Intermittent Aortic Valve Opening and Risk of Thrombosis in Ventricular Assist Device Patients

Physiological control for left ventricular assist devices based on deep reinforcement learning

BACKGROUND

The improvement of controllers of left ventricular assist device (LVAD) technology supporting heart failure (HF) patients has enormous impact, given the high prevalence and mortality of HF in the population. The use of reinforcement learning for control applications in LVAD remains minimally explored. This work introduces a preload-based deep reinforcement learning control for LVAD based on the proximal policy optimization algorithm.

METHODS

The deep reinforcement learning control is built upon data derived from a deterministic high-fidelity cardiorespiratory simulator exposed to variations of total blood volume, heart rate, systemic vascular resistance, pulmonary vascular resistance, right ventricular end-systolic elastance, and left ventricular end-systolic elastance, to replicate realistic inter- and intra-patient variability of patients with a severe HF supported by LVAD. The deep reinforcement learning control obtained in this work is trained to avoid ventricular suction and allow aortic valve opening by using left ventricular pressure signals: end-diastolic pressure, maximum pressure in the left ventricle (LV), and maximum pressure in the aorta.

RESULTS

The results show controller obtained in this work, compared to the constant speed LVAD alternative, assures a more stable end-diastolic volume (EDV), with a standard deviation of 5 mL and 9 mL, respectively, and a higher degree of aortic flow, with an average flow of 1.1 L/min and 0.9 L/min, respectively.

CONCLUSION

This work implements a deep reinforcement learning controller in a high-fidelity cardiorespiratory simulator, resulting in increases of flow through the aortic valve and increases of EDV stability, when compared to a constant speed LVAD strategy.

Aortic Root Vortex Formation During Left Ventricular Assist Device Support

The vortex that forms in the aortic sinus plays a vital role in optimizing blood flow. Disruption of the vortex can result in flow stagnation and activate thrombus formation in the aortic root, especially when aortic valve flow is reduced as during left ventricular assist device (LVAD) support. Our goal in this study was to visualize vortex formation in an experimental model of the aortic root as flow is progressively reduced. A mock circulatory loop that reproduces heart failure hemodynamics was combined with a HeartMate II LVAD and velocity measured in a transparent aortic root with a bioprosthetic valve. The aortic valve sinus vortices are clearly visible as counter-rotating structures in the velocity field at baseline and for all conditions with flow through the aortic valve. As LVAD speed increases, the central jet narrows but the vortices persist, disappearing only when the valve is completely closed. The vortices preserve fluid momentum and generate shear stress along the tissue surfaces which disrupts flow stasis. These features underscore the importance of maintaining "intermittent" aortic valve opening, as recommended for LVAD patients. This study is the first to report vortex formation in the aortic root during LVAD support, providing a motivation for further evaluation.

Optimization design of semi-open impeller based on thrombogenicity in a blood pump

BACKGROUND

Blood clots are composed of aggregated fibrin and platelets, and thrombosis is the body's natural response to repairing injured blood vessels or stopping bleeding. However, when this process is activated abnormally, such as in a mechanical blood pump, it can lead to excessive thrombus formation. Therefore, how to avoid or reduce the probability of thrombus formation is an important indicator of the stable operation of a blood pump.

METHODS

In this paper, Lagrangian particle tracking trajectories are simulated to study platelet transport in a blood pump. The design of the thrombus blood pump was optimized using an orthogonal design method based on three factors: inlet angle, outlet angle, and blade number. The effect of blood pump pressure, rotational speed, impeller outlet angle, inlet angle, and number of blades on thrombus formation was analysed using Fluent software. The thrombogenic potential was derived by analyzing the trajectory and flow parameters of platelet particles in the blood pump, as well as the statistical parameters of residence time and stress accumulation thrombus in the platelet pump.

RESULTS

When the impeller inlet angle is 30°, the outlet angle is 20°, and the number of blades is 6, the probability of thrombus formation is minimized in the orthogonal design method, aligning with the requirements for blood pump performance.

CONCLUSIONS

These design parameters serve as a numerical guideline for optimizing the geometry of the semi-open impeller in blood pumps and provide a theoretical foundation for subsequent in vitro experiments.

Aortic Root Thrombosis in patients with HeartMate 3 left ventricular assist device support

BACKGROUND

Aortic root thrombosis(ART) is a complication of continuous-flow left ventricular assist device therapy. However, the incidence and related complications of ART in HeartMate 3 (HM3) patients remain unknown.

METHODS

Patients who underwent HM3 implantation from November 2014 to August 2020 at a quaternary academic medical center were included. Demographics and outcomes were abstracted from the medical record. Echocardiograms and contrast-enhanced computed tomography studies were reviewed to identify patients who developed ART and/or moderate or greater aortic insufficiency (AI) on HM3 support.

RESULTS

The study cohort included 197 HM3 patients with a median postimplant follow-up of 17.5 months. Nineteen patients (9.6%) developed ART during HM3 support, and 15 patients (7.6%) developed moderate or greater AI. Baseline age, gender, race, implantation strategy, and INTERMACS classification were similar between the ART and no-ART groups. ART was associated with an increased risk of death, stroke, or aortic valve (AV) intervention (subhazard ratio [SHR] 3.60 [95% confidence interval (CI) 1.71-7.56]; p = 0.001) and moderate or greater AI (SHR 11.1 [CI 3.60-34.1]; p < 0.001) but was not associated with a statistically significantly increased risk of death or stroke on HM3 support (2.12 [0.86-5.22]; p = 0.10). Of the 19 patients with ART, 6 (31.6%) developed moderate or greater AI, necessitating more frequent AV interventions (ART: 5 AV interventions [3 surgical repairs, 1 surgical replacement, 1 transcatheter replacement; 26.3%]; no-ART: 0).

CONCLUSIONS

Nearly 10% of HM3 patients developed ART during device support. ART was associated with increased risk of a composite end-point of death, stroke, or AV intervention as well as moderate or greater AI.

Atrial fibrillation increases thrombogenicity of LVAD therapy

BACKGROUND

This study investigates the hypothesis that presence of atrial fibrillation (AF) in LVAD patients increases thrombogenicity in the left ventricle (LV) and exacerbates stroke risk.

METHODS

Using an anatomical LV model implanted with an LVAD inflow cannula, we analyze thrombogenic risk and blood flow patterns in either AF or sinus rhythm (SR) using unsteady computational fluid dynamics (CFD). To analyze platelet activation and thrombogenesis in the LV, hundreds of thousands of platelets are individually tracked to quantify platelet residence time (RT) and shear stress accumulation history (SH).

RESULTS

The irregular and chaotic mitral inflow associated with AF results in markedly different intraventricular flow patterns, with profoundly negative impact on blood flow-induced stimuli experienced by platelets as they traverse the LV. Twice as many platelets accumulated very high SH in the LVAD + AF case, resulting in a 36% increase in thrombogenic potential score, relative to the LVAD + SR case.

CONCLUSIONS

This supports the hypothesis that AF results in unfavorable blood flow patterns in the LV adding to an increased stroke risk for LVAD + AF patients. Quantification of thrombogenic risk associated with AF for LVAD patients may help guide clinical decision-making on interventions to mitigate the increased risk of thromboembolic events.

Enhancing the implantation of mechanical circulatory support devices using computational simulations

Introduction: Patients with end-stage heart failure (HF) may need mechanical circulatory support such as a left ventricular assist device (LVAD). However, there are a range of complications associated with LVAD including aortic regurgitation (AR) and thrombus formation. This study assesses whether the risk of developing aortic conditions can be minimised by optimising LVAD implantation technique. Methods: In this work, we evaluate the aortic flow patterns produced under different geometrical parameters for the anastomosis of the outflow graft (OG) to the aorta using computational fluid dynamics (CFD). A three-dimensional aortic model is created and the HeartMate III OG positioning is simulated by modifying (i) the distance from the anatomic ventriculo-arterial junction (AVJ) to the OG, (ii) the cardinal position around the aorta, and (iii) the angle between the aorta and the OG. The continuous LVAD flow and the remnant native cardiac cycle are used as inlet boundaries and the three-element Windkessel model is applied at the pressure outlets. Results: The analysis quantifies the impact of OG positioning on different haemodynamic parameters, including velocity, wall shear stress (WSS), pressure, vorticity and turbulent kinetic energy (TKE). We find that WSS on the aortic root (AoR) is around two times lower when the OG is attached to the coronal side of the aorta using an angle of 45° ± 10° at a distance of 55 mm. Discussion: The results show that the OG placement may significantly influence the haemodynamic patterns, demonstrating the potential application of CFD for optimising OG positioning to minimise the risk of cardiovascular complications after LVAD implantation.

Hemodynamics Indicates Differences Between Patients With And Without A Stroke Outcome After Left Ventricular Assist Device Implantation

Stroke remains a leading cause of complications and mortality in heart failure patients treated with LVAD circulatory support. Hemodynamics plays a central role in affecting risk and etiology of stroke during LVAD support. Yet, detailed quantitative assessment of hemodynamic variables and their relation to stroke outcomes in patients with an implanted LVAD remains a challenge. We present an in silico hemodynamics analysis in a set of 12 patients on LVAD support; 6 with reported stroke outcomes and 6 without. We conducted patient-specific hemodynamics simulations for models with the LVAD outflow graft reconstructed from cardiac-gated CT images. A pre-implantation baseline flow model was virtually generated for each case by removing the LVAD outflow graft and driving flow from the aortic root. Hemodynamics was characterized using quantitative descriptors for helical flow, vortex generation, and wall shear stress. Our analysis showed higher average values for descriptors of positive helical flow, vortex generation, and wall shear stress, across the 6 cases with stroke outcomes on LVAD support, when compared with cases without stroke. When the descriptors for LVAD-driven flow were compared against estimated baseline flow pre-implantation, extent of positive helicity was higher, and vorticity and wall shear were lower in cases with stroke compared to those without. The study suggests that quantitative analysis of hemodynamics after LVAD implantation; and hemodynamic alterations from a pre-implant flow scenario, can potentially reveal hidden information linked to stroke outcomes during LVAD support. This has broad implications on understanding stroke etiology, LVAD treatment planning, surgical optimization, and efficacy assessment.

The Relation Between Viscous Energy Dissipation and Pulsation for Aortic Hemodynamics Driven by a Left Ventricular Assist Device

Left ventricular assist device (LVAD) provides mechanical circulatory support for patients with advanced heart failure. Treatment using LVAD is commonly associated with complications such as stroke and gastro-intestinal bleeding. These complications are intimately related to the state of hemodynamics in the aorta, driven by a jet flow from the LVAD outflow graft that impinges into the aorta wall. Here we conduct a systematic analyses of hemodynamics driven by an LVAD with a specific focus on viscous energy transport and dissipation. We conduct a complementary set of analysis using idealized cylindrical tubes with diameter equivalent to common carotid artery and aorta, and a patient-specific model of 27 different LVAD configurations. Results from our analysis demonstrate how energy dissipation is governed by key parameters such as frequency and pulsation, wall elasticity, and LVAD outflow graft surgical anastomosis. We find that frequency, pulsation, and surgical angles have a dominant effect, while wall elasticity has a weaker effect, in determining the state of energy dissipation. For the patient-specific scenario, we also find that energy dissipation is higher in the aortic arch and lower in the abdominal aorta, when compared to the baseline flow without an LVAD. This further illustrates the key hemodynamic role played by the LVAD outflow jet impingement, and subsequent aortic hemodynamics during LVAD operation.

Aortic Thrombosis in Patients on Mechanical Circulatory Support: A Systematic Literature Review

BACKGROUND

Aortic valve (AV) thrombosis is an uncommon but ominous complication in patients managed with mechanical circulatory support (MCS) devices. In this systematic review, we summarised the data on clinical presentations and outcomes in such patients.

METHODS

We searched articles on PubMed and Google Scholar, reporting at least one adult patient with aortic thrombosis on MCS support and where the individual patient data could be extracted. We grouped the patients by the type of MCS (temporary or durable), and the type of the AV (prosthetic, surgically modified, or native) RESULTS: We identified reports on six patients with aortic thrombus on short-term MCS, and on 41 patients on durable left ventricular assist devices (LVADs). On temporary MCS, AV thrombus typically causes no symptoms and is found incidentally pre- or intra-operatively. For those with durable MCS, the occurrence of aortic thrombus forming on prosthetic or surgically modified valves appears to be more related to the intervention on the valve, rather than from the presence of LVAD. The mortality in this group was 18%. In patients with native AV on durable LVAD support, 60% of patients presented with acute myocardial infarction, acute stroke, or acute heart failure, and mortality in this cohort was 45%. In terms of management, heart transplantation was most successful.

CONCLUSIONS

While the outcomes of aortic thrombosis were good in patients where temporary MCS was used in the setting of aortic valve surgery, patients with native AV who develop this complication on durable LVAD have high morbidity and mortality. Cardiac transplantation should be strongly considered in eligible candidates because other therapies provide inconsistent results.

In Vitro Investigation of the Effect of the Timing of Left Ventricular Assist Device Speed Modulation on Intraventricular Flow Patterns

Thromboembolic events remain a common complication for left ventricular assist device (LVAD) patients. To prevent in-pump thrombosis, third-generation LVADs use speed modulation, which is not synchronized with the native left ventricle (LV) contractility. This study aims to investigate the effect of speed modulation on intraventricular flow patterns, and specifically, the impact of timing relative to pressure variations in the LV. Stereo-particle image velocimetry measurements were performed in a patient-derived LV implanted with an LVAD, for different timings of the speed modulation and speed. Speed modulation has a strong effect on instantaneous afterload and flowrate (-16% and +20%). The different timings of the speed modulation resulted in different flowrate waveforms, exhibiting different maxima (5.3-5.9 L/min, at constant average flowrate). Moreover, the timing of the speed modulation was found to strongly influence intraventricular flow patterns, specifically, stagnation areas within the LV. These experiments highlight, once more, the complex relationship between LVAD speed, hemodynamic resistance, and intraventricular pressure. Overall, this study demonstrates the importance of considering native LV contractility in future LVAD controls, to improve hemocompatibility and reduce the risk of thromboembolic complications.

Quantitative Assessment of Aortic Hemodynamics for Varying Left Ventricular Assist Device Outflow Graft Angles and Flow Pulsation

Left ventricular assist devices (LVADs) comprise a primary treatment choice for advanced heart failure patients. Treatment with LVAD is commonly associated with complications like stroke and gastro-intestinal (GI) bleeding, which adversely impacts treatment outcomes, and causes fatalities. The etiology and mechanisms of these complications can be linked to the fact that LVAD outflow jet leads to an altered state of hemodynamics in the aorta as compared to baseline flow driven by aortic jet during ventricular systole. Here, we present a framework for quantitative assessment of aortic hemodynamics in LVAD flows realistic human vasculature, with a focus on quantifying the differences between flow driven by LVAD jet and the physiological aortic jet when no LVAD is present. We model hemodynamics in the aortic arch proximal to the LVAD outflow graft, as well as in the abdominal aorta away from the LVAD region. We characterize hemodynamics using quantitative descriptors of flow velocity, stasis, helicity, vorticity and mixing, and wall shear stress. These are used on a set of 27 LVAD scenarios obtained by parametrically varying LVAD outflow graft anastomosis angles, and LVAD flow pulse modulation. Computed descriptors for each of these scenarios are compared against the baseline flow, and a detailed quantitative characterization of the altered state of hemodynamics due to LVAD operation (when compared to baseline aortic flow) is compiled. These are interpreted using a conceptual model for LVAD flow that distinguishes between flow originating from the LVAD outflow jet (and its impingement on the aorta wall), and flow originating from aortic jet during aortic valve opening in normal physiological state.

Pathophysiology and management of valvular disease in patients with destination left ventricular assist devices

Over the last two decades, implantable continuous flow left ventricular assist devices (LVAD) have proven to be invaluable tools for the management of selected advanced heart failure patients, improving patient longevity and quality of life. The presence of concomitant valvular pathology, including that involving the tricuspid, mitral, and aortic valve, has important implications relating to the decision to move forward with LVAD implantation. Furthermore, the presence of concomitant valvular pathology often influences the surgical strategy for LVAD implantation. Concomitant valve repair or replacement is not uncommonly required in such circumstances, which increases surgical complexity and has demonstrated prognostic implications both short and longer term following LVAD implantation. Beyond the index operation, it is also well established that certain valvular pathologies may develop or worsen over time following LVAD support. The presence of pre-existing valvular pathology or that which develops following LVAD implant is of particular importance to the destination therapy LVAD patient population. As these patients are not expected to have the opportunity for heart transplantation in the future, optimization of LVAD support including ameliorating valvular disease is critical for the maximization of patient longevity and quality of life. As collective experience has grown over time, the ability of clinicians to effectively address concomitant valvular pathology in LVAD patients has improved in the pre-implant, implant, and post-implant phase, through both medical management and procedural optimization. Nevertheless, there remains uncertainty over many facets of concomitant valvular pathology in advanced heart failure patients, and the understanding of how to best approach these conditions in the LVAD patient population continues to evolve. Herein, we present a comprehensive review of the current state of the field relating to the pathophysiology and management of valvular disease in destination LVAD patients.

A Computational Hemodynamics Approach to Left Ventricular Assist Device (LVAD) Optimization Validated in a Large Patient Cohort

With increasing use of left ventricular assist devices (LVAD) it is critical to devise strategies to optimize LVAD speed while controlling mean arterial pressure (MAP) and flow according to patient physiology. The complex interdependency between LVAD speed, MAP, and flow frequently makes optimization difficult under clinical conditions. We propose a method to guide this procedure in silico, narrowing the conditions to test clinically. A computational model of the circulatory network that simulates HF and LVAD support, incorporating LVAD pressure-flow curves was applied retrospectively to anonymized patient hemodynamics data from the University of Washington Medical Center. MAP management on 61 patient-specific computational models with a target of 70 mm Hg, resulting flow for a given LVAD speed was analyzed, and compared to a target output of 5 L/min. Before performing virtual MAP management, 51% had a MAP>70 mm Hg and CO>5 L/min, and 33% had a MAP>70 mm Hg and CO<5 L/min. After changing systemic resistance to meet the MAP target (without adjusting LVAD speed), 84% of cases resulted in CO higher than 5 L/min, with a median CO of 6.79 L/min, using the computational predictive model. Blood pressure management alone is insufficient in meeting both MAP and CO targets, due to the risk of hypervolemia, and requires appropriate LVAD speed optimization to achieve both targets, while preserving right heart health. Such computational tools can narrow down conditions to be tested for each patient, providing significant insight into the pump-patient interplay. LVAD hemodynamic optimization has the potential to reduce complications and improve outcomes.

Left Ventricular Flow Dynamics with the HeartMate3 Left Ventricular Assist Device: Effect of Inflow Cannula Position and Speed Modulation

Improper left ventricular assist device (LVAD) inflow cannula (IC) positioning creates areas of stasis and low pulsatility that predispose thromboembolism, but may be mitigated with LVAD speed modulation. A mock loop study was performed to assess the sensitivity of left ventricle (LV) flow architecture to IC position and speed modulation during HeartMate3 support. System pressure, flow, and the time-resolved velocity field were measured within a transparent silicone LV for three IC angles and three IC insertion depths at matched levels of cardiac function and LVAD speed. Inflow cannula angulation towards the septum increased the resistance to LVAD flow as well as increasing the size and energy of the counter-clockwise (CCW) vortex. Apical velocity was reduced compared to IC angulation towards the mitral valve, but regional pulsatility was maintained across all angles and LVAD speeds. Increased IC protrusion decreased LVAD flow resistance, increasing velocity within the IC but reducing flow and pulsatility in the adjacent apical region. Increasing LVAD flow resistance improves aortic valve opening and strengthens the CCW vortex which directs inflow towards the septum, producing higher blood residence time and shear activation potential. Despite this impact on flow architecture, pulsatility reduction with increased LVAD speed was minimal with the HeartMate3 speed modulation feature.

Effect of Timings of the Lavare Cycle on the Ventricular Washout in an In Vitro Flow Visualization Setup

Left ventricular assist devices inherently alter the intraventricular flow field and create areas of blood stasis with potential thrombus formation. The Lavare cycle of the Medtronic HeartWare HVAD was designed to improve ventricular washout. This study aims to evaluate its effects on ventricular washout in a pulsatile in vitro setting with a focus on the timing of pump speed changes. Ventricular flow fields were obtained via particle image velocimetry in two modes: With constant left ventricular assist devices speed and with the Lavare cycle applied. The start of the Lavare cycle was shifted over an entire cardiac cycle, and ventricular washout was evaluated based on velocity fields, kinetic energy, and normalized pulsatility of flow fields. The ventricular flow fields showed dependence on the timing of the Lavare cycle and interaction between speed changes and the cardiac phase. Higher apical velocity was observed for speed decreases at the late E wave and for increases at mid systole by 29% (P = 0.002) and 61% (P < 0.001), respectively. Mean apical kinetic energy for these phases also increased by 21% (P = 0.0013) and 46% (P < 0.001). The Lavare cycle generally promotes higher apical washout and can specifically generate further improved washout if speed steps are applied at the correct timing on the cardiac cycle.

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.

Pump position and thrombosis in ventricular assist devices: Correlation of radiographs and CT data

Malpositioning of left ventricular assist devices (LVAD) is a risk factor for thrombosis, but its identification from clinical imaging remains challenging. X-rays and CT scans were analyzed and parameters identified that correlated to pump thrombosis. Retrospective imaging data of patients (n = 115) with HeartmateII (HMII) or HVAD were analyzed in two groups (pump-thrombosis PT, n = 15 vs matched control group NT, n = 15) using routine X-rays and CT scans. In CT, directional deviations of the inflow cannula in three-chamber and two-chamber view (α and β angles) were identified. In HVAD PT frontal radiographs showed reduced pump body area and smaller minor axis (PT 41.3 ± 4.8 mm vs NT 34.9 ± 6.0 mm, p = 0.026), and in the lateral radiographs the visibility of the inflow cannula served as a predictive parameter for PT. In HMII patients, no parameters were associated with PT. The angle α differed significantly (NT −1.2 ± 7.5°, PT −22.0 ± 4.7°, p = 0.006) in HVAD patients. Further, correlations of x-ray parameters with CT angles α and β showed that radiographs can be used to identify malpositioned pumps. Well-aligned inflow cannula positions are essential. HVAD patients with a posterior rotation of the inflow cannula have a higher risk of pump thrombosis. This risk can reliably be identified from routine radiographs.

In Vitro Investigation of the Effect of Left Ventricular Assist Device Speed and Pulsatility Mode on Intraventricular Hemodynamics

Stroke has become the main cause of mortality and morbidity in patients treated with Left Ventricular Assist Devices (LVADs). The hemodynamics of the left ventricle are altered by the implantation of an LVAD, with the increase of thrombogenic flow patterns, such as stagnation regions. Time-resolved stereo particle image velocimetry (Stereo-PIV) measurements of the flow inside a patient-specific model of the left ventricle (LV) implanted with an LVAD were performed. The effects of LVAD speed, peripheral resistance and afterload were investigated. The impact of activating the LVAD pulsatility mode (periodic speed modulation) was also evaluated. Analysis of the velocity measurements in two orthogonal planes revealed stagnation zones which may be favorable to thrombus formation. Increasing LVAD speed, despite increasing the flow rate through the inflow cannula, does not automatically result in smaller stagnation regions. These results demonstrated the strong interdependence of peripheral resistance, afterload and flow through the LVAD. As a consequence, the pulsatility mode showed very limited effect on overall flow rate. However, it did reduce the size of high stagnation areas. This study showed how LVAD speed, peripheral resistance and afterload impact the complex intraventricular flow patterns in a ventricle implanted with an LVAD and quantify their thrombogenic risk.

Thrombotic Risk of Rotor Speed Modulation Regimes of Contemporary Centrifugal Continuous-flow Left Ventricular Assist Devices

Contemporary centrifugal continuous-flow left ventricular assist devices (LVADs) incorporate dynamic speed modulation algorithms. Hemocompatibility of these periodic unsteady pump operating conditions has been only partially explored. We evaluated whether speed modulation induces flow alterations associated with detrimental prothrombotic effects. For this aim, we evaluated the thrombogenic profile of the HeartWare ventricular assist device (HVAD) Lavare Cycle (LC) and HeartMate3 (HM3) artificial pulse (AP) via comprehensive numerical evaluation of (i) pump washout, (ii) stagnation zones, (iii) shear stress regimens, and (iv) modeling of platelet activation status via the platelet activity state (PAS) model. Data were compared between different simulated operating scenarios, including: (i) constant rotational speed and pump pressure head, used as reference; (ii) unsteady pump pressure head as induced by cardiac pulsatility; and (iii) unsteady rotor speed modulation of the LC (HVAD) and AP (HM3). Our results show that pump washout did not improve across the different simulated scenarios in neither the HVAD nor the HM3. The LC reduced but did not eliminate flow stagnation (-57%) and did not impact metrics of HVAD platelet activation (median PAS: +0.4%). The AP reduced HM3 flow stagnation by up to 91% but increased prothrombotic shear stress and simulated platelet activation (median PAS: +124%). Our study advances understanding of the pathogenesis of LVAD thrombosis, suggesting mechanistic implications of rotor speed modulation. Our data provide rationale criteria for the future design optimization of next generation LVADs to further reduce hemocompatibility-related adverse events.

Left Ventricular Assist Device Outflow Graft Obstruction: A Complication Specific to Polytetrafluoroethylene Covering. A Word of Caution!

Pump thrombosis is an established complication of left ventricular assist devices (LVADs). Outflow graft obstruction has been previously described as one cause of LVAD thrombosis. We identified four cases of outflow graft obstruction that were attributed to a commonly applied polytetrafluoroethylene (PTFE) covering of the outflow graft. In this set of patients, the outflow graft was obstructed by a thrombus which formed between the outflow graft and its external PTFE covering, leading to impingement of the outflow graft. Patients typically presented after a median duration of 26 months (range 23-41 months) of support with gradual increase of heart failure symptoms and low pump flows. Computed tomography angiography was found to be the best diagnostic modality. Treatments included surgical LVAD replacement as well as percutaneous intraluminal stenting of the outflow graft. Our findings indicate that PTFE graft covering of the LVAD outflow graft can lead to graft occlusion and should be reconsidered as a potentially harmful modification to the approved device implant technique.

Small Left Ventricular Size Is an Independent Risk Factor for Ventricular Assist Device Thrombosis

The prevalence of ventricular assist device (VAD) therapy has continued to increase due to a stagnant donor supply and growing advanced heart failure (HF) population. We hypothesize that left ventricular (LV) size strongly influences biocompatibility and risk of thrombosis. Unsteady computational fluid dynamics (CFD) was used in conjunction with patient-derived computational modeling and virtual surgery with a standard, apically implanted inflow cannula. A dual-focus approach of evaluating thrombogenicity was employed: platelet-based metrics to characterize the platelet environment and flow-based metrics to investigate hemodynamics. Left ventricular end-diastolic dimensions (LVEDds) ranging from 4.5 to 6.5 cm were studied and ranked according to relative thrombogenic potential. Over 150,000 platelets were individually tracked in each LV model over 15 cardiac cycles. As LV size decreased, platelets experienced markedly increased shear stress histories (SHs), whereas platelet residence time (RT) in the LV increased with size. The complex interplay between increased SH and longer RT has profound implications on thrombogenicity, with a significantly higher proportion of platelets in small LVs having long RT times and being subjected to high SH, contributing to thrombus formation. Our data suggest that small LV size, rather than decreased VAD speed, is the primary pathologic mechanism responsible for the increased incidence of thrombosis observed in VAD patients with small LVs.

Pulsatile arterial blood pressure mimicking aortic valve opening during continuous-flow LVAD support: a case report

BACKGROUND

Left ventricular assist devices (LVAD) have become a common treatment option in advanced heart failure. Lack of aortic valve opening during left ventricular unloading is a common complication and associated with a worse outcome. Maintaining a minimum pulse pressure is an important goal during the early postoperative period after LVAD implantation since it is commonly seen as secure sign of aortic valve opening.

AIMS/OBJECTIVE

We report a case of an LVAD-supported patient with early permanent closure of the aortic valve despite a pulse pressure > 15 mmHg at all times following LVAD implantation. We demonstrate how careful assessment of the invasive arterial blood pressure curve can indicate aortic valve closure irrespective of pulsatile blood flow.

METHOD

A 69-year old male patient with terminal ischemic cardiomyopathy was referred for long-term mechanical circulatory support. Due to mild aortic regurgitation both an aortic bioprosthesis and a continuous-flow left ventricular assist device were implanted. Postoperative echocardiography documented a patent aortic bioprosthesis and an acceptable residual systolic left ventricular contractility. During invasive arterial blood pressure monitoring repetitive transient slight blood pressure decreases followed by slight blood pressure increases coincided with programmed LVAD flushing cycles. Permanent pulsatile flow with a pulse pressure of ≥15 mmHg conveyed systolic opening of the aortic valve. Echocardiography, however, proved early permanent aortic valve closure. In retrospect, transformation of the automated LVAD flushing cycles into visible changes of the arterial blood pressure curve during invasive blood pressure monitoring is indicative of ejection of the complete cardiac output through LVAD itself, and therefore an early clinical sign of aortic valve closure.

DISCUSSION/CONCLUSION

We present this interesting didactic case to highlight caveats during the early postoperative period after LVAD implantation. Moreover, this case demonstrates that careful and differentiated observation of the arterial blood pressure waveform provides crucial information in this unique and growing patient population of continuous-flow LVAD support.

A Novel Control Method for Rotary Blood Pumps as Left Ventricular Assist Device Utilizing Aortic Valve State Detection

A novel control method for rotary blood pumps is proposed relying on two different objectives: regulation of pump flow in accordance with desired value and the maintenance of partial support with an open aortic valve by the variation of pump speed. The estimation of pump flow and detection of aortic valve state was performed with mathematical models describing the first- and second generation of Sputnik rotary blood pumps. The control method was validated using a cardiovascular system model. The state of the aortic valve was detected with a mean accuracy of 91% for Sputnik 1 and 96.2% for Sputnik 2 when contractility, heart rate, and systemic vascular resistance was changed. In silico results for both pumps showed that the proposed control method can achieve the desired pump flow level and maintain the open state of the aortic valve by periodically switching between two objectives under contractility, heart rate, and systemic vascular resistance changes. The proposed method showed its potential for safe operation without adverse events and for the improvement of chances for myocardial recovery.

Ventricular Flow Field Visualization During Mechanical Circulatory Support in the Assisted Isolated Beating Heart

Investigations of ventricular flow patterns during mechanical circulatory support are limited to in vitro flow models or in silico simulations, which cannot fully replicate the complex anatomy and contraction of the heart. Therefore, the feasibility of using echocardiographic particle image velocimetry (Echo-PIV) was evaluated in an isolated working heart setup. Porcine hearts were connected to an isolated, working heart setup and a left ventricular assist device (LVAD) was implanted. During different levels of LVAD support (unsupported, partial support, full support), microbubbles were injected and echocardiographic images were acquired. Iterative PIV algorithms were applied to calculate flow fields. The isolated heart setup allowed different hemodynamic situations. In the unsupported heart, diastolic intra-ventricular blood flow was redirected at the heart’s apex towards the left ventricular outflow tract (LVOT). With increasing pump speed, large vortex formation was suppressed, and blood flow from the mitral valve directly entered the pump cannula. The maximum velocities in the LVOT were significantly reduced with increasing support. For the first time, cardiac blood flow patterns during LVAD support were visualized and quantified in an ex vivo model using Echo-PIV. The results reveal potential regions of stagnation in the LVOT and, in future the methods might be also used in clinical routine to evaluate intraventricular flow fields during LVAD support.

Left Ventricular Assist Device Inflow Cannula Angle and Thrombosis Risk

BACKGROUND

As heart failure prevalence continues to increase in the setting of a static donor supply, left ventricular assist device (LVAD) therapy for end-stage heart failure continues to grow. Anecdotal evidence suggests that malalignment of the LVAD inflow cannula may increase thrombosis risk, but this effect has not been explored mechanistically or quantified statistically. Our objective is to elucidate the impact of surgical angulation of the inflow cannula on thrombogenicity.

METHODS AND RESULTS

Unsteady computational fluid dynamics is used in conjunction with computational modeling and virtual surgery to model flow through the left ventricle for 5 different inflow cannula angulations. We use a holistic approach to evaluate thrombogenicity: platelet-based (Lagrangian) metrics to evaluate the platelet mechanical environment, combined with flow-based (Eulerian) metrics to investigate intraventricular hemodynamics. The thrombogenic potential of each LVAD inflow cannula angulation is quantitatively evaluated based on platelet shear stress history and residence time. Intraventricular hemodynamics are strongly influenced by LVAD inflow cannula angulation. Platelet behavior indicates elevated thrombogenic potential for certain inflow cannula angles, potentially leading to platelet activation. Our analysis demonstrates that the optimal range of inflow angulation is within 0±7° of the left ventricular apical axis.

CONCLUSIONS

Angulation of the inflow cannula >7° from the apical axis (axis connecting mitral valve and ventricular apex) leads to markedly unfavorable hemodynamics as determined by computational fluid dynamics. Computational hemodynamic simulations incorporating Lagrangian and Eulerian metrics are a powerful tool for studying optimization of LVAD implantation strategies, with the long-term potential of improving outcomes.

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.

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.