SUMMARYIt is known that the recently introduced unsymmetric 8-node quadrilateral element (US-QUAD8) is highly tolerant to mesh distortions. The high distortion tolerance of US-QUAD8 is due to the ability of the element to satisfy all the desirable compatibility and completeness requirements of the test and trial functions. The element uses isoparametric and metric shape functions as the test and trial functions, respectively. Despite its high distortion tolerance, the element has two defects. The use of metric shape functions as trial functions causes the element to exhibit rotational frame dependence as well as interpolation failure under certain conditions. A detailed investigation into both these problems is reported and remedies are proposed.
Recent developments in aortic valve replacement include the truly stentless pericardial bioprostheses with single point attached commissures (SPAC) implantation technique. The leaflet geometry available for the SPAC valves can either be a simple tubular or a complex three-dimensional structure molded using specially designed molds. Our main objective was to compare these two leaflet designs, the tubular vs. the molded, by dynamic finite element simulation. Time-varying physiological pressure loadings over a full cardiac cycle were simulated using ABAQUS. Dynamic leaflet behavior, leaflet coaptation parameters, and stress distribution were compared. The maximum effective valve orifice area during systole is 633.5 mm(2) in the molded valve vs. 400.6 mm(2) in the tubular valve, and the leaflet coaptation height during diastole is 4.5 mm in the former, in contrast to 1.6 mm in the latter. Computed compressive stress indicates high magnitudes at the commissures and inter-leaflet margins of the tubular valve, the highest being 3.83 MPa, more than twice greater than 1.80 MPa in the molded valve. The molded leaflet design which resembles the native valve exerts a positive influence on the mechanical performance of the SPAC pericardial valves compared with the simple tubular design. This may suggest enhanced valve efficacy and durability.
SUMMARYThis paper presents a distortion resistant 20-node hexahedron element that employs two different sets of shape functions for the trial and test functions. The formulation seeks to satisfy the continuity and completeness requirements by exploiting the intrinsic properties of these two sets of shape functions. Several test problems are used to assess the performance of the element under various mesh distortions. The ability of the proposed as well as the classical 20-node element to maintain solution accuracy under severe mesh distortions has been studied. The proposed element exhibits a very high tolerance to mesh distortions. In particular, for problems involving linear and quadratic displacement fields, the element is capable of reproducing exact solution under all admissible geometrical distortions of the mesh. For test problems involving higher-order displacement fields, the performance of the present element is in general better than that of the classical element.
BackgroundThe aim of this study is to provide better insights into the cerebral perfusion patterns and collateral mechanism of the circle of Willis (CoW) under anatomical and pathological variations.MethodsIn the current study, a patient-specific three-dimensional computational model of the CoW was reconstructed based on the computed tomography (CT) images. The Carreau model was applied to simulate the non-Newtonian property of blood. Flow distributions in five common anatomical variations coexisting with different degrees of stenosis in the right internal carotid artery (RICA) were investigated to obtain detailed flow information.ResultsWith the development of stenosis in unilateral internal carotid artery (ICA), the cerebral blood supply decreased when the degree of stenosis increased. The blood supply of the ipsilateral middle cerebral artery (MCA) was most affected by the stenosis of ICA. The anterior communicating artery (ACoA) and ipsilateral posterior communicating artery (PCoA) functioned as the important collateral circulation channels when unilateral stenosis occurred. The blood flow of the anterior circulation and the total cerebral blood flow (CBF) reached to the minimum in the configuration of the contralateral proximal anterior cerebral artery (A1) absence coexisting with unilateral ICA stenosis.ConclusionsCommunicating arteries provided important collateral channels in the complete CoW when stenosis in unilateral ICA occurred. The cross-flow in the ACoA is a sensitive indicator of the morphological change of the ICA. The collateral function of the PCoA on the affected side will not be fully activated until a severe stenosis occurred in unilateral ICA. The absence of unilateral A1 coexisting with the stenosis in the contralateral ICA could be the most dangerous configuration in terms of the total cerebral blood supply. The findings of this study would enhance the understanding of the collateral mechanism of the CoW under different anatomical variations.
The MediWatch is a wrist-mounted noninvasive blood pressure monitor designed to capture the radial pulse waveform using arterial tonometry and yield blood pressure measurements when the waveform is calibrated. An early prototype of this monitor uses a pulse-sensing system with a cylindrical plunger to applanate the radial artery. This prototype was evaluated against simulated blood pressure generated by a pneumatic pressure-pulse generator. The simulation-based results show that the prototype gave accurate pressure measurements when the MediWatch waveforms were calibrated against the simulator's pressure, indicating that the pulse-sensing system was able to measure force accurately. The prototype was clinically evaluated against intra-arterial pressure on post-open heart surgery patients. The results show that, under stationary conditions, for short periods of time and when the MediWatch waveforms were calibrated against the intra-arterial pressure, the prototype gave measurements that satisfy some of the statistical criteria of the 1993 Association for the Advancement of Medical Instrumentation standard, the 1993 British Hypertension Society protocol and the 2002 European Society of Hypertension protocol. These clinical results indicate that, under the stated test conditions, the prototype was able to accurately track changes in the patients' systolic and diastolic pressures. The MediWatch is being developed into an ambulatory device that provides a macroscopic view of the patient's blood pressure through measurement at preprogrammed intervals over 24 h, as well as a microscopic view of the patient's pressure through the pulse waveform captured during each measurement cycle. The design features of the MediWatch are being adapted for other applications that require the arterial pulse waveform, calibrated beat-to-beat blood pressure or both. An improved MediWatch prototype has been developed that provides memory storage for measurement data and functions as an integral part of a Web-based system that allows measurement data to be accessed over the Internet. A pulse-wave analyser has been developed that allows the radial pulse waveform to be captured, calibrated and viewed in real time on a personal computer. A continuous noninvasive blood pressure monitoring system based on arterial tonometry is being developed for use as an alternative to the arterial line in invasive blood pressure monitoring.
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