104 research outputs found
Material transport in the left ventricle with aortic valve regurgitation
This experimental in vitro work investigates material transport properties in
a model left ventricle in the case of aortic regurgitation, a valvular disease
characterized by a leaking aortic valve and consequently double-jet filling
within the elastic left ventricular geometry. This study suggests that material
transport phenomena are strongly determined by the motion of the
counterrotating vortices driven by the regurgitant aortic and mitral jets. The
overall particle residence time appears to be significantly longer with
moderate aortic regurgitation, attributed to the dynamics resulting from the
timing between the onset of the two jets. Increasing regurgitation severity is
shown to be associated with higher frequencies in the time-frequency spectra of
the velocity signals at various points in the flow, suggesting nonlaminar
mixing past moderate regurgitation. Additionally, a large part of the
regurgitant inflow is retained for at least one cardiac cycle. Such an increase
in particle residence time accompanied by the occurrence and persistence of a
number of attracting Lagrangian coherent structures presents favorable
conditions and locations for activated platelets to agglomerate within the left
ventricle, potentially posing an additional risk factor for patients with
aortic regurgitation
Evaluating Uncertainties in CFD Simulations of Patient-Specific Aorta Models using Grid Convergence Index Method
Cardiovascular diseases are among the most important causes of global
mortality. Computational Fluid Dynamics (CFD) is a powerful research tool that
analyzes the hemodynamics of artery and blood flow patterns. In this study, CFD
simulations are performed to assess the patient-specific healthy aorta,
fusiform, and saccular aneurysm with various mesh types, including tetrahedral,
polyhedral, and poly-hexacore. The aim of this study is to explore how
different mesh types and grid densities impact the hemodynamic properties of
physiological flows, with the goal of identifying the most cost-effective
meshing approach. A mesh independence study is carried out to ensure the
precision of the results, considering the wall shear stress distribution. For
this, five different mesh resolutions are generated for each geometry. The
uncertainties of the simulations associated with the discretization techniques
and solutions are evaluated using the Grid Convergence Index (GCI) method. The
findings showed that increasing the mesh density provides smaller uncertainty.
GCI values for the wall shear stress are in the range of convergence,
indicating that the results are reliable and accurate. Mesh type selection
affects the accuracy and computational cost of our simulations. The polyhedral
and poly-hexacore meshes lead to a good compromise between precision and
computational cost, while the tetrahedral mesh style gives the most precise
results with fluctuation. This work provides a systematic approach based on the
Grid Convergence Index method in order to select the most appropriate mesh type
for evaluating uncertainties in CFD simulations of patient-specific healthy
aortas and aortas with abdominal aneurysms. According to the findings and GCI
analysis, the polyhedral mesh type was chosen for all patient-specific aorta
models. The study clearly demonstrated its superiority over other mesh types,
...Comment: Research pape
On the evaluation of vorticity using cardiovascular magnetic resonance velocity measurements.
Vorticity and vortical structures play a fundamental role affect-
ing the evaluation of energetic aspects (mainly left ventricle
work) of cardiovascular function. Vorticity can be derived from
cardiovascular magnetic resonance (CMR) imaging velocity
measurements. However, several numerical schemes can be used
to evaluate the vorticity field. The main objective of this work is
to assess different numerical schemes used to evaluate the vor-
ticity field derived from CMR velocity measurements. We com-
pared the vorticity field obtained using direct differentiation
schemes (eight-point circulation and Chapra) and derivate dif-
ferentiation schemes (Richardson 4* and compact Richardson
4*) from a theoretical velocity field and in vivo CMR velocity
measurements. In all cases, the effect of artificial spatial resolu-
tion up-sampling and signal-to-noise ratio (SNR) on vorticity
computation was evaluated. Theoretical and in vivo results
showed that the eight-point circulation method underestimated
vorticity. Up-sampling evaluation showed that the artificial
improvement of spatial resolution had no effect on mean abso-
lute vorticity estimation but it affected SNR for all methods. The
Richardson 4* method and its compact version were the most
accurate and stable methods for vorticity magnitude evaluation.
Vorticity field determination using the eight-point circulation
method, the most common method used in CMR, has reduced ac-
curacy compared to other vorticity schemes. Richardson 4* and
its compact version showed stable SNR using both theoretical
and in vivo data
Mixing Within Patterned Vortex Core
The video shows the flow dynamics within inner and outer regions of a vortex
core. The observed phenomena mimic a transport process occurring within the
Antarctic vortex. The video shows two distinct regions: a strongly mixed core
and broad ring of weakly mixed region extending out the vortex core boundaries.
The two regions are separated by a thin layer that isolates the weakly and
strongly mixed regions; this thin layer behaves as barrier to the mixing of the
two regions. The video shows that the barriers deplete when a swirl of the
vortex core increases and the vortex core espouses a triangular pattern.Comment: 62nd Annual Meeting of the APS Division of Fluid Dynamics, Fluid
Dynamics Vide
An in vitro model of aortic stenosis for the assessment of transcatheter aortic valve implantation
A significant number of elderly patients with severe symptomatic aortic stenosis are denied surgical aortic valve replacement (SAVR) because of high operative risk. Transcatheter aortic valve implantation (TAVI) has emerged as a valid alternative to SAVR in these patients. One of the main characteristics of TAVI, when compared to SAVR, is that the diseased native aortic valve remains in place. For hemodynamic testing of new percutaneous valves and clinical training, one should rely on animal models. However, the development of an appropriate animal model of severe aortic stenosis is not straightforward. This work aims at developing and testing an elastic model of the ascending aorta including a severe aortic stenosis. The physical model was built based on a previous silicone model and tested experimentally in this study. Experimental results showed that the error between the computer-aided design (CAD) file and the physical elastic model was <5%, the compliance of the ascending aorta was 1.15 ml/mm Hg, the effective orifice area (EOA) of the stenotic valve was 0.86 cm2, the peak jet velocity was 4.9 m/s and mean transvalvular pressure gradient was 50 mm Hg, consistent with as severe. An EDWARDS-SAPIEN 26 mm valve was then implanted in the model leading to a significant increase in EOA (2.22 cm2) and a significant decrease in both peak jet velocity (1.29 m/s) and mean transvalvular pressure gradient (3.1 mm Hg). This model can be useful for preliminary in vitro testing of percutaneous valves before more extensive animal and in vivo tests
Impact of prosthesis-patient mismatch in the mitral position on left atrial and pulmonary arterial pressures: a numerical study
Prosthesis-patient mismatch (PPM) occurs when the effective orifice area (EOA) of the prosthesis is too small in relation to the body size and thus to the cardiac output requirement of the patient. A recent retrospective study from our group suggests that mitral PPM defined as an indexed EOA < 1.2 cm2/m2 is associated with lesser regression of pulmonary hypertension after mitral valve replacemen
Comparison between cardiovascular magnetic resonance and transthoracic doppler echocardiography for the estimation of effective orifice area in aortic stenosis
<p>Abstract</p> <p>Background</p> <p>The effective orifice area (EOA) estimated by transthoracic Doppler echocardiography (TTE) via the continuity equation is commonly used to determine the severity of aortic stenosis (AS). However, there are often discrepancies between TTE-derived EOA and invasive indices of stenosis, thus raising uncertainty about actual definite severity. Cardiovascular magnetic resonance (CMR) has emerged as an alternative method for non-invasive estimation of valve EOA. The objective of this study was to assess the concordance between TTE and CMR for the estimation of valve EOA.</p> <p>Methods and results</p> <p>31 patients with mild to severe AS (EOA range: 0.72 to 1.73 cm<sup>2</sup>) and seven (7) healthy control subjects with normal transvalvular flow rate underwent TTE and velocity-encoded CMR. Valve EOA was calculated by the continuity equation. CMR revealed that the left ventricular outflow tract (LVOT) cross-section is typically oval and not circular. As a consequence, TTE underestimated the LVOT cross-sectional area (A<sub>LVOT</sub>, 3.84 ± 0.80 cm<sup>2</sup>) compared to CMR (4.78 ± 1.05 cm<sup>2</sup>). On the other hand, TTE overestimated the LVOT velocity-time integral (VTI<sub>LVOT</sub>: 21 ± 4 vs. 15 ± 4 cm). Good concordance was observed between TTE and CMR for estimation of aortic jet VTI (61 ± 22 vs. 57 ± 20 cm). Overall, there was a good correlation and concordance between TTE-derived and CMR-derived EOAs (1.53 ± 0.67 vs. 1.59 ± 0.73 cm<sup>2</sup>, r = 0.92, bias = 0.06 ± 0.29 cm<sup>2</sup>). The intra- and inter- observer variability of TTE-derived EOA was 5 ± 5% and 9 ± 5%, respectively, compared to 2 ± 1% and 7 ± 5% for CMR-derived EOA.</p> <p>Conclusion</p> <p>Underestimation of A<sub>LVOT </sub>by TTE is compensated by overestimation of VTI<sub>LVOT</sub>, thereby resulting in a good concordance between TTE and CMR for estimation of aortic valve EOA. CMR was associated with less intra- and inter- observer measurement variability compared to TTE. CMR provides a non-invasive and reliable alternative to Doppler-echocardiography for the quantification of AS severity.</p
- …