Flow Visualization in Arteriovenous Fistula and Aneurysm Using Computational Fluid Dynamics

The arteriovenous fistula (AVF) is characterized by enhanced blood flow and is the most widely used vascular access for chronic haemodialysis (Sivanesan et al., 1998). A large proportion of the AVF late failures are related to local haemodynamics (Sivanesan et al., 1999a). As in AVF, blood flow dynamics plays an important role in growth, rupture, and surgical treatment of aneurysm. Several techniques have been used to study the flow patterns in simplified models of vascular anastomose and aneurysm. In the present investigation, Computational Fluid Dynamics (CFD) is used to analyze the flow patterns in AVF and aneurysm through the velocity waveform obtained from experimental surgeries in dogs (Galego et al., 2000), as well as intra-operative blood flow recordings of patients with radiocephalic AVF ( Sivanesan et al., 1999b) and physiological pulses (Aires, 1991), respectively. The flow patterns in AVF for dog and patient surgeries data are qualitatively similar. Perturbation, recirculation and separation zones appeared during cardiac cycle, and these were intensified in the diastole phase for the AVF and aneurysm models. The values of wall shear stress presented in this investigation of AVF and aneurysm models oscillated in the range that can both cause damage to endothelial cells and develop atherosclerosis.

Residence time distribution in holding tubes using generalized convection model and numerical convolution for non-ideal tracer detection

A procedure is proposed for the determination of the residence time distribution (RTD) of curved tubes taking into account the non-ideal detection of the tracer. The procedure was applied to two holding tubes used for milk pasteurization in laboratory scale. Experimental data was obtained using an ionic tracer. The signal distortion caused by the detection system was considerable because of the short residence time. Four RTD models, namely axial dispersion, extended tanks in series, generalized convection and PER + CSTR association, were adjusted after convolution with the E-curve of the detection system. The generalized convection model provided the best fit because it could better represent the tail on the tracer concentration curve that is Caused by the laminar velocity profile and the recirculation regions. Adjusted model parameters were well cot-related with the now rate. (C) 2010 Elsevier Ltd. All rights reserved.

Arteriovenous Fistula Puncture: An Essential Factor for Hemodialysis Efficiency

Objective. To determine the blood recirculation ratio in the vascular access of patients on hemodialysis, and to calculate the Kt/Vs obtained with the different techniques of arteriovenous fistula punctures. Materials and Methods. A total of 174 patients were divided according to the technique used for arteriovenous fistula puncture: group 1, needles in opposite directions and with a distance of 5 cm or more between them; group 2, needles in opposite directions but with a distance of less than 5 cm; group 3, unidirectional needles with both directed to the heart and with a distance of 5 cm or more; group 4, unidirectional needles but separated by a distance of less than 5 cm between needles; and group 5, patients carrying a temporary venous catheter. Blood samples were collected for urea analysis, pre and post-dialysis for Kt/V rate, and other samples for calculation of the access recirculation. Results. Group 1 presented the lowest rate of access recirculation (8.51 +/- 4.90%) and the best Kt/V (1.71 +/- 0.36), while group 4 presented the worst access recirculation (20.68 +/- 4.92%) and Kt/V (1.16 +/- 0.26). All groups differed significantly from group 4 (p < 0.05), except group 5 with regard for Kt/V parameter. Discussion. The technique of arteriovenous fistula puncture is an essential factor to decrease the access recirculation and assure better results of measurement of hemodialysis adequacy. On the basis of the results obtained, insertion of the needles in the same direction and with a distance of less than 5 cm between them should be avoided.

Can eccentric arterial plaques alone cause flow stagnation points and favour thrombus incorporation?

We have used an experimental model of aorta stenosis, with a Plexiglas plug, simulating a stable atheromatous plaque that promotes local turbulence and thrombosis. With animal survival of more than 24 h, we followed the partial fibrinolysis of the thrombus as well as its posterior organization and incorporation to the arterial wall as a neointima for up to 30 days. The mushroom plug form permitted the development of recirculation and stasis areas around it, favouring this evolution. Despite noted limitations, this study demonstrates that thrombus incorporation can contribute to plaque extension, as it can promote recirculation and stasis areas.

Culture and exopolysaccharide production from sugarcane molasses by Gordonia polyisoprenivorans CCT 7137, isolated from contaminated groundwater in Brazil

Gordonia polyisoprenivorans CCT 7137 was isolated from groundwater contaminated with leachate in an old controlled landfill (Sauo Paulo, Brazil), and cultured in GYM medium at different concentrations of sugarcane molasses (2%, 6%, and 10%). The strain growth was analyzed by monitoring the viable cell counts (c.f.u. mL(-1)) and optical density and EPS production was evaluated at the end of the exponential phase and 24 h after it. The analysis of the viable cell counts showed that the medium that most favored bacterial growth was not the one that favored EPS production. The control medium (GYM) was the one that most favored the strain growth, at the maximum specific growth rate of 0.232 h(-1). Differences in bacterial growth when cultured at three different concentrations of molasses were not observed. Production of EPS, in all culture media used, began during the exponential phase and continued during the growth stationary phase. The highest total EPS production, after 24 h of stationary phase, was observed in 6% molasses medium (172.86 g L(-1)) and 10% (139.47 g L(-1)) and the specific total EPS production was higher in 10% molasses medium (39.03 x10(-11)g c.f.u.(-1)). After the exponential phase, in 2%, 6%, and 10% molasses media, a higher percentage of free exopolysaccharides (EPS) was observed, representing 88.4%, 62.4%, and 64.2% of the total, respectively. A different result was observed in pattern medium, which presented EPS made up of higher percentage of capsular EPS (66.4% of the total). This work is the first study on EPS production by G. polyisoprenivorans strain in GYM medium and in medium utilizing sugarcane molasses as the sole nutrient source and suggests its potential use for EPS production by G. polyisoprenivorans CCT 7137 aiming at application in biotechnological processes.

OZONATION OF SEDIMENTS FROM AN URBAN LAKE: AN EXPLORATORY INVESTIGATION

An exploratory investigation was conducted on the effects of application of ozone on the removal of organic and inorganic contaminants and the reduction of settleable solids in urban lake sediments. Homogenized sediment samples were treated in a batch reactor with an external recirculation loop and ozone feed from a Venturi injector. The ozone generating system was fed with ambient air with small footprint and operational simplicity. Ozone mass application (g/h) and contact time (min) were varied over wide ranges during testing. The effects of the ozone mass applied per unit time and the contact time on contaminant removal efficiencies were analyzed and a trade-off between the costs of ozonation and of solids treatment and disposal was proposed. The minimum ozone mass application required for total contaminant removal apparently depended on the type of organic contaminant present. An apparent influence of inorganic contaminant speciation on the removal efficiency was found and discussed.

Electrocombustion of humic acid and removal of algae from aqueous solutions

This paper reports experiments involving the electrochemical combustion of humic acid (HA) and removal of algae from pond water. An electrochemical flow reactor with a boron-doped diamond film anode was used and constant current experiments were conducted in batch recirculation mode. The mass transfer characteristics of the electrochemical device were determined by voltammetric experiments in the potential region of water stability, followed by a controlled current experiment in the potential region of oxygen evolution. The average mass transfer coefficient was 5.2 x 10(-5) m s(-1). The pond water was then processed to remove HA and algae in the conditions in which the reaction combustion occurred under mass transfer control. To this end, the mass transfer coefficient was used to estimate the initial limiting current density applied in the electrolytic experiments. As expected, all the parameters analyzed here-solution absorbance at 270 nm, total phenol concentration and total organic carbon concentration-decayed according to first-order kinetics. Since the diamond film anode successfully incinerated organic matter, the electrochemical system proved to be predictable and programmable.