Device thrombogenicity emulation: a novel methodology for optimizing the thromboresistance of cardiovascular devices

Thrombogenic Risk Assessment of Transcatheter Prosthetic Heart Valves Using a Fluid-Structure Interaction Approach

Background and Objective

Prosthetic heart valve interventions such as TAVR have surged over the past decade, but the associated complication of long-term, life-threatening thrombotic events continues to undermine patient outcomes. Thus, improving thrombogenic risk analysis of TAVR devices is crucial. In vitro studies for thrombogenicity are typically difficult to perform. However, revised ISO testing standards include computational testing for thrombogenic risk assessment of cardiovascular implants. We present a fluid-structure interaction (FSI) approach for assessing thrombogenic risk of prosthetic heart valves.

Methods

An FSI framework was implemented via the incompressible computational fluid dynamics multi-physics solver of the Ansys LS-DYNA software. The numerical modeling approach for flow analysis was validated by comparing the derived flow rate of the 29-mm CoreValve device from benchtop testing and orifice areas of commercial TAVR valves in the literature to in silico results. Thrombogenic risk was analyzed by computing stress accumulation (SA) on virtual platelets seeded in the flow fields via Ansys EnSight. The integrated FSI-thrombogenicity methodology was subsequently employed to examine hemodynamics and thrombogenic risk of TAVR devices with two approaches: 1) engineering optimization and 2) clinical assessment.

Results

The simulated effective orifice areas of the commercial devices were in the range reported in the literature. The flow rates from the in vitro flow testing matched well with the in silico results. The approach was used to analyze the effect of various TAVR leaflet designs on hemodynamics. Platelets experienced different magnitudes of SA along their trajectories as they flowed past each design. Post-TAVR deployment hemodynamics in patient-specific bicuspid aortic valve anatomies revealed varying degrees of thrombogenic risk for these patients, despite being clinically defined as "mild" paravalvular leak.

Conclusions

Our methodology can be used to improve the thromboresistance of prosthetic valves from the initial design stage to the clinic. It allows for unparalleled optimization of devices, uncovering key TAVR leaflet design parameters that can be used to mitigate thrombogenic risk, in addition to patient-specific modeling to evaluate device performance. This work demonstrates the utility of advanced in silico analysis of TAVR devices that can be utilized for thrombogenic risk assessment of other blood recirculating devices.

Effect of Sinotubular Junction Size on TAVR Leaflet Thrombosis: A Fluid-Structure Interaction Analysis

TAVR has emerged as a standard approach for treating severe aortic stenosis patients. However, it is associated with several clinical complications, including subclinical leaflet thrombosis characterized by Hypoattenuated Leaflet Thickening (HALT). A rigorous analysis of TAVR device thrombogenicity considering anatomical variations is essential for estimating this risk. Clinicians use the Sinotubular Junction (STJ) diameter for TAVR sizing, but there is a paucity of research on its influence on TAVR devices thrombogenicity. A Medtronic Evolut® TAVR device was deployed in three patient models with varying STJ diameters (26, 30, and 34 mm) to evaluate its impact on post-deployment hemodynamics and thrombogenicity, employing a novel computational framework combining prosthesis deployment and fluid-structure interaction analysis. The 30 mm STJ patient case exhibited the best hemodynamic performance: 5.94 mmHg mean transvalvular pressure gradient (TPG), 2.64 cm2 mean geometric orifice area (GOA), and the lowest mean residence time (TR)-indicating a reduced thrombogenic risk; 26 mm STJ exhibited a 10 % reduction in GOA and a 35% increase in mean TPG compared to the 30 mm STJ; 34 mm STJ depicted hemodynamics comparable to the 30 mm STJ, but with a 6% increase in TR and elevated platelet stress accumulation. A smaller STJ size impairs adequate expansion of the TAVR stent, which may lead to suboptimal hemodynamic performance. Conversely, a larger STJ size marginally enhances the hemodynamic performance but increases the risk of TAVR leaflet thrombosis. Such analysis can aid pre-procedural planning and minimize the risk of TAVR leaflet thrombosis.

Mild Paravalvular Leak May Pose an Increased Thrombogenic Risk in Transcatheter Aortic Valve Replacement (TAVR) Patients-Insights from Patient Specific In Vitro and In Silico Studies

In recent years, the treatment of aortic stenosis with TAVR has rapidly expanded to younger and lower-risk patients. However, persistent thrombotic events such as stroke and valve thrombosis expose recipients to severe clinical complications that hamper TAVR's rapid advance. We presented a novel methodology for establishing a link between commonly acceptable mild paravalvular leak (PVL) levels through the device and increased thrombogenic risk. It utilizes in vitro patient-specific TAVR 3D-printed replicas evaluated for hydrodynamic performance. High-resolution µCT scans are used to reconstruct in silico FSI models of these replicas, in which multiple platelet trajectories are studied through the PVL channels to quantify thrombogenicity, showing that those are highly dependent on patient-specific flow conditions within the PVL channels. It demonstrates that platelets have the potential to enter the PVL channels multiple times over successive cardiac cycles, increasing the thrombogenic risk. This cannot be reliably approximated by standard hemodynamic parameters. It highlights the shortcomings of subjectively ranked PVL commonly used in clinical practice by indicating an increased thrombogenic risk in patient cases otherwise classified as mild PVL. It reiterates the need for more rigorous clinical evaluation for properly diagnosing thrombogenic risk in TAVR patients.

In vitro thrombogenicity evaluation of rotary blood pumps by thromboelastometry

In vitro thrombogenicity tests for rotary blood pumps (RBPs) could benefit from assessing coagulation kinematics, as RBP design improves. In this feasibility study, we investigated if the method of thromboelastometry (TEM) is able to assess coagulation kinematics under the in vitro conditions of RBP tests. We conducted in vitro thrombogenicity tests (n=4) by placing Deltastream^® DP3 pumps into test loops that were filled with 150 mL of slightly anti-coagulated porcine blood, adjusted to an activated clotting time (ACT) well below clinically recommended levels. Blood samples were taken at certain time points during the experiment until a continuous decrease in pump flow indicated major thrombus formation. Blood samples were analyzed for ACT, platelet count (PLT), and several TEM parameters. While visible thrombus formation was observed in three pumps, ACT indicated an ongoing activation of coagulation, PLT might have indicated platelet consumption. Unexpectedly, most TEM results gave no clear indications. Nonetheless, TEM clotting time obtained by non-anticoagulated and chemically non-activated whole blood (HEPNATEM-CT) appeared to be more sensitive for the activation of coagulation in vitro than ACT, which might be of interest for future pump tests. However, more research regarding standardization of thrombogenicity pump tests is urgently required.

On-Chip Platelet Activation Assessment: Microfluidic Emulation of Shear Stress Profiles Induced by Mechanical Circulatory Support Devices

Mechanical circulatory support devices (MCSDs), although proved to be a pillar in the clinical setting of advanced heart failure, are afflicted by thromboembolic complications. Shear-mediated platelet activation has been recognized to drive thromboembolic events in patients implanted with MCSDs. Despite this, to date, a clinically reliable diagnostic test for assessing platelet response to stress stimuli is still missing. Here, we describe and apply the previously developed device thrombogenicity emulation methodology to the design of a microfluidic platform able to replicate shear stress profiles representative of MCSDs. The device-specific shear-mediated platelet activation is finally assessed by the platelet activity state assay, which measures real-time thrombin production, as a marker of platelet activation level. This technique can be employed to emulate the shear stress patterns of different MCSDs, such as mechanical heart valves, ventricular assist devices, and stents.

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.

Introducing 3D-potting: a novel production process for artificial membrane lungs with superior blood flow design

Currently, artificial-membrane lungs consist of thousands of hollow fiber membranes where blood flows around the fibers and gas flows inside the fibers, achieving diffusive gas exchange. At both ends of the fibers, the interspaces between the hollow fiber membranes and the plastic housing are filled with glue to separate the gas from the blood phase. During a uniaxial centrifugation process, the glue forms the “potting.” The shape of the cured potting is then determined by the centrifugation process, limiting design possibilities and leading to unfavorable stagnation zones associated with blood clotting. In this study, a new multiaxial centrifugation process was developed, expanding the possible shapes of the potting and allowing for completely new module designs with potentially superior blood flow guidance within the potting margins. Two-phase simulations of the process in conceptual artificial lungs were performed to explore the possibilities of a biaxial centrifugation process and determine suitable parameter sets. A corresponding biaxial centrifugation setup was built to prove feasibility and experimentally validate four conceptual designs, resulting in good agreement with the simulations. In summary, this study shows the feasibility of a multiaxial centrifugation process allowing greater variety in potting shapes, eliminating inefficient stagnation zones and more favorable blood flow conditions in artificial lungs.

Graphic abstract

Design, Modeling, and Experimental Characterization of A Valveless Pulsatile Flow Mechanical Circulatory Support Device

Mechanical circulatory support (MCS) devices, i.e., ventricular assist devices (VADs) and total artificial hearts (TAHs), while effective and vital in restoring hemodynamics in patients with circulatory compromise in advanced heart failure, remain limited by significant adverse thrombotic, embolic and bleeding events. Many of these complications relate to chronic exposure, via these devices, to nonpulsatile flow and the high shear stress created by current methods of blood propulsion or use of prosthetic valves. Here we propose a novel noncompressing single sliding vane MCS device to: 1) dramatically reduce pump operating speed thus potentially lowering the shear stress imparted to blood; 2) eliminate utilization of prosthetic valves thus diminishing potential shear stress generations; 3) allow direct flow rate control to generate physically desired blood flow rate include pulsatile flow; and 4) achieve compactness to fit into the majority of patients. The fundamental working principle and governing design equations are introduced first with multiple design and performance objectives presented. A first prototype was fabricated and experimental tests were conducted to validate the model with a 93.10% match between theoretical and experimental flow rate results. After model validation, the proposed MCS was tested to illustrate the ability of pulsatile flow generation. Finally, it was compared with some representative MCS pumps to discuss its potential of improving current MCS design. The presented work offers a novel MCS design and paves the way for next steps in device hemocompatibility testing.

In Vitro Durability and Stability Testing of a Novel Polymeric Transcatheter Aortic Valve

Transcatheter aortic valve replacement (TAVR) has emerged as an effective therapy for the unmet clinical need of inoperable patients with severe aortic stenosis (AS). Current clinically used tissue TAVR valves suffer from limited durability that hampers TAVR's rapid expansion to younger, lower risk patients. Polymeric TAVR valves optimized for hemodynamic performance, hemocompatibility, extended durability, and resistance to calcific degeneration offer a viable solution to this challenge. We present extensive in vitro durability and stability testing of a novel polymeric TAVR valve (PolyNova valve) using 1) accelerated wear testing (AWT, ISO 5840); 2) calcification susceptibility (in the AWT)-compared with clinically used tissue valves; and 3) extended crimping stability (valves crimped to 16 Fr for 8 days). Hydrodynamic testing was performed every 50M cycles. The valves were also evaluated visually for structural integrity and by scanning electron microscopy for evaluation of surface damage in the micro-scale. Calcium and phosphorus deposition was evaluated using micro-computed tomography (μCT) and inductive coupled plasma spectroscopy. The valves passed 400M cycles in the AWT without failure. The effective orifice area kept stable at 1.8 cm with a desired gradual decrease in transvalvular pressure gradient and regurgitation (10.4 mm Hg and 6.9%, respectively). Calcium and phosphorus deposition was significantly lower in the polymeric valve: down by a factor of 85 and 16, respectively-as compared to a tissue valve. Following the extended crimping testing, no tears nor surface damage were evident. The results of this study demonstrate the potential of a polymeric TAVR valve to be a viable alternative to tissue-based TAVR valves.

Characterizing Complications of Intracranial Responsive Neurostimulation Devices for Epilepsy Through a Retrospective Analysis of the Federal MAUDE Database

OBJECTIVES

Responsive neurostimulation is an innovative modality in the treatment of medication-refractory epilepsy for patients who are not suitable candidates for surgical intervention. While being a potentially life-changing treatment option for many individuals with epilepsy, little is known about the system's complications aside from its performance in initial clinical trials. Therefore, the goal of this study was to characterize all reported complications of the RNS system made to the Food & Drug Administration since its approval.

MATERIALS AND METHODS

The Manufacturer and User Facility Device Experience (MAUDE) database was queried for entries reported under "implanted brain stimulator for epilepsy" through the dates of November 1, 2013 to March 1, 2020. After correction of duplicate entries, each was sorted into complication types based on the entries' narrative descriptions.

RESULTS

The searched yielded 241 unique complication events. The most common complications were attributed to infections (40%) and lead breaks (12%). Other reported complications included poor wound healing (10%) and intrinsic device failure (7%). Focal neurological deficits were found in 2%. Over half (67%) of the reported complications required return to the operating room for revision or explant. The remainder of the adverse events were self-resolved or treated with either medication or software adjustment.

CONCLUSIONS

Future research endeavors should attempt to optimize the implantable device for preventing infections. The data of complications provided by this review will also aid physicians in providing the most accurate informed consent for patients when deciding to undergo implantation with the responsive neurostimulation system.

Circulatory loop design and components introduce artifacts impacting in vitro evaluation of ventricular assist device thrombogenicity: A call for caution

Mechanical circulatory support (MCS) devices continue to be hampered by thrombotic adverse events (AEs), a consequence of device-imparted supraphysiologic shear stresses, leading to shear-mediated platelet activation (SMPA). In advancing MCS devices from design to clinical use, in vitro circulatory loops containing the device under development and testing are utilized as a means of assessing device thrombogenicity. Physical characteristics of these test circulatory loops may also contribute to inadvertent platelet activation through imparted shear stress, adding inadvertent error in evaluating MCS device thrombogenicity. While investigators normally control for the effect of a loop, inadvertent addition of what are considered innocuous connectors may impact test results. Here, we tested the effect of common, additive components of in vitro circulatory test loops, that is, connectors and loop geometry, as to their additive contribution to shear stress via both in silico and in vitro models. A series of test circulatory loops containing a ventricular assist device (VAD) with differing constituent components, were established in silico including: loops with 0~5 Luer connectors, a loop with a T-connector creating 90° angulation, and a loop with 90° angulation. Computational fluid dynamics (CFD) simulations were performed using a k - ω shear stress transport turbulence model to platelet activation index (PAI) based on a power law model. VAD-operated loops replicating in silico designs were assembled in vitro and gel-filtered human platelets were recirculated within (1 hour) and SMPA was determined. CFD simulations demonstrated high shear being introduced at non-smooth regions such as edge-connector boundaries, tubing, and at Luer holes. Noticeable peaks' shifts of scalar shear stress (sss) distributions toward high shear-region existed with increasing loop complexity. Platelet activation also increased with increasing shear exposure time, being statistically higher when platelets were exposed to connector-employed loop designs. The extent of platelet activation in vitro could be successfully predicted by CFD simulations. Loops employing additional components (non-physiological flow pattern connectors) resulted in higher PAI. Loops with more components (5-connector loop and 90° T-connector) showed 63% and 128% higher platelet activation levels, respectively, versus those with fewer (0-connector (P = .023) and a 90° heat-bend loop (P = .0041). Our results underscore the importance of careful consideration of all component elements, and suggest the need for standardization in designing in vitro circulatory loops for MCS device evaluation to avoid inadvertent additive SMPA during device evaluation, confounding overall results. Specifically, we caution on the use and inadvertent introduction of additional connectors, ports, and other shear-generating elements which introduce artifact, clouding primary device evaluation via introduction of additive SMPA.

The blood compatibility challenge. Part 1: Blood-contacting medical devices: The scope of the problem

Blood-contacting medical devices are an integral part of modern medicine. Such devices may be used for only a few hours or may be implanted for life. Despite advances in biomaterial science, clotting on medical devices remains a common problem. Systemic administration of antiplatelet drugs or anticoagulants is often needed to reduce the risk of clotting. Although effective, such therapy increases the risk of bleeding, which can be fatal. This chapter (a) describes some of the commonly used blood-contacting devices and their potential complications, (b) provides an overview of the mechanisms that drive device-associated clotting, and (c) reviews the strategies employed to attenuate clotting on blood-contacting medical devices. STATEMENT OF SIGNIFICANCE: This paper is part 1 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

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.

Stenosis Indicators Applied to Patient-Specific Renal Arteries without and with Stenosis

Pulsatile flow in the abdominal aorta and the renal arteries of three patients was studied numerically. Two of the patients had renal artery stenosis. The aim of the study was to assess the use of four types of indicators for determining the risk of new stenosis after revascularization of the affected arteries. The four indicators considered include the time averaged wall shear stress (TAWSS), the oscillatory shear index (OSI), the relative reference time (RRT) and a power law model based in platelet activation modeling but applied to the endothelium, named endothelium activation indicator (EAI). The results show that the indicators can detect the existing stenosis but are less successful in the revascularized cases. The TAWSS and, more clearly, the EAI approach seem to be better in predicting the risk for stenosis relapse at the original location and close to the post-stenotic dilatation. The shortcomings of the respective indicators are discussed along with potential improvements to endothelial activation modeling and its use as an indicator for risks of restenosis.

Principles of TAVR valve design, modelling, and testing

INTRODUCTION

Transcatheter aortic valve replacement (TAVR) has emerged as an effective minimally-invasive alternative to surgical valve replacement in medium- to high-risk, elderly patients with calcific aortic valve disease and severe aortic stenosis. The rapid growth of the TAVR devices market has led to a high variety of designs, each aiming to address persistent complications associated with TAVR valves that may hamper the anticipated expansion of TAVR utility.

AREAS COVERED

Here we outline the challenges and the technical demands that TAVR devices need to address for achieving the desired expansion, and review design aspects of selected, latest generation, TAVR valves of both clinically-used and investigational devices. We further review in detail some of the up-to-date modeling and testing approaches for TAVR, both computationally and experimentally, and additionally discuss those as complementary approaches to the ISO 5840-3 standard. A comprehensive survey of the prior and up-to-date literature was conducted to cover the most pertaining issues and challenges that TAVR technology faces.

EXPERT COMMENTARY

The expansion of TAVR over SAVR and to new indications seems more promising than ever. With new challenges to come, new TAV design approaches, and materials used, are expected to emerge, and novel testing/modeling methods to be developed.

Large eddy simulation in a rotary blood pump: Viscous shear stress computation and comparison with unsteady Reynolds-averaged Navier-Stokes simulation

PURPOSE:

Numerical flow analysis (computational fluid dynamics) in combination with the prediction of blood damage is an important procedure to investigate the hemocompatibility of a blood pump, since blood trauma due to shear stresses remains a problem in these devices. Today, the numerical damage prediction is conducted using unsteady Reynolds-averaged Navier-Stokes simulations. Investigations with large eddy simulations are rarely being performed for blood pumps. Hence, the aim of the study is to examine the viscous shear stresses of a large eddy simulation in a blood pump and compare the results with an unsteady Reynolds-averaged Navier-Stokes simulation.

METHODS:

The simulations were carried out at two operation points of a blood pump. The flow was simulated on a 100M element mesh for the large eddy simulation and a 20M element mesh for the unsteady Reynolds-averaged Navier-Stokes simulation. As a first step, the large eddy simulation was verified by analyzing internal dissipative losses within the pump. Then, the pump characteristics and mean and turbulent viscous shear stresses were compared between the two simulation methods.

RESULTS:

The verification showed that the large eddy simulation is able to reproduce the significant portion of dissipative losses, which is a global indication that the equivalent viscous shear stresses are adequately resolved. The comparison with the unsteady Reynolds-averaged Navier-Stokes simulation revealed that the hydraulic parameters were in agreement, but differences for the shear stresses were found.

CONCLUSION:

The results show the potential of the large eddy simulation as a high-quality comparative case to check the suitability of a chosen Reynolds-averaged Navier-Stokes setup and turbulence model. Furthermore, the results lead to suggest that large eddy simulations are superior to unsteady Reynolds-averaged Navier-Stokes simulations when instantaneous stresses are applied for the blood damage prediction.

Computational Fluid Dynamics Assessment Associated with Transcatheter Heart Valve Prostheses: A Position Paper of the ISO Working Group

The governing international standard for the development of prosthetic heart valves is International Organization for Standardization (ISO) 5840. This standard requires the assessment of the thrombus potential of transcatheter heart valve substitutes using an integrated thrombus evaluation. Besides experimental flow field assessment and ex vivo flow testing, computational fluid dynamics is a critical component of this integrated approach. This position paper is intended to provide and discuss best practices for the setup of a computational model, numerical solving, post-processing, data evaluation and reporting, as it relates to transcatheter heart valve substitutes. This paper is not intended to be a review of current computational technology; instead, it represents the position of the ISO working group consisting of experts from academia and industry with regards to considerations for computational fluid dynamic assessment of transcatheter heart valve substitutes.

Implantable Device-Related Infection

Over half of the nearly two million healthcare-associated infections can be attributed to indwelling medical devices. In this review, we highlight the difficulty in diagnosing implantable device-related infection and how this leads to a likely underestimate of the prevalence. We then provide a length-scale conceptualization of device-related infection pathogenesis. Within this conceptualization we focus specifically on biofilm formation and the role of host immune and coagulation systems. Using this framework, we describe how current and developing preventative strategies target specific processes along the entire length-scale. In light of the significant time horizon for the development and translation of new preventative technologies, we also emphasize the need for parallel development of in situ treatment strategies. Specific examples of both preventative and treatment strategies and how they align with the length-scale conceptualization are described.

Anti-fouling strategies for central venous catheters

Central venous catheters (CVCs) are ubiquitous in the healthcare industry and carry two common complications, catheter related infections and occlusion, particularly by thrombus. Catheter-related bloodstream infections (CRBSI) are an important cause of nosocomial infections that increase patient morbidity, mortality, and hospital cost. Innovative design strategies for intravenous catheters can help reduce these preventable infections. Antimicrobial coatings can play a major role in preventing disease. These coatings can be divided into two major categories: drug eluting and non-drug eluting. Much of these catheter designs are targeted at preventing the formation of microbial biofilms that make treatment of CRBSI nearly impossible without removal of the intravenous device. Exciting developments in catheter impregnation with antibiotics as well as nanoscale surface design promise innovative changes in the way that physicians manage intravenous catheters. Occlusion of a catheter renders the catheter unusable and is often treated by tissue plasminogen activator administration or replacement of the line. Prevention of this complication requires a thorough understanding of the mechanisms of platelet aggregation, signaling and cross-linking. This article will look at the advances in biomaterial design specifically drug eluting, non-drug eluting, lubricious coatings and micropatterning as well as some of the characteristics of each as they relate to CVCs.

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.

Rotary mechanical circulatory support systems

A detailed survey of the current trends and recent advances in rotary mechanical circulatory support systems is presented in this paper. Rather than clinical reports, the focus is on technological aspects of these rehabilitating devices as a reference for engineers and biomedical researchers. Existing trends in flow regimes, flow control, and bearing mechanisms are summarized. System specifications and applications of the most prominent continuous-flow ventricular assistive devices are provided. Based on the flow regime, pumps are categorized as axial flow, centrifugal flow, and mixed flow. Unique characteristics of each system are unveiled through an examination of the structure, bearing mechanism, impeller design, flow rate, and biocompatibility. A discussion on the current limitations is provided to invite more studies and further improvements.

Dielectrophoresis-Mediated Electrodeformation as a Means of Determining Individual Platelet Stiffness

Platelets, essential for hemostasis, are easily activated via biochemical and mechanical stimuli. Cell stiffness is a vital parameter modulating the mechano-transduction of exogenous mechanical stimuli. While methods exist to measure cell stiffness, no ready method exists for measuring platelet stiffness that is both minimally-contacting, imparting minimal exogenous force and non-activating. We developed a minimal-contact methodology capable of trapping and measuring the stiffness of individual platelets utilizing dielectrophoresis (DEP)-mediated electrodeformation. Parametric studies demonstrate a non-uniform electric field in the MHz frequency range (0.2-20 MHz) is required for generating effective DEP forces on platelets, suspended in isotonic buffer with conductivity ~100-200 μS/cm. A nano-Newton DEP force (0.125-4.5 nN) was demonstrated to be essential for platelet electrodeformation, which could be generated with an electric field with strength of 1.5-9 V/μm. Young's moduli of platelets were calculated using a Maxwell stress tensor model and stress-deformation relationship. Platelet stiffness was determined to be in the range of 3.5 ± 1.4 and 8.5 ± 1.5 kPa for resting and 0.4% paraformaldehyde-treated cells, respectively. The developed methodology fills a gap in approaches of measuring individual platelet stiffness, free of inadvertent platelet activation, which will facilitate further studies of mechanisms involved in mechanically-mediated platelet activation.

Microfluidic approaches for the assessment of blood cell trauma: a focus on thrombotic risk in mechanical circulatory support devices

INTRODUCTION

Mechanical circulatory support devices (MCSDs) are emerging as a valuable therapeutic option for the management of end-stage heart failure. However, although recipients are routinely administered with anti-thrombotic (AT) drugs, thrombosis persists as a severe post-implant complication. Conventional clinical assays and coagulation markers demonstrate partial ability in preventing the onset of thrombosis. Through years, different laboratory techniques have been proposed as potential tools for the evaluation of platelets' hemostatic response in MCSD recipients. Most rely on platelet aggregation tests; they are performed in static or low shear conditions, neglecting the prominent contribution of MCSD shear-induced mechanical load in enhancing platelet activation (PA). On the other hand, those tests able to account for shear-induced PA have limited possibility of effective clinical translation.

AIMS AND METHODS

Advances on this side have been addressed by microfluidic technology. Microfluidic devices have been developed for AT drug monitoring under flow, able to replicate physiological and/or constant shear flow conditions in vitro. In this paper, we present a newly developed microfluidic platform able to expose platelets to MCSD-specific dynamic shear stress patterns. We performed in vitro tests circulating human platelets in the microfluidic platform and quantifying the dynamics of PA by means of the Platelet Activity State (PAS) assay.

RESULTS

Our results prove the feasibility of using microfluidics for the diagnosis of MCSD-related thrombotic risk. This study paves the way for the development of a miniaturized point-of-care device for monitoring AT drug regimen. Such a system may have significant impact on limiting the incidence of thrombosis in MCSD recipients.

Computational fluid dynamics modelling in cardiovascular medicine

This paper reviews the methods, benefits and challenges associated with the adoption and translation of computational fluid dynamics (CFD) modelling within cardiovascular medicine. CFD, a specialist area of mathematics and a branch of fluid mechanics, is used routinely in a diverse range of safety-critical engineering systems, which increasingly is being applied to the cardiovascular system. By facilitating rapid, economical, low-risk prototyping, CFD modelling has already revolutionised research and development of devices such as stents, valve prostheses, and ventricular assist devices. Combined with cardiovascular imaging, CFD simulation enables detailed characterisation of complex physiological pressure and flow fields and the computation of metrics which cannot be directly measured, for example, wall shear stress. CFD models are now being translated into clinical tools for physicians to use across the spectrum of coronary, valvular, congenital, myocardial and peripheral vascular diseases. CFD modelling is apposite for minimally-invasive patient assessment. Patient-specific (incorporating data unique to the individual) and multi-scale (combining models of different length- and time-scales) modelling enables individualised risk prediction and virtual treatment planning. This represents a significant departure from traditional dependence upon registry-based, population-averaged data. Model integration is progressively moving towards ‘digital patient’ or ‘virtual physiological human’ representations. When combined with population-scale numerical models, these models have the potential to reduce the cost, time and risk associated with clinical trials. The adoption of CFD modelling signals a new era in cardiovascular medicine. While potentially highly beneficial, a number of academic and commercial groups are addressing the associated methodological, regulatory, education- and service-related challenges.

Thromboembolic stroke in patients with a HeartMate-II left ventricular assist device - the role of anticoagulation

BACKGROUND AND PURPOSE

It is unknown what the optimal anticoagulant level is to prevent thromboembolic stroke in patients with left ventricular assist device (LVAD) support. We aimed to evaluate the relation between coagulation status and the occurrence of thromboembolic stroke in HeartMate-II LVAD assisted patients.

METHODS

Thirty-eight consecutive patients with a HeartMate-II LVAD were included. Coagulation status was classified according to INR and aPTT ratio at: 1) the moment of first thromboembolic stroke; and 2) during the two weeks preceding the first thromboembolic stroke to assess long-term coagulation status. In patients without stroke, coagulation status was determined just before heart transplant, VAD explantation or death, whichever came first, and at two weeks preceding these surrogate endpoints. Based on coagulation status, patients were divided in two groups: Group I (reference group) was defined as INR below 2 and aPTT ratio below 1.5; Group II (adequate anticoagulation) as INR above 2 or aPTT ratio above 1.5. Logistic regression analysis was performed to assess the odds ratio for developing stroke for patients with adequate anticoagulation compared to the reference Group.

RESULTS

Thromboembolic stroke occurred in six (16 %) patients, none within 2 weeks after LVAD implantation. Considering coagulation status at the time of event, patients in coagulation Group II had no decreased risk for thromboembolic stroke (OR 0.78; 95 % CI 0.12-5.0). Results for coagulation status 2 weeks prior of event could not be calculated as all six strokes occurred in Group II.

CONCLUSION

In our experience anticoagulation within predefined targets is not associated with a reduced thromboembolic stroke risk in patients with a HeartMate-II LVAD on antiplatelet therapy. However, no firm statement about the effect of either anticoagulant or antiaggregant therapy can be made based on our study. A larger randomized study is needed to support the hypothesis that there may be no additional benefit of coumarin or heparin therapy compared with antiplatelet therapy alone.

Hemodynamic and thrombogenic analysis of a trileaflet polymeric valve using a fluid-structure interaction approach

Surgical valve replacement in patients with severe calcific aortic valve disease using either bioprosthetic or mechanical heart valves is still limited by structural valve deterioration for the former and thrombosis risk mandating anticoagulant therapy for the latter. Prosthetic polymeric heart valves have the potential to overcome the inherent material and design limitations of these valves, but their development is still ongoing. The aim of this study was to characterize the hemodynamics and thrombogenic potential of the Polynova polymeric trileaflet valve prototype using a fluid-structure interaction (FSI) approach. The FSI model replicated experimental conditions of the valve as tested in a left heart simulator. Hemodynamic parameters (transvalvular pressure gradient, flow rate, maximum velocity, and effective orifice area) were compared to assess the validity of the FSI model. The thrombogenic footprint of the polymeric valve was evaluated using a Lagrangian approach to calculate the stress accumulation (SA) values along multiple platelet trajectories and their statistical distribution. In the commissural regions, platelets were exposed to the highest SA values because of highest stress levels combined with local reverse flow patterns and vortices. Stress-loading waveforms from representative trajectories in regions of interest were emulated in our hemodynamic shearing device (HSD). Platelet activity was measured using our platelet activation state (PAS) assay and the results confirmed the higher thrombogenic potential of the commissural hotspots. In conclusion, the proposed method provides an in depth analysis of the hemodynamic and thrombogenic performance of the polymer valve prototype and identifies locations for further design optimization.

Multi-objective optimization of pulsatile ventricular assist device hemocompatibility based on neural networks and a genetic algorithm

PURPOSE

Given the benefit of pulsatile blood flow for perfusion of coronary arteries and end organs, pulsatile ventricular assist devices (VADs) are still widely used as paracorporeal mechanical circulatory support devices in clinical applications. However, poor hemocompatibility limits the service period of the VADs. Most previous improvements on VAD hemocompatibility were conducted by trial-and-error CFD analysis, which does not easily arrive at the best solution.

METHODS

In this paper, a multi-objective optimization method integrating neural networks and NSGA-II (Non-dominated Sorted Genetic Algorithm-II) based on FSI simulation was developed and applied to a pulsatile VAD to optimize its hemocompatibility. First, the VAD blood chamber was parameterized with the principal geometrical parameters. Three hemocompatibility indices including hemolysis, platelet activation, and platelet deposition were chosen as goal functions. The neural networks were built to fit the nonlinear relationship between goal functions and geometrical parameters. Next, a multi-objective optimization algorithm (NSGA-II) was used to search out the Pareto optimal solutions in the built neural networks. Finally, the best compromise solution was selected from the Pareto optimal solutions by a fuzzy membership approach and validated by FSI simulation.

RESULTS

The best compromise solution simultaneously possesses an acceptable hemolysis index, platelet activation index, and platelet deposition index, and the corresponding relative errors between the indices predicted by optimization algorithm and the one calculated by FSI simulations are all less than 5%.

CONCLUSIONS

The results suggest that the proposed multi-objective optimization method has the potential for application in optimizing pulsatile VAD hemocompatibility, and may also be applied to other blood-wetted devices.

Medical device-induced thrombosis: what causes it and how can we prevent it?

Blood-contacting medical devices, such as vascular grafts, stents, heart valves, and catheters, are often used to treat cardiovascular diseases. Thrombus formation is a common cause of failure of these devices. This study (i) examines the interface between devices and blood, (ii) reviews the pathogenesis of clotting on blood-contacting medical devices, (iii) describes contemporary methods to prevent thrombosis on blood-contacting medical devices, (iv) explains why some anticoagulants are better than others for prevention of thrombosis on medical devices, and (v) identifies future directions in biomaterial research for prevention of thrombosis on blood-contacting medical devices.

Comparative efficacy of in vitro and in vivo metabolized aspirin in the DeBakey ventricular assist device

Ventricular assist devices (VADs) are implanted in patients with end-stage heart failure to provide both short- and long-term hemodynamic support. Unfortunately, bleeding and thromboembolic complications due to the severely disturbed, dynamic flow conditions generated within these devices require complex, long-term antiplatelet and anticoagulant therapy. While several studies have examined the effectiveness of one such agent, aspirin, under flow conditions, data comparing the efficacy of in vitro and in vivo metabolized aspirin is lacking. Two sets of studies were conducted in vitro with purified human platelets circulating for 30 min in a flow loop containing the DeBakey VAD (MicroMed Cardiovascular, Houston, TX, USA): (a) 20 μM aspirin was added exogenously in vitro to platelets isolated from aspirin-free subjects, and (b) platelets were obtained from donors 2 h (n = 14) and 20 h (n = 13) after ingestion of 1,000 mg aspirin. Near real-time platelet activation state (PAS) was measured with a modified prothrombinase-based assay. Platelets exposed to aspirin in vitro and in vivo (metabolized) showed 28.2 and 25.3 % reduction in platelet activation rate, respectively, compared to untreated controls. Our results demonstrate that in vitro treatment with antiplatelet drugs such as aspirin is as effective as in vivo metabolized aspirin in testing the effect of reducing shear-induced platelet activation in the VAD. Using the PAS assay provides a practical in vitro alternative to in vivo testing of antiplatelet efficacy, as well as for testing the thrombogenic performance of devices during their research and development.

Thromboresistance comparison of the HeartMate II ventricular assist device with the device thrombogenicity emulation- optimized HeartAssist 5 VAD

Approximately 7.5 × 106 patients in the US currently suffer from end-stage heart failure. The FDA has recently approved the designations of the Thoratec HeartMate II ventricular assist device (VAD) for both bridge-to-transplant and destination therapy (DT) due to its mechanical durability and improved hemodynamics. However, incidence of pump thrombosis and thromboembolic events remains high, and the life-long complex pharmacological regimens are mandatory in its VAD recipients. We have previously successfully applied our device thrombogenicity emulation (DTE) methodology for optimizing device thromboresistance to the Micromed Debakey VAD, and demonstrated that optimizing device features implicated in exposing blood to elevated shear stresses and exposure times significantly reduces shear-induced platelet activation and significantly improves the device thromboresistance. In the present study, we compared the thrombogenicity of the FDA-approved HeartMate II VAD with the DTE-optimized Debakey VAD (now labeled HeartAssist 5). With quantitative probability density functions of the stress accumulation along large number of platelet trajectories within each device which were extracted from numerical flow simulations in each device, and through measurements of platelet activation rates in recirculation flow loops, we specifically show that: (a) Platelets flowing through the HeartAssist 5 are exposed to significantly lower stress accumulation that lead to platelet activation than the HeartMate II, especially at the impeller-shroud gap regions (b) Thrombus formation patterns observed in the HeartMate II are absent in the HeartAssist 5 (c) Platelet activation rates (PAR) measured in vitro with the VADs mounted in recirculation flow-loops show a 2.5-fold significantly higher PAR value for the HeartMate II. This head to head thrombogenic performance comparative study of the two VADs, one optimized with the DTE methodology and one FDA-approved, demonstrates the efficacy of the DTE methodology for drastically reducing the device thrombogenic potential, validating the need for a robust in silico/in vitro optimization methodology for improving cardiovascular devices thromboresistance.

Real time visualization and characterization of platelet deposition under flow onto clinically relevant opaque surfaces

Although the thrombogenic nature of the surfaces of cardiovascular devices is an important aspect of blood biocompatibility, few studies have examined platelet deposition onto opaque materials used for these devices in real time. This is particularly true for the metallic surfaces used in current ventricular assist devices (VADs). Using hemoglobin depleted red blood cells (RBC ghosts) and long working distance optics to visualize platelet deposition, we sought to perform such an evaluation. Fluorescently labeled platelets mixed with human RBC ghosts were perfused across six opaque materials (a titanium alloy (Ti6Al4V), silicon carbide (SiC), alumina (Al2O3, 2-methacryloyloxyethyl phosphorylcholine polymer coated Ti6Al4V (MPC-Ti6Al4V), yttria partially stabilized zirconia (YZTP), and zirconia toughened alumina (ZTA)) for 5 min at wall shear rates of 400 and 1000 s(-1). Ti6Al4V had significantly increased platelet deposition relative to MPC-Ti6Al4V, Al2 O3 , YZTP, and ZTA at both wall shear rates (p < 0.01). For all test surfaces, increasing the wall shear rate produced a trend of decreased platelet adhesion. The described system can be a utilized as a tool for comparative analysis of candidate blood-contacting materials with acute blood contact.

Association between cell-derived microparticles and adverse events in patients with nonpulsatile left ventricular assist devices

BACKGROUND

Continuous-flow left ventricular assist devices (LVADs) expose blood cells to high shear stress, potentially resulting in the production of microparticles that express phosphatidylserine (PS+) and promote coagulation and inflammation. In this prospective study, we attempted to determine whether PS+ microparticle levels correlate with clinical outcomes in LVAD-supported patients.

METHODS

We enrolled 20 patients undergoing implantation of the HeartMate II LVAD (Thoratec Corp, Pleasanton, CA) and 10 healthy controls who provided reference values for the microparticle assays. Plasma was collected before LVAD implantation, at discharge, at the 3-month follow-up, and when an adverse clinical event occurred. We quantified PS+ microparticles in the plasma using flow cytometry.

RESULTS

During the study period, 8 patients developed adverse clinical events: ventricular tachycardia storm in 1, non-ST-elevation myocardial infarction in 2, arterial thrombosis in 2, gastrointestinal bleeding in 2, and stroke in 3. Levels of PS+ microparticles were higher in patients at baseline than in healthy controls (2.11% ± 1.26% vs 0.69% ± 0.46%, p = 0.007). After LVAD implantation, patient PS+ microparticle levels increased to 2.39% ± 1.22% at discharge and then leveled to 1.97% ± 1.25% at the 3-month follow-up. Importantly, levels of PS+ microparticles were significantly higher in patients who developed an adverse event than in patients with no events (3.82% ± 1.17% vs 1.57% ± 0.59%, p < 0.001), even though the 2 patient groups did not markedly differ in other clinical and hematologic parameters.

CONCLUSIONS

Our results suggest that an elevation of PS+ microparticle levels may be associated with adverse clinical events. Thus, measuring PS+ microparticle levels in LVAD-supported patients may help identify patients at increased risk for adverse events.

Toward optimization of a novel trileaflet polymeric prosthetic heart valve via device thrombogenicity emulation

Aortic stenosis is the most prevalent and life-threatening form of valvular heart disease. It is primarily treated via open-heart surgical valve replacement with either a tissue or a mechanical prosthetic heart valve (PHV), each prone to degradation and thrombosis, respectively. Polymeric PHVs may be optimized to eliminate these complications, and they may be more suitable for the new transcatheter aortic valve replacement procedure and in devices like the total artificial heart. However, the development of polymer PHVs has been hampered by persistent in vivo calcification, degradation, and thrombosis. To address these issues, we have developed a novel surgically implantable polymer PHV composed of a new thermoset polyolefin called cross-linked poly(styrene-block-isobutylene-block-styrene), or xSIBS, in which key parameters were optimized for superior functionality via our device thrombogenicity emulation methodology. In this parametric study, we compared our homogeneous optimized polcymer PHV to a prior composite polymer PHV and to a benchmark tissue valve. Our results show significantly improved hemodynamics and reduced thrombogenicity in the optimized polymer PHV compared to the other valves. These results indicate that our new design may not require anticoagulants and may be more durable than its predecessor, and validate the improvement, toward optimization, of this novel polymeric PHV design.