this model showed excellent correlation with experimental and clinical data. It is important for determining the degree of platelet activation in VAD and identifying regions prone to thrombus formation, as well as guiding the optimal design of VAD and clinical treatment.

A Hemodynamic Analysis of the Thrombosis Within Occluded Coronary Arterial Fistulas With Terminal Aneurysms Using a Blood Stasis Model

Objective: The aim of this study is to numerically evaluate thrombosis risk within occluded coronary arterial fistulas (CAF) with terminal aneurysms, and provide guidance in choosing occlusion positions, with clinical observations as reference. Method: Four patients with CAF were studied, with different occlusion positions in actual treatments. Hemodynamics simulations were conducted, with blood residue predicted using the blood stasis model. Three types of models (untreated model, aneurysm-reserved model and aneurysm-removed model) were studeid for each patient. Four metrics, i.e., proportion of high oscillatory shear index (OSI), area of high OSI, old blood volume fraction (OBVF)) and old blood volume (OBV) was obtained to distinguish the thrombosis risk of different treatments (proximal or distal occlusion), comparing with the follow-up CTA. Results: For all the postopertive models, the high OBVF, high OSI(>0.3) and low time-averaged wall shear stress (TAWSS) regions were mainly at the distal fistula, indicating these regions were prone to thrombosis. The regions where blood residue remains are roughly regions of high OSI, corresponding well with clinical observations. In contrast, TAWSS failed to distinguish the difference in thrombosis risk. Absolute values (area of high OSI, OBV) can better reflect the degree of thrombosis risk between treatment types compared with percentage values (proportion of high OSI, OBVF). By comparing with the actual clinical treatments and observations, the OBV is superior to the area of high OSI in determining treatment type. Conclusion: The OBV, a volumetric parameter for blood stasis, can better account for the CAF thrombosis and reflect the degree of blood stasis compared with OSI or TAWSS, is a more appropriate metric for thrombosis in the fistula. Together with morphological parameters, the OBV could guide clinicians to formulate more appropriate surgical plans, which is of great significance for the preoperative evaluation and treatment prognosis of CAF patients.

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.

An Accelerated Thrombosis Model for Computational Fluid Dynamics Simulations in Rotary Blood Pumps

Purpose

Thrombosis ranks among the major complications in blood-carrying medical devices and a better understanding to influence the design related contribution to thrombosis is desirable. Over the past years many computational models of thrombosis have been developed. However, numerically cheap models able to predict localized thrombus risk in complex geometries are still lacking. The aim of the study was to develop and test a computationally efficient model for thrombus risk prediction in rotary blood pumps.

Methods

We used a two-stage approach to calculate thrombus risk. The first stage involves the computation of velocity and pressure fields by computational fluid dynamic simulations. At the second stage, platelet activation by mechanical and chemical stimuli was determined through species transport with an Eulerian approach. The model was compared with existing clinical data on thrombus deposition within the HeartMate II. Furthermore, an operating point and model parameter sensitivity analysis was performed.

Results

Our model shows good correlation (R² > 0.93) with clinical data and identifies the bearing and outlet stator region of the HeartMate II as the location most prone to thrombus formation. The calculation of thrombus risk requires an additional 10–20 core hours of computation time.

Conclusion

The concentration of activated platelets can be used as a surrogate and computationally low-cost marker to determine potential risk regions of thrombus deposition in a blood pump. Relative comparisons of thrombus risk are possible even considering the intrinsic uncertainty in model parameters and operating conditions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13239-021-00606-y.

A two-fluid blood stasis model for false lumen thrombosis after type B dissection repair

The formation of thrombosis is a major concern in the false lumen (FL) for post-TEVAR (thoracic endovascular aortic repair) patients. Blood stasis is one of the key factors which lead to the formation of thrombosis in the arterial systems. This study proposed a computational model for blood stasis, using a two-fluid principle to track the locations of blood residual over time. The current study applied this novel model to evaluate blood stasis and thrombosis potential in four patient-specific post-TEVAR FLs of type B aortic dissection, with their follow-up in-vivo observations two years after TEVAR. The locations and topologies of residual blood in the FL predicted by the model agreed well with the in-vivo observations of thrombus. In addition, the results corresponded better with clinical observations in terms of interpatient comparison of degree of thrombosis, compared with conventional hemodynamic parameters. The blood stasis model serves as a valuable addition to conventional metrics to better predict thrombosis potential. Collectively, these metrics can provide an efficient non-invasive method for evaluating blood stasis and thrombosis potential in arterial system, and useful guidance for clinicians' operative planning and postoperative evaluation.

Patient-specific hemodynamic analysis of IVCS-induced DVT

The aim of this study is to perform patient-specific hemodynamic simulations of patients with iliac vein compression syndrome (IVCS) and evaluate the deep venous thrombosis (DVT) potential, with clinical observations as reference. 15 patient-specific IVCS models were reconstructed from computed tomography venography (CTV) data, and divided into three groups, i.e. two groups with thrombosis: Group A (complete obstruction) and Group B (incomplete obstruction), and a third group without DVT, Group C. Hemodynamic simulations were conducted with patient-specific inlet flow rates. The blood residue was predicted using the blood stasis model. Time histories of old blood volume fraction (OBVF) was obtained, in addition to conventional hemodynamic parameters such as wall shear stress (WSS). The mean area-averaged WSS of the stenosis region for Group A and Group B were 3.68 Pa and 1.78 Pa, respectively. For the telecentric end region, the WSS were 0.76 Pa and 0.58 Pa, respectively. For Group C, the WSS at these two regions were 4.61 Pa and 1.57 Pa, respectively. The OBVF was 74.0% at the stenosis region and 76.2% at the telecentric end region for Group A, much higher than 4.8% and 43.1% of Group B. For Group C, the OBVF at the two regions were close to 0. This corresponded well with clinical observations. The potential of DVT can be predicted through patient-specific hemodynamic simulations in combination of blood stasis model. The findings of this study are of great significance for the preoperative evaluation and treatment prognosis of IVCS patients with DVT.

Large eddy simulation as a fast and accurate engineering approach for the simulation of rotary blood pumps

An accurate representation of the flow field in blood pumps is important for the design and optimization of blood pumps. The primary turbulence modeling methods applied to blood pumps have been the Reynolds-averaged Navier-Stokes (RANS) or URANS (unsteady RANS) method. Large eddy simulation (LES) method has been introduced to simulate blood pumps. Nonetheless, LES has not been widely used to assist in the design and optimization of blood pumps to date due to its formidable computational cost. The purpose of this study is to explore the potential of the LES technique as a fast and accurate engineering approach for the simulation of rotary blood pumps. The performance of "Light LES" (using the same time and spatial resolutions as the URANS) and LES in two rotary blood pumps was evaluated by comparing the results with the URANS and extensive experimental results. This study showed that the results of both "Light LES" and LES are superior to URANS, in terms of both performance curves and key flow features. URANS could not predict the flow separation and recirculation in diffusers for both pumps. In contrast, LES is superior to URANS in capturing these flows, performing well for both design and off-design conditions. The differences between the "Light LES" and LES results were relatively small. This study shows that with less computational cost than URANS, "Light LES" can be considered as a cost-effective engineering approach to assist in the design and optimization of rotary blood pumps.

Influence of Distal Re-entry Tears on False Lumen Thrombosis After Thoracic Endovascular Aortic Repair in Type B Aortic Dissection Patients: A Computational Fluid Dynamics Simulation

PURPOSE

Distal re-entry tears play a significant role in false lumen (FL) thrombosis, which will strongly affect the postoperative long-term survival of patients with type B aortic dissection (TBAD) after thoracic endovascular aortic repair (TEVAR). This study aimed to investigate the influence of a peculiar morphological parameter of the residual re-entry tears in TBAD patients after TEVAR on long-term FL thrombosis using the computational fluid dynamics.

METHODS

Ideal population-based three-dimensional models of post-operative TBAD were established. Numerical simulation was performed to investigate the hemodynamic differences caused by different tear features, including the tear count, the maximum distance between tears, and the tear area.

RESULTS

Although the low relative residence time (RRT) area did not change significantly when the tear distance was fixed, the area of oscillatory shear index (OSI) > 0.45 and endothelial cell activation potential (ECAP) > 1.5 decreased significantly with the tear count and area increased and a dramatic increase in blood flow into the FL was also observed. When tear count and total area were fixed, for each 10-mm increase in the maximum distance between tears, the area of low RRT in the FL increased significantly, while the average pressure difference increased by 10.85%.

CONCLUSION

The different morphology of the re-entry tears had different effects on the thrombosis-related hemodynamic parameters in FL following TEVAR. and the number of re-entry tears was most crucial to the potential thrombosis in the post-TEVAR FL of TBAD patients.