Torsten Schenkel

MRI-Based CFD analysis of flow in a human left ventricle: Methodology and application to a healthy heart

A three-dimensional computational fluid dynamics (CFD) method has been developed to simulate the flow in a pumping left ventricle. The proposed method uses magnetic resonance imaging (MRI) technology to provide a patient specific, time dependent geometry of the ventricle to be simulated. Standard clinical imaging procedures were used in this study. A two-dimensional time-dependent orifice representation of the heart valves was used. The location and size of the valves is estimated based on additional long axis images through the valves. A semi-automatic grid generator was created to generate the calculation grid. Since the time resolution of the MR scans does not fit the requirements of the CFD calculations a third order bezier approximation scheme was developed to realize a smooth wall boundary and grid movement. The calculation was performed by a Navier–Stokes solver using the arbitrary Lagrange–Euler (ALE) formulation. Results show that during diastole, blood flow through the mitral valve forms an asymmetric jet, leading to an asymmetric development of the initial vortex ring. These flow features are in reasonable agreement with in vivo measurements but also show an extremely high sensitivity to the boundary conditions imposed at the inflow. Changes in the atrial representation severely alter the resulting flow field. These shortcomings will have to be addressed in further studies, possibly by inclusion of the real atrial geometry, and imply additional requirements for the clinical imaging processes.

Fluid-structure coupled CFD simulation of the left ventricular flow during filling phase.

The fluid-structure coupled simulation of the heart, though at its developing stage, has shown great prospect in heart function investigations and clinical applications. The purpose of this paper is to verify a commercial software based fluid-structure interaction scheme for the left ventricular filling. The scheme applies the finite volume method to discretize the arbitrary Lagrangian–Eulerian formulation of the Navier–Stokes equations for the fluid while using the nonlinear finite element method to model the structure. The coupling of the fluid and structure is implemented by combining the fluid and structure equations as a unified system and solving it simultaneously at every time step. The left ventricular filling flow in a three-dimensional ellipsoidal thin-wall model geometry of the human heart is simulated, based on a prescribed time-varying Young’s modulus. The coupling converges smoothly though the deformation is very large. The pressure–volume relation of the model ventricle, the spatial and temporal distributions of pressure, transient velocity vectors as well as vortex patterns are analyzed, and they agree qualitatively and quantitatively well with the existing data. This preliminary study has verified the feasibility of the scheme and shown the possibility to simulate the left ventricular flow in a more realistic way by adding a myocardial constitutive law into the model and using a more realistic heart geometry.

Heart rate reduction with ivabradine promotes shear stress-dependent anti-inflammatory mechanisms in arteries

Blood flow generates wall shear stress (WSS) which alters endothelial cell (EC) function. Low WSS promotes vascular inflammation and atherosclerosis whereas high uniform WSS is protective. Ivabradine decreases heart rate leading to altered haemodynamics. Besides its cardio-protective effects, ivabradine protects arteries from inflammation and atherosclerosis via unknown mechanisms. We hypothesised that ivabradine protects arteries by increasing WSS to reduce vascular inflammation. Hypercholesterolaemic mice were treated with ivabradine for seven weeks in drinking water or remained untreated as a control. En face immunostaining demonstrated that treatment with ivabradine reduced the expression of pro-inflammatory VCAM-1 (p<0.01) and enhanced the expression of anti-inflammatory eNOS (p<0.01) at the inner curvature of the aorta. We concluded that ivabradine alters EC physiology indirectly via modulation of flow because treatment with ivabradine had no effect in ligated carotid arteries in vivo, and did not influence the basal or TNFα-induced expression of inflammatory (VCAM-1, MCP-1) or protective (eNOS, HMOX1, KLF2, KLF4) genes in cultured EC. We therefore considered whether ivabradine can alter WSS which is a regulator of EC inflammatory activation. Computational fluid dynamics demonstrated that ivabradine treatment reduced heart rate by 20 % and enhanced WSS in the aorta. In conclusion, ivabradine treatment altered haemodynamics in the murine aorta by increasing the magnitude of shear stress. This was accompanied by induction of eNOS and suppression of VCAM-1, whereas ivabradine did not alter EC that could not respond to flow. Thus ivabradine protects arteries by altering local mechanical conditions to trigger an anti-inflammatory response.

The Karlsruhe Heart Model KaHMo: A modular framework for numerical simulation of cardiac hemodynamics

Numerical methods are rapidly gaining importance for answering medical questions. One field in which these answer are especially valuable is cardiology. The understanding of the cardiac function on a detailed, physical level can help to improve diagnostics, prognosis and therapy for a large number of pathologies.

Multi-physical simulation of left-ventricular blood flow based on patient-specific MRI data

Statistically, heart disease has been the major cause of death in the recent past, which emphasizes the need for computational heart models. In this context, the so-called KaHMo (Karlsruhe Heart Model) is specialized on the innerventricular blood flow and its influence on the overall heart conditions. Both healthy and diseased hearts are simulated in order to analyze characteristic flow patterns and pressure losses. The patient-specific fluid domain movement is realized by time-dependent geometries out of MRI data.

The Progressive Wave Pump: Numerical Multiphysics Investigation of a Novel Pump Concept With Potential to Ventricular Assist Device Application

This article describes the numerical fluid-structure interaction (FSI) validation of a new pumping concept and the possibility for application of a further developed type, as an implantable ventricular assist device (VAD). The novel principle of the so-called progressive wave pump is based on the interaction of an elastic membrane actuated by forced excitation with a surrounding fluid and the pump housing. By applying forced vibrations to one end of the membrane, a transversal wave builds up and progresses to the far end generating both a positive pressure gradient and flow rate. Among others, two axisymmetric geometrical configurations are possible, namely the discoidal and the tubular design. The first one has been built as a physical prototype and is experimentally investigated. In addition, a corresponding numerical FSI model is set up and validated against the experimental findings. Based on this validated numerical method, further numerical investigations are conducted focusing on the development of a tubular progressive wave pump concept with regard to its potential for application as a VAD in the future. To address VAD-relevant issues such as size, hydraulic performance, and blood trauma, corresponding numerical simulations involving macroscopic blood trauma models have been performed. Although being still in an early phase of development, the results are promising and indicate that the wave pump concept in its present state is feasible and can be further developed and investigated as a new type of blood pump.

Effects of VAD placement on 3D fluid flow in a patient specific numerical model of the left ventricle with ischemic heart failure

This paper focuses on the interaction between a generic ventricular assist device (VAD) system and the three dimensional flow structure in a pathological left ventricle (LV). The MRI derived geometry motion data of the ventricular endothelial wall has been used as boundary condition for a three dimensional CFD model incorporating the placement of a VAD between the ventricular apex and the ascending aorta. In a first step, the pump action is implemented by simple momentum source terms, which is yet a placeholder for an arbitrary realistic pump system in future work. A two element circulatory system model is adjusted to physiological conditions and serves as an adaptive pressure boundary condition representing the capacity and resistance of the arterial circulatory system. The heart valves are modeled two dimensionally with a projected, realistic opening area. The computational model therefore accounts on the one hand for complex pressure flow interaction in time, like lumped parameter models do. On the other hand, the detailed three dimensional fluid flow structure is solved to analyze the effects of pump action on intra ventricular, intra aortic and pump cannula flow patterns. First simulation results indicate significant changes in the flow structure and therefore show relevance for the estimation of flow induced blood trauma caused directly and indirectly by VAD placement.

Dietary Docosahexaenoic Acid Reduces Oscillatory Wall Shear Stress, Atherosclerosis, and Hypertension, Most Likely Mediated via an IL‐1–Mediated Mechanism

Background Hypertension is a complex condition and a common cardiovascular risk factor. Dietary docosahexaenoic acid (DHA) modulates atherosclerosis and hypertension, possibly via an inflammatory mechanism. IL‐1 (interleukin 1) has an established role in atherosclerosis and inflammation, although whether IL‐1 inhibition modulates blood pressure is unclear. Methods and Results Male apoE−/− (apolipoprotein E–null) mice were fed either a high fat diet or a high fat diet plus DHA (300 mg/kg per day) for 12 weeks. Blood pressure and cardiac function were assessed, and effects of DHA on wall shear stress and atherosclerosis were determined. DHA supplementation improved left ventricular function, reduced wall shear stress and oscillatory shear at ostia in the descending aorta, and significantly lowered blood pressure compared with controls (119.5±7 versus 159.7±3 mm Hg, P<0.001, n=4 per group). Analysis of atheroma following DHA feeding in mice demonstrated a 4‐fold reduction in lesion burden in distal aortas and in brachiocephalic arteries (P<0.001, n=12 per group). In addition, DHA treatment selectively decreased plaque endothelial IL‐1β (P<0.01). Conclusions Our findings revealed that raised blood pressure can be reduced by inhibiting IL‐1 indirectly by administration of DHA in the diet through a mechanism that involves a reduction in wall shear stress and local expression of the proinflammatory cytokine IL‐1β.

192 Dietary Docosahexaenoic Acid Reduced Experimental Atherosclerosis by Inducing Protective Haemodynamic Conditions

Introduction Dietary omega-3 fatty acids have been associated with protection from atherosclerosis. However, the underlying mechanisms are incompletely understood. Blood flow generate a frictional force on endothelial cells called wall shear stress (WSS) that alters vascular wall function. The aim of this study was to determine whether docosahexaenoic acid (DHA), an omega-3 fatty acid, modulates vascular wall inflammation, blood flow velocity and WSS in experimental atherosclerosis. Methods ApoE–/– mice fed either high fat diet (control) or high fat diet plus DHA (300 mg/kg/day) for 12 weeks (n = 12/group). Blood flow velocity was assessed using pulsed wave doppler echocardiography and blood pressure was monitored using Visitech tail-cuff system. Atherosclerosis was measured in whole aorta using enface Oil red O stain, and in aortic roots and brachiocephalic sections stained with Alcian Blue & Elastic Van Gieson stain. Computational flow dynamics (CFD) was used to map WSS magnitude and oscillation in the aorta. Plasma cholesterol levels were quantified by gas chromatography. Results Plasma high density lipoprotein/total cholesterol ratio was significantly increased in DHA treated mice compared to controls (10.77 ± 1.86 vs. 6.63 ± 1.04, p < 0.05). DHA fed mice exhibited a 4–5 fold reduction in distal aortic and brachiocephalic atherosclerosis (p < 0.01) whereas lesion burden in the aortic arch was similar between groups. Dietary supplementation using DHA led to a reduction in blood pressure (119.5 ± 7.33 mmHg (+DHA) vs. 159.7 ± 2.482 mmHg (controls), p < 0.001 and a 12% decrease in aortic blood flow. CFD revealed that oscillatory shear in the descending aorta was reduced in DHA-fed mice compared to controls. Conclusions/Implications Our study suggests that dietary DHA can act systemically by enhancing levels of HDL. It can also act locally by reducing oscillatory shear stress in the descending aorta. Dietary DHA reduced lesion formation specifically in the descending aorta, an effect that can be explained by its dual effects on oscillatory shear and HDL. Therefore, the current study suggests novel and interacting protective mechanisms for DHA actions in atherogenesis with implications for the development of dietary interventions to prevent cardiovascular disease.

Biofluidmechanics of Avian Flight: Recent Numerical and Experimental Investigations

A three dimensional mechanical and numerical avian model with identical geometry was developed to investigate the aerodynamic performance of flapping flight for varying flow velocities and wing beat frequencies. The corresponding reduced frequencies range from k=0.22 to k=1.0 providing turbulent and unsteady flow. The model consists of a rigid body and elastic wings. Its shape was inspired by birds, but restricted by manufacturing and numerical specifications. Using a sinusoidal flapping about an off-centre axis parallel to the body axis and a phase-shifted pitching about the moving lateral wing axis the wing beat motion was realized. Wind tunnel tests with Particle Image Velocimetry (PIV) were performed to capture the velocity field around and behind the mechanical model for different reduced frequencies. Furthermore, simulations for the corresponding numerical model have been conducted by means of fluid-structure-interaction (FSI) simulation techniques providing a fully resolved flow field. The results were used to analyze the flow configurations and to validate the numerical and experimental setup for further investigations. The results of the numerical simulations and wind tunnel experiments are in good agreement and facilitate a reconstruction of the three dimensional vortex structures in the wake. The results show, that for all reduced frequencies, the wakes consist of a chain of interlocked vortex rings behind each wing. For high reduced frequencies, a shedding of small-scale vortices composing vortex sheets generates oppositely rotating upstroke (UVS) and downstroke (DVS) vortex structures which contain starting, stopping, tip and root vortices. For decreasing reduced frequencies, the upstroke becomes more aerodynamically active leading to a diffusion of the upstroke vortex structures.