Marcus Hormes

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.

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.

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.

Transient, three-dimensional flow field simulation through a mechanical, trileaflet heart valve prosthesis.

Thromboembolic complications are one of the major challenges faced by designers and researchers in development of artificial organs with blood-contacting devices such as heart valve prostheses, especially mechanical valves. Besides increasing the thrombogenic potential, these valves change the hydrodynamic performance of the heart. In this study, the flow through a trileaflet, mechanical heart valve prosthesis was modeled with transient computational fluid dynamics to analyze flow patterns causing thrombus formations on valves. The valve was simulated under conditions of a test rig (THIA II), which was specially designed to analyze different valves with respect to thrombosis. The main goal of this study was to mimic the exact conditions of the test rig to be able to compare numerical and experimental results. The boundary conditions were obtained from experimental data as leaflet kinematics and pressure profiles. One complete cycle of the valve was simulated. Numerical flow and pressure results were analyzed and compared with experimental results. Shear stress and shear rates were determined with respect to thrombogenic potential, especially in the pivot regions, which seem to be the main influence for activation and deposition of thrombocytes. Approximately 0.7% of the blood volume moving through the fluid domain of the valve was exposed to shear rates high enough to cause platelet activation. However, shear rates of up to 20,000 s−1 occurred in pivot regions. The pressure differences between the simulation and experimental data were approximately 2.5% during systole and increased up to 25% during diastole. The presented method, however, can be used to gain more information about the flow through different heart valve prostheses and, thus, improve the development process.

A validated CFD model to predict O2 and CO2 transfer within hollow fiber membrane oxygenators

Hollow fiber oxygenators provide gas exchange to and from the blood during heart surgery or lung recovery. Minimal fiber surface area and optimal gas exchange rate may be achieved by optimization of hollow fiber shape and orientation (1). In this study, a modified CFD model is developed and validated with a specially developed micro membrane oxygenator (MicroMox). The MicroMox was designed in such a way that fiber arrangement and bundle geometry are highly reproducible and potential flow channeling is avoided, which is important for the validation. Its small size (VFluid=0.04 mL) allows the simulation of the entire bundle of 120 fibers. A non-Newtonian blood model was used as simulation fluid. Physical solubility and chemical bond of O2 and CO2 in blood was represented by the numerical model. Constant oxygen partial pressure at the pores of the fibers and a steady state flow field was used to calculate the mass transport. In order to resolve the entire MicroMox fiber bundle, the mass transport was simulated for symmetric geometry sections in flow direction. In vitro validation was achieved by measurements of the gas transfer rates of the MicroMox. All measurements were performed according to DIN EN 12022 (2) using porcine blood. The numerical simulation of the mass transfer showed good agreement with the experimental data for different mass flows and constant inlet partial pressures. Good agreement could be achieved for two different fiber configurations. Thus, it was possible to establish a validated model for the prediction of gas exchange in hollow fiber oxygenators.

Vascular electromagnetic tracking: experiences in phantom and animal cadaveric models

This study assesses the feasibility of navigated vascular interventions in a rapid prototyped anthropomorphic phantom and swine cadaveric model using electromagnetic tracking (EMT) in combination with a CT image data set. In the phantom model overall feasibility and handling of the EMT navigation system were evaluated and first experiments performing visceral vessel catheterization were carried out. In the cadaveric model a predefined structure was targeted using an electromagnetically tracked guidewire. In the phantom model the catheterization of visceral vessels was reproducibly possible. In the cadaveric model a catheter placement accuracy of 0.97 ± 0.88 mm was achieved. The feasibility of EMT based vascular interventions could be shown and a landmark concerning the accuracy of such systems in a model setting could be determined. Further ex- and in-vivo experiments are needed to preevaluate applicability of such systems in clinical settings.

CFD simulation of a novel bileaflet mechanical heart valve prosthesis : an estimation of the Venturi passage formed by the leaflets

The aim of this study was to validate the flow characteristics of the novel Helmholtz-Institute Aachen Bileaflet (HIA-BL) heart valve prosthesis. The curved leaflets of the HIA-BL valve form a Venturi passage between the leaflets at peak systole. By narrowing the cross section the flow accelerates and the static pressure at the central passage decreases according to the Venturi effect. The low-pressure zone between the leaflets is expected to stabilize the leaflets in fully open position at peak systole. To investigate the Venturi passage, the flow fields of two valve geometries were investigated by CFD (Computational Fluid Dynamics): one geometry exhibits curved leaflets resulting in a Venturi passage; the other geometry features straight leaflets. The flow profiles, pressure distribution and resulting torque of both passages were compared and investigated. Although flow profiles downstream of both valves were similar, the flow passages between the leaflets were different for the investigated leaflet geometries. The straight leaflet passage showed a large boundary layer separation zone near the leaflets and the lowest pressure at the leading edge of the leaflet. The Venturi passage showed a reduction of the boundary layer separation zones and the lowest pressure between the leaflets could be found in the narrowest flow cross section of the Venturi passage. Additionally, the resulting torque showed that the Venturi passage produced an opening momentum. The results demonstrate that the Venturi passage stabilizes the leaflets in open position at peak systole.

Comparison of different cannulation approaches for Cardio Pulmonary Bypass

The cerebral perfusion can be insufficient during Cardio Pulmonary Bypass (CPB) operations. The outflow cannula position influences the flow field in the cardiovascular system, whereby end organ perfusion is affected. There are different cannulation approaches for CPB. A Computational Fluid Dynamics study was initially undertaken to compare cannulation of the ascending aorta, regarding different cannula positions, and cannulation of the right subclavian artery, with regard to the distance between the cannula tip and the vertebral artery. Usual CPB conditions were assumed for both approaches. The models were created from CT/MRI records of young to middle-aged healthy patients. The cannula jet appears to be the main reason for the loss in cerebral perfusion seen clinically during CPB. Also, the results suggest that cannulation of the subclavian artery provides the best flow distribution if the cannula tip is a few mm away from the vertebral artery branch. For cannulation of the ascending aorta, the cannula should be placed sagital from the left body side. The presented method can be adapted for other clinical applications, e.g. support conditions of Ventricular Assist Devices. Flow fields, pressure, stress and shear rates can be analysed, which may be used for evaluation and development of new products and applications.