Amy L. Throckmorton

Quantitative Evaluation of Blood Damage in a Centrifugal VAD by Computational Fluid Dynamics

This study explores a quantitative evaluation of blood damage that occurs in a continuous flow left ventricular assist device (LVAD) due to fluid stress. Computational fluid dynamics (CFD) analysis is used to track the shear stress history of 388 particle streaklines. The accumulation of shear and exposure time is integrated along the streaklines to evaluate the levels of blood trauma. This analysis, which includes viscous and turbulent stresses, provides a statistical estimate of possible damage to cells flowing through the pump. Since experimental data for hemolysis levels in our LVAD are not available, in vitro normalized index of hemolysis values for clinically available ventricular assist devices were compared to our damage indices. This approach allowed for an order of magnitude comparison between our estimations and experimentally measured hemolysis levels, which resulted in a reasonable correlation. This work ultimately demonstrates that CFD is a convenient and effective approach to analyze the Lagrangian behavior of blood in a heart assist device

Pediatric circulatory support systems.

Ventricular assist devices (VADs) are a valid option for long-term circulatory support in pediatric patients with postoperative myocardial failure or debilitating heart defects. Most clinical experience to date has involved the short-term support of patients weighing 6 kg and larger. For cases of VAD implementation in pediatric patients, the assist device showed tremendous promise in reversing cardiac failure and providing adequate support as a bridge to cardiac transplantation. The Medos-HIA system, Berlin Heart, Medtronic Bio-Medicus Pump, Abiomed BVS 5000, Toyobo-Zeon pumps, and Hemopumps have proven successful for short-term circulatory support for the pediatric population. The Jarvik 2000 and Pierce-Donachy pediatric system further demonstrate the potential to be used for pediatric circulatory support. The clinical and experimental success of these support systems provide encouragement to believe that long-term support is possible.

Intravascular Mechanical Cavopulmonary Assistance for Patients With Failing Fontan Physiology

To provide a viable bridge-to-transplant, bridge-to-recovery, or bridge-to-surgical reconstruction for patients with failing Fontan physiology, we are developing a collapsible, percutaneously inserted, magnetically levitated axial flow blood pump to support the cavopulmonary circulation in adolescent and adult patients. This unique blood pump will augment pressure and thus flow in the inferior vena cava through the lungs and ameliorate the poor hemodynamics associated with the univentricular circulation. Computational fluid dynamics analyses were performed to create the design of the impeller, the protective cage of filaments, and the set of diffuser blades for our axial flow blood pump. These analyses included the generation of pressure-flow characteristics, scalar stress estimations, and blood damage indexes. A quasi-steady analysis of the diffuser rotation was also completed and indicated an optimal diffuser rotational orientation of approximately 12 degrees. The numerical predictions of the pump performance demonstrated a pressure generation of 2-25 mm Hg for 1-7 L/min over 3000-8000 rpm. Scalar stress values were less than 200 Pa, and fluid residence times were found to be within acceptable ranges being less than 0.25 s. The maximum blood damage index was calculated to be 0.068%. These results support the continued design and development of this cavopulmonary assist device, building upon previous numerical work and experimental prototype testing.

Pediatric Circulatory Support: Current Strategies and Future Directions. Biventricular and Univentricular Mechanical Assistance

Mechanical circulatory support is gaining increased recognition as a viable treatment option for pediatric patients who suffer from congenital or acquired heart disease. Historically, the treatment options have been very limited for pediatric patients, but recent technological advances, combined with new research into circulatory support devices, are seeking alternative therapeutics options for infants and children. We present a review of the technological advances of mechanical circulatory support in the pediatric population, including the recent emergence of a new class of circulatory support devices for pediatric patients with single ventricle physiology. The National Heart, Lung, and Blood Institute pediatric circulatory support program is discussed, in addition to the use of adult devices in pediatric applications, the Berlin Heart Excor, and several other blood pumps in development for bridge-to-transplant and bridge-to-recovery support. These devices have the potential to generate a paradigm shift in the treatment of the pediatric patients with heart failure—a shift is likely already be underway.

Numerical design and experimental hydraulic testing of an axial flow ventricular assist device for infants and children.

Mechanical circulatory support options for infants and children are very limited in the United States. Existing circulatory support systems have proven successful for short-term pediatric assist, but are not completely successful as a bridge-to-transplant or bridge-to-recovery. To address this substantial need for alternative pediatric mechanical assist, we are developing a novel, magnetically levitated, axial flow pediatric ventricular assist device (PVAD) intended for longer-term ventricular support. Three major numerical design and optimization phases have been completed. A prototype was built based on the latest numerical design (PVAD3) and hydraulically tested in a flow loop. The plastic PVAD prototype delivered 0.5–4 lpm, generating pressure rises of 50–115 mm Hg for operating speeds of 6,000–9,000 rpm. The experimental testing data and the numerical predictions correlated well. The error between these sets of data was found to be generally 7.8% with a maximum deviation of 24% at higher flow rates. The axial fluid forces for the numerical simulations ranged from 0.5 to 1 N and deviated from the experimental results by generally 8.5% with a maximum deviation of 12% at higher flow rates. These hydraulic results demonstrate the excellent performance of the PVAD3 and illustrate the achievement of the design objectives.

Computational Design and Experimental Testing of a Novel Axial Flow LVAD

Thousands of cardiac failure patients per year in the United States could benefit from long-term mechanical circulatory support as destination therapy. To provide an improvement over currently available devices, we have designed a fully implantable axial-flow ventricular assist device with a magnetically levitated impeller (LEV-VAD). In contrast to currently available devices, the LEV-VAD has an unobstructed blood flow path and no secondary flow regions, generating substantially less retrograde and stagnant flow. The pump design included the extensive use of conventional pump design equations and computational fluid dynamics (CFD) modeling for predicting pressure-flow curves, hydraulic efficiencies, scalar fluid stress levels, exposure times to such stress, and axial fluid forces exerted on the impeller for the suspension design. Flow performance testing was completed on a plastic prototype of the LEV-VAD for comparison with the CFD predictions. Animal fit trials were completed to determine optimum pump location and cannulae configuration for future acute and long-term animal implantations, providing additional insight into the LEV-VAD configuration and implantability. Per the CFD results, the LEV-VAD produces 6 l/min and 100 mm Hg at a rotational speed of approximately 6300 rpm for steady flow conditions. The pressure-flow performance predictions demonstrated the VAD’s ability to deliver adequate flow over physiologic pressures for reasonable rotational speeds with best efficiency points ranging from 25% to 30%. The CFD numerical estimations generally agree within 10% of the experimental measurements over the entire range of rotational speeds tested. Animal fit trials revealed that the LEV-VAD’s size and configuration were adequate, requiring no alterations to cannulae configurations for future animal testing. These acceptable performance results for LEV-VAD design support proceeding with manufacturing of a prototype for extensive mock loop and initial acute animal testing.

Computational design and experimental performance testing of an axial-flow pediatric ventricular assist device.

The Virginia Artificial Heart Institute continues to design and develop an axial-flow pediatric ventricular assist device (PVAD) for infants and children in the United States. Our research team has created a database to track potential PVAD candidates at the University of Virginia Childrens Hospital. The findings of this database aided with need assessment and design optimization of the PVAD. A numerical analysis of the optimized PVAD1 design (PVAD2 model) was also completed using computational fluid dynamics (CFD) to predict pressure-flow performance, fluid force estimations, and blood damage levels in the flow domain. Based on the PVAD2 model and after alterations to accommodate manufacturing, a plastic prototype for experimental flow testing was constructed via rapid prototyping techniques or stereolithography. CFD predictions demonstrated a pressure rise range of 36–118 mm Hg and axial fluid forces of 0.8–1.7 N for flows of 0.5–3 l/min over 7,000–9,000 rpm. Blood damage indices per CFD ranged from 0.24% to 0.35% for 200 massless and inert particles analyzed. Approximately 187 (93.5%) of the particles took less than 0.14 seconds to travel completely through the PVAD. The mean residence time was 0.105 seconds with a maximum time of 0.224 seconds. Additionally, in a water/glycerin blood analog solution, the plastic prototype produced pressure rises of 20–160 mm Hg for rotational speeds of 5,960 ± 18 rpm to 9,975 ± 31 rpm over flows from 0.5 to 4.5 l/min. The numerical results for the PVAD2 and the prototype hydraulic testing indicate an acceptable design for the pump, represent a significant step in the development phase of this device, and encourage manufacturing of a magnetically levitated prototype for animal experiments.

Design and Transient Computational Fluid Dynamics Study of a Continuous Axial Flow Ventricular Assist Device

A ventricular assist device (VAD), which is a miniaturized axial flow pump from the point of view of mechanism, has been designed and studied in this report. It consists of an inducer, an impeller, and a diffuser. The main design objective of this VAD is to produce an axial pump with a streamlined, idealized, and nonobstructing blood flow path. The magnetic bearings are adapted so that the impeller is completely magnetically levitated. The VAD operates under transient conditions because of the spinning movement of the impeller and the pulsatile inlet flow rate. The design method, procedure, and iterations are presented. The VADs performance under transient conditions is investigated by means of computational fluid dynamics (CFD). Two reference frames, rotational and stationary, are implemented in the CFD simulations. The inlet and outlet surfaces of the impeller, which are connected to the inducer and diffuser respectively, are allowed to rotate and slide during the calculation to simulate the realistic spinning motion of the impeller. The flow head curves are determined, and the variation of pressure distribution during a cardiac cycle (including systole and diastole) is given. The axial oscillation of impeller is also estimated for the magnetic bearing design. The transient CFD simulation, which requires more computer resources and calculation efforts than the steady simulation, provides a range rather than only a point for the VADs performance. Because of pulsatile flow phenomena and virtual spinning movement of the impeller, the transient simulation, which is realistically correlated with the in vivo implant scenarios of a VAD, is essential to ensure an effective and reliable VAD design.

Mechanical cavopulmonary assist for the univentricular Fontan circulation using a novel folding propeller blood pump.

A blood pump specifically designed to operate in the unique anatomic and physiologic conditions of a cavopulmonary connection has never been developed. Mechanical augmentation of cavopulmonary blood flow in a univentricular circulation would reduce systemic venous pressure, increase preload to the single ventricle, and temporarily reproduce a scenario analogous to the normal two-ventricle circulation. We hypothesize that a folding propeller blood pump would function optimally in this cavopulmonary circulation. The hydraulic performance of a two-bladed propeller prototype was characterized in an experimental flow loop using a blood analog fluid for 0.5–3.5 lpm at rotational speeds of 3,600–4,000 rpm. We also created five distinctive blood pump designs and evaluated their hydraulic performance using computational fluid dynamics (CFD). The two-bladed prototype performed well over the design range of 0.5–3.5 lpm, producing physiologic pressure rises of 5–18 mm Hg. Building upon this proof-of-concept testing, the CFD analysis of the five numerical models predicted a physiologic pressure range of 5–40 mm Hg over 0.5–4 lpm for rotational speeds of 3,000–7,000 rpm. These preliminary propeller designs and the two-bladed prototype achieved the expected hydraulic performance. Optimization of these configurations will reduce fluid stress levels, remove regions of recirculation, and improve the hydraulic performance of the folding propeller. This propeller design produces the physiologic pressures and flows that are in the ideal range to mechanically support the cavopulmonary circulation and represents an exciting new therapeutic option for the support of a univentricular Fontan circulation.

The medical physics of ventricular assist devices

Millions of patients, from infants to adults, are diagnosed with congestive heart failure each year all over the world. A limited number of donor hearts available for these patients results in a tremendous demand for alternative, supplemental circulatory support in the form of artificial heart pumps or ventricular assist devices (VADs). The development procedure for such a device requires careful consideration of biophysical factors, such as biocompatibility, haemolysis, thrombosis, implantability, physiologic control feasibility and pump performance. Conventional pump design equations based on Newtons law and computational fluid dynamics (CFD) are readily used for the initial design of VADs. In particular, CFD can be employed to predict the pressure-flow performance, hydraulic efficiencies, flow profile through the pump, stress levels and biophysical factors, such as possible blood cell damage. These computational flow simulations may involve comprehensive steady and transient flow analyses. The transient simulations involve time-varying boundary conditions and virtual modelling of the impeller rotation in the blood pumps. After prototype manufacture, laser flow measurements with sophisticated optics and mock circulatory flow loop testing assist with validation of pump design and identification of irregular flow patterns for optimization. Additionally, acute and chronic animal implants illustrate the blood pumps ability to support life physiologically. These extensive design techniques, coupled with fundamental principles of physics, ensure a reliable and effective VAD for thousands of heart failure patients each year.