In vitro evaluation of the hydraulic permeability of polysulfone dialysers

On the Temporal Evolution of Key Hemofilter Parameters-In Vitro Study under Co-Current Flow

Effective permeability KP, the ultrafiltration coefficient (KUF), the sieving coefficient (SC), and the loss/permeation of proteins (primarily albumin) are key parameters/specifications characterizing hemofilter (HF) performance. However, there are uncertainties regarding their determination. This work aims (a) to demonstrate that the co-current flow (of blood and dialysate) can lead to beneficial unidirectional filtration (from blood/plasma to dialysate) under a fairly uniform local trans-membrane pressure (TMP), unlike the presently employed counter-current flow; (b) to study the temporal evolution of key HF performance parameters under co-current flow, particularly during the important early stage of hemocatharsis (HC). Experiments with human plasma and BSA solutions in co-current flow mode (for which a fluid mechanical model is developed) show a fairly uniform local/axial TMP, which also improves the local/axial uniformity of protein membrane fouling, particularly under (currently favored) high convective flux operation. Due to incipient membrane fouling, a significant temporal variability/decline in the effective KP is observed, and, in turn, of other parameters (i.e., the Kuf, SC, and permeation/mass flux Mm for albumin and total proteins). A satisfactory correlation of the albumin/protein mass flux Mm with permeability KP is obtained, indicating strong inter-dependence. In conclusion, co-current flow, allowing for a fair local TMP axial uniformity, enables the acquisition of accurate/representative data on the evolution of HF parameters, facilitating their interpretation and correlation. The new results provide a basis for exploring the clinical application of the co-current flow.

Physical Processes in Polymeric Filters Used for Dialysis

The key physical processes in polymeric filters used for the blood purification include transport across the capillary wall and the interaction of blood cells with the polymer membrane surface. Theoretical modeling of membrane transport is an important tool which provides researchers with a quantification of the complex phenomena involved in dialysis. In the paper, we present a dense review of the most successful theoretical approaches to the description of transport across the polymeric membrane wall as well as the cell⁻polymer surface interaction, and refer to the corresponding experimental methods while studying these phenomena in dialyzing filters.

Combining SPECT Medical Imaging and Computational Fluid Dynamics for Analyzing Blood and Dialysate Flow in Hemodialyzers

For a better insight in dialyzer efficiency with respect to local mass transport in a low flux dialyzer (Fresenius F6HPS), blood and dialysate flow distributions were visualized with computational fluid dynamic (CFD) simulations, which were validated with single photon emission computed tomography (SPECT) imaging. To visualize blood-side flow while avoiding transport through the fiber membrane, a bolus of 99m-Technetium labeled MAA (Macro Aggregated Albumin) was injected in the flow using an electronic valve. Water was used to simulate blood, but flow rate was adjusted according to laws of dynamic similarity to account for the viscosity difference (factor 2.75). For the visualization of dialysate flow, a bolus of 99m-Technetium labeled DMSA (Dimercaptosuccinic Acid) was injected, while pressurized air in the blood compartment avoided transmembrane flow. For each test series, 3D acquisitions were made on a two respectively three-headed SPECT camera. By evaluating the images at different time steps, dynamic 3D intensity plots were obtained, which were further used to derive local flow velocities. Additionally, three-dimensional CFD models were developed for simulating the overall blood and dialysate flow, respectively. In both models, the whole fiber compartment was defined as a porous medium with overall axial and radial permeability derived theoretically and from in vitro tests. With the imaging as well as with the computational technique, a homogeneous blood flow distribution was found, while vortices and fluid stagnation were observed in the dialyzer inlet manifold. The non-homogeneous dialysate distribution, as found with SPECT imaging, implies the occurrence of non-efficient sites with respect to mass transfer. The discrepancy between the dialysate results of both techniques indicated that the assumption of a constant fiber bundle permeability in the CFD model was too optimistic. In conclusion, medical imaging techniques like SPECT are very helpful to validate CFD models, which can be further applied for dialyzer design and optimization.

Assessment of the association between increasing membrane pore size and endotoxin permeability using a novel experimental dialysis simulation set-up

Background

Membranes with increasing pore size are introduced to enhance removal of large uremic toxins with regular hemodialysis. These membranes might theoretically have higher permeability for bacterial degradation products. In this paper, permeability for bacterial degradation products of membranes of comparable composition with different pore size was investigated with a new in vitro set-up that represents clinical flow and pressure conditions.

Methods

Dialysis was simulated with an AK200 machine using a low-flux, high-flux, medium cut-off (MCO) or high cut-off (HCO) device (n = 6/type). A polyvinylpyrrolidone-solution (PVP) was recirculated at blood side. At dialysate side, a challenge solution containing a filtrated lysate of two water-borne bacteria (Pseudomonas aeruginosa and Pelomononas saccharophila) was infused in the dialysate flow (endotoxin ≥ 4EU/ml). Blood and dialysate flow were set at 400 and 500 ml/min for 60 min. PVP was sampled before (PVP_pre) and after (PVP_post) the experiment and dialysate after 5 and 55 min. Limulus Amebocyte Lysate (LAL) test was performed. Additionally, samples were incubated with a THP-1 cell line (24 h) and IL-1β levels were measured evaluating biological activity.

Results

The LAL-assay confirmed presence of 9.5 ± 7.4 EU/ml at dialysate side. For none of the devices the LAL activity in PVP_pre vs. PVP_post was significantly different. Although more blood side PVP solutions had a detectable amount of endotoxin using a highly sensitive LAL assay in the more open vs traditional membranes, the permeability for endotoxins of the 4 tested dialysis membranes was not significantly different but the number of repeats is small. None of the PVP solutions induced IL-1β in the THP-1 assay.

Conclusions

A realisitic in vitro dialysis was developed to assess membrane translocation of bacterial products. LAL activity on the blood side after endotoxin exposure did not change for all membranes. Also, none of the PVP_post solutions induced IL-1β in the THP-1 bio-assay.

Optimisation of solute transport in dialysers using a three-dimensional finite volume model

Dialyser manufacturers only provide limited information about mass removal under well-defined flow and solute conditions in commercially available dialysers for hemodialysis. This computational study aimed at assessing the solute transport efficiency in a dialyser for different geometries (fiber lengths and diameters). A three-dimensional finite volume model of a single fiber in a high flux polysulphone dialyser (Fresenius F60) was developed. Different equations describe blood and dialysate flow (Navier-Stokes), radial filtration flow (Darcy) and solute transport (convection-diffusion). Fluid and membrane properties were derived from in vitro and in vivo tests as well as from literature data. Urea (MW60) was used as marker to simulate small molecule removal, while middle molecule transport was modelled using vitamin B12 (MW1355) and inulin (MW5200). Keeping the fluid velocity in a single fiber constant, fiber diameter and length were changed in a wide range for evaluation of solute removal efficiency. Clearances were found enhanced by 13% (urea), 50% (vitamin B12) and 89% (inulin) for a fiber twice as long as a standard one and by 5.5% (vitamin B12) and 21% (inulin) for a fiber diameter of 150 mum instead of 200 mum. The impact of fiber dimensions was more pronounced for the middle molecules compared to urea.

Middle molecule removal in low-flux polysulfone dialyzers: Impact of flows and surface area on whole-body and dialyzer clearances

Some studies found that the removal of middle molecules has a long-term effect on mortality and, even more, is enhanced by high-flux dialysis. In order to enhance middle molecule removal in a low-flux dialyzer, the present study aimed at investigating the combined impact of dialyzer flows and membrane surface area. Blood and dialysate flows were varied within the clinical range 300-500 and 500-800 mL/min, respectively, while the ultrafiltration rate was kept constant at 0.1 L/hr. Single-pass tests were performed in vitro in a single Fresenius F6HPS dialyzer (3 tests) and serially (5 tests) and parallel (3 tests) connected dialyzers. The blood substitution fluid consisted of dialysis fluid in which radioactive-labeled vitamin B12 (molecular weight 1355 Da) was dissolved. Dialyzer clearance as well as whole-body clearance was calculated from radioactivity concentrations of samples taken from the inlet and outlet bloodline. Adding a second dialyzer in series or parallel ameliorated the overall dialyzer and whole-body clearance significantly, except for the highest applied blood flows of 500 mL/min. Better solute removal was also obtained with higher dialysate flows, while the use of higher blood flows seemed advantageous only when using a single dialyzer. Analysis of the ultrafiltration profiles in the different configurations illustrated that enhancing the internal filtration rate ameliorates convective transport of middle molecules. Adequate solute removal results from a number of interactions, as there are blood and dialysate flows, membrane surface area, filtration profile and concentration profiles in the blood and dialysate compartment.

Computational flow modeling in hollow-fiber dialyzers

A three-dimensional finite volume model of the blood-dialysate interface over the complete length of the dialyzer was developed. Different equations govern dialyzer flow and pressure distribution (Navier-Stokes) and radial transport (Darcy). Blood was modeled as a non-Newtonian fluid with a viscosity varying in radial and axial direction determined by the local hematocrit, the diameter of the capillaries, and the local shear rate. The dialysate flow was assumed to be an incompressible, isothermal laminar Newtonian flow with a constant viscosity. The permeability characteristics of the membrane were calculated from laboratory tests for forward and backfiltration. The oncotic pressure induced by the plasma proteins was implemented as well as the reduction of the overall permeability caused by the adhesion of proteins to the membrane. From the calculated pressure distribution, the impact of flow, hematocrit, and capillary dimensions on the presence and localization of backfiltration can be investigated.