Sequential School Choice with Public and Private Schools

Motivated by school admission systems used in, e.g., Turkey and Sweden, this paper investigates a sequential two-stage admission system with public and private schools. To perform the analysis, relevant axioms and equilibrium notions need to be tailored for the considered dynamic setting. In particular, a notion of truthfulness, referred to as straightforwardness, is introduced. In sharp contrast to classic one-stage admission systems, sequentiality leads to a trade-off between the existence of a straightforward (i.e., truthful) equilibrium and non-wastefulness. Given this insight, we identify the unique set of rules for two-stage admission systems that guarantees the existence of a straightforward equilibrium and, at the same time, reduces the number of wasted school seats. Several existing admission systems are also theoretically analyzed within our general framework and empirically evaluated using school choice data from Sweden. The latter analysis allows us to quantify various trade-offs in sequential admission systems

Right ventricular outflow tract reconstruction with bicuspid valved polytetrafluoroethylene conduit.

BACKGROUND: In general, all conduits available for right ventricular outflow tract (RVOT) reconstruction eventually become stenotic or insufficient. Owing to the lack of an ideal conduit and with the hope of reducing the incidence of reoperations, we have developed and utilized a bicuspid valved polytetrafluoroethylene (PTFE) conduit for the reconstruction of the RVOT. The purpose of this study was to review our early experience with this conduit. METHODS: From October 2008 to September 2009, we have implanted bicuspid valved PTFE conduits in 18 patients with a median age of 1.7 years (range 6 days to 16 years). Their diagnoses include tetralogy of Fallot with pulmonary atresia in 8, truncus arteriosus in 6, congenital aortic stenosis in 2, transposition of great arteries in 1, and interrupted aortic arch with a ventricular septal defect in 1. In 16 patients, a complete biventricular repair was performed. In another 2 cases, the conduit was used for palliative RVOT reconstruction. The conduit sizes varied from 10 mm to 24 mm in diameter. Three-dimensional flow fields obtained from computational fluid dynamics studies were utilized in the conduit design process. RESULTS: There was no surgical mortality or reinterventions associated with the PTFE conduit placement in our series. At the time of discharge, none of the patients had any echocardiographic findings consistent with significant conduit stenosis or insufficiency. During the follow-up period of 6.2 ± 3.9 months, all patients were alive and only 3 had more than mild pulmonary insufficiency. CONCLUSIONS: Our bicuspid valved PTFE conduit has an acceptable early performance, with a low incidence of valve insufficiency and no conduit stenosis. Certainly, longer follow-up is necessary to fully assess its long-term benefits.</p

Computer modeling for the prediction of thoracic aortic stent graft collapse.

<p>OBJECTIVE: To assess the biomechanical implications of excessive stent protrusion into the aortic arch in relation to thoracic aortic stent graft (TASG) collapse by simulating the structural load and quantifying the fluid dynamics on the TASG wall protrusion extended into a model arch.</p> <p>METHODS: One-way coupled fluid-solid interaction analyses were performed to investigate the flow-induced hemodynamic and structural loads exerted on the proximal protrusion of the TASG and aortic wall reconstructed from a patient who underwent traumatic thoracic aortic injury repair. Mechanical properties of a Gore TAG thoracic endoprosthesis (W. L. Gore and Assoc, Flagstaff, Ariz) were assessed via experimental radial compression testing and incorporated into the computational modeling. The TASG wall protrusion geometry was characterized by the protrusion extension (PE) and by the angle (θ) between the TASG and the lesser curvature of the aorta. The effect of θ was explored with the following four models with PE fixed at 1.1 cm: θ = 10 degrees, 20 degrees, 30 degrees, and 40 degrees. The effect of PE was evaluated with the following four models with θ fixed at 10 degrees: PE = 1.1 cm, 1.4 cm, 1.7 cm and 2.0 cm.</p> <p>RESULTS: The presence of TASG wall protrusion into the aortic arch resulted in the formation of swirling, complex flow regions in the proximal luminal surface of the endograft. High PE values (PE = 2.0 cm) led to a markedly reduced left subclavian flow rate (0.27 L/min), low systolic perfusion pressure (98 mm Hg), and peak systolic TASG diameter reduction (2 mm). The transmural pressure load across the TASG was maximum for the model with the highest PE and θ, 15.2 mm Hg for the model with PE = 2.0 cm and θ = 10 degrees, and 11.6 mm Hg for PE = 1.1 cm and θ = 40 degrees.</p> <p>CONCLUSIONS: The findings of this study suggest that increased PE imparts an apparent risk of distal end-organ malperfusion and proximal hypertension and that both increased PE and θ lead to a markedly increased transmural pressure across the TASG wall, a load that would portend TASG collapse. Patient-specific computational modeling may allow for identification of patients with high risk of TASG collapse and guide preventive intervention.</p> <p>CLINICAL RELEVANCE: A potentially devastating complication that may occur after endovascular repair of traumatic thoracicaortic injuries is stent graft collapse. Although usually asymptomatic, stent graft collapse may be accompanied by adverse hemodynamic consequences. Numerous anatomic and device-related factors contribute to the development of collapse, but predictive factors have not yet been clearly defined. In the present study, we assessed the relevant hemodynamics and solid mechanics underlying stent graft collapse using a computational fluid-structure interaction framework of stent graft malapposition. Our findings suggest that both increased stent graft angle and extension into the aortic arch lead to a markedly increased transmural pressure across the stent graft wall, portending collapse. Patient-specific computational modeling may allow for identification of patients at high risk for collapse and aid in planning for an additional, prophylactic intervention to avert its occurrence.</p

The relationship between anemia and recurrence of ischemic stroke in patients with Trousseau's syndrome: A retrospective cross-sectional study

Objectives: The relationship between cancer and thrombosis was first recognized by the French internist Armand Trousseau in 1865. Trousseau's syndrome is a spectrum of symptoms that result from recurrent thromboembolism associated with cancer or malignancy-related hypercoagulability. In this study, we investigated whether demographics, clinical features, or laboratory findings were able to predict recurrent stroke episodes in patients with Trousseau's syndrome. Methods: In total, 178 adult patients were enrolled in this retrospective cross-sectional study. All patients had been admitted to the emergency room of our hospital between January 2011 and September 2014 and were diagnosed with acute ischemic stroke. Patients were divided into two groups: patients with malignancy (Trousseau's syndrome), and patients without malignancy. Results: There were several significant differences between the laboratory results of the two patient groups. For patients with Trousseau's, the hemoglobin levels for those with one stroke was 12.29 ± 1.81, while those in patients who had experienced more than one stroke was 10.94 ± 2.14 (p = 0.004). Conclusions: Trousseau's syndrome is a cancer-associated coagulopathy associated with high morbidity and mortality rates. In this study, anemia was associated with increased stroke recurrence in patients with malignancy (Trousseau's syndrome). Keywords: Anemia, Ischemic stroke, Malignancy, Trousseau's syndrom

Computational hemodynamic optimization predicts dominant aortic arch selection is driven by embryonic outflow tract orientation in the chick embryo.

In the early embryo, a series of symmetric, paired vessels, the aortic arches, surround the foregut and distribute cardiac output to the growing embryo and fetus. During embryonic development, the arch vessels undergo large-scale asymmetric morphogenesis to form species-specific adult great vessel patterns. These transformations occur within a dynamic biomechanical environment, which can play an important role in the development of normal arch configurations or the aberrant arch morphologies associated with congenital cardiac defects. Arrested migration and rotation of the embryonic outflow tract during late stages of cardiac looping has been shown to produce both outflow tract and several arch abnormalities. Here, we investigate how changes in flow distribution due to a perturbation in the angular orientation of the embryonic outflow tract impact the morphogenesis and growth of the aortic arches. Using a combination of in vivo arch morphometry with fluorescent dye injection and hemodynamics-driven bioengineering optimization-based vascular growth modeling, we demonstrate that outflow tract orientation significantly changes during development and that the associated changes in hemodynamic load can dramatically influence downstream aortic arch patterning. Optimization reveals that balancing energy expenditure with diffusive capacity leads to multiple arch vessel patterns as seen in the embryo, while minimizing energy alone led to the single arch configuration seen in the mature arch of aorta. Our model further shows the critical importance of the orientation of the outflow tract in dictating morphogenesis to the adult single arch and accurately predicts arch IV as the dominant mature arch of aorta. These results support the hypothesis that abnormal positioning of the outflow tract during early cardiac morphogenesis may lead to congenital defects of the great vessels due to altered hemodynamic loading.</p

Fontan conversion templates: patient-specific hemodynamic performance of the lateral tunnel versus the intraatrial conduit with fenestration.

<p>Intraatrial-conduit Fontan is considered a modification of both extracardiac and lateral-tunnel Fontan. In this study, the patient-specific hemodynamic performance of intraatrial-conduit and lateral-tunnel Fontan with fenestration, considered as conversion templates, was investigated based on the authors' patient cohort. Pulsatile computational fluid dynamics simulations were performed using patient-specific models of intraatrial-conduit and lateral-tunnel Fontan patients. Real-time "simultaneous" inferior and superior vena cava, pulmonary artery, and fenestration flow waveforms were acquired from ultrasound. Multiple hemodynamic performance indices were investigated, with particular focus on evaluation of the pulsatile flow performance. Power loss inside the lateral-tunnel Fontan appeared to be significantly higher than with the intraatrial-conduit Fontan for patient-specific cardiac output and normalized connection size. Inclusion of the 4-mm fenestration at a 0.24 L/min mean flow resulted in a lower cavopulmonary pressure gradient and less time-averaged power loss for both Fontan connections. Flow structures within the intraatrial conduit were notability more uniform than within the lateral tunnel. Hepatic flow majorly favored the left lung in both surgical connections: conversion from lateral-tunnel to intraatrial-conduit Fontan resulted in better hemodynamics with less power loss, a lower pressure gradient, and fewer stagnant flow zones along the conduit. This patient-specific computational case study demonstrated superior hemodynamics of intraatrial-conduit Fontan over those of lateral-tunnel Fontan with or without fenestration and improved performance after conversion of the lateral tunnel to the intraatrial conduit. The geometry-specific effect of the nonuniform hepatic flow distribution may motivate new rationales for the surgical design.</p

Computer-Aided Patient-Specific Coronary Artery Graft Design Improvements Using CFD Coupled Shape Optimizer.

This study aims to (i) demonstrate the efficacy of a new surgical planning framework for complex cardiovascular reconstructions, (ii) develop a computational fluid dynamics (CFD) coupled multi-dimensional shape optimization method to aid patient-specific coronary artery by-pass graft (CABG) design and, (iii) compare the hemodynamic efficiency of the sequential CABG, i.e., raising a daughter parallel branch from the parent CABG in patient-specific 3D settings. Hemodynamic efficiency of patient-specific complete revascularization scenarios for right coronary artery (RCA), left anterior descending artery (LAD), and left circumflex artery (LCX) bypasses were investigated in comparison to the stenosis condition. Multivariate 2D constraint optimization was applied on the left internal mammary artery (LIMA) graft, which was parameterized based on actual surgical settings extracted from 2D CT slices. The objective function was set to minimize the local variation of wall shear stress (WSS) and other hemodynamic indices (energy dissipation, flow deviation angle, average WSS, and vorticity) that correlate with performance of the graft and risk of re-stenosis at the anastomosis zone. Once the optimized 2D graft shape was obtained, it was translated to 3D using an in-house "sketch-based" interactive anatomical editing tool. The final graft design was evaluated using an experimentally validated second-order non-Newtonian CFD solver incorporating resistance based outlet boundary conditions. 3D patient-specific simulations for the healthy coronary anatomy produced realistic coronary flows. All revascularization techniques restored coronary perfusions to the healthy baseline. Multi-scale evaluation of the optimized LIMA graft enabled significant wall shear stress gradient (WSSG) relief (~34%). In comparison to original LIMA graft, sequential graft also lowered the WSSG by 15% proximal to LAD and diagonal bifurcation. The proposed sketch-based surgical planning paradigm evaluated the selected coronary bypass surgery procedures based on acute hemodynamic readjustments of aorta-CA flow. This methodology may provide a rational to aid surgical decision making in time-critical, patient-specific CA bypass operations before in vivo execution.</p