Effect of left ventricular assist device outflow conduit anastomosis location on flow patterns in the native aorta

Management of aortic valve insufficiency in patients with continuous-flow left ventricular assist device: a republication of the review published in Japanese Journal of Artificial Organs

Since 2011, implantable ventricular assist devices have been a standard treatment for severe heart failure alongside heart transplantation in Japan. However, the limited availability of donors has led to a prolonged wait for transplants, now averaging 1719 days, intensifying the issue of aortic insufficiency in patients with continuous flow ventricular assist devices. These devices limit the opening of the aortic valve, leading to sustained closure and increased shear stress, which accelerates valve degradation. Risk factors for aortic insufficiency include having a smaller body surface area, being of advanced age, and the presence of mild aortic insufficiency prior to device implantation. In patients presenting with mild or moderate aortic regurgitation at the time of ventricular assist device implantation, interventions such as aortic valve repair or bioprosthetic valve replacement are performed with the aim of halting its progression. The choice of surgical procedure should be tailored to each patient's individual condition. The management of de novo aortic insufficiency in patients with continuous flow ventricular assist devices remains challenging, with no clear consensus on when to intervene. Interventions for significant aortic insufficiency typically consider the patient's symptoms and aortic insufficiency severity. De novo aortic insufficiency progression in continuous flow ventricular assist devices patients necessitates careful monitoring and intervention based on individual patient assessments and valve condition. This review was created based on a translation of the Japanese review written in the Japanese Journal of Artificial Organs in 2023 (Vol. 52, No. 1, pp. 77-80), with some modifications.

Durable left ventricular assist devices as a bridge to transplantation: what to expect along the way?

INTRODUCTION

The scarcity of donors coupled with the improvements in left ventricular assist devices (LVAD) technology has led to the use of LVAD as a bridge to transplantation (BTT).

AREAS COVERED

The authors provide an overview of the current status of LVAD BTT implantation with special focus ranging from patient selection and pre-implantation optimization to post-transplant outcomes.

EXPERT OPINION

The United Network for Organ Sharing 2018 policy amendment resulted in a significant reduction in the number of LVADs used for BTT in the US. To overcome this issue, modifications in the US allocation policy to consider factors such as days on device support, age, and type of complications may be necessary to potentially increase implantation rates.

Development of idealized human aortic models for in vitro and in silico hemodynamic studies

Background

The aorta, a central component of the cardiovascular system, plays a pivotal role in ensuring blood circulation. Despite its importance, there is a notable lack of idealized models for experimental and computational studies.

Objective

This study aims to develop computer-aided design (CAD) models for the idealized human aorta, intended for studying hemodynamics or solid mechanics in both in vitro and in silico settings.

Methods

Various parameters were extracted from comprehensive literature sources to evaluate major anatomical characteristics of the aorta in healthy adults, including variations in aortic arch branches and corresponding dimensions. The idealized models were generated based on averages weighted by the cohort size of each study for several morphological parameters collected and compiled from image-based or cadaveric studies, as well as data from four recruited subjects. The models were used for hemodynamics assessment using particle image velocimetry (PIV) measurements and computational fluid dynamics (CFD) simulations.

Results

Two CAD models for the idealized human aorta were developed, focusing on the healthy population. The CFD simulations, which align closely with the PIV measurements, capture the main global flow features and wall shear stress patterns observed in patient-specific cases, demonstrating the capabilities of the designed models.

Conclusions

The collected statistical data on the aorta and the two idealized aorta models, covering prevalent arch variants known as Normal and Bovine types, are shown to be useful for examining the hemodynamics of the aorta. They also hold promise for applications in designing medical devices where anatomical statistics are needed.

Aortic insufficiency in the patient on contemporary durable left ventricular assist device support: A state-of-the-art review on preoperative and postoperative assessment and management

The development of aortic insufficiency (AI) during HeartMate 3 durable left ventricular assist device (dLVAD) support can lead to ineffective pump output and recurrent heart failure symptoms. Progression of AI often comingles with the occurrence of other hemodynamic-related events encountered during LVAD support, including right heart failure, arrhythmias, and cardiorenal syndrome. While data on AI burdens and clinical impact are still insufficient in patients on HeartMate 3 support, moderate or worse AI occurs in approximately 8% of patients by 1 year and studies suggest AI continues to progress over time and is associated with increased frequency of right heart failure. The first line intervention for AI management is prevention, undertaking surgical intervention on the insufficient valve at the time of dLVAD implant and avoiding excessive device flows and hypertension during long-term support. Device speed augmentation may then be undertaken to try and overcome the insufficient lesion, but the progression of AI should be anticipated over the long term. Surgical or transcatheter aortic valve interventions may be considered in dLVAD patients with significant persistent AI despite medical management, but neither intervention is without risk. It is imperative that future studies of dLVAD support capture AI in clinical end-points using uniform assessment and grading of AI severity by individuals trained in AI assessment during dLVAD support.

Stress Load and Ascending Aortic Aneurysms: An Observational, Longitudinal, Single-Center Study Using Computational Fluid Dynamics

Ascending aortic aneurysm (AAoA) is a silent disease with high mortality; however, the factors associated with a worse prognosis are not completely understood. The objective of this observational, longitudinal, single-center study was to identify the hemodynamic patterns and their influence on AAoA growth using computational fluid dynamics (CFD), focusing on the effects of geometrical variations on aortic hemodynamics. Personalized anatomic models were obtained from angiotomography scans of 30 patients in two different years (with intervals of one to three years between them), of which 16 (53%) showed aneurysm growth (defined as an increase in the ascending aorta volume by 5% or more). Numerically determined velocity and pressure fields were compared with the outcome of aneurysm growth. Through a statistical analysis, hemodynamic characteristics were found to be associated with aneurysm growth: average and maximum high pressure (superior to 100 Pa); average and maximum high wall shear stress (superior to 7 Pa) combined with high pressure (>100 Pa); and stress load over time (maximum pressure multiplied by the time interval between the exams). This study provides insights into a worse prognosis of this serious disease and may collaborate for the expansion of knowledge about mechanobiology in the progression of AAoA.

The Impact of Left Ventricular Assist Device Outflow Graft Positioning on Aortic Hemodynamics: Improving Flow Dynamics to Mitigate Aortic Insufficiency

Heart failure is a global health concern with significant implications for healthcare systems. Left ventricular assist devices (LVADs) provide mechanical support for patients with severe heart failure. However, the placement of the LVAD outflow graft within the aorta has substantial implications for hemodynamics and can lead to aortic insufficiency during long-term support. This study employs computational fluid dynamics (CFD) simulations to investigate the impact of different LVAD outflow graft locations on aortic hemodynamics. The introduction of valve morphology within the aorta geometry allows for a more detailed analysis of hemodynamics at the aortic root. The results demonstrate that the formation of vortex rings and subsequent vortices during the high-velocity jet flow from the graft interacted with the aortic wall. Time-averaged wall shear stress (TAWSS) and oscillatory shear index (OSI) indicate that modification of the outflow graft location changes mechanical states within the aortic wall and aortic valve. Among the studied geometric factors, both the height and inclination angle of the LVAD outflow graft are important in controlling retrograde flow to the aortic root, while the azimuthal angle primarily determines the rotational direction of blood flow in the aortic arch. Thus, precise positioning of the LVAD outflow graft emerges as a critical factor in optimizing patient outcomes by improving the hemodynamic environment.

Reciprocal interferences of the left ventricular assist device and the aortic valve competence

Patients suffering from end-stage heart failure tend to have high mortality rates. With growing numbers of patients progressing into severe heart failure, the shortage of available donors is a growing concern, with less than 10% of patients undergoing cardiac transplantation (CTx). Fortunately, the use of left ventricular assist devices (LVADs), a variant of mechanical circulatory support has been on the rise in recent years. The expansion of LVADs has led them to be incorporated into a variety of clinical settings, based on the goals of therapy for patients ailing from heart failure. However, with an increase in the use of LVADs, there are a host of complications that arise with it. One such complication is the development and progression of aortic regurgitation (AR) which is noted to adversely influence patient outcomes and compromise pump benefits leading to increased morbidity and mortality. The underlying mechanisms are likely multifactorial and involve the aortic root-aortic valve (AV) complex, as well as the LVAD device, patient, and other factors, all of them alter the physiological mechanics of the heart resulting in AV dysfunction. Thus, it is imperative to screen patients before LVAD implantation for AR, as moderate or greater AR requires a concurrent intervention at the time of LVADs implantation. No current strict guidelines were identified in the literature search on how to actively manage and limit the development and/or progression of AR, due to the limited information. However, some recommendations include medical management by targeting fluid overload and arterial blood pressure, along with adjusting the settings of the LVADs device itself. Surgical interventions are to be considered depending on patient factors, goals of care, and the underlying pathology. These interventions include the closure of the AV, replacement of the valve, and percutaneous approach via percutaneous occluding device or transcatheter aortic valve implantation. In the present review, we describe the interaction between AV and LVAD placement, in terms of patient management and prognosis. Also it is provided a comprehensive echocardiographic strategy for the precise assessment of AV regurgitation severity.

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.

Design and execution of a verification, validation, and uncertainty quantification plan for a numerical model of left ventricular flow after LVAD implantation

Background

Left ventricular assist devices (LVADs) are implantable pumps that act as a life support therapy for patients with severe heart failure. Despite improving the survival rate, LVAD therapy can carry major complications. Particularly, the flow distortion introduced by the LVAD in the left ventricle (LV) may induce thrombus formation. While previous works have used numerical models to study the impact of multiple variables in the intra-LV stagnation regions, a comprehensive validation analysis has never been executed. The main goal of this work is to present a model of the LV-LVAD system and to design and follow a verification, validation and uncertainty quantification (VVUQ) plan based on the ASME V&V40 and V&V20 standards to ensure credible predictions.

Methods

The experiment used to validate the simulation is the SDSU cardiac simulator, a bench mock-up of the cardiovascular system that allows mimicking multiple operation conditions for the heart-LVAD system. The numerical model is based on Alya, the BSC’s in-house platform for numerical modelling. Alya solves the Navier-Stokes equation with an Arbitrary Lagrangian-Eulerian (ALE) formulation in a deformable ventricle and includes pressure-driven valves, a 0D Windkessel model for the arterial output and a LVAD boundary condition modeled through a dynamic pressure-flow performance curve. The designed VVUQ plan involves: (a) a risk analysis and the associated credibility goals; (b) a verification stage to ensure correctness in the numerical solution procedure; (c) a sensitivity analysis to quantify the impact of the inputs on the four quantities of interest (QoIs) (average aortic root flow Q A o a v g, maximum aortic root flow Q A o m a x, average LVAD flow Q V A D a v g, and maximum LVAD flow Q V A D m a x); (d) an uncertainty quantification using six validation experiments that include extreme operating conditions.

Results

Numerical code verification tests ensured correctness of the solution procedure and numerical calculation verification showed a grid convergence index (GCI)95% <3.3%. The total Sobol indices obtained during the sensitivity analysis demonstrated that the ejection fraction, the heart rate, and the pump performance curve coefficients are the most impactful inputs for the analysed QoIs. The Minkowski norm is used as validation metric for the uncertainty quantification. It shows that the midpoint cases have more accurate results when compared to the extreme cases. The total computational cost of the simulations was above 100 [core-years] executed in around three weeks time span in Marenostrum IV supercomputer.

Conclusions

This work details a novel numerical model for the LV-LVAD system, that is supported by the design and execution of a VVUQ plan created following recognised international standards. We present a methodology demonstrating that stringent VVUQ according to ASME standards is feasible but computationally expensive.

Left Ventricular Assist Device Support-Induced Alteration of Mechanical Stress on Aortic Valve and Aortic Wall

The aim of this study was to evaluate the fluid dynamics in the aortic valve and proximal aorta during continuous-flow left ventricular assist device (LVAD) support using epiaortic echocardiography and vector flow mapping technology. A total of 12 patients who underwent HeartMate 3 implantation between December 2018 and February 2020 were prospectively examined. The wall shear stress (WSS) on the ascending aorta, aortic root, and aortic valve was evaluated before and after LVAD implantation. The median age of the cohort was 62 years and 17% were women. The peak WSS on the ascending aorta (Pre 1.48 [0.86-1.69] [Pascal {Pa}] vs. Post 0.33 [0.21-0.58] [Pa]; p = 0.002), aortic root (Pre 0.46 [0.31-0.58] (Pa) vs. Post 0.18 [0.12-0.25] (Pa); p = 0.001), and ventricularis of the aortic valve (Pre 1.76 [1.59-2.30] (Pa) vs. Post 0.30 [0.10-0.61] (Pa); p = 0.001) was significantly lower after LVAD implantation. No difference in WSS was observed on the fibrosa of the aortic valve (Pre 0.36 [0.22-0.53] (Pa) vs. Post 0.38 [0.38-0.52] (Pa); p = 0.850) before and after implantation. The WSS on the ascending aorta, aortic root, and ventricularis of the aortic valve leaflets was significantly altered by LVAD implantation, providing preliminary data on the potential contribution of fluid dynamics to LVAD-induced aortic insufficiency and root thrombus.

Exercise capacity following ventricular assist device implantation via thoracotomy with outflow cannula anastomosis to the descending aorta

Left ventricular assist device (LVAD) implantation via left lateral thoracotomy with outflow cannula anastomosis to the descending aorta is an alternative technique that avoids anterior mediastinal planes and requires a single incision. This study compares changes in exercise capacity following LVAD implantation with outflow cannula anastomosis to the descending aorta versus ascending aorta. Adult patients who received a continuous flow centrifugal LVAD implantation and completed both pre- and postimplantation cardiopulmonary exercise tests (CPETs) and or 6-minute walk tests (6MWT) were included. Change in CPET parameters (maximum oxygen intake: vO₂ max, oxygen uptake efficiency ratio: OUES, ventilatory efficiency ratio: vE/vCO₂ Slope) and 6MWT distance were compared between ascending and descending aorta anastomosis groups. Ascending and descending aorta anastomosis cohorts included 59 and 14 patients, respectively. Pre- and postimplantation CPETs were performed 63 ± 12 days before and 216 ± 17 days following implantation. The improvement in CPET parameters (vO₂ max, OUES, vE/vCO₂ Slope) or 6MWT distance was not significantly different between the ascending and descending aorta anastomosis groups. This study found no significant difference in the improvement of CPET parameters or 6MWT distance between LVAD implantation via thoracotomy with outflow cannula anastomosis to descending aorta and standard implantation via sternotomy with outflow cannula anastomosis to ascending aorta.

Ventricular assist devices implantation: surgical assessment and technical strategies

Along with the worldwide increase in continuous left ventricular assist device (LVAD) strategy adoption, more and more patients with demanding anatomical and clinical features are currently referred to heart failure (HF) departments for treatment. Thus surgeons have to deal, technically, with re-entry due to previous cardiac surgery procedures, porcelain aorta, peripheral vascular arterial disease, concomitant valvular or septal disease, biventricular failure. New surgical techniques and surgical tools have been developed to offer acceptable postoperative outcomes to all mechanical circulatory support recipients. Several less invasive and/or thoracotomic approaches for surgery combined with various LVAD inflow and outflow graft alternative anastomotic sites for system placement have been reported and described to solve complex clinical scenarios. Surgical techniques have been upgraded with further technical tips to preserve the native anatomy in case of re-entry for heart transplantation, myocardial recovery or device explant. The current continuous-flow miniaturized and intrapericardial devices provide versatility and technical advantages. However, the surgical planning requires a careful multidisciplinary evaluation which must be driven by a dedicated and well-trained Heart Failure team. Biventricular assist device (BVAD) implantation by adoption of the newer radial pumps might be a challenge. However, the results are encouraging thus remaining a valid option. This paper reviews and summarizes LVAD preoperative assessment and current surgical techniques for implantation.

Biomechanical effects of the working modes of LVADs on the aortic valve: A primary numerical study

Aortic valve diseases caused by the support from left ventricular assist devices (LVADs) have attracted increasing attention due to the wide application of the LVADs. However, the biomechanical effects of the working modes of LVADs on the aortic valve are still poorly understood. Hence, in this study, these biomechanical effects are investigated using a novel fluid-structure interaction method, which combines the lattice Boltzmann and the finite element methods. On the basis of the clinical practice, three working modes of LVADs, namely, the constant flow, co-pulse, and counter pulse modes, are chosen. Results demonstrate that the working mode of LVADs is an important factor as it can change the biomechanical states of the aortic valve and the hemodynamic environment in the aortic root directly. Compared with the constant flow mode, the two other working modes can provide better biomechanical effects on the aortic valve. However, the advantages of the co-pulse and the counter pulse modes on the aortic valve are not the same. The LVADs in the co-pulse mode can remarkable reduce the pressure load of the leaflets during the diastolic phase (maximum stress: co-pulse mode, 0.85 MPa; constant flow mode, 1.23 MPa; counter pulse mode, 1.50 MPa). By contrast, the LVADs in the counter pulse mode can achieve the highest effective orifice area of the aortic valve (co-pulse mode: 0.12 cm², constant flow mode: 0.17 cm², counter pulse mode: 0.25 cm²). In sum, the co-pulse mode is suitable for patients with certain cardiac function, because this mode keeps the valve open intermittently and reduces the pressure load on the aortic leaflets during the diastolic phase to prevent valve remodeling. By contrast, the counter pulse mode is suitable for patients with severely impaired cardiac function, because this mode keeps the valve open as much as possible and provides high blood perfusion.

Use of patient-specific computational models for optimization of aortic insufficiency after implantation of left ventricular assist device

OBJECTIVE

Aortic incompetence (AI) is observed to be accelerated in the continuous-flow left ventricular assist device (LVAD) population and is related to increased mortality. Using computational fluid dynamics (CFD), we investigated the hemodynamic conditions related to the orientation of the LVAD outflow in these patients.

METHOD

We identified 10 patients with new aortic regurgitation, and 20 who did not, after LVAD implantation between 2009 and 2018. Three-dimensional models of patients' aortas were created from their computed tomography scans. The geometry of the LVAD outflow graft in relation to the aorta was quantified using azimuth angles (AA), polar angles (PAs), and distance from aortic root. The models were used to run CFD simulations, which calculated the pressures and wall shear stress (rWSS) exerted on the aortic root.

RESULTS

The AA and PA were found to be similar. However, for combinations of high values of AA and low values of PA, there were no patients with AI. The distance from aortic root to the outflow graft was also smaller in patients who developed AI (3.39 ± 0.7 vs 4.07 ± 0.77 cm, P = .04). There was no significant difference in aortic root pressures in the 2 groups. The rWSS was greater in AI patients (4.60 ± 5.70 vs 2.37 ± 1.20 dyne/cm², P < .001). Qualitatively, we observed a trend of greater perturbations, regions of high rWSS, and flow eddies in the AI group.

CONCLUSIONS

Using CFD simulations, we demonstrated that patients who developed de novo AI have greater rWSS at the aortic root, and their outflow grafts were placed closer to the aortic roots than those patients without de novo AI.

Aortic Insufficiency During Contemporary Left Ventricular Assist Device Support: Analysis of the INTERMACS Registry

OBJECTIVES

This study sought to evaluate the impact of moderate to severe aortic insufficiency (AI) on outcomes in patients with continuous flow left ventricular assist devices (CF-LVADs).

BACKGROUND

Development of worsening AI is a common complication of prolonged CF-LVAD support and portends poor prognosis in single-center studies. Predictors of worsening AI and its impact on clinical outcomes have not been examined in a large cohort.

METHODS

We conducted a retrospective analysis of patients with CF-LVAD in the INTERMACS (Interagency Registry for Mechanically Assisted Circulatory Support) study. Development of significant AI was defined as the first instance of at least moderate AI. Primary outcomes of interest were survival after development of significant AI and time to adverse events, including device complications and rehospitalizations.

RESULTS

Among 10,603 eligible patients, 1,399 patients on CF-LVAD support developed moderate to severe AI. Prevalence of significant AI progressively increased over time. Predictors of worsening AI included older age, female sex, smaller body mass index, mild pre-implantation AI, and destination therapy strategy. Moderate to severe AI was associated with significantly higher left ventricular end-diastolic diameter, reduced cardiac output, and higher levels of brain natriuretic peptide. Significant AI was associated with higher rates of rehospitalization (32.1% vs. 26.6%, respectively, at 2 years; p = 0.015) and mortality (77.2% vs. 71.4%, respectively, at 2 years; p = 0.005), conditional upon survival to 1 year.

CONCLUSIONS

Development of moderate to severe AI has a negative impact on hemodynamics, hospitalizations, and survival on CF-LVAD support. Pre- and post-implantation management strategies should be developed to prevent and treat this complication.

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.

Hurdles to Cardioprotection in the Critically Ill

Cardiovascular disease is the largest contributor to worldwide mortality, and the deleterious impact of heart failure (HF) is projected to grow exponentially in the future. As heart transplantation (HTx) is the only effective treatment for end-stage HF, development of mechanical circulatory support (MCS) technology has unveiled additional therapeutic options for refractory cardiac disease. Unfortunately, despite both MCS and HTx being quintessential treatments for significant cardiac impairment, associated morbidity and mortality remain high. MCS technology continues to evolve, but is associated with numerous disturbances to cardiac function (e.g., oxidative damage, arrhythmias). Following MCS intervention, HTx is frequently the destination option for survival of critically ill cardiac patients. While effective, donor hearts are scarce, thus limiting HTx to few qualifying patients, and HTx remains correlated with substantial post-HTx complications. While MCS and HTx are vital to survival of critically ill cardiac patients, cardioprotective strategies to improve outcomes from these treatments are highly desirable. Accordingly, this review summarizes the current status of MCS and HTx in the clinic, and the associated cardiac complications inherent to these treatments. Furthermore, we detail current research being undertaken to improve cardiac outcomes following MCS/HTx, and important considerations for reducing the significant morbidity and mortality associated with these necessary treatment strategies.

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.

The angle of the outflow graft to the aorta can affect recirculation due to aortic insufficiency under left ventricular assist device support

Aortic insufficiency (AI) is a crucial complication during continuous-flow left ventricular assist device (LVAD) support. Our previous clinical study suggested that a larger angle between the outflow graft and the aorta (O-A angle) could cause AI progression. This study examined the effect of the O-A angle on the hemodynamics of AI under LVAD support in an acute animal experimental model. An LVAD was installed in seven calves, with the inflow cannula inserted from the LV apex and with the outflow graft sutured at the ascending aorta. The AI model was made using a temporary inferior vena cava filter inserted from the LV apex and placed at the aortic valve. Cardiac dysfunction was induced by continuous beta-blocker infusion. Hemodynamic values and the myocardial oxygen extraction rate (O₂ER) were evaluated at three O-A angles (45°, 90°, and 135°) over three levels of AI (none, Sellers I-II AI, and Sellers III-IV AI). The recirculation rate, defined as the percentage of regurgitation flow to LVAD output, was calculated. Systemic flow tended to decrease with a larger O-A angle. The recirculation rate was significantly increased with a larger O-A angle (22, 23, and 31% at 45°, 90°, and 135° in Sellers III-IV AI, respectively). Coronary artery flow was decreased at a larger O-A angle (86, 76 and 75 mL/min at 45°, 90°, and 135° in Sellers I-II AI, respectively, and 77, 67, and 56 mL/min at 45°, 90°, and 135° in Sellers III-IV AI, respectively). O₂ER tended to increase with a larger O-A angle (40, 43, and 49% at 45°, 90°, and 135° in Sellers III-IV AI, respectively). A larger O-A angle can increase the recirculation due to AI and can be disadvantageous to LVAD-AI hemodynamics and myocardial oxygen metabolism.

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

The current study evaluates quantitatively the impact that intermittent aortic valve (AV) opening has on the thrombogenicity in the aortic arch region for patients under left ventricular assist device (LVAD) therapy. The influence of flow through the AV, opening once every five cardiac cycles, on the flow patterns in the ascending aortic is measured in a patient-derived computed tomography image-based model, after LVAD implantation. The mechanical environment of flowing platelets is investigated, by statistical treatment of outliers in Lagrangian particle tracking, and thrombogenesis metrics (platelet residence times and activation state characterized by shear stress accumulation) are compared for the cases of closed AV versus intermittent AV opening. All hemodynamics metrics are improved by AV opening, even at a reduced frequency and flow rate. Residence times of platelets or microthrombi are reduced significantly by transvalvular flow, as are the shear stress history experienced and the shear stress magnitude and gradients on the aortic root endothelium. The findings of this device-neutral study support the multiple advantages of management that enables AV opening, providing a rationale for establishing this as a standard in long-term treatment and care for advanced heart failure patients.

Impact of LVAD Implantation Site on Ventricular Blood Stagnation

Treatment of end-stage heart failure includes cardiac transplantation or ventricular assist device (VAD) therapy. Although increasingly prevalent, current VAD therapy has inherent complications, including thrombosis. Studies have demonstrated that VAD implantation alters intracardiac blood flow, creating areas of stagnation that predispose to thrombus formation. Two potential surgical configurations exist for VAD implantation: through the apical or diaphragmatic surfaces of the heart. We hypothesized that diaphragmatic implantation causes more stagnation than apical implantation. We also hypothesized that intermittent aortic valve (AV) opening reduces stagnation of blood inside the left ventricle (LV) when compared with a closed AV. To test these hypotheses, a human LV geometry was recreated in silico and a VAD inflow cannula was virtually implanted in each configuration. A computational indicator-dilution study was conducted where "virtually dyed blood" was washed out of the LV by injecting blood with no dye. Simulations demonstrated a substantial reduction in stagnation with intermittent AV opening. In addition, virtual dye was cleared slightly faster in the apical configuration. Simulations from our study demonstrate the clinical importance of VAD management to allow intermittent opening of the AV to prevent subvalvular stagnation, and also suggests that apical configuration might be more hemodynamically favorable.

Outflow graft anastomosis site design could be correlated to aortic valve regurgitation under left ventricular assist device support

Aortic valve regurgitation (AR) is a critical complication during circulatory support with a left ventricular assist device (LVAD). The time-course of AR and related factors, including outflow graft anastomosis site design, were investigated. Twenty-three patients who had continuous-flow LVAD implantation and were supported for more than 6 months were investigated. AR grade (none, 0; trivial, 0.5; mild, 1; mild-moderate, 1.5; moderate, 2; moderate-severe, 2.5; severe, 3) and aortic valve opening were evaluated with echocardiography. Computed tomography was performed to all the patients postoperatively. The angle of the outflow graft to the aorta (O-A angle, parallel 0; tangent 90°, 0-180°), aortic diameter at the anastomosis site, sino-tubular junction (STJ) diameter, distance between the STJ and the anastomosis site, and distance between the anastomosis site and the brachiocephalic artery were measured. The patients' age was 38 ± 11 years. Support duration was 686 ± 354 days. Mean AR grade after continuous-flow LVAD implantation was increased to around mild and was maintained thereafter. No patient needed any intervention to the aortic valve. The aortic valves of 82.6% of patients were closed continuously. The O-A angle (83 ± 14) was positively correlated with maximum AR grade (p = 0.0095). The O-A angle was significantly smaller in patients with maximum AR grade of 1 or less (77 ± 9°) than in those with 1.5 or greater (94 ± 15°, p = 0.021). The other CT measurements had no correlation with AR grade. In conclusion, the O-A angle was correlated with AR grade progression. The O-A angle appears to be one of the important factors related to AR under continuous-flow LVAD support.

LVAD Outflow Graft Angle and Thrombosis Risk

This study quantifies thrombogenic potential (TP) of a wide range of left ventricular assist device (LVAD) outflow graft anastomosis angles through state-of-the-art techniques: 3D imaged-based patient-specific models created via virtual surgery and unsteady computational fluid dynamics with Lagrangian particle tracking. This study aims at clarifying the influence of a single parameter (outflow graft angle) on the thrombogenesis associated with flow patterns in the aortic root after LVAD implantation. This is an important and poorly-understood aspect of LVAD therapy, because several studies have shown strong inter and intrapatient thrombogenic variability and current LVAD implantation strategies do not incorporate outflow graft angle optimization. Accurate platelet-level investigation, enabled by statistical treatment of outliers in Lagrangian particle tracking, demonstrates a strong influence of outflow graft anastomoses angle on thrombogenicity (platelet residence times and activation state characterized by shear stress accumulation) with significantly reduced TP for acutely-angled anastomosed outflow grafts. The methodology presented in this study provides a device-neutral platform for conducting comprehensive thrombogenicity evaluation of LVAD surgical configurations, empowering optimal patient-focused surgical strategies for long-term treatment and care for advanced heart failure patients.

Current status of mechanical circulatory support for treatment of advanced end-stage heart failure: successes, shortcomings and needs

INTRODUCTION

Heart failure (HF) remains a major global burden in terms of morbidity and mortality. Despite advances in pharmacological and resynchronization device therapy, many patients worsen to end-stage HF. Although the gold-standard treatment for such patients is heart transplantation, there will always be a shortage of donor hearts. Areas covered: A left ventricular assist device (LVAD) is a valuable option for these patients as a bridge measure (to recovery, to candidacy for transplant, or to transplant itself) or as destination therapy. This review describes the current indications for and complications of the most commonly implanted LVADs. In addition, we review the potential and promising new LVADs, including the HeartMate 3, MVAD, and other LVADs. Studies investigating each were identified through a combination of online database and direct extraction of studies cited in previously identified articles. Expert commentary: The goal of LVADs has been to fill the gap between patients with end-stage HF who would likely not benefit from heart transplantation and those who could benefit from a donor heart. As of now, the use of LVADs has been limited to patients with end-stage HF, but next-generation LVAD therapy may improve both survival and quality of life in less sick patients.

The hemodynamic effects of the LVAD outflow cannula location on the thrombi distribution in the aorta: A primary numerical study

Although a growing number of patients undergo LVAD implantation for heart failure treatment, thrombi are still the devastating complication for patients who used LVAD. LVAD outflow cannula location and thrombi generation sources were hypothesized to affect the thrombi distribution in the aorta. To test this hypothesis, numerical studies were conducted by using computational fluid dynamic (CFD) theory. Two anastomotic configurations, in which the LVAD outflow cannula is anastomosed to the anterior and lateral ascending aortic wall (named as anterior configurations and lateral configurations, respectively), are designed. The particles, whose sized are same as those of thrombi, are released at the LVAD output cannula and the aortic valve (named as thrombiP and thrombiL, respectively) to calculate the distribution of thrombi. The simulation results demonstrate that the thrombi distribution in the aorta is significantly affected by the LVAD outflow cannula location. In anterior configuration, the thrombi probability of entering into the three branches is 23.60%, while that in lateral configuration is 36.68%. Similarly, in anterior configuration, the thrombi probabilities of entering into brachiocephalic artery, left common carotid artery and left subclavian artery, is 8.51%, 9.64%, 5.45%, respectively, while that in lateral configuration it is 11.39%, 3.09%, 22.20% respectively. Moreover, the origins of thrombi could affect their distributions in the aorta. In anterior configuration, the thrombiP has a lower probability to enter into the three branches than thrombiL (12% vs. 25%). In contrast, in lateral configuration, the thrombiP has a higher probability to enter into the three branches than thrombiL (47% vs. 35%). In brief, the LVAD outflow cannula location significantly affects the distribution of thrombi in the aorta. Thus, in the clinical practice, the selection of outflow location of LVAD and the risk of thrombi formed in the left ventricle should be paid more attention than before.

Aortic insufficiency in continuous-flow left ventricular assist device support patients is common but does not impact long-term mortality

BACKGROUND

Aortic insufficiency (AI) is a significant long-term complication of continuous-flow left ventricular assist device (CF-LVAD) implantation. We sought to evaluate its impact on clinical outcomes and mortality in CF-LVAD recipients.

METHODS

We retrospectively analyzed 237 patients implanted with HeartMate II CF-LVADs at our institution from June 2005 through June 2013. We evaluated recipients' baseline characteristics and annual echocardiograms, grading AI severity as either none, trace, mild, moderate or severe. Only moderate or severe AI was considered clinically significant. Recipients who underwent concomitant aortic valve surgery or who had undergone previous prosthetic aortic valve implantation were excluded.

RESULTS

Moderate or severe AI occurred in 32 (15.2%) patients. Risk factors that significantly affected the development of AI included older age at the time of implantation, female gender, longer duration of LVAD support and destination therapy designation. Freedom from moderate or severe AI was 94%, 76% and 65% of patients at 1, 3 and 5 years, respectively. Overall cohort survival based on Kaplan-Meier analysis was 78%, 59% and 42% at 1, 3 and 5 years, respectively. There was no difference in survival between recipients who developed significant AI and those who did not (log-rank test, p = 0.73).

CONCLUSIONS

In this large, single-institution study, the overall rate of AI was low, but increased in frequency with longer duration of LVAD support. Although AI development remains a concern for patients on long-term CF-LVAD support, AI development does not appear to impact long-term mortality.

Challenges faced in long term ventricular assist device support

INTRODUCTION

The development of ventricular assist device (VAD) has been one of the revolutionary advancements in end-stage heart failure management. Although the device has developed and improved significantly over the last few decades, we still face multiple challenges.

AREAS COVERED

This review will discuss quality of life, survival, and clinically encountered complications in patients with VAD support. The literature was extensively reviewed for studies describing the above topic area. We describe the impact of major challenges faced in VAD support and discuss their future and expectations. Expert commentary: The technological advancement of VADs has contributed to major improvement of overall survival, enhancement of quality of life and decrease of incidence of complications. It is expected that technologies will continue to evolve. At the same time, the indications for and timing of device implantation, and selection of device type are continuously important in clinical practice setting.

Continuous and Pulsatile Pediatric Ventricular Assist Device Hemodynamics with a Viscoelastic Blood Model

To investigate the effects of pulsatile and continuous pediatric ventricular assist (PVAD) flow and pediatric blood viscoelasticity on hemodynamics in a pediatric aortic graft model. Hemodynamic parameters of pulsatility, along with velocity and wall shear stress (WSS), are analyzed and compared between Newtonian and viscoelastic blood models at a range of physiological pediatric hematocrits using computational fluid dynamics. Both pulsatile and continuous PVAD flow lead to a decrease in pulsatility (surplus hemodynamic energy, ergs/cm(3)) compared to healthy aortic flow but with continuous PVAD pulsatility up to 2.4 times lower than pulsatile PVAD pulsatility at each aortic outlet. Significant differences are also seen between the two flow modes in velocity and WSS. The higher velocity jet during systole with pulsatile flow leads to higher WSSs at the anastomotic toe and at the aortic branch bifurcations. The lower velocity but continuous flow jet leads to a much different flow field and higher WSSs into diastole. Under a range of physiological pediatric hematocrit (20-60%), both velocity and WSS can vary significantly with the higher hematocrit blood model generally leading to higher peak WSSs but also lower WSSs in regions of flow separation. The large decrease in pulsatility seen from continuous PVAD flow could lead to complications in pediatric vascular development while the high WSSs during peak systole from pulsatile PVAD flow could lead to blood damage. Both flow modes lead to similar regions prone to intimal hyperplasia resulting from low time-averaged WSS and high oscillatory shear index.

Review of recent results using computational fluid dynamics simulations in patients receiving mechanical assist devices for end-stage heart failure

Many end-stage heart failure patients are not eligible to undergo heart transplantation due to organ shortage, and even those under consideration for transplantation might suffer long waiting periods. A better understanding of the hemodynamic impact of left ventricular assist devices (LVAD) on the cardiovascular system is therefore of great interest. Computational fluid dynamics (CFD) simulations give the opportunity to study the hemodynamics in this patient population using clinical imaging data such as computed tomographic angiography. This article reviews a recent study series involving patients with pulsatile and constant-flow LVAD devices in which CFD simulations were used to qualitatively and quantitatively assess blood flow dynamics in the thoracic aorta, demonstrating its potential to enhance the information available from medical imaging.

Late Complications Following Continuous-Flow Left Ventricular Assist Device Implantation

Left ventricular assist devices have become standard therapy for patients with end-stage heart failure. They represent potential long-term solutions for a growing public health problem. However, initial enthusiasm for this technology has been tempered by challenges posed by long-term support. This review examines these challenges and out current understanding of their etiologies.

Computational analysis of pediatric ventricular assist device implantation to decrease cerebral particulate embolization

Stroke is the most devastating complication after ventricular assist device (VAD) implantation with a 19% incidence and 65% mortality in the pediatric population. Current pediatric VAD technology and anticoagulation strategies alone are suboptimal. VAD implantation assisted by computational methods (CFD) may contribute reducing the risk of cerebral embolization. Representative three-dimensional aortic arch models of an infant and a child were generated. An 8 mm VAD outflow-graft (VAD-OG) anastomosed to the aorta was rendered and CFD was applied to study blood flow patterns. Particle tracks, originating in the VAD, were computed with a Lagrangian phase model and the percentage of particles entering the cerebral vessels was calculated. Eight implantation configurations (infant = 5 and child = 3) and 5 particle sizes (0.5, 1, 2, 3, and 4 mm) were considered. For the infant model, percentage of particles entering the cerebral vessels ranged from 15% for a VAD-OG anastomosed at 90° to the aorta, to 31% for 30° VAD-OG anastomosis (overall percentages: X(2) = 10,852, p < 0.0001). For the child model, cerebral embolization ranged from 9% for the 30° VAD-OG anastomosis to 15% for the 60° anastomosis (overall percentages: χ(2) = 10,323, p < 0.0001). Using detailed CFD calculations, we demonstrate that the risk of stroke depends significantly on the VAD implantation geometry. In turn, the risk probably depends on patient-specific anatomy. CFD can be used to optimize VAD implantation geometry to minimize stroke risk.

Complications of Continuous-Flow Mechanical Circulatory Support Devices

Left ventricular assist devices (LVADs), more importantly the continuous-flow subclass, have revolutionized the medical field by improving New York Heart Association (NYHA) functional class status, quality of life, and survival rates in patients with advanced systolic heart failure. From the first pulsatile device to modern day continuous-flow devices, LVADs have continued to improve, but they are still associated with several complications. These complications include infection, bleeding, thrombosis, hemolysis, aortic valvular dysfunction, right heart failure, and ventricular arrhythmias. In this article, we aim to review these complications to understand the most appropriate approach for their prevention and to discuss the available therapeutic modalities.

Blood pressure and adverse events during continuous flow left ventricular assist device support

BACKGROUND

Adverse events (AEs), such as intracranial hemorrhage, thromboembolic event, and progressive aortic insufficiency, create substantial morbidity and mortality during continuous flow left ventricular assist device support yet their relation to blood pressure control is underexplored.

METHODS AND RESULTS

A multicenter retrospective review of patients supported for at least 30 days and ≤18 months by a continuous flow left ventricular assist device from June 2006 to December 2013 was conducted. All outpatient Doppler blood pressure (DOPBP) recordings were averaged up to the time of intracranial hemorrhage, thromboembolic event, or progressive aortic insufficiency. DOPBP was analyzed as a categorical variable grouped as high (>90 mm Hg; n=40), intermediate (80-90 mm Hg; n=52), and controlled (<80 mm Hg; n=31). Cumulative survival free from an AE was calculated using Kaplan-Meier curves and Cox hazard ratios were derived. Patients in the high DOPBP group had worse baseline renal function, lower angiotensin-converting enzyme inhibitor or angiotensin receptor blocker usage during continuous flow left ventricular assist device support, and a more prevalent history of hypertension. Twelve (30%) patients in the high DOPBP group had an AE, in comparison with 7 (13%) patients in the intermediate DOPBP group and only 1 (3%) in the controlled DOPBP group. The likelihood of an AE increased in patients with a high DOPBP (adjusted hazard ratios [95% confidence interval], 16.4 [1.8-147.3]; P=0.012 versus controlled and 2.6 [0.93-7.4]; P=0.068 versus intermediate). Overall, a similar association was noted for the risk of intracranial hemorrhage (P=0.015) and progressive aortic insufficiency (P=0.078) but not for thromboembolic event (P=0.638). Patients with an AE had a higher DOPBP (90±10 mm Hg) in comparison with those without an AE (85±10 mm Hg; P=0.05).

CONCLUSIONS

In a population at risk, higher DOPBP during continuous flow left ventricular assist device support was significantly associated with a composite of AEs.

Investigation of hemodynamics in an in vitro system simulating left ventricular support through the right subclavian artery using 4-dimensional flow magnetic resonance imaging

OBJECTIVES

Left ventricular assist devices are an important treatment option for patients with heart failure alter the hemodynamics in the heart and great vessels. Because in vivo magnetic resonance studies of patients with ventricular assist devices are not possible, in vitro models represent an important tool to investigate flow alterations caused by these systems. By using an in vitro magnetic resonance-compatible model that mimics physiologic conditions as close as possible, this work investigated the flow characteristics using 4-dimensional flow-sensitive magnetic resonance imaging of a left ventricular assist device with outflow via the right subclavian artery as commonly used in cardiothoracic surgery in the recent past.

METHODS

An in vitro model was developed consisting of an aorta with its supra-aortic branches connected to a left ventricular assist device simulating the pulsatile flow of the native failing heart. A second left ventricular assist device supplied the aorta with continuous flow via the right subclavian artery. Four-dimensional flow-sensitive magnetic resonance imaging was performed for different flow rates of the left ventricular assist device simulating the native heart and the left ventricular assist device providing the continuous flow. Flow characteristics were qualitatively and quantitatively evaluated in the entire vessel system.

RESULTS

Flow characteristics inside the aorta and its upper branching vessels revealed that the right subclavian artery and the right carotid artery were solely supported by the continuous-flow left ventricular assist device for all flow rates. The flow rates in the brain-supplying arteries are only marginally affected by different operating conditions. The qualitative analysis revealed only minor effects on the flow characteristics, such as weakly pronounced vortex flow caused by the retrograde flow via the brachiocephalic artery.

CONCLUSIONS

The results indicate that, despite the massive alterations in natural hemodynamics due to the retrograde flow via the right subclavian and brachiocephalic arteries, there are no drastic consequences on the flow in the brain-feeding arteries and the flow characteristics in the ascending and descending aortas. It may be beneficial to adjust the operating condition of the left ventricular assist device to the residual function of the failing heart.

A computational fluid dynamics comparison between different outflow graft anastomosis locations of Left Ventricular Assist Device (LVAD) in a patient-specific aortic model

Left ventricular assist devices (LVADs) are mechanical supports used in case of heart failure. Little is known as the height of the anastomosis in aorta might influence the hemodynamic. The aim of the study was to evaluate the fluid dynamic behavior due to the outflow graft placement of a continuous flow LVAD in ascending aorta and to identify the insertion site with the best hemodynamic profile. Computational fluid dynamic studies were carried out to analyze 4 different anastomosis locations in a patient-specific aorta 3D model coupled with a lumped parameters model: 1 cm (case 1), 2 cm (case 2), 3 cm (case 3) and 4 cm (case 4) above the ST junction. In cases 1 and 2, epiaortic vessels presented a steady flow, while in cases 3 and 4 the flow was whirling. Moreover, maximum velocity occurred before: brachiocephalic trunk (case 1), brachiocephalic and left carotid arteries (case 2), left carotid and left subclavian artery (case 3) and left subclavian vessel and upper wall of aortic arch (case 4). Maximum time averaged wall shear stress (TAWSS) was located in: the ascending aorta (cases 1 and 2), the inferior curvature of the arch (case 3); at the origin of epiaortic vessels (case 4). Furthermore, a flow recirculation (cases 1 and 2), a blood stagnation and chaotic flow (cases 3 and 4) occurred above the aortic valve. The results suggested that the placement of the outflow graft at 2 cm above the ST junction gave the most favorable hemodynamic profile.