1,513 research outputs found
Local vs. Global Blood Flow Modulation in Artificial Microvascular Networks: Effects on Red Blood Cell Distribution and Partitioning
Get PDFOur understanding of cerebral blood flow (CBF) regulation during functional activation is still limited. Alongside with the accepted role of smooth muscle cells in controlling the arteriolar diameter, a new hypothesis has been recently formulated suggesting that CBF may be modulated by capillary diameter changes mediated by pericytes. In this study, we developed in vitro microvascular network models featuring a valve enabling the dilation of a specific micro-channel. This allowed us to investigate the non-uniform red blood cell (RBC) partitioning at microvascular bifurcations (phase separation) and the hematocrit distribution at rest and for two scenarios modeling capillary and arteriolar dilation. RBC partitioning showed similar phase separation behavior during baseline and activation. Results indicated that the RBCs at diverging bifurcations generally enter the high-flow branch (classical partitioning). Inverse behavior (reverse partitioning) was observed for skewed hematocrit profiles in the parent vessel of bifurcations, especially for high RBC velocity (i.e., arteriolar activation). Moreover, results revealed that a local capillary dilation, as it may be mediated in vivo by pericytes, led to a localized increase of RBC flow and a heterogeneous hematocrit redistribution within the whole network. In case of a global increase of the blood flow, as it may be achieved by dilating an arteriole, a homogeneous increase of RBC flow was observed in the whole network and the RBCs were concentrated along preferential pathways. In conclusion, overall increase of RBC flow could be obtained by arteriolar and capillary dilation, but only capillary dilation was found to alter the perfusion locally and heterogeneously
In vitro investigations of red blood cell phase separation in a complex microchannel network
Get PDFMicrovascular networks feature a complex topology with multiple bifurcating vessels. Nonuniform partitioning (phase separation) of red blood cells (RBCs) occurs at diverging bifurcations, leading to a heterogeneous RBC distribution that ultimately affects the oxygen delivery to living tissues. Our understanding of the mechanisms governing RBC heterogeneity is still limited, especially in large networks where the RBC dynamics can be nonintuitive. In this study, our quantitative data for phase separation were obtained in a complex in vitro network with symmetric bifurcations and 176 microchannels. Our experiments showed that the hematocrit is heterogeneously distributed and confirmed the classical result that the branch with a higher blood fraction received an even higher RBC fraction (classical partitioning). An inversion of this classical phase separation (reverse partitioning) was observed in the case of a skewed hematocrit profile in the parent vessels of bifurcations. In agreement with a recent computational study [P. Balogh and P. Bagchi, Phys. Fluids 30,051902 (2018)], a correlation between the RBC reverse partitioning and the skewness of the hematocrit profile due to sequential converging and diverging bifurcations was reported. A flow threshold below which no RBCs enter a branch was identified. These results highlight the importance of considering the RBC flow history and the local RBC distribution to correctly describe the RBC phase separation in complex networks
Data Assimilation by Stochastic Ensemble Kalman Filtering to Enhance Turbulent Cardiovascular Flow Data From Under-Resolved Observations
Get PDFWe propose a data assimilation methodology that can be used to enhance the spatial and temporal resolution of voxel-based data as it may be obtained from biomedical imaging modalities. It can be used to improve the assessment of turbulent blood flow in large vessels by combining observed data with a computational fluid dynamics solver. The methodology is based on a Stochastic Ensemble Kalman Filter (SEnKF) approach and geared toward pulsatile and turbulent flow configurations. We describe the observed flow fields by a mean value and its covariance. These flow fields are combined with forecasts obtained from a direct numerical simulation of the flow field. The method is validated against canonical pulsatile and turbulent flows. Finally, it is applied to a clinically relevant configuration, namely the flow downstream of a bioprosthetic valve in an aorta phantom. It is demonstrated how the 4D flow field obtained from experimental observations can be enhanced by the data assimilation algorithm. Results show that the presented method is promising for future use with in vivo data from 4D Flow Magnetic Resonance Imaging (4D Flow MRI). 4D Flow MRI returns spatially and temporally averaged flow fields that are limited by the spatial and the temporal resolution of the tool. These averaged flow fields and the associated uncertainty might be used as observation data in the context of the proposed methodology
Characterization of turbulent flow behind a transcatheter aortic valve in different implantation positions
Get PDFThe development of turbulence after transcatheter aortic valve (TAV) implantation may have detrimental effects on the long-term performance and durability of the valves. The characterization of turbulent flow generated after TAV implantation can provide fundamental insights to enhance implantation techniques. A self-expandable TAV was tested in a pulse replicator and the three-dimensional flow field was extracted by means of tomographic particle image velocimetry. The valve was fixed inside a silicone phantom mimicking the aortic root and the flow field was studied for two different supra-annular axial positions at peak systole. Fluctuating velocities and turbulent kinetic energy were compared between the two implantations. Velocity spectra were derived at different spatial positions in the turbulent wakes to characterize the turbulent flow. The valve presented similar overall flow topology but approximately 8% higher turbulent intensity in the lower implantation. In this configuration, axial views of the valve revealed smaller opening area and more corrugated leaflets during systole, as well as more accentuated pinwheeling during diastole. The difference arose from a lower degree of expansion of the TAV's stent inside the aortic lumen. These results suggest that the degree of expansion of the TAV in-situ is related to the onset of turbulence and that a smaller and less regular opening area might introduce flow instabilities that could be detrimental for the long-term performance of the valve. The present study highlights how implantation mismatches may affect the structure and intensity of the turbulent flow in the aortic root
Planning in Brazil, India and Germany
Get PDFPlanning is a fundamental cognitive ability that helps in organizing and structuring events unfolding in a person\u27s daily life. Two studies are presented that analyze planning behavior in different cultures: Brazil, India, and Germany. The first is a cross-cultural psychological study in which students develop plans for uncertain problem scenarios. The second study follows a cultural psychological tradition. Workers from different domains are interviewed about their life problems and plans. The strengths and the weaknesses of both approaches become obvious in the description and discussion of these two studies. The cross-cultural study sheds light on cross-cultural similarities and differences in planning in Brazil, India, and Germany. The cultural psychological approach yields data regarding a theoretical model on the specific cultural influences on planning
Relation between hematocrit partitioning and red blood cell lingering in a microfluidic network.
Get PDFDespite increased interest in the effect of lingering red blood cells (LRBCs) on the heterogeneous hematocrit distribution in the microcirculation, quantitative data on LRBCs before and after the lingering event is still limited. The aim of the study was to investigate the relation between RBC lingering and hematocrit partitioning in a microfluidic model of a microvascular bifurcation in the limit of low hematocrit conditions (tube hematocrit < 10%). To this end, the classification of lingering RBCs was performed based on timing, position, and velocity of the RBCs. The investigation provided statistical information on the velocity, shape, and orientation of lingering RBCs as well as on their lateral distribution in the parent and daughter vessels. LRBCs traveled predominantly close to the centerline of the parent vessel, but they marginated close to the distal wall in the daughter vessels. Differently than the RBC flow observed in the smallest vessels, no influence of lingering events on the local hematocrit partitioning was observed in our experiments. However, we importantly found that lingering RBCs flowing in the daughter vessel after lingering may be connected to reverse hematocrit partitioning in downstream bifurcations by influencing the skewness of the hematocrit distribution in the daughter vessel which relates to the so-called network history effect
8th World Congress of Biomechanics O1707: Transition to Turbulent Flow in Heart Valve Prostheses
Get PDFIntroduction
Turbulent aortic flow has long been recognised as a symptom of aortic valve stenosis [1]. It is believed that turbulence is physiologically unfavourable in the aorta, as it correlates with elevated drag and shear stress levels. It can lead to endothelial dysfunction and blood trauma. Quantitative turbulent flow analysis, as the core concept of in-vivo, in-vitro and in-silico studies, can be directly used for valvular disease diagnosis and testing of existing prostheses. Understanding the transition process to turbulence in the aorta can further act as a cornerstone for the optimal design of aortic valve prostheses, in particular, for mechanical prostheses, for which, blood damage and coagulation are major limitations. Recent post-operative survey studies [2] report lower mortality rates and re-operation need for the recipients of mechanical prostheses, as compared to biologic ones. However, mechanical valves are connected with higher risks of post-operative trauma, such as bleeding and stroke. Preventing the turbulence in and past a mechanical prosthesis can reduce such adverse events, and offer a design with reliable performance and longevity.
Methods
Herein, we report our ongoing work on understanding the mechanisms of transition to turbulence in such prostheses. We resort to high-order direct numerical simulations (DNS) of systolic blood flow in the proximity of a bileaflet mechanical valve. In order to capture the unstable modes of blood motion and their interaction in the complex wall-bounded geometry of valve and sinuses, scale-resolving simulations (25 micrometer grid spacing) are performed on massive number of CPUs/GPUs of a Cray XC50 supercomputer (CSCS’s Piz Daint).
Results and Discussion
DNS results have unveiled critical vortex shedding on the valve leaflets. It becomes evident only by ultra- high resolutions that there exists a stagnation point on the leaflet’s thin leading attachment surface. (Figure 1). The significant transverse flow near the leading edge forms a vortex which elongates over the leaflet’s inner side by gaining systolic momentum and eventually separates at a second stagnation point on the leaflet. Vortices are then shed and grow spatially as they move downstream. Such boundary layer modes then appear to interact with Burger vortices developing at the trailing edge of the leaflets. This interaction seems to trigger transition to turbulence. In contrast, the outer sides of the valve leaflets, feature stable Falkner-Skan type boundary layers. In order to study the critical inter-leaflet instabilities, linear stability theory is locally applied. Efforts are then made towards suppressing such unstable modes, predicted by hydrodynamic stability theory.
References
[1] Roberts, W.C., et al., 1971, The American journal of cardiology, 27(5), pp.497-506.
[2] Goldstone, A.B., et al., 2017, New England Journal of Medicine, 377(19), pp.1847-1857
3D flow topology behind an aortic valve bioprosthesis
Get PDFWe have developed a hydraulic flow loop to replicate the flow conditions in the ascending aorta. The setup features a measurement cell with a silicone phantom of the aortic root. We use a refraction index matched fluid and multi-view camera setup for recording of the pulsatile fluid flow within the silicone phantom. The fluid is seeded with fluorescent particles. Tomographic particle image velocimetry is applied to measure the three-dimensional instantaneous velocity field at specific phases of the pulse. We present results of the instantaneous and the phase averaged velocity field past an aortic bioprosthesis. Based on a Reynolds decomposition of the velocity field the root-mean-square velocity fluctuation can be computed, revealing regions with increased turbulent shear stresses. In general a specific flow topology that can be related to the features of the valve is observed. With this paper we aim to demonstrate the great potential of the presented method to investigate the performance of the valve
Study of the kinetic energy anisotropy over spherical shells downstream of stenotic and bioprosthetic aortic valves
No full textHaemodynamic transition-to-turbulence due to aortic stenoses or non-optimal bioprosthetic aortic valves (BioAVs) may contribute to a variety of pathophysiological effects (Stein & Sabbah, 1976). Therefore, the thorough study of the behaviour of large-scale eddies, of their interactions with laminar regions and of their spatio-temporal distribution is fundamental to elucidate how turbulence is triggered and develops in aortic flows at peak systole. We thus intend to investigate the anisotropy in the modal kinetic energy carried by the flow structures at various wavenumbers over time. The anisotropy intensity is calculated over equispaced spherical shells placed in the vicinity of the valve orifice. To conduct such a study, data are obtained from a direct numerical simulation of the incompressible flow downstream of a fixed-wall aortic stenosis (Corso et al., 2021) using a high-order spectral element Navier-Stokes solver (Fischer et al., 2008) and from three-dimensional fluid-structure interaction (FSI) simulations of BioAVs with unstably moving leaflets (Nestola et al, 2019). With regard to the spatial distribution of the anisotropy intensity, in the stenotic case, the region where the flow velocity is smaller (that is, outside of the jet) is characterised by high anisotropy intensity values, which is a direct consequence of the highly asymmetric and eccentric stenotic orifice. In the case of the BioAVs with moving leaflets, the highest intensities are found close to the leaflets’ free edges as a result of the time-varying orifice area. It is also noted that the ring post of the bioprosthetic valve tends to be aligned with peaks in the anisotropy field. The novelty in this work lies in the method developed to quantify the anisotropy in the kinetic energy for complex flows as the ones found downstream of the investigated aortic valve configurations. The conclusions drawn are instrumental for the improvement of BioAVs design
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