Hemodialyzer Performance Research Articles

Determinants of Hemodialysis Performance: Modeling Fluid and Solute Transport in Hollow-Fiber Dialyzers.

Hemodialysis constitutes the lifeline of patients with end stage renal disease, yet the parameters that affect hemodialyzer performance remain incompletely understood. We developed a computational model of mass transfer and solute transport in a hollow-fiber dialyzer to gain greater insight into the determinant factors. The model predicts fluid velocity, pressure, and solute concentration profiles for given geometric characteristics, membrane transport properties, and inlet conditions. We examined the impact of transport and structural parameters on uremic solute clearance by varying parameter values within the constraints of standard clinical practice. The model was validated by comparison with published experimental data. Our results suggest solute clearance can be significantly altered by changes in blood and dialysate flow rates, fiber radius and length, and net ultrafiltration rate. Our model further suggests that the main determinant of the clearance of unreactive solutes is their diffusive permeability. The clearance of protein-bound toxins is also strongly determined by blood hematocrit and plasma protein concentrations. Results from this model may serve to optimize hemodialyzer operating conditions in clinical practice to achieve better clearance of pathogenic uremic solutes.

Computational Modeling of Effects of Mechanical Shaking on Hemodynamics in Hollow Fibers

Blood-membrane interaction during hemodialysis develops a secondary protein layer on the dialysis membrane surface, resulting in reduction of hemodialyzer performance. Wall shear stress at the surface of the hollow-fiber membrane is one of the determinant factors able to influence dialysis efficiency. Shaking of hemodialyzer during treatment could increase the wall shear stress of the membrane, which could enhance hemodialyzer performance. In this study, hemodynamic changes in hollow fibers were analyzed using computational fluid dynamics software for various shaking conditions of hemodialyzer (longitudinal, transverse, rotational motions). Longitudinal motion induced reverse flow, while transverse motion induced symmetric swirling inside the hollow fiber. During rotational motions, nonuniform vortices were developed according to the rotational radius of the hollow fiber. These changes in flow pathlines induced by different shaking profiles increased the relative motion of blood, transmembrane pressure, and wall shear stress on dialysis membrane surfaces. Both longitudinal and transverse shaking profiles showed a linear relationship between shaking velocity (the product of amplitude and frequency) and wall shear stress. Performance of hemodialyzer can be enhanced with simple mechanical shaking motions, and optimal shaking profiles for clinical application can be investigated and predicted with the computational fluid dynamics model proposed in this study.

Data acquisition system for dialysis machines. A model for membrane hydraulic permeability.

Membrane permeability is a key determinant of dialyzer performance; in vivo, membrane hydraulic permeability is affected by the formation of a protein cake on its surface, reducing ultrafiltration and convective fluxes. The purpose of this work was to evaluate the real hydraulic permeability of high flux polysulfone membrane under conditions of hemodiafiltration, and to consequently develop a mathematical model to estimate ultrafiltration Kuf and protein adsorption Kc coefficients. The DIB08 data acquisition system adapted to the Fresenius 2008E dialysis machine (Fresenius, Bad Homburg, Germany) allowed the recording of useful information for dialysis quantification, which was then processed by a bedside computer. The system was able to evaluate Kuf(t) profile, by calculation from the transmembrane pressure over time (TMP(t)) and ultrafiltration rate (Quf):Kuf (t) = Quf/TMP (t). Subsequent modeling of Kuf involved the determination of two key parameters: Kufhd (dialyzer permeability during diffusion only) (in mL/h/mmHg), and Kc (protein adsorption coefficient) (in mL/h/mmHg2). The model chosen was the following: Kuf (t) = Kuf0 x (1 - (Kc/Kuf0) x In(t + 1)) where Kuf0 represents the initial Kuf obtained at the beginning of the session. Thirty-one sessions were evaluated by real kinetic analysis, from which the mathematical model was derived. It included 27 postdilutional on-line hemodiafiltration and four hemodialysis sessions performed in four patients with nonreused HF80s dialyzers. For the analysis, three subgroups were defined: Group 1, first session of the week (Monday or Tuesday); Group 2, second session of the week (Wednesday or Thursday); and Group 3, third session of the week (Friday or Saturday). Results of Kuf and Kc obtained by real kinetic analysis are presented. The midweek session was associated with a higher membrane hydraulic permeability, most likely relative to lesser ultrafiltration rates and an associated relative decrease in membrane protein coating, represented by Kc. The described data acquisition system allowed the assessment of real time membrane hydraulic permeability and the subsequent development of a mathematical model to estimate this fundamental parameter as it functions to hemodialyzer performance.

The Influence of the Dialysate Flow Rate on Hollow Fiber Hemodialyzer Performance

The clearance characteristics of two sizes of hemodialyzers (0.9 m2 and 1.5 m2) from the same range of products have been studied over the dialysate flow range of 500-3,000 ml/min to establish the device's overall mass transfer resistance characteristics. The results obtained demonstrate a difference in the overall mass transfer resistance which is most marked at the commonly used dialysate flow rate of 500 ml/min. This difference suggests that the increase in size results in the introduction of flow imperfections which reduces the benefit that might be gained by the use of a larger surface area. Results established indicate a reduction in the overall mass transfer resistance with an increasing dialysate flow rate. This reduction is attributed to the presence of turbulence in the dialysate pathway at higher flow rates. The presence of such turbulence was confirmed by visual inspection of the dialyzer after the completion of the study when it was noted that the original well-ordered configuration present in a new dialyzer had been substantially disturbed. Correlation of the dialysate flow rate with overall mass transfer resistance by the use of a Wilson plots indicates a nonlinear relationship. This nonlinearity is attributed to a nonfully developed turbulent flow profile in the dialysate pathway.

Hemodialyzer Performance

Artificial OrgansVolume 19, Issue 11 p. 1112-1112 Hemodialyzer Performance Nicholas Andrew Hoenich, Nicholas Andrew Hoenich Guest EditorSearch for more papers by this author Nicholas Andrew Hoenich, Nicholas Andrew Hoenich Guest EditorSearch for more papers by this author First published: November 1995 https://doi.org/10.1111/j.1525-1594.1995.tb02271.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume19, Issue11November 1995Pages 1112-1112 RelatedInformation

Further discussions on the representation of hemodialyzer performance.

To express the solute-removing capacity of a dialyzer, two diagrams are used in combination: one to represent the relation between the mass transfer coefficient and the solute molecular weight (M), and the other between the extraction ratio and the number of mass transfer units. It is shown that membrane permeation properties can be deduced from the first diagram, a limiting factor of removal can be understood from the second diagram, both can be used to compare performances with different blood or dialysate flow rate (QB or QD) or membrane area, and this method of representing dialyzer performance is more informative than the conventional ones in which dialysance (DB) is plotted against QB or M. Discussion is also given on the conditions under which the relation between DB and log M, or log DB and log M, becomes linear. Numerical evaluation of DB/QB for cross-flow geometry is presented in the Appendix.

A note on the representation of the performance of hemodialyzers.

The performance of hemodialyzers was expressed schematically using three dimensionless quantities. Compared with the usual representation in which the dialysance is plotted against the blood flow rate for each of the solutes of interest, this expression is considered to have the following advantages: (1) the function of a dialyzer to remove different solutes can be expressed by a single curve, (2) the effect of changing the membrane area, its permeability, and the flow rates on the performance can easily be understood, and (3) the effect of changing the blood flow rate on the ratio of the transport efficiency of two or more kinds of solutes can be evaluated. An estimate of the overall mass transfer coefficient is the only procedure necessary to move from the usual to the present expression. These analyses were applied to the performance of a commercial hemodialyzer, and it was verified that this representation could denote its operation fairly well.

Hemodialyzer performance: an assessment of currently available units.

The performance characteristics of nine currently available disposable parallel-flow dialyzers were assessed, both before and immediately following dialysis, by measuring: (i) in vitro clearances of C14 labeled urea, creatinine, and sucrose and H3 labeled vitamin B12 at standard conditions, (ii) ultrafiltration rates in saline at 37C, and (iii) loss in the mass transfer coefficient-membrane area product (hoA), following a single dialysis. Parallel-plate dialyzers underwent considerable membrane stretching during dialysis and exhibited greater variability, particularly regarding vitamin B12 clearance and ultrafiltration ability, than did capillary dialyzers. The variability in plate units was greater for large-area dialyzers where there were 25 to 26 blood channels. However, despite considerable differences in performance characteristics, all dialyzers were judged to be clinically acceptable in terms of both solute clearance and ultrafiltration characteristics.

Mass transfer in parallel-plate dialyzers—A conjugated boundary value problem

Mass transfer in semi-infinite parallel-plate dialysers has been analyzed by solving the convective diffusion equations for the two fluids (blood and dialysate) for arbitrary interfacial concentration distributions, and linking the two solutions via a continuity-of-flux boundary condition at the membrane surfaces. This yields expressions for the true interfacial concentration distributions which, when substituted for the arbitrary distributions, produces complete solutions for the concentration fields in both fluid phases. This model was applied to typical parallel-plate Kiil hemodialyzers. A parametric study was performed to determine which variables influence hemodialyzer performance most strongly. Our computed results provide substantial information about mass transfer in such systems without introducing empirical constants, which are not known a prior.

Photoelectric measurement of blood flow during hemodialysis

Accurate measurements of blood flow rate during hemodialysis are essential for determinations of hemodialyzer performance. Despite the availability of a variety of electronic blood flow meters, use of such instruments for determination of blood flow in hemodialysis has never been satisfactory, a situation caused by such factors as the need to recalibrate frequently, the necessity of incorporating transducers into the dialyzer blood circuit and the effect which variables such as hematocrit may have on the accuracy of determination. Furthermore, such devices are expensive and often require the periodic services of an electronics specialist to maintain them in an operable condition. An alternative technique for measuring flow rate in the extracorporeal blood tubing consists of timing the passage of an injected air bubble between two points in the tubing. Since the advent of maintenance dialysis, the racetrack and bubble time technique [1] has been widely used for routine determinations of blood flow rate. Data derived from research studies in which this method of flow determination was employed have been published widely, in spite of the relative inaccuracies of the method.