1,720,984 research outputs found
Sublethal Supraphysiological Shear Stress Alters Erythrocyte Dynamics in Subsequent Low-Shear Flows
No full textBlood is a non-Newtonian, shear-thinning fluid owing to the physical properties and behaviors of red blood cells (RBCs). Under increased shear flow, pre-existing clusters of cells disaggregate, orientate with flow, and deform. These essential processes enhance fluidity of blood, although accumulating evidence suggests that sublethal blood trauma—induced by supraphysiological shear exposure—paradoxically increases the deformability of RBCs when examined under low-shear conditions, despite obvious decrement of cellular deformation at moderate-to-higher shear stresses. Some propose that rather than actual enhancement of cell mechanics, these observations are “pseudoimprovements” and possibly reflect altered flow and/or cell orientation, leading to methodological artifacts, although direct evidence is lacking. This study thus sought to explore RBC mechanical responses in shear flow using purpose-built laser diffractometry in tandem with direct optical visualization to address this problem. Freshly collected RBCs were exposed to a mechanical stimulus known to drastically alter cell deformability (i.e., prior shear exposure (PSE) to 100 Pa × 300 s). Samples were subsequently transferred to a custom-built slit-flow chamber that combined laser diffractometry with direct cell visualization. Cell suspensions were sheared in a stepwise manner (between 0.3 and 5.0 Pa), with each step being maintained for 15 s. Deformability and cell orientation indices were recorded for small-scatter Fraunhofer diffraction patterns and also visualized RBCs. PSE RBCs had significantly decreased visualized and laser-derived deformability at any given shear stress ≥1 Pa. Novel, to our knowledge, observations demonstrated that PSE RBCs had increased heterogeneity of direct visualized orientation with flow vector at any shear, which may be due to greater vorticity and thus instability in 5-Pa flow compared with unsheared control. These findings indicate that shear exposure and stress-strain history can alter subsequent RBC behavior in physiologically relevant low-shear flows. These findings may yield insight into microvascular disorders in recipients of mechanical circulatory support and individuals with hematological diseases that alter physical properties of blood.Full Tex
Rotary Ventricular Assist Device Control With a Fiber Bragg Grating Pressure Sensor
No full textIEEE Current ventricular assist devices (VADs) are rotary blood pumps used to treat end-stage heart failure. VADs are operated at a constant speed that is manually adjusted by a clinician based on the patient's cardiac demand during routine medical examinations. VADs operated at a constant speed have inadequate passive flow regulation due to the inherent mechanical pressure-flow characteristics of the pump; this can lead to harmful situations where the VAD is overpumping or underpumping. Typically, patients on long-term VAD support are discharged to an outpatient setting where the VAD speed can remain the same for weeks or months at a time, impacting patient safety and quality of life. Previously, physiological controllers for VADs have been proposed, which automatically adjust VAD speed to meet patient cardiac demand. Clinical implementation of physiological control is currently hindered by the lack of clinically available, implantable, and continuous hemodynamic sensors. This study describes the physiological control of a VAD using a fiber Bragg grating (FBG) sensor previously developed for measuring VAD inlet pressure. The FBG sensor was used as a feedback to a Starling-like physiological controller, and the control quality was compared against the same controller with feedback from a nonimplantable industrial pressure sensor (Omega sensor). Experiments were conducted in a bench-top cardiovascular simulator under various simulated patient scenarios. The average steady-state difference in VAD flow across all experiments was 0.1 L/min with a maximum difference of -0.4 L/min. Similarly, the average steady-state difference in left ventricular end-diastolic pressure was 0.02 mmHg with a maximum difference of -0.2 mmHg. The clinically insignificant differences found between the two feedback methods indicate that the FBG pressure sensor is viable for the physiological control of VADs.Full Tex
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
In Silico Analysis of Pulsatile Flow Veno-Arterial Extracorporeal Membrane Oxygenation on Human Aorta Model
No full textElectrocardiogram (ECG)—synchronized pulsatile veno-arterial extracorporeal membrane oxygenation (V-A ECMO) is a recent development in extracorporeal therapy for patients with severe cardiogenic shock. Although preclinical studies have shown benefits of pulsatile flow relative to continuous ECMO flow, none have explored the effects of the timing of ECMO pulses with respect to the cardiac cycle and its possible implications on ECMO complications. This study aimed to develop a computational fluid dynamics (CFD) model of V-A ECMO in a patient-specific human aorta and evaluate the effect of ECMO timing on cardiac unloading, surplus hemodynamic energy delivery, and mixing zone position. Using direct flow measurements from cardiogenic shock patients and an ECMO device, the model revealed that maximal left ventricular (LV) unloading occurred when the ECMO pulse was in early diastole (35–40% from LV peak systolic flow). Maximum surplus hemodynamic energy transmission to aortic branches occurred at 20% from LV peak systolic flow. This indicates a trade-off between heart afterload and hemodynamic energy delivery in selecting ECMO pulse timing. The mixing zone was primarily located in the aortic arch across timing configurations. Therefore, selecting ECMO pulse timing is crucial to maximizing the benefits of pulsatile flow in V-A ECMO treatment.Full Tex
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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Computational investigation of turbulent, non-Newtonian flow in heart valve conduits
Full text linkHeart valve conduit prosthesis design has, to date, simply incorporated an existing aortic or mitral valve prosthesis into a length of cylindrical graft tubing. However, as these valves are developed to control flow between the large chambers of the heart, their use in conduits ignores the potential benefits that might be afforded by a "purpose-built" conduit valve.
The purpose of this thesis is to describe the design and assessment of haemodynamics of such a valve, aided via the application of computational fluid Mechanics (CFM).
First a computational model of blood flow was developed which was based on a k - ϵ model of turbulence coupled with a constitutive equation to describe the non-Newtonian flow behaviour of blood. This was applied to a ball valve conduit with a diverging/converging form, and appraisal was made of pressure drop, flow fields, occluder stability and shear components. Behavioural differences between blood analogue solution and blood were limited, thereby endorsing the use of analogues for in-vitro experiments.
The valve finally developed exhibited excellent pressure/flow characteristics, in particular a pressure drop less than half that of an ideal annulus-mounted valve. Furthermore the computational technique allows it to be sized to minimise the thromboembolic and haemolytic potentials (based on predicted shear rate and shear stresses) for any individual patient, rather than simply to fit an annulus.
The major conclusions are: i) the additional flow parameter information, especially thromboembolic and haemolytic potentials, gained from applying CFM at the design stage of valve development is invaluable ii) employment of a realistic constitutive function for non-Newtonian modelling of blood flow provides the standard against which other flow models should be compared iii) greatly improved flow characteristics may be realised with purpose built valved conduits
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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