Michael Neidlin

Implementation of intrinsic lumped parameter modeling into computational fluid dynamics studies of cardiopulmonary bypass

Stroke and cerebral hypoxia are among the main complications during cardiopulmonary bypass (CPB). The two main reasons for these complications are the cannula jet, due to altered flow conditions and the sandblast effect, and impaired cerebral autoregulation which often occurs in the elderly. The effect of autoregulation has so far mainly been modeled using lumped parameter modeling, while Computational Fluid Dynamics (CFD) has been applied to analyze flow conditions during CPB. In this study, we combine both modeling techniques to analyze the effect of lumped parameter modeling on blood flow during CPB. Additionally, cerebral autoregulation is implemented using the Baroreflex, which adapts the cerebrovascular resistance and compliance based on the cerebral perfusion pressure. The results show that while a combination of CFD and lumped parameter modeling without autoregulation delivers feasible results for physiological flow conditions, it overestimates the loss of cerebral blood flow during CPB. This is counteracted by the Baroreflex, which restores the cerebral blood flow to native levels. However, the cerebral blood flow during CPB is typically reduced by 10-20% in the clinic. This indicates that either the Baroreflex is not fully functional during CPB, or that the target value for the Baroreflex is not a full native cerebral blood flow, but the plateau phase of cerebral autoregulation, which starts at approximately 80% of native flow.

Hemodynamic analysis of outflow grafting positions of a ventricular assist device using closed-loop multiscale CFD simulations: Preliminary results

Subclavian arteries are a possible alternate location for left ventricular assist device (LVAD) outflow grafts due to easier surgical access and application in high risk patients. As vascular blood flow mechanics strongly influence the clinical outcome, insights into the hemodynamics during LVAD support can be used to evaluate different grafting locations. In this study, the feasibility of left and right subclavian artery (SA) grafting was investigated for the HeartWare HVAD with a numerical multiscale model. A 3-D CFD model of the aortic arch was coupled to a lumped parameter model of the cardiovascular system under LVAD support. Grafts in the left and right SA were placed at three different anastomoses angles (90°, 60° and 30°). Additionally, standard grafting of the ascending and descending aorta was modelled. Full support LVAD (5l/min) and partial support LVAD (3l/min) in co-pulsation and counter-pulsation mode were analysed. The grafting positions were investigated regarding coronary and cerebral perfusion. Furthermore, the influence of the anastomosis angle on wall shear stress (WSS) was evaluated. Grafting of left or right subclavian arteries has similar hemodynamic performance in comparison to standard cannula positions. Angularity change of the graft anastomosis from 90° to 30° slightly increases the coronary and cerebral blood flow by 6-9% while significantly reduces the WSS by 35%. Cannulation of the SA is a feasible anastomosis location for the HVAD in the investigated vessel geometry.

In vitro flow investigations in the aortic arch during cardiopulmonary bypass with stereo-PIV

The cardiopulmonary bypass is related to complications like stroke or hypoxia. The cannula jet is suspected to be one reason for these complications, due to the sandblast effect on the vessel wall. Several in silico and in vitro studies investigated the underlying mechanisms, but the applied experimental flow measurement techniques were not able to address the highly three-dimensional flow character with a satisfying resolution. In this work in vitro flow measurements in a cannulated and a non-cannulated aortic silicone model are presented. Stereo particle image velocimetry measurements in multiple planes were carried out. By assembling the data of the different measurement planes, quasi 3D velocity fields with a resolution of~1.5×1.5×2.5 mm(3) were obtained. The resulting velocity fields have been compared regarding magnitude, streamlines and vorticity. The presented method shows to be a suitable in vitro technique to measure and address the three-dimensional aortic CPB cannula flow with a high temporal and spatial resolution.

A multiscale 0-D/3-D approach to patient-specific adaptation of a cerebral autoregulation model for computational fluid dynamics studies of cardiopulmonary bypass.

Neurological complication often occurs during cardiopulmonary bypass (CPB). One of the main causes is hypoperfusion of the cerebral tissue affected by the position of the cannula tip and diminished cerebral autoregulation (CA). Recently, a lumped parameter approach could describe the baroreflex, one of the main mechanisms of cerebral autoregulation, in a computational fluid dynamics (CFD) study of CPB. However, the cerebral blood flow (CBF) was overestimated and the physiological meaning of the variables and their impact on the model was unknown. In this study, we use a 0-D control circuit representation of the Baroreflex mechanism, to assess the parameters with respect to their physiological meaning and their influence on CBF. Afterwards the parameters are transferred to 3D-CFD and the static and dynamic behavior of cerebral autoregulation is investigated. The parameters of the baroreflex mechanism can reproduce normotensive, hypertensive and impaired autoregulation behavior. Further on, the proposed model can mimic the effects of anesthetic agents and other factors controlling dynamic CA. The CFD simulations deliver similar results of static and dynamic CBF as the 0-D control circuit. This study shows the feasibility of a multiscale 0-D/3-D approach to include patient-specific cerebral autoregulation into CFD studies.

Design Modifications and Computational Fluid Dynamic Analysis of an Outflow Cannula for Cardiopulmonary Bypass

Cardiopulmonary bypass is a well-established technique during open heart surgeries. However, neurological complications due to insufficient cerebral oxygen supply occur and the severe consequences must not be neglected. Recent computational fluid dynamics (CFD) studies showed that during axillary cannulation the cerebral perfusion is strongly affected by the distance between the cannula tip and the vertebral artery branch. In this study we use two modifications of the cannula design to analyze the flow characteristics by means of CFD and experimental validation with particle image velocimetry (PIV). One approach applies a spin to the blood stream with a helical surface in the cannula cross section. Another approach uses radial bores in a constricted cannula tip to split the outflow jet. The additional helicity improves the perfusion of the cerebral vessels and suppresses the blood suction in the right vertebral artery observed with a standard cannula. The cannula with a helix throughout the entire length changes the blood flow from −124 to 32xa0mL/min in comparison with an unmodified design and has the lowest prediction of blood damage. Separating the blood stream does not deliver satisfying results. The PIV measurements validate the simulations and correspond with the velocity distribution as well as vortex locations.

Numerical prediction of thrombus risk in an anatomically dilated left ventricle: the effect of inflow cannula designs

BackgroundImplantation of a rotary blood pump (RBP) can cause non-physiological flow fields in the left ventricle (LV) which may trigger thrombosis. Different inflow cannula geometry can affect LV flow fields. The aim of this study was to determine the effect of inflow cannula geometry on intraventricular flow under full LV support in a patient specific model.MethodsComputed tomography angiography imaging of the LV was performed on a RBP candidate to develop a patient-specific model. Five inflow cannulae were evaluated, which were modelled on those used clinically or under development. The inflow cannulae are described as a crown like tip, thin walled tubular tip, large filleted tip, trumpet like tip and an inferiorly flared cannula. Placement of the inflow cannula was at the LV apex with the central axis intersecting the centre of the mitral valve. Full support was simulated by prescribing 5xa0l/min across the mitral valve. Thrombus risk was evaluated by identifying regions of stagnation. Rate of LV washout was assessed using a volume of fluid model. Relative haemolysis index and blood residence time was calculated using an Eulerian approach.ResultsThe inferiorly flared inflow cannula had the lowest thrombus risk due to low stagnation volumes. All cannulae had similar rates of LV washout and blood residence time. The crown like tip and thin walled tubular tip resulted in relatively higher blood damage indices within the LV.ConclusionChanges in intraventricular flow due to variances in cannula geometry resulted in different stagnation volumes. Cannula geometry does not appreciably affect LV washout rates and blood residence time. The patient specific, full support computational fluid dynamic model provided a repeatable platform to investigate the effects of inflow cannula geometry on intraventricular flow.

Investigation of hemodynamics during cardiopulmonary bypass: A multiscale multiphysics fluid–structure-interaction study

Neurological complications often occur during cardiopulmonary bypass (CPB). Hypoperfusion of brain tissue due to diminished cerebral autoregulation (CA) and thromboembolism from atherosclerotic plaque reduce the cerebral oxygen supply and increase the risk of perioperative stroke. To improve the outcome of cardiac surgeries, patient-specific computational fluid dynamic (CFD) models can be used to investigate the blood flow during CPB. In this study, we establish a computational model of CPB which includes cerebral autoregulation and movement of aortic walls on the basis of in vivo measurements. First, the Baroreflex mechanism, which plays a leading role in CA, is represented with a 0-D control circuit and coupled to the 3-D domain with differential equations as boundary conditions. Additionally a two-way coupled fluid-structure interaction (FSI) model with CA is set up. The wall shear stress (WSS) distribution is computed for the whole FSI domain and a comparison to rigid wall CFD is made. Constant flow and pulsatile flow CPB is considered. Rigid wall CFD delivers higher wall shear stress values than FSI simulations, especially during pulsatile perfusion. The flow rates through the supraaortic vessels are almost not affected, if considered as percentages of total cannula output. The developed multiphysic multiscale framework allows deeper insights into the underlying mechanisms during CPB on a patient-specific basis.

Ventricular flow dynamics with varying LVAD inflow cannula lengths: In-silico evaluation in a multiscale model

Left ventricular assist devices are associated with thromboembolic events, which are potentially caused by altered intraventricular flow. Due to patient variability, differences in apical wall thickness affects cannula insertion lengths, potentially promoting unfavourable intraventricular flow patterns which are thought to be correlated to the risk of thrombosis. This study aimed to present a 3D multiscale computational fluid dynamic model of the left ventricle (LV) developed using a commercial software, Ansys, and evaluate the risk of thrombosis with varying inflow cannula insertion lengths in a severely dilated LV. Based on a HeartWare HVAD inflow cannula, insertion lengths of 5, 19, 24 and 50u202fmm represented cases of apical hypertrophy, typical ranges of apical thicknesses and an experimental length, respectively. The risk of thrombosis was evaluated based on blood washout, residence time, instantaneous blood stagnation and a pulsatility index. By introducing fresh blood to displace pre-existing blood in the LV, after 5 cardiac cycles, 46.7%, 45.7%, 45.1% and 41.8% of pre-existing blood remained for insertion lengths of 5, 19, 24 and 50u202fmm, respectively. Compared to the 50u202fmm insertion, blood residence time was at least 9%, 7% and 6% higher with the 5, 19 and 24u202fmm insertion lengths, respectively. No instantaneous stagnation at the apex was observed directly after the E-wave. Pulsatility indices adjacent to the cannula increased with shorter insertion lengths. For the specific scenario studied, a longer insertion length, relative to LV size, may be advantageous to minimise thrombosis by increasing LV washout and reducing blood residence time.

Design of a right ventricular mock circulation loop as a test bench for right ventricular assist devices.

Abstract Right heart failure (RHF), e.g. due to pulmonary hypertension (PH), is a serious health issue with growing occurrence and high mortality rate. Limited efficacy of medication in advanced stages of the disease constitutes the need for mechanical circulatory support of the right ventricle (RV). An essential contribution to the process of developing right ventricular assist devices (RVADs) is the in vitro test bench, which simulates the hemodynamic behavior of the native circulatory system. To model healthy and diseased arterial-pulmonary hemodynamics in adults (mild and severe PH and RHF), a right heart mock circulation loop (MCL) was developed. Incorporating an anatomically shaped silicone RV and a silicone atrium, it not only enables investigations of hemodynamic values but also suction events or the handling of minimal invasive RVADs in an anatomical test environment. Ventricular pressure-volume loops of all simulated conditions as well as pressure and volume waveforms were recorded and compared to literature data. In an exemplary test, an RVAD was connected to the apex to further test the feasibility of studying such devices with the developed MCL. In conclusion, the hemodynamic behavior of the native system was well reproduced by the developed MCL, which is a useful basis for future RVAD tests.

Multiscale Multiphysic Approaches in Vascular Hemodynamics

Vascular hemodynamics are an important part of many medical and engineering applications. The advances in imaging technologies, modeling methods and the increasing computational power allow for sophisticated in-depth studies of the fluid dynamical behavior of blood. The integration of patient-specific geometries, the consideration of dynamical boundary conditions and the interaction between fluid and solid structures play the major role in all numerical modeling frameworks. The following chapter gives a brief summary on each of these topics providing a concise guideline for the interested modeler. In the end three applications are provided. Studies on Fluid-Structure-Interaction in the aortic arch, multiscale CFD simulations of heart support and the vascular hemodynamics in the cerebral arteries (Circle of Willis) are shown. The cases are chosen to emphasize the emerging complexity of fluid dynamics in biological systems and the methods to tackle these obstacles.