11,833 research outputs found
Treating a 20 mm Hg Gradient Alleviates Myocardial Hypertrophy in Experimental Aortic Coarctation
Background Children with coarctation of the aorta (CoA) can have a hyperdynamic and remodeled left ventricle (LV) from increased afterload. Literature from an experimental model suggests the putative 20 mm Hg blood pressure gradient (BPG) treatment guideline frequently implemented in CoA studies may permit irreversible vascular changes. LV remodeling from pressure overload has been studied, but data are limited following correction and using a clinically representative BPG. Materials and methods Rabbits underwent CoA at 10 weeks to induce a 20 mm Hg BPG using permanent or dissolvable suture thereby replicating untreated and corrected CoA, respectively. Cardiac function was evaluated at 32 weeks by magnetic resonance imaging using a spoiled cine GRE sequence (TR/TE/FA 8/2.9/20), 14 × 14-cm FOV, and 3-mm slice thickness. Images (20 frames/cycle) were acquired in 6-8 short axis views from the apex to the mitral valve annulus. LV volume, ejection fraction (EF), and mass were quantified. Results LV mass was elevated for CoA (5.2 ± 0.55 g) versus control (3.6 ± 0.16 g) and corrected (4.0 ± 0.44 g) rabbits, resulting in increased LV mass/volume ratio for CoA rabbits. A trend toward increased EF and stroke volume was observed but did not reach significance. Elevated EF by volumetric analysis in CoA rabbits was supported by concomitant increases in total aortic flow by phase-contrast magnetic resonance imaging. Conclusions The indices quantified trended toward a persistent hyperdynamic LV despite correction, but differences were not statistically significant versus control rabbits. These findings suggest the current putative 20 mm Hg BPG for treatment may be reasonable from the LV\u27s perspective
Numerical and Experimental Investigation of Circulation in Short Cylinders
In preparation for an experimental study of magnetorotational instability
(MRI) in liquid metal, we explore Couette flows having height comparable to the
gap between cylinders, centrifugally stable rotation, and high Reynolds number.
Experiments in water are compared with numerical simulations. Simulations show
that endcaps corotating with the outer cylinder drive a strong poloidal
circulation that redistributes angular momentum. Predicted azimuthal flow
profiles agree well with experimental measurements. Spin-down times scale with
Reynolds number as expected for laminar Ekman circulation; extrapolation from
two-dimensional simulations at agrees remarkably well with
experiment at . This suggests that turbulence does not dominate
the effective viscosity. Further detailed numerical studies reveal a strong
radially inward flow near both endcaps. After turning vertically along the
inner cylinder, these flows converge at the midplane and depart the boundary in
a radial jet. To minimize this circulation in the MRI experiment, endcaps
consisting of multiple, differentially rotating rings are proposed. Simulations
predict that an adequate approximation to the ideal Couette profile can be
obtained with a few rings
Computational Simulations for Aortic Coarctation: Representative Results From a Sampling of Patients
Treatments for coarctation of the aorta (CoA) can alleviate blood pressure (BP) gradients(D), but long-term morbidity still exists that can be explained by altered indices of hemodynamics and biomechanics. We introduce a technique to increase our understanding of these indices for CoA under resting and nonresting conditions, quantify their contribution to morbidity, and evaluate treatment options. Patient-specific computational fluid dynamics (CFD) models were created from imaging and BP data for one normal and four CoA patients (moderate native CoA: D12 mmHg, severe native CoA: D25 mmHg and postoperative end-to-end and end-to-side patients: D0 mmHg). Simulations incorporated vessel deformation, downstream vascular resistance and compliance. Indices including cyclic strain, time-averaged wall shear stress (TAWSS), and oscillatory shear index (OSI) were quantified. Simulations replicated resting BP and blood flow data. BP during simulated exercise for the normal patient matched reported values. Greatest exercise-induced increases in systolic BP and mean and peak DBP occurred for the moderate native CoA patient (SBP: 115 to 154 mmHg; mean and peak DBP: 31 and 73 mmHg). Cyclic strain was elevated proximal to the coarctation for native CoA patients, but reduced throughout the aorta after treatment. A greater percentage of vessels was exposed to subnormal TAWSS or elevated OSI for CoA patients. Local patterns of these indices reported to correlate with atherosclerosis in normal patients were accentuated by CoA. These results apply CFD to a range of CoA patients for the first time and provide the foundation for future progress in this area
Rossby Wave Instability and Long-Term Evolution of Dead Zones in Protoplanetary Discs
The physical mechanism of angular momentum transport in poorly ionized
regions of protoplanetary discs, the dead zones (DZs), is not understood. The
presence of a DZ naturally leads to conditions susceptible to the Rossby wave
instability (RWI), which produces vortices and spiral density waves that may
revive the DZ and be responsible for observed large-scale disc structures. We
present a series of two-dimensional hydrodynamic simulations to investigate the
role of the RWI in DZs, including its impact on the long-term evolution of the
disc and its morphology. The nonlinear RWI can generate Reynolds stresses
(effective parameter) as large as in the DZ, helping to
sustain quasi-steady accretion throughout the disc. It also produces novel disc
morphologies, including azimuthal asymmetries with , and atypical
vortex shapes. The angular momentum transport strength and morphology are most
sensitive to two parameters: the radial extent of the DZ and the disc
viscosity. The largest Reynolds stresses are produced when the radial extent of
the DZ is less than its distance to the central star. Such narrow DZs lead to a
single vortex or two coherent antipodal vortices in the quasi-steady state. The
edges of wider DZs evolve separately, resulting in two independent vortices and
reduced angular momentum transport efficiency. In either case, we find that,
because of the Reynolds stresses generated by the nonlinear RWI, gravitational
instability is unlikely to play a role in angular momentum transport across the
DZ, unless the accretion rate is sufficiently high.Comment: 15 pages, 15 figures, submitted to MNRA
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