Ventricular Assist Device Implantation Configurations Impact Overall Mechanical Circulatory Support System Thrombogenic Potential

A Computational Investigation of the Effects of Temporal Synchronization of Left Ventricular Assist Device Speed Modulation with the Cardiac Cycle on Intraventricular Hemodynamics

Patients with advanced heart failure are implanted with a left ventricular assist device (LVAD) as a bridge-to-transplantation or destination therapy. Despite advances in pump design, the risk of stroke remains high. LVAD implantation significantly alters intraventricular hemodynamics, where regions of stagnation or elevated shear stresses promote thrombus formation. Third generation pumps incorporate a pulsatility mode that modulates rotational speed of the pump to enhance in-pump washout. We investigated how the timing of the pulsatility mode with the cardiac cycle affects intraventricular hemodynamic factors linked to thrombus formation. Computational fluid dynamics simulations with Lagrangian particle tracking to model platelet behavior in a patient-specific left ventricle captured altered intraventricular hemodynamics due to LVAD implantation. HeartMate 3 incorporates a pulsatility mode that modulates the speed of the pump every two seconds. Four different timings of this pulsatility mode with respect to the cardiac cycle were investigated. A strong jet formed between the mitral valve and inflow cannula. Blood stagnated in the left ventricular outflow tract beneath a closed aortic valve, in the near-wall regions off-axis of the jet, and in a large counterrotating vortex near the anterior wall. Computational results showed good agreement with particle image velocimetry results. Synchronization of the pulsatility mode with peak systole decreased stasis, reflected in the intraventricular washout of virtual contrast and Lagrangian particles over time. Temporal synchronization of HeartMate 3 pulsatility with the cardiac cycle reduces intraventricular stasis and could be beneficial for decreasing thrombogenicity.

Resistance valves in circulatory loops have a significant impact on in vitro evaluation of blood damage caused by blood pumps: a computational study

Background: Hemolysis and its complications are major concerns during the clinical application of blood pumps. In-vitro circulatory testing loops have been employed as the key procedure to evaluate the hemolytic and thrombogenic performance of blood pumps during the development phase and before preclinical in-vivo animal studies. Except for the blood damage induced by the pump under test, blood damage induced by loop components such as the resistance valve may affect the accuracy, reproducibility, and intercomparability of test results. Methods: This study quantitatively investigated the impact of the resistance valve on in vitro evaluation of blood damage caused by blood pumps under different operating points. A series of idealized tubing models under the resistance valve with different openings were created. Three pumps - the FDA benchmark pump, the HeartMate 3 LVAD, and the CH-VAD - were involved in hypothetical tests. Eight operating points were chosen to cover a relatively wide spectrum of testing scenarios. Computational fluid dynamics (CFD) simulations of the tubing and pump models were conducted at the same operating points. Results and Conclusion: Overall, hemolysis and platelet activation induced by a typical resistance valve are equivalent to 17%-45% and 14%-60%, respectively, of those induced by the pump itself. Both ratios varied greatly with flow rate, valve opening and pump models. Differences in blood damage levels between different blood pumps or working conditions can be attenuated by up to 45%. Thus, hemolysis and platelet activation induced by the resistance valve significantly affect the accuracy of in-vitro hemocompatibility evaluations of blood pumps. A more accurate and credible method for hemocompatibility evaluations of blood pumps will benefit from these findings.

Preclinical Device Thrombogenicity Assessments: Key Messages From the 2018 FDA, Industry, and Academia Forum

Device-related thrombosis and thromboembolic complications remain a major clinical concern and often impact patient morbidity and mortality. Thus, improved preclinical thrombogenicity assessment methods that better predict clinical outcomes and enhance patient safety are needed. However, there are several challenges and limitations associated with developing and performing preclinical thrombogenicity assessments on the bench and in animals (e.g., the clinical relevance of most in vitro tests has not been established, animal studies may not accurately predict clinical thrombotic events). To facilitate a discussion on how to overcome some of these challenges and to promote collaboration between the Food and Drug Administration (FDA), industry, and academia for the development of more reliable test methods, a scientific forum was organized by FDA and held in Washington, DC, on June 15, 2018 at the ASAIO 64th Annual Conference. Three subject matter experts from the medical device industry and FDA presented their perspectives at this forum, and several audience experts provided input during the open dialogue session. This article summarizes the key messages from the forum regarding the current status and challenges of preclinical thrombogenicity testing, important areas of needed research, and mechanisms for working with FDA to further improve thrombogenicity evaluations of medical devices.

New Insights into Valve Hemodynamics

Heart valve diseases are common disorders with five million annual diagnoses being made in the United States alone. All heart valve disorders alter cardiac hemodynamic performance; therefore, treatments aim to restore normal flow. This paper reviews the state-of-the-art clinical and engineering advancements in heart valve treatments with a focus on hemodynamics. We review engineering studies and clinical literature on the experience with devices for aortic valve treatment, as well as the latest advancements in mitral valve treatments and the pulmonic and tricuspid valves on the right side of the heart. Upcoming innovations will potentially revolutionize treatment of heart valve disorders. These advancements, and more gradual enhancements in the procedural techniques and imaging modalities, could improve the quality of life of patients suffering from valvular disease who currently cannot be treated.

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.

Aortic Hemodynamics of Spiral-Flow-Generated Mechanical Assistance

BACKGROUND

Mechanical circulatory support devices are being increasingly used as destination therapy in end-stage heart failure patients. Although current devices have significantly improved survival rates, the resulting hemodynamics remains nonphysiological. Spiral forms of blood flow are known to exist in the large arteries (eg, aorta) and serve as a biomimetic-motivation for generating these physiologically adapted flow regimes. We aimed to study the potential benefits of generating spiral flow at the mechanical circulatory support outflow graft and the resultant flow-fields in the aorta, including recirculation zones and endothelial wall shear stress (WSS) areas.

METHODS

A three-dimensional model of an outflow graft virtually anastomosed end-to-side to an image-derived aortic arch was used in computational fluid dynamic simulations. To study the impact of both spiral flow modulation (clockwise/counterclockwise helical-flow content) and the outflow graft anastomosis angle (inferiorly/superiorly directed, anteriorly/posteriorly directed), flow velocities were measured, low/high WSS were computed, and fluid streamlines were visualized.

RESULTS

Increased helical-flow content reduced regions of low velocity (<5 cm/s), minimized areas exhibiting low WSS (<3 dyn/cm²), and concomitantly increased areas of high WSS (>80 dyn/cm²). The outflow graft anastomosis angle was a key determinant of aortic root washout and fluid-jet wall impingement. Despite counterclockwise spiral flow predominance in diminishing the size of recirculation/stasis zones compared to straight/clockwise flow, exceptions to this were noted with the superiorly directed and posteriorly directed graft placements.

CONCLUSIONS

Spiral flow-forms better tailored to the underlying three-dimensional aortic curvature and graft angle positioning is expected to help attenuate atherogenesis, preventing vascular remodeling and minimizing plaque formation/erosion in mechanically assisted circulation.

Recurrent pump thrombosis is common after axial continuous-flow left ventricular assist device exchange

In selected patients with left ventricular assist device-associated infection or malfunction, pump exchange may become necessary after conservative treatment options fail and heart transplantation is not readily available. We examined the survival and complication rate in patients (⩾19 years of age) who underwent HeartMate II to HeartMate II exchange at our institution from 1 January 2010 to 28 February 2018. Clinical outcomes were analyzed and compared for patients who underwent exchange for pump thrombosis (14 patients), breach of driveline integrity (5 patients), and device-associated infection (2 patients). There were no differences in 30-day mortality (p = 0.58), need for temporary renal replacement therapy (p = 0.58), right ventricular mechanical support (p = 0.11), and postoperative stroke (p = 0.80) among groups. Survival at 1 year was 90% ± 7% for the whole cohort and 85% ± 10% for those who underwent exchange for pump thrombosis. In patients exchanged for device thrombosis, freedom from re-thrombosis and survival free from pump re-thrombosis at 1 year were 49% ± 16% and 42% ± 15%, respectively. No association of demographic and clinical variables with the risk of recurrent pump thrombosis after the first exchange was identified. Survival after left ventricular assist device exchange compares well with published results after primary left ventricular assist device implantation. However, recurrence of thrombosis was common among patients who required a left ventricular assist device exchange due to pump thrombosis. In this sub-group, consideration should be given to alternative strategies to improve the outcomes.

Device Thrombogenicity Emulation: An In Silico Predictor of In Vitro and In Vivo Ventricular Assist Device Thrombogenicity

Ventricular assist devices (VAD), a mainstay of therapy for advanced and end-stage heart failure, remain plagued by device thrombogenicity. Combining advanced in silico and in vitro methods, Device Thrombogenicity Emulation (DTE) is a device design approach for enhancing VAD thromboresistance. Here we tested DTE efficacy in experimental VAD designs. DTE incorporates iterative design modifications with advanced CFD to compute the propensity of large populations of platelets to activate by flow-induced stresses (statistically representing the VAD ‘Thrombogenic Footprint’). The DTE approach was applied to a VAD (MIN_DTE) design with a favorable thromboresistance profile and compared against a design (MAX_DTE) that generated an intentionally poor thromboresistance profile. DTE predictions were confirmed by testing physical prototypes in vitro by measuring VAD thrombogenicity using the modified prothrombinase assay. Chronic in vivo studies in VAD implanted calves, revealed MIN_DTE calf surviving well with low platelet activation, whereas the MAX_DTE animal sustained thromboembolic strokes. DTE predictions were confirmed, correlating with in vitro and in vivo thrombogenicity, supporting utility in guiding device development, potentially reducing the need for animal studies.

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.

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.

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

Introduction:

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

Methods:

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

Results:

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

Conclusion:

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