Ulrich Steinseifer

A review of computational fluid dynamics analysis of blood pumps

Ventricular Assist Devices (VADs) provide long- and short-term support to chronically-ill heart disease patients; these devices are expected to match the remarkable functionality of the natural heart, which makes their design a very challenging task. Blood pumps, the principal component of the VADs, must operate over a wide range of flowrates and pressure heads, and minimize the damage to blood cells in the process. They should also be small to allow easy implantation in both children and adults. Mathematical methods and Computational Fluid Dynamics (CFD) have recently emerged as a powerful design tool in this context; a review of the recent advances in the field is presented here. This review focuses on the CFD-based design strategies applied to blood flow in blood pumps and other blood-handling devices. Both simulation methods for blood flow and blood damage models are reviewed. The literature is put into context with a discussion of the chronological development in the field. The review is illustrated with specific examples drawn from our group’s Galerkin/LeastSquares (GLS) finite element simulations of the basic Newtonian flow problem for the continuous-flow centrifugal GYRO blood pump. The GLS formulation is outlined, and modifications to include models that better represent blood rheology are shown. Hemocompatibility analysis of the pump is reviewed in the context of hemolysis estimations based on different blood damage models. Our strainbased blood damage model that accounts for the viscoleasticity associated with the red blood cells is reviewed in detail. The viability of trial-and-error based design improvement and complete simulation-based design optimization schemes are also discussed.

In vitro comparison of dabigatran, unfractionated heparin, and low-molecular-weight heparin in preventing thrombus formation on mechanical heart valves

INTRODUCTION Lifelong oral anticoagulation (OAC) therapy is required for the prevention of thromboembolic events after implantation of an artificial heart valve. Thromboembolism and anticoagulant-related bleedings account for approximately 75% of all complications experienced by heart valve recipients (2-9% of patients per year). The present study investigated the efficacy of dabigatran, a new direct thrombin inhibitor for oral use, as compared to unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) in preventing thrombus formation on mechanical heart valves in vitro. MATERIAL AND METHODS Blood (230 ml) from healthy young male volunteers was anticoagulated either by dabigatran (1 micromol/l), UFH (150 IU), or LMWH (100 IU). Mechanical heart valve prostheses were placed in an in vitro thrombosis tester and exposed to the anticoagulated blood samples under continuous circulation at a rate of 75 beats per minute. RESULTS In whole blood with no anticoagulant, the apparatus completely clotted in 15-20 minutes. When blood was treated with dabigatran, the mean thrombus weight was 164+/-55 mg, in the UFH group 159+/-69 mg, and in the LMWH group 182+/-82 mg (p-value: 0.704). Electron microscopy showed no significant difference in thrombus formation in any group. CONCLUSIONS Dabigatran was as effective as UFH and LMWH in preventing thrombus formation on mechanical heart valves in our in vitro investigation. Thus, we hypothesize that dabigatran etexilate might potentially be a useful and competitive orally administered alternative to UFH and LMWH for recipients of alloplastic heart valve prostheses.

Flow Analysis of Ventricular Assist Device Inflow and Outflow Cannula Positioning Using a Naturally Shaped Ventricle and Aortic Branch

Tip geometry and placement of rotary blood pump inflow and outflow cannulae influence the dynamics of flow within the ventricle and aortic branch. Cannulation, therefore, directly influences the potential for thrombus formation and end-organ perfusion during ventricular assist device (VAD) support or cardiopulmonary bypass (CPB). The purpose of this study was to investigate the effect of various inflow/outflow cannula tip geometries and positions on ventricular and greater vessel flow patterns to evaluate ventricular washout and impact on cerebral perfusion. Transparent models of a dilated cardiomyopathic ventricle and an aortic branch were reconstructed from magnetic resonance imaging data to allow flow measurements using particle image velocimetry (PIV). The contractile function of the failing ventricle was reproduced pneumatically, and supported with a rotary pump. Flow patterns were visualized around VAD inflow cannulae, with various tip geometries placed in three positions in the ventricle. The outflow cannula was placed in the subclavian artery and at several positions in the aorta. Flow patterns were measured using PIV and used to validate an aortic flow computational fluid dynamic study. The PIV technique indicated that locating the inflow tip in the left ventricular outflow tract improved complete ventricular washout while the tip geometry had a smaller influence. However, side holes in the inflow cannula improved washout in all cases. The PIV results confirmed that the positioning and orientation of the outflow cannula in the aortic branch had a high impact on the flow pattern in the vessels, with a negative blood flow in the right carotid artery observed in some cases. Cannula placement within the ventricle had a high influence on chamber washout. The positioning of the outflow cannula directly influences the flow through the greater vessels, and may be responsible for the occasional reduction in cerebral perfusion seen in clinical CPB.

Flow Distribution During Cardiopulmonary Bypass in Dependency on the Outflow Cannula Positioning

Oxygen deficiency in the right brain is a common problem during cardiopulmonary bypass (CPB). This is linked to an insufficient perfusion of the carotid and vertebral artery. The flow to these vessels is strongly influenced by the outflow cannula position, which is traditionally located in the ascending aorta. Another approach however is to return blood via the right subclavian artery. A computational fluid dynamics (CFD) study was performed for both methods and validated by particle image velocimetry (PIV). A 3-dimensional computer aided design model of the cardiovascular (CV) system was generated from realtime computed tomography and magnetic resonance imaging data. Mesh generation (CFD) and rapid prototyping (PIV) were used for the further model creation. The simulations were performed assuming usual CPB conditions, and the same boundary conditions were applied for the PIV validation. The flow distribution was analyzed for 55 cannula positions inside the aorta and in relation to the distance between the cannula tip and the vertebral artery branch for subclavian cannulation. The study reveals that the Venturi effect due to the cannula jet appears to be the main reason for the loss in cerebral perfusion seen clinically. It provides a PIV-validated CFD method of analyzing the flow distribution in the CV system and can be transferred to other applications.

Mechanical Circulatory Support for Right Heart Failure: Current Technology and Future Outlook

The increasing global prevalence of congestive heart failure is a major healthcare concern, accounting for a high morbidity rate worldwide. In particular, isolated right heart dysfunction after cardiotomy has a poor prognosis and is associated with a high mortality rate. The occurrence of postoperative right heart failure may develop in more than 40% of patients undergoing implantation of a left ventricular assist device (LVAD) and cardiac transplantation. To date, mechanical cardiac assistance in the form of VADs has become accepted as a therapeutic solution for end-stage patients when a donor heart is not available. However, right ventricular (RV) assistance is still in the early phase of development when compared with LVAD technology. State-of-the-art RVADs, both in clinical use and under development, are reviewed in this manuscript. Clinical RVADs include the extracorporeal pulsatile Abiomed BVS 5000 and AB5000, Thoratec PVAD, MEDOS VAD, BerlinHeart Excor, the percutaneous continuous flow CentriMag and TandemHeart systems, and the implantable Thoratec IVAD. Devices on the horizon, including the wear-free implantable DexAide and the minimally invasive Impella RD, are additionally reviewed. In addition to the current status of RV assistance, as well as the device categorization, the outlook and considerations for successful development of future RVADs were discussed.

Polyurethane heart valves: past, present and future

Replacement cardiac valves have been in use since the 1950s, and today represent the most widely used cardiovascular devices. One type of replacement cardiac valve, the polyurethane heart valve, has been around since the first stages of prosthesis development, and has made advances along with the development of biological and mechanical heart valves over the past 60 years. During this time, problems with durability and biocompatibility have held back polyurethane valves, but progress in materials and manufacturing techniques can lead the way to a brighter future for these devices and their huge potential. This article describes previous efforts to manufacture polyurethane heart valves, highlights the challenges of manufacturing and explains the factors influencing durability and successful functioning of such a device.

The Impact of Aortic/Subclavian Outflow Cannulation for Cardiopulmonary Bypass and Cardiac Support: A Computational Fluid Dynamics Study

Approximately 100 000 cases of oxygen deficiency in the brain occur during cardiopulmonary bypass (CPB) procedures each year. In particular, perfusion of the carotid and vertebral arteries is affected. The position of the outflow cannula influences the blood flow to the cardiovascular system and thus end organ perfusion. Traditionally, the cannula returns blood into the ascending aorta. But some surgeons prefer cannulation to the right subclavian artery. A computational fluid dynamics study was initially undertaken for both approaches. The vessel model was created from real computed tomography/magnetic resonance imaging data of young healthy patients. The simulations were run with usual CPB conditions. The flow distribution for different cannula positions in the aorta was studied, as well as the impact of the cannula tip distance to vertebral artery for the subclavian position. The study presents a fast method of analyzing the flow distribution in the cardiovascular system, and can be adapted for other applications such as ventricular assist device support. It revealed that two effects cause the loss of perfusion seen clinically: a vortex under the brachiocephalic trunk and low pressure regions near the cannula jet. The results suggest that cannulation to the subclavian artery is preferred if the cannula tip is sufficiently far away from the branch of the vertebral artery. For the aortic positions, however, the cannula should be injected from the left body side.

Fluid-Structure Interaction Model of a Percutaneous Aortic Valve: Comparison with an In Vitro Test and Feasibility Study in a Patient-Specific Case.

Transcatheter aortic valve replacement (TAVR) represents an established recent technology in a high risk patient base. To better understand TAVR performance, a fluid–structure interaction (FSI) model of a self-expandable transcatheter aortic valve was proposed. After an in vitro durability experiment was done to test the valve, the FSI model was built to reproduce the experimental test. Lastly, the FSI model was used to simulate the virtual implant and performance in a patient-specific case. Results showed that the leaflet opening area during the cycle was similar to that of the in vitro test and the difference of the maximum leaflet opening between the two methodologies was of 0.42%. Furthermore, the FSI simulation quantified the pressure and velocity fields. The computed strain amplitudes in the stent frame showed that this distribution in the patient-specific case is highly affected by the aortic root anatomy, suggesting that the in vitro tests that follow standards might not be representative of the real behavior of the percutaneous valve. The patient-specific case also compared in vivo literature data on fast opening and closing characteristics of the aortic valve during systolic ejection. FSI simulations represent useful tools in determining design errors or optimization potentials before the fabrication of aortic valve prototypes and the performance of tests.

New Linear Motor Concepts for Artificial Hearts

Total artificial hearts (TAHs), available in todays market, have the disadvantage of wear-prone components. Thus, their expectation of life is limited and the devices can only be used for temporary and not destination therapy. Durability- and wear-free operations are the critical requirements, as failure is an immediate threat to the patients life. These attributes are combined in linear motors. In this paper, the potential of a linear motor as TAHs drive is shown by a prototype. On the basis of this prototype, different motor concepts are employed. The dimensions of each concepts geometry are first roughly determined by analytical optimization, and in a second step, more finely tuned by means of finite-element (FE) calculations. After optimization, two concepts achieve the requirements, provided by the natural heart of the human body. The first motor consists of moving coils and static permanent magnets, which are embedded in a flux concentrating geometry. To avoid the disadvantage of wear-prone power connection of the coils, the other concept consists of static coils and moving permanent magnets, arranged in a Halbach array. After constructing and testing both concepts in laboratory, animal experiments will follow to identify the superior one.

NeonatOx: a pumpless extracorporeal lung support for premature neonates.

Gas exchange in premature neonates is regularly impaired by structural and functional immaturity of the lung. Mechanical ventilation, which is vitally important to sustain oxygenation and CO(2) elimination, causes, at the same time, mechanical and inflammatory destruction of lung tissue. To date, extracorporeal oxygenation is not a treatment option, one reason among others being the size of available oxygenators and cannulas. We hypothesized that a substantial improvement in gas exchange can be achieved by maintenance of the fetal cardiopulmonary bypass and interposition of a suitable passively driven (arteriovenous) membrane oxygenator. In close cooperation between engineers and neonatologists, we developed a miniaturized oxygenator and adapted cannulas to be used as a pumpless extracorporeal lung support that is connected to the circulation via cannulation of the umbilical cord vessels. First in vitro and in vivo studies show promising results. We regard this as one step on the way to clinical application of the artificial placenta.