A new method to evaluate the local clearance at different annular rings inside hemodialyzers.

Recent research indicated that the dialysate flow distribution inside a hemodialyzer was not uniform ("channeling" of the dialysate flows). However, effect of the channeling on the solute clearance has not been directly and quantitatively examined. In this report, a novel experiment approach is presented to test the hypothesis that hollow fibers in different regions within a given hemodialyzer may contribute differently to the solute clearance. Water solution with urea (molecular weight 60) and creatinine (molecular weight 113) were made as "blood," and pure water was used as dialysate. Two high flux dialyzers, dialyzer A (cellulose triacetate) and dialyzer B (polyethersulfone), were used in this study. The hollow fiber potting area at the blood inlet of a dialyzer was divided into equal area concentric rings. In each experiment, only one of the rings was open for blood flow, and the other rings were blocked by epoxy. The "blood" was pumped at 120 ml/min while the dialysate flow rate (Qd) varied at 500, 800, and 1,000 ml/min, respectively. The solute clearance with a specific ring open (local clearance) was determined by measuring solute (urea/creatinine) concentration at the "blood" inlet and outlet. For dialyzer A, local clearance of urea and creatinine were significantly higher in the outer ring than in the inner ring. With increasing Qd, local solute clearance increased significantly for all rings. For dialyzer B, at any given Qd, solutes local clearance also increased from the inner to outer rings. In comparison, the effect of increasing Qd on solute clearance was greater for dialyzer B than for dialyzer A. In conclusion, using the new experimental method, the authors quantitatively evaluated the solute clearance contributed by the hollow fibers at different locations (concentric rings) in dialyzers. Hollow fibers at different locations did contribute differently to the solute clearance, which may be caused by the channeling of dialysate flow. A careful design of the dialyzer to minimize the channeling is needed.

Effect of spacer yarns on the dialysate flow distribution of hemodialyzers: a magnetic resonance imaging study.

Blood side and dialysate side flow distributions play an important role in determining the optimal use of dialysis membrane in hemodialyzers for the removal of uremic solutes. In this article, we used two nonintrusive magnetic resonance imaging (MRI) techniques called the two-dimensional phase contrast (2DPC) and two-dimensional Fourier transform (2DFT) velocity imaging techniques to (1) study the effect of space yarns on the dialysate side flow distribution, (2) investigate the effect of flow baffle on the dialysate side flow distribution, and (3) characterize the blood side and dialysate side flow profiles of hemodialyzers with flow rates ranging from 200 to 1000 ml/min. We investigated two types of hollow fiber hemodialyzers: hemodialyzers A (with spacer yarns) and B (without spacer yarns). We used a 3 mmol cupric sulfate solution as the compartmental fluid and imaged five transverse cross sections of these hemodialyzers. The hemodialyzer with spacer yarns had a more uniform dialysate side spatial flow distribution than that without spacer yarns. In addition, the design of flow baffle in these hemodialyzers can be further improved to promote uniform dialysate side flow distribution, and the blood side flow had a fully developed laminar flow profile. Our experimental results showed that these velocity imaging techniques provide an innovative, nonintrusive tool for characterizing flow distribution in hemodialyzers.

A numerical and experimental study of mass transfer in the artificial kidney.

To develop a more efficient and optimal artificial kidney, many experimental approaches have been used to study mass transfer inside, outside, and cross hollow fiber membranes with different kinds of membranes, solutes, and flow rates as parameters. However, these experimental approaches are expensive and time consuming. Numerical calculation and computer simulation is an effective way to study mass transfer in the artificial kidney, which can save substantial time and reduce experimental cost. This paper presents a new model to simulate mass transfer in artificial kidney by coupling together shell-side, lumen-side, and transmembrane flows. Darcy's equations were employed to simulate shell-side flow, Navier-Stokes equations were employed to simulate lumen-side flow, and Kedem-Katchalsky equations were used to compute transmembrane flow. Numerical results agreed well with experimental results within 10% error. Numerical results showed the nonuniform distribution of flow and solute concentration in shell-side flow due to the entry/exit effect and Darcy permeability. In the shell side, the axial velocity in the periphery is higher than that in the center. This numerical model presented a clear insight view of mass transfer in an artificial kidney and may be used to help design an optimal artificial kidney and its operation conditions to improve hemodialysis.

Effect of flow baffles on the dialysate flow distribution of hollow-fiber hemodialyzers: a nonintrusive experimental study using MRI.

We used an innovative, nonintrusive MRI technique called the two-dimensional (2D) Phase-Contrast (2DPC) velocity-imaging technique to investigate the effect of flow baffles on the dialysate-side flow distribution in two different hollow-fiber hemodialyzers (A and B); each with flow rates between 200 and 1000 mL/min (3.33 x 10(-6) and 1.67 x 10(-5) m3/s). Our experimental results show that (1) the dialysate-side flow distribution was nonuniform with channeling flow occurred at the peripheral cross section of these hollow-fiber hemodialyzers, and (2) the existing designs of flow baffles failed to promote uniform dialysate-side flow distribution for all flow rates studies.

Kinetic comparison of different acute dialysis therapies.

Recent clinical data indicate both ultrafiltration rate (Qf) and timing of treatment initiation in continuous renal replacement therapy (CRRT) and therapy frequency in intermittent hemodialysis (HD) influence survival in critically ill patients with acute renal failure (ARF). In this study, kinetic modeling is used to compare effective dose delivery by three acute dialysis therapies: continuous venovenous hemofiltration (CVVH), daily HD, and sustained low-efficiency dialysis (SLED). A modified equivalent renal clearance (EKR) approach to account for the initial unsteady-state stage during dialysis is employed. Effective small solute clearance in CVVH is found to be 8% and 60% higher than in SLED and daily HD, respectively. Differences are more pronounced for middle and large solute categories, and EKR in CVVH is approximately 2-fold and 4-fold greater than the corresponding values in daily HD and SLED, respectively. The superior middle and large solute removal for CVVH is due to the powerful combination of convection and continuous operation. In CVVH, a decrease in the initial BUN from 150 to 50 mg/dL is predicted to decrease TAC and, therefore, increase EKR by approximately 35%. After clinical validation, the quantification method presented in this article could be a useful tool to assist in the dialytic management of critically ill ARF patients.