A new scintigraphic method to characterize ultrafiltration in hollow fiber dialyzers

Basic prerequisites for on-line, high-volume hemodiafiltration

High-volume hemodiafiltration involves filtration of >23 L/treatment and its replacement by sterile non-pyrogenic substitution fluid, while maintaining the patient's fluid balance. That volume of substitution fluid precludes the use of prepackaged sterile fluid. Instead, substitution fluid must be prepared on-line using machines that incorporate a series of bacteria- and endotoxin-retentive filters. The sterilizing ultrafilters are validated to deliver sterile, non-pyrogenic fluid to the patient when operated according to the machine manufacturer's instructions and in compliance with international standards and regulatory oversight. A successful hemodiafiltration program also places important responsibilities on the user. Specifically, the user is responsible for ensuring that the dialysis water or dialysis fluid delivered to the sterilizing filters of the hemodiafiltration machine meets the machine manufacturer's specifications and is consistent with the quality used in the sterilization validation process. The user is also responsible for ensuring that the treatment prescription allows a filtration volume >23 L/treatment to be achieved by careful selection of a dialyzer, blood flow rate and treatment time. Questions related to assurance that the substitution fluid will routinely be sterile and non-pyrogenic have limited the uptake of on-line hemodiafiltration as a therapeutic option in some countries, such as the United States.

Supportive Effects of Online Hemodiafiltration Therapy on the Nutritional State and Lipid Profile in Very Elderly Dialysis Patients

INTRODUCTION

Online hemodiafiltration (HDF) therapy has been recognized as one of the potential dialysis modalities. However, the long-term effects of online HDF therapy on very elderly dialysis patients older than 75 years have yet to be fully elucidated.

METHODS

Seventy-four very elderly patients older than 75 years undergoing maintenance dialysis therapy were studied retrospectively. Twenty-four (mean ± SE, 81.5 ± 1.0 years) were treated by predilution online HDF, and fifty (81.2 ± 0.6 years) were treated by conventional hemodialysis (HD) for 3 years. Laboratory data related to the nutritional state and lipid profile were collected. Body composition was measured by a bioelectrical impedance method.

RESULTS

Dry weight and body mass index decreased in HD patients (2.9%, p = 0.003 and 3.1%, p = 0.001, respectively), while no significant changes were found in online HDF patients. Serum albumin levels reduced in both HD and online HDF groups (3.5%, p = 0.003 and 2.9%, p = 0.026, respectively). The geriatric nutritional risk index decreased in HD patients (3.0%, p < 0.001), while no significant change was shown in online HDF patients. Body composition analysis demonstrated a significant decrease in intracellular water and increases in extracellular water and edema ratio in both groups. Fat mass and %fat showed significant decreases in HD patients (8.1%, p = 0.003 and 7.3%, p = 0.003, respectively), but no significant changes in online HDF patients. Among laboratory data, serum high-density lipoprotein cholesterol levels did not change in HD patients. However, the levels elevated significantly (10.6%, p = 0.03) in online HDF patients.

DISCUSSION/CONCLUSION

These results indicated that the time-dependent deterioration of the nutritional state in very elderly dialysis patients was inevitable; however, such deterioration was not prominent in online HDF patients. Moreover, the lipid profile showed unique changes in online HDF patients. In order to treat very elderly dialysis patients, online HDF should preferentially be taken into consideration because the maintenance of general condition seems to be a practical goal against the natural time-dependent deterioration.

Flow Dynamic Analysis by Contrast-Enhanced Imaging Techniques of Medium Cutoff Membrane Hemodialyzer

INTRODUCTION

Medium cutoff (MCO) membranes represent an interesting innovation in the field of hemodialysis. Given the correlation between large (PM >25 kDa) middle molecules (LMM) and clinical outcomes, the possibility to broaden the spectrum of solutes removed in hemodialysis with MCO membranes introduces a new perspective for end-stage kidney disease patients. Due to low diffusion coefficients of LMM, the use of convection is required to maximize extracorporeal clearance. High convective rates are achieved with high-flux membranes in hemodiafiltration, a technique not available in the US. In case of the MCO membrane, remarkable clearances of LMM are achieved combining the permeability of the membrane with a significant amount of internal convection. The mechanism of filtration-backfiltration inside the dialyzer enables effective removal of LMM in a technique called expanded hemodialysis (HDx). Given such theoretical explanation, it is important to demonstrate the blood and ultrafiltration rheology inside the MCO dialyzer.

METHOD

This study for the first time describes flow dynamic parameters and internal cross-filtration, thanks to specific radiology and nuclear imaging techniques.

RESULTS

Flow dynamic analysis of the blood and dialysate compartment confirms excellent distribution of velocities and an excellent matching of blood and dialysate. Average blood flow velocity allows for wall shear rates adequate to avoid protein stagnation at the blood membrane interface and increase in blood viscosity. Cross-filtration analysis demonstrates a remarkable filtration/backfiltration flux reaching values >30 mL/min at a blood flow of 300 mL/min and zero net filtration.

CONCLUSION

The MCO dialyzer Theranova 400 appears to have a design optimized to perform expanded hemodialysis (HDx).

Ultrafiltration in critically ill patients treated with kidney replacement therapy

Management of fluid overload is one of the most challenging problems in the care of critically ill patients with oliguric acute kidney injury. Various clinical practice guidelines support fluid removal using ultrafiltration during kidney replacement therapy. However, ultrafiltration is associated with considerable risks. Emerging evidence from observational studies suggests that both slow and fast rates of net fluid removal (that is, net ultrafiltration (UFNET)) during continuous kidney replacement therapy are associated with increased mortality compared with moderate UFNET rates. In addition, fast UFNET rates are associated with an increased risk of cardiac arrhythmias. Experimental studies in patients with kidney failure who were treated with intermittent haemodialysis suggest that fast UFNET rates are also associated with ischaemic injury to the heart, brain, kidney and gut. The UFNET rate should be prescribed based on patient body weight in millilitres per kilogramme per hour with close monitoring of patient haemodynamics and fluid balance. Dialysate cooling and sodium modelling may prevent haemodynamic instability and facilitate large volumes of fluid removal in patients with kidney failure who are treated with intermittent haemodialysis, but the effects of this strategy on organ injury are less well studied in critically ill patients treated with continuous kidney replacement therapy. Randomized trials are required to examine whether moderate UFNET rates are associated with a reduced risk of haemodynamic instability, organ injury and improved outcomes in critically ill patients.

Expanded haemodialysis: from operational mechanism to clinical results

Recent advances in chemical composition and new production techniques resulted in improved biocompatibility and permeability of dialysis membranes. Among these, the creation of a new class of membranes called medium cut-off (MCO) represents an important step towards improvement of clinical outcomes. Such membranes have been developed to improve the clearance of medium to high molecular weight (MW) solutes (i.e. uraemic toxins in the range of 5–50 kDa). MCO membranes have peculiar retention onset and cut-off characteristics. Due to a modified sieving profile, MCO membranes have also been described as high-retention onset. The significant internal filtration achieved in MCO haemodialysers provides a remarkable convective clearance of medium to high MW solutes. The marginal loss of albumin observed in MCO membranes compared with high cut-off membranes is considered acceptable, if not beneficial, producing a certain clearance of protein-bound solutes. The application of MCO membranes in a classic dialysis modality characterizes a new technique called expanded haemodialysis. This therapy does not need specific software or dedicated hardware, making its application possible in every setting where the quality of dialysis fluid meets current standards.

Measuring intradialyser transmembrane and hydrostatic pressures: pitfalls and relevance in haemodialysis and haemodiafiltration

Background

Post-dilutional haemodiafiltration (HDF) with high convection volumes (HCVs) could improve survival. HCV-HDF requires a significant pressure to be applied to the dialyser membrane. The aim of this study was to assess the pressure applied to the dialysers in HCV-HDF, evaluate the influence of transmembrane pressure (TMP) calculation methods on TMP values and check how they relate to the safety limits proposed by guidelines.

Methods

Nine stable dialysis patients were treated with post-dilutional HCV-HDF with three different convection volumes [including haemodialysis (HD)]. The pressures at blood inlet (B_i), blood outlet (B_o) and dialysate outlet (D_o) were continuously recorded. TMP was calculated using two pressures (TMP2: B_o, D_o) or three pressures (TMP3: B_o, D_o, B_i). Dialysis parameters were analysed at the start of the session and at the end of treatment or at the first occurrence of a manual intervention to decrease convection due to TMP alarms.

Results

During HD sessions, TMP2 and TMP3 remained stable. During HCV-HDF, TMP2 remained stable while TMP3 clearly increased. For the same condition, TMP3 could be 3-fold greater than TMP2. This shows that the TMP limit of 300 mmHg as recommended by guidelines could have different effects according to the TMP calculation method. In HCV-HDF, the pressure at the B_i increased over time and exceeded the safety limits of 600 mmHg provided by the manufacturer, even when respecting TMP safety limits.

Conclusions

This study draws our attention to the dangers of using a two-pressure points TMP calculation, particularly when performing HCV-HDF.

Regulatory Considerations for Hemodiafiltration in the United States

Online hemodiafiltration provides greater removal of higher molecular weight uremic retention solutes than conventional high-flux hemodialysis. However, online hemodiafiltration is used sparsely in the United States in part because of a paucity of delivery systems cleared for clinical use by the US Food and Drug Administration. Although a pathway for regulatory approval exists in the United States, concerns remain, particularly regarding online production of the large volumes of sterile, nonpyrogenic substitution fluid infused directly into the bloodstream to maintain fluid balance. Clearly defined testing protocols, acceptable to Food and Drug Administration, will be useful to show that an online hemodiafiltration system is capable of routinely achieving a sterility assurance level of 10-6 and nonpyrogenic levels of endotoxin. Large-scale clinical experience has shown that systems providing this level of performance when combined with certain design features, such as redundancy, and an appropriate quality management process can routinely and safely produce substitution fluid for online hemodiafiltration.

Quantification of Internal Filtration in Hollow Fiber Hemodialyzers with Medium Cut-Off Membrane

Background: Inadequate removal of molecules between 5 and 50 KDa may cause long-term complication in chronic hemodialysis. Medium cut-off (MCO) is a new class of membranes with enhanced sieving properties and negligible albumin loss. MCO membrane makes it possible to perform expanded hemodialysis (HDx), a technique based on high internal filtration (IF).The present study is designed to quantify IF in 2 MCO dialyzers (Theranova 400 and 500, Baxter, Deerfield, USA) using a nuclear imaging technique previously validated. Methods: Blood and dialysate compartment pressure drop along with transmembrane pressure; they were measured in a closed in vitro circuit with human blood (blood flow [QB] = 300 and 400 mL/min; dialysate flow 500 mL/min; net ultrafiltration rate 0 mL/min). A non-diffusible marker molecule (albumin macro-aggregates labeled with 99Tc metastable) was injected in the blood compartment and nuclear emission was recorded by a gamma camera. Relative variations in the concentration of the marker molecule along the length of the filter were used to calculate local cross filtration. Results: Based on marker concentration profiles, IF was estimated. For Theranova 400, IF were 29.7 and 41.6 mL/min for QB of 300 and 400 mL/min. For Theranova 500, IF were 31.6 and 53.1 mL/min for QB of 300 and 400 mL/min respectively. Conclusions: MCO membrane provides significant amounts of IF due to the particular combination between hydraulic permeability of the membrane and reduced inner diameter of the fibers. High IF combined with enhanced sieving profile of MCO membrane leads to improved removal of a wider spectrum of uremia retention molecules in HDx, without requiring complex equipment.

Technical and Clinical Evaluation of a New Asymmetric Polysulfone Membrane (Biosulfane®)

First generation asymmetric polysulfone membranes had high hydraulic permeability (kf=40 ml/h/mmHg/sqm) but a low diffusive permeability due to the hydrophobic nature and wall thickness of 75–100 microns. We have tested a new polysulfone membrane with a wall thickness of 40 microns in a series of in vitro and in vivo dialysis session experiments. The new “Biosulfane®” membrane presented a Kf of 45.8 with constant performance up to 240 mins. The koA was 760 and the clearance value at 350 ml/min of Qb in hemodiafiltration was 255 ml/min for urea, 210 for creatinine, 225 for phosphate, 76 for inulin. In high flux dialysis the clearances were similar except for inulin which was 32% lower due to the lower convection amount. Beta-2 microglobulin clearance was 22 ml/min in high flux dialysis and 37 in hemodiafiltration. Solute sieving coefficients were close to 1 for the majority of the studied solutes in a wide range of molecular weights and slight variations were observed for charged solutes due to Donnan's effect. The sieving for Inulin was 0.96 while that for Beta-2 microglobulin was not measurable due to a large molecule adsorption on the inner structure of the fibres. The good performances of this membrane are probably due to reduced wall thickness and a consequent improvement in diffusive permeability to small size solutes.

Dialysate Flow Distribution in Hollow Fiber Hemodialyzers with Different Dialysate Pathway Configurations

The efficiency of a hemodialyzer is largely dependent on its ability to facilitate diffusion, since this is the main mechanism by which small solutes are removed. The diffusion process can be impaired if there is a mismatch between blood and dialysate flow distribution in the dialyzer. The objective of the paper was to study the impact of different dialysate compartment designs on dialysate flow distribution and urea clearances.

Eighteen hollow fiber 1.3 m2 hemodialyzers were studied, 6 each of 3 designs: Type A- standard fiber bundle (PAN 65DX Asahi Medical, Tokyo, Japan); Type B - spacing filaments external to the fibers (PAN 65SF Asahi Medical, Tokyo, Japan); Type C - fibers waved to give Moiré structure (FB130 Nissho-Nipro, Osaka, Japan). In vitro studies: 3 dialyzers of each type were studied following dye injection into the dialysate compartment. Dynamic sequential imaging of longitudinal sections of the dialyzer were undertaken, using a new generation helical CT scanner (X-Press/HS1 Toshiba Corporation, Tokyo, Japan). In vivo studies: 3 dialyzers of each type were studied, in randomized sequence, in 3 different patients under standardized dialysis conditions. Blood- and dialysate-side urea clearances were measured at 30 and 150 minutes of treatment.

Macroscopic and densitometrical analysis revealed that flow distribution was most homogeneous in the dialyzer with Moiré structure (Type C) and least homogeneous in the standard dialyzer (Type A). Space yarns (Type B) gave an intermediate dialysate flow distribution. Significantly increased urea clearances (p<0.001) were seen with Types B and C, compared to the standard dialyzer. Type C (Moiré) had the highest clearances although these were not significantly greater than Type B (space yarns).

In conclusion, more homogeneous dialysate flow distribution and improved small solute clearances can be achieved by use of spacing yarns or waved (Moiré structure) patterns of fiber packing in the dialyzer. These effects are achieved probably as a result of reduced dialysate channeling resulting in a lower degree of mismatch between blood and dialysate flows. The new radiological technique using the helical CT scanner allows detailed flow distribution analysis and has the potential for testing future modifications to dialyzer design.

In Vitro and in Vivo Evaluation of a New Polysulfone Membrane for Hemodialysis. Reference Methodology and Clinical Results: (Part 1: In Vitro Study)

Different high flux membranes have been recently developed. The present study is aimed at describing the technical features and the clinical performances of a new high flux polysulfone membrane (T-sulfone, Toray Japan). The study has been carried out on two different dialyzers (surface area = 1.3 and 1.8 m2). The filters have been tested in vitro under definite experimental conditions. The hydraulic flow resistance, the pressure drop in the blood compartment and the hydraulic permeability have been determined in a wide range of in vitro experimental conditions. The in vitro sieving coefficients for various solutes have also been determined utilizing human blood. Hydraulic permeability was found in the range of 28.4 ml/h/mmHg/m2 and sieving coefficients were between 0.96 and 1.0 for all low molecular weight solutes. The sieving coefficient for inulin was 0.95. The pressure drop in the filter at 300 ml/min of blood flow was 95 mmHg for the 1.3 m2 and 57 mmHg for the 1.8 m2. The filters are then designed to operate in the presence of high blood flows without excessive resistance in the blood compartment. The blood compartment analyzed by means of a special radiological sequence obtained with a helical scanner after dye injection confirmed the homogeneous distribution of the blood flow in several cross sections of the bundle. Adequate distribution of dialysate was confirmed with a similar method applied to the dialysate compartment. The new imaging techniques utilized were greatly helpful to determine adequacy of filter design and flows distribution.

Impact of Spacing Filaments External to Hollow Fibers on Dialysate flow Distribution and Dialyzer Performance

A new type of dialyzer (PAN 650 SF Asahi) is analyzed in terms of hydraulic properties, solute clearances and dialysate flow distribution. The new type of dialyzer is a polyacrylonitrile hollow fiber filter, equipped with spacing filaments placed externally to the fibers to facilitate dialysate distribution and avoid channeling. The new filter is compared with a similar filter without spacing filaments. For this purpose, blood and dialysate side clearances have been measured in sequential dialysis session carried out randomly in the same patients. Furthermore, a last generation helical scanner (X-Press / HS1, Toshiba) has been utilized to analyze in vitro the flow distribution of dialysate inside the dialyzer. A contrast medium was injected and a sequence of images has been achieved on a longitudinal section of the dialyzer. This new method permits to avoid any bias due to the cylindrical shape of the dialyzer, since a 10 mm thick rectangular section is analyzed and not the entire body of the filter. The dialyzers equipped with spacing filaments displayed a significant improvement of the dialysate distribution as demonstrated by the radiological pattern. In detail, despite a channeling phenomenon in the peripherical region of the bundle is still present, this is remarkably reduced in comparison with the channelling phenomenon observed in the standard dialyzers. This improved distribution is confirmed by a significant improvement of the solute clearances.

Experimental quantification of the fluid dynamics in blood-processing devices through 4D-flow imaging: A pilot study on a real oxygenator/heat-exchanger module

The performance of blood-processing devices largely depends on the associated fluid dynamics, which hence represents a key aspect in their design and optimization. To this aim, two approaches are currently adopted: computational fluid-dynamics, which yields highly resolved three-dimensional data but relies on simplifying assumptions, and in vitro experiments, which typically involve the direct video-acquisition of the flow field and provide 2D data only. We propose a novel method that exploits space- and time-resolved magnetic resonance imaging (4D-flow) to quantify the complex 3D flow field in blood-processing devices and to overcome these limitations. We tested our method on a real device that integrates an oxygenator and a heat exchanger. A dedicated mock loop was implemented, and novel 4D-flow sequences with sub-millimetric spatial resolution and region-dependent velocity encodings were defined. Automated in house software was developed to quantify the complex 3D flow field within the different regions of the device: region-dependent flow rates, pressure drops, paths of the working fluid and wall shear stresses were computed. Our analysis highlighted the effects of fine geometrical features of the device on the local fluid-dynamics, which would be unlikely observed by current in vitro approaches. Also, the effects of non-idealities on the flow field distribution were captured, thanks to the absence of the simplifying assumptions that typically characterize numerical models. To the best of our knowledge, our approach is the first of its kind and could be extended to the analysis of a broad range of clinically relevant devices.

The Influence of Colloid Osmotic Pressure on Hydrostatic Pressures in High- and Low-Flux Hemodialyzers

The aim of this study was to examine the relationship between hydrostatic trans-membrane pressure (TMP_h ) and colloid osmotic pressure (COP) in low-flux (LF) and high-flux (HF) dialyzers. Hydrostatic pressures were measured in dialyzers distinguished by their ultrafiltration coefficient K_uf (16 and 85 mL/h/mm Hg) under constant dialysate flow and variable blood flow (Q_b ) ranging from 0 to 400 mL/min using (i) alginate (70 kDa) dissolved in dialysate, (ii) diluted, undiluted, and concentrated plasma, or (iii) whole blood at different hematocrit, all in absence of ultrafiltration (UF). For a given fluid, TMP_h linearly increased with increasing Q_b . The intercept of the linear TMP_h to Q_b relationship correlated with measured COP with an average bias of 1.00 ± 2.26 mm Hg and a concordance correlation coefficient of 0.98. The slope of the linear TMP_h to Q_b relationship increased with increasing sample viscosity and was much larger in HF dialyzers under otherwise identical operating conditions, most likely because of increased internal filtration. The TMP_h to Q_b relationship measured in dialyzers in absence of UF can be described by the intercept related to measured COP and the slope related to internal filtration. This relationship could be of interest to estimate internal filtration and COP under in vivo conditions.

Clinical Evaluation of Internal Hemodiafiltration (iHDF): A Diffusive-Convective Technique Performed with Internal Filtration Enhanced High-Flux Dialyzers

Aim

Efficiency in removing middle molecules such as ß2-microglobulin (ß2-MG) is one of the main purposes of modern dialytic therapy. In order to achieve this, techniques requiring complex machines and substitution fluid have been developed over recent years. Alternatively, the internal filtration / back filtration phenomenon can be used. The recent development of a so-called “internal filtration enhanced dialyser” prompted us to compare the removal of ß2-MG together with other small molecules when the dialyser was used either in standard hemodiafiltration (HDF) or internal hemodiafiltration (iHDF).

Methods

Ten stable, anuric, hemodialysis (HD) patients treated by thrice weekly standard bicarbonate HD using low-flux synthetic membrane entered the study. A new high-flux polysulfone dialyser designed with the specific aim of enhancing internal filtration (BS-1.6 UL, 1.6 m2, Toray Industries) was used. Post dilution HDF (2.5 l/hour of substitution fluid, dialysate flow 500 ml/min) was compared with iHDF (dialysate flow 750 ml/min), with blood flow at 300 ml/min. Samples were obtained at the start and at the end of the session in order to measure the % removal of urea, creatinine, uric acid, phosphate and ß2-MG (corrected for total protein concentration). In addition, after 20 min of dialysis the clearances of the same molecules were measured. A mathematical model has been developed for the description of the hydrodynamic phenomena taking place within the dialyser and of fluid filtration across the membrane.

Results

No significant differences have been observed in removal rate switching from HDF to iHDF except for ß2-MG removal, which was slightly higher in HDF than in iHDF. Phosphate clearance is significantly higher than those obtained with creatinine in both HDF (p<0.005) and iHDF (p<0.01) modalities. The total convection calculated with the model is reduced with respect to HDF only by 24% (4100 ml/h vs. 5400 ml/h on the average).

Conclusions

iHDF is a high flux dialysis method, which, if performed with a dialyser designed to enhance internal filtration, obtains a much higher removal rate in comparison with dialysers in traditional high flux dialysis, as previously reported in the literature. Provided that the dialyser is used on a dialysis machine working with ultra pure dialysate and UF control, this dialyser line can perform reliable internal HDF without the need for replacement solution. Considering the narrow difference in performance observed between iHDF and HDF, and the increasing number (and age) of patients leading to higher dialysis costs, iHDF represents a cost-effective alternative to other diffusive-convective techniques.

Prolonged Daily Intermittent Renal Replacement Therapy in ICU Patients by ICU Nurses and ICU Physicians

Objectives

Prolonged daily intermittent renal replacement therapy (PDIRRT) has been proposed as a new form of treatment for severe acute renal failure (ARF). However, this treatment has so far implied a) full dependence on nephrological input, b) lack of any convective clearance and c) limited purification of dialysate water. The aim of this study was to establish the feasibility and safety of performing PDIRRT in the ICU with a) no nephrological input, b) the addition of some convective clearance with on-line fluid replacement and c) a new advanced water purification system.

Design

Prospective observational study.

Patients

Fourteen patients treated with PDIRRT.

Setting

ICU of tertiary institution.

Interventions

Treatment of patients with severe ARF and critical illness with PDIRRT. Prescription of treatment by ICU physicians. Conduct of treatment by ICU nurses. Use of combined convective and diffusive therapy with on-line generation of fluid replacement, application of a double-filtration water purification system.

Measurements and Main Results

We prospectively collected demographic, biochemical, hemodynamic and clinical data in 14 patients, who received 30 PDIRRT treatments for a cumulative treatment time of 205.4 hours. The mean age was 57.9 ± 16.0. Eight patients were male and 6 female. Their mean APACHE II score was 24.6 ± 5.9 and their SAPS II score was 41.7 ± 18.8. PDIRRT was used after at least 24 hours of initial stabilization with continuous veno-venous hemofiltration (CVVH). Blood flow was kept at 100ml/min dialysate flow at 200 ml/min and convective clearance varied from 21 ml/min to 33 ml/min. All patients were either anuric or oliguric (UO < 400 ml/day). Ten patients were on mechanical ventilation and 11 patients on vasopressor support. Mean treatment session time was 6.9 ± 1.8 hours. The mean pre-PDIRRT urea was 19.2 ± 6.9 mmol/L and the creatinine was 274 ± 116 μmol/L. The mean pre-PDIRRT lactate was 2.95 ± 2.24 mmol/L. Following treatment, all had significantly decreased to 13.2 ± 6.3 mmol/L, 215 ± 95 μmol/L and 2.25 ± 1.61 mmol/L, respectively (p=<0.0001, <0.0001, <0.05). Bicarbonate levels remained stable during treatment (23.0 ± 3.8 mmol/L to 23.1 ± 2.5 mmol/L). Mean norepinephrine dose changed from 8.8 ± 11.9 μg/min to 12.9 ± 27.0 μg/min after treatment (NS). There were no complications of therapy. Patient ICU survival was 71.4%.

Conclusions

PDIRRT with combined diffusive and convective clearance is an efficacious form of renal replacement, which can be safely and effectively conducted by ICU nurses following prescription by ICU physicians without any nephrological involvement and with adequate double filtration water purification.

The Rise of Expanded Hemodialysis

The low water permeability feature of original cellulosic membranes was considered an advantage in the absence of dialysis equipment that are capable of controlling water removal. The advent of ultrafiltration control systems led to the development and use of high-flux (HF) membranes that allowed improved middle molecule removal including β-2 microglobulin. Further advances in technology allowed better control over the structure and permeability of membranes. Different polymers and improved spinning modalities led to significant advances in solute removal and hemocompatibility. Inner surface modification produced a reduction in membrane thrombogenicity and protein-membrane interaction with a less tendency to fouling and permeability decay. Further evolution in technology led to the development of a new class of membranes referred to as protein-leaking membranes or super-flux or high cutoff (HCO). These membranes are more permeable than conventional HF membranes and allow some passage of proteins, including albumin. The rationale for these membranes is the need for increased clearance of low molecular weight proteins and protein-bound solutes. However, albumin loss in protein-leaking HCO membranes represents a limitation whose effect in patients is still controversial. The last evolution in the field of membranes is the development of a new class defined as "high retention onset" (HRO) due to the peculiar high sieving value in the middle to high molecular weight range. The introduction of HRO membranes in the clinical routine has enabled the development of a new concept therapy called "expanded hemodialysis." Its simple set up and application offer the possibility to use it even in patients with suboptimal vascular access or even with an indwelling catheter. The system does not require particular hardware or unusual nursing skill. The quality of dialysis fluid is, however, mandatory to ensure a safe conduction of the dialysis session. This new therapy is likely to modify the outcome of end-stage kidney disease patients, thanks to the enhanced removal of molecules traditionally retained by current dialysis techniques.

Optimization of the convection volume in online post-dilution haemodiafiltration: practical and technical issues

In post-dilution online haemodiafiltration (ol-HDF), a relationship has been demonstrated between the magnitude of the convection volume and survival. However, to achieve high convection volumes (>22 L per session) detailed notion of its determining factors is highly desirable. This manuscript summarizes practical problems and pitfalls that were encountered during the quest for high convection volumes. Specifically, it addresses issues such as type of vascular access, needles, blood flow rate, recirculation, filtration fraction, anticoagulation and dialysers. Finally, five of the main HDF systems in Europe are briefly described as far as HDF prescription and optimization of the convection volume is concerned.

Internal filtration in a high-flux dialyzer quantified by mean transit time of an albumin-bound indicator

Internal filtration in high-flux (HF) dialyzers significantly contributes to convective solute removal of molecules with poor diffusibility, but it is difficult to quantify. The aim of this study was to present the theory and to develop a method for measuring internal filtration and backfiltration in HF dialyzers, which also could be applied to patient studies. In a series of lab-bench experiments, the mean transit times (τd) of indocyanine green (ICG) passing the dialyzer were optically measured under different operating conditions and compared with mean transit times calculated from the known volume of the blood compartment (τV) using a mathematical model. τd was always larger than τV. The relative difference in mean transit times (1 - τV/τd) was related to the average cumulative filtration rate (Qfil). The internal filtration fraction Fb = Qfil/Qb was largely independent of blood flow (Qb) and not different from theoretical predictions obtained from a mathematical model. The dispersion of a nondiffusible indicator such as ICG can be used to quantify the magnitude of internal filtration and backfiltration in HF dialyzers using available technology. This information could be useful for testing the HF dialyzers in everyday situations.

Engineering perspective on the evolution of push/pull-based dialysis treatments

The incidence of kidney disease is rapidly increasing worldwide, and techniques and devices for treating end-stage renal disease (ESRD) patients have been evolving. Better outcomes achieved by convective treatment have encouraged the use of synthetic membranes with high water permeability in clinical setups, and high-flux hemodialysis (HD) and hemodiafiltration (HDF) are now preferred forms of convective therapy in ESRD patients. Push/pull-based dialysis strategies have also been examined to increase convective mass transfer in ESRD patients. The push/pull technique uses the entire membrane as a forward filtration domain for a period of time. However, backfiltration must accompany the forward filtration to compensate for the fluid depletion resulting from the forward filtration, making it necessary to switch the membranes to a backfiltration domain. This paper attempts to describe the advancement of push/pull-based renal supportive treatments in terms of their technical description, hemodialytic efficacy including fluid management accuracy and applicability for clinical use. How the optimization of push and pull actions could translate into better convective efficiency will also be discussed in depth.

Pilot study to assess increased dialysis efficiency in patients with limited blood flow rates due to vascular access problems

In the last few years, the number of hemodialysis patients with inadequate blood flow (Qb) rates has increased due to vascular access problems. To avoid a clinical status of underdialysis, these patients need long-lasting dialysis sessions. However, other factors aimed to optimize the dialysis dose have to be considered. High-efficiency convective therapies, such as online hemodiafiltration (HDF), are claimed to be superior to high-flux hemodialysis (HF-HD) in improving the dialysis efficacy, but treatment efficacy is strongly related to blood flow rate and infusion volumes. Online mid-dilution (HDF-MD) with the Nephros OL-pure MD190 represents a new HDF concept to increase the removal of middle molecules. In a cross-over clinical trial, 8 patients, with Qb eff <300 mL/min, received either online HDF-MD or HF-HD; Qd was 700 mL/min, the time duration was 240 min, and the filtration volume in HDF-MD was 112+/-7 mL/min. No differences were found for Kt/V, urea, and creatinine clearances. Clearance of both small phosphate (P) large beta(2)-microglobulin (beta(2)m), and leptin (L) solutes was significantly greater for MD (P 217+/-32, beta(2)m 85.5+/-10, L 42.6+/-18 mL/min) than for HF-HD (P 178+/-32, beta(2)m 71.9+/-13, L 32.1+/-12 mL/min). The results of this study indicate that HDF remains the best means of providing increased removal of large-molecular weight solutes even in patients with vascular access problems.

Flow distribution analysis by helical scanning in polysulfone hemodialyzers: Effects of fiber structure and design on flow patterns and solute clearances

The efficiency of a hemodialyzer is largely dependent on its ability to facilitate diffusion, as this is the main mechanism by which small solutes are removed. The diffusion process can be impaired if there is a mismatch between blood and dialysate flow distribution in the dialyzer. The objective of the paper was to study the impact of different fiber bundle configurations on blood and dialysate flow distribution and urea clearances. The Optiflux 200 NR hemodialyzer was studied and the standard F 80 A hemodialyzer was used as a control for the study. Six dialyzers of each type were studied in vitro in the radiology department utilizing a new generation of helical computed tomography (CT) scan following contrast medium injection into the blood and dialysate compartment. Dynamic sequential imaging of longitudinal sections of the dialyzer was undertaken to detect flow distribution, average and peak velocities, and calculate wall shear rates. Six patients were dialyzed with 2 different dialyzers in random consecutive sequence. In these patients, 2 consecutive dialyses were carried out with identical operational parameters (Qb = 300 mL/min, Qd = 500 mL/min). In each session, blood and dialysate side urea clearances were measured at 30 and 150 min of treatment. Macroscopic and densitometrical analysis revealed that flow distribution was most homogeneous in the dialyzer with a new bundle configuration. Significantly increased urea clearances (p < 0.001) were seen with the Optiflux dialyzer compared with the standard dialyzer. In conclusion, more homogeneous dialysate blood and dialysate flow distribution and improved small solute clearances can be achieved by modifying the configuration of the filter bundle. These effects are achieved probably as a result of reduced blood to dialysate mismatch with reduction of flow channeling. The used radiological technique allows detailed flow distribution analysis and has the potential for testing future modifications to dialyzer design.

Performance of polysulfone membrane dialyzers and dialysate flow pattern

BACKGROUND

It is important to observe the flow pattern of dialysate when evaluating dialyzer function and developing the most appropriate design. We investigated dialysate flow through two polysulfone membrane dialyzers (TS-UL [Toray Medical] and APS-S [Asahi Medical]) by computed tomography (CT), with barium sulfate as the contrast medium. We also performed a clinical comparison of these two dialyzers.

METHODS

For the in vitro experiment, after confirming the steady-state flow of mock blood (xanthan gum solution; 200 ml/min) and dialysate (500 ml/min), fresh dialysate, containing 5% (w/v) barium sulfate was perfused, and longitudinal CT scans of the dialyzer were obtained. Then the concentration of barium sulfate was measured (in Hounsfield units) in three fixed regions of interest. For the in vivo experiment, 12 patients on stable hemodialysis who had been using the APS-S for more than 1 month were switched to the TS-UL for 1 month and changes in various parameters were assessed.

RESULTS

The distribution of dialysate was homogeneous on CT scans of the TS-UL, but not on scans of the APS-S. The dialysate concentration curves for the three regions of interest were similar with the TS-UL, but not with the APS-S. Clearance of urea nitrogen and albumin loss were both significantly higher with the TS-UL than with the APS-S. The decrease in alpha 1-microglobulin was larger with the TS-UL than with the APS-S, but not significantly.

CONCLUSIONS

Clearance of substances over a wide range of molecular weights was higher with the TS-UL than with the APS-S, and differences in the design of the dialysate compartment may have been involved in this feature.

Thermography as potential real-time technique to assess changes in flow distribution in hemofiltration

Flow distributions are critical determinants in the function of hemofilters. Despite their importance, however, flow distributions cannot currently be measured in filters during experimental or clinical applications. Here, we demonstrate that the thermal conduction properties of extracorporeal circuits may provide a tool to overcome this limitation. More specifically, we show that thermography provides an indirect approach to visualize differences in regional perfusion rates through temperature profiles on the filter surface. Thermograms were recorded using a TVS700 system (Ca. Goratec) during recirculating in vitro hemofiltration of porcine blood. Different test protocols were executed to characterize the contribution of thermal conduction and convection to the measurable changes in the temperature at the surface of the filter housing. For comparison and validation, these experiments were supplemented by computer tomography (CT) of filters after dye injection. Thermography enabled real-time visualization of the flow distributions in a hemofilter. Moreover, 'point' trends taken from different regions of the filter provided quantitative information about changes of flow distributions in response to changing experimental conditions. Our preliminary data suggest that thermography is a promising new approach for assessing the principles and time-related changes in flow distributions in hemofiltration. As expected, resolution is lower than that in CT measurements and further studies will be necessary to determine the smallest temperature gradient that still identifies differences in regional perfusion rates. Given its potential to develop into an inexpensive tool for the 'bedside' level monitoring of flow distributions during clinical studies, further investigation of thermography is highly desirable.

Clinical effects of online dialysate and infusion fluids

Online hemodiafiltration appears to be the most effective technique of renal replacement therapy in many respects. Removal of small and high-molecular weight substances is enhanced. Modern technology ensures a safe, online production of reinfusion fluids. Nonetheless, stringent maintenance rules are required for the production of sterile and nonpyrogenic-dialysate solutions. In this review, we will critically review the state of the art of the clinical effects derived from the use of ultrapure dialysate and the online production of dialysate fluids in high-flux hemodiafiltration.

Membrane fouling and dialysate flow pattern in an internal filtration-enhancing dialyzer

For efficient removal of large molecular weight solutes by dialysis, several types of internal filtration-enhancing dialyzers (IFEDs) are commercially available. However, in a pressure-driven membrane separation process (i.e., filtration), membrane fouling caused by adhesion of plasma proteins is a severe problem. The objective of the present study is to investigate the effects of internal filtration on membrane fouling based on the membrane's pure-water permeability, diffusive permeability, and sieving coefficient. Hemodialysis experiments were performed with two different dialyzers, IFEDs and non-IFEDs. Local membrane fouling in each dialyzer was evaluated by measuring the pure-water permeability, the diffusive permeability, and the sieving coefficient of native membranes and membranes treated with bovine blood. The effects of packing ratio on dialysate flow pattern were also evaluated by measuring the time required for an ion tracer to reach electrodes placed in the dialyzers. In the IFED, membrane fouling caused by protein adhesion is increased because of enhanced internal filtration only at the early stage of dialysis, and this fouling tends to occur only near the dialysate outlet port. However, enhanced internal filtration has little effect on measured membrane transfer parameters.

Protein-Leaking Membranes for Hemodialysis: A New Class of Membranes in Search of an Application?

A new class of membranes that leak protein has been developed for hemodialysis. These membranes provide greater clearances of low molecular weight proteins and small protein-bound solutes than do conventional high-flux dialysis membranes but at the cost of some albumin loss into the dialysate. Protein-leaking membranes have been used in a small number of clinical trials. The results of these trials suggest that protein-leaking membranes improve anemia correction, decrease plasma total homocysteine concentrations, and reduce plasma concentrations of glycosylated and oxidized proteins. However, it is not clear yet that routine use of protein-leaking membranes is warranted. Specific uremic toxins that are removed by protein-leaking membranes but not conventional high-flux membranes have not been identified. It is also unclear whether protein-leaking membranes offer benefits beyond those obtained with conventional high-flux membranes used in convective therapies, such as hemofiltration and hemodiafiltration. Finally, the amount of albumin loss that can be tolerated by hemodialysis patients in a long-term therapy has yet to be determined. Protein-leaking membranes offer a new approach to improving outcomes in hemodialysis, but whether their benefits will outweigh their disadvantages will require more basic and clinical research.

Effect of hollow fiber length on solute removal and quantification of internal filtration rate by Doppler ultrasound

Renal replacement therapy with dialyzers capable of enhanced internal filtration (IF) can be an alternative to standard hemodiafiltration, as it provides convective solute removal comparable to that of hemodiafiltration by a simple procedure. In this study, we clinically evaluated the effect of the hollow fiber length in the dialyzer, a crucial factor influencing the rate of IF, by comparing two commercial dialyzers (BS-1.6U, BS-1.6UL, Toray, Japan) which differed in the fiber length, but had the same surface area and inner diameter of their hollow fibers. We showed that in the dialyzer with the longer fibers, the pressure profile along the dialyzer was significantly altered, and the solute clearance tended to be increased. In addition, we successfully quantified the IF rate with a Doppler ultrasound in the experimental circuit, by measuring the blood flow velocities along the bundle of fibers. We showed that the changes in the blood flow velocity were more marked in the dialyzer with the longer fibers; the calculated IF rates in the dialyzers with the shorter and longer fibers were 11.1 mL/min and 37.7 mL/min, respectively, which seemed to be compatible with the solute clearances. This simple and readily applicable method is expected to be useful in the development of modified dialyzers to fully exploit the benefits of IF in renal replacement therapy.

The Acute Dialysis Quality Initiative--part VII: fluid composition and management in CRRT

Fluid composition and management are important parts of continuous renal replacement therapy (CRRT). Most commercially available CRRT solutions are able to reestablish electrolyte homeostasis provided some phosphate supplementation is given. Supraphysiologic glucose concentrations should be avoided. Predilution fluid replacement allows higher ultrafiltration rates and can be considered as an adjunct to the anticoagulation regimen. Lactate is an effective buffer in most CRRT patients. Bicarbonate is preferred in patients with lactic acidosis and/or liver failure. When citrate is used as anticoagulant, frequent monitoring of pH is required. The clinical consequences of CRRT-induced decreases of body temperature are not clear. Substitution fluid should be sterile, but the bacteriologic requirements for CRRT dialysate are less clear. There is no consensus on the optimal parameters to monitor fluid management. Integrated balancing systems have theoretical advantages over adaptive use of intravenous fluid pumps. Although there is evidence that volume overload is associated with adverse outcome, there is no evidence that fluid removal per se improves outcome in critically ill patients with or without acute renal failure.

Hemodialyzer: from macro-design to membrane nanostructure; the case of the FX-class of hemodialyzers

Very few innovations have characterized the different components of the hemodialyzers in the past 20 years. Most improvements have concerned membrane biocompatibility. In this article, we focus our attention on the most recent advances in hemodialyzer components from the macro design of the unit to the nanostructure of the membrane. For this purpose, we took as an example the FX class of hemodialyzers (FMC, Bad Homburg, Germany). The studied devices were chosen as an example representing some of the most recent hemodialyzers and are well suited to describe technical innovations occurring in the field of dialyzer technology. In vitro and in vivo studies were performed to characterize hemodynamic parameters of three models (1.4-1, 8, and 2.2 m2) and to determine membrane permeability, sieving coefficients, and solute clearances. The units were characterized by a relatively high resistance of the blood and dialysate compartments, leading to an increased internal filtration if compared with similar hemodialyzers of other series. Nevertheless, the flow distribution in both compartments was homogeneous and well balanced. This effect was obtained by the improved blood and dialysate ports design, the increased packing density of the fibers and a reduction of the inner diameter of the fibers from 200 to 180 microm. At the same time, the sieving coefficients for middle-large solutes such as beta2 microglobulin and insulin were higher than those observed in standard high flux dialysers. The same effect was noted for the clearance values of these solutes. This was observed in the absence of significant albumin leakage. These results were obtained thanks to a new nano-controlled spinning technology applied to the fiber. The innermost layer of the membrane is in fact characterized by a homogeneous porosity, with increased number of pores of large dimension but a sharp cutoff of the membrane excluding albumin losses. In conclusion, new technologies and new diagnostic tools today allow for improvement in hemodialyzer design from its macro-components to its nano-structure. The application of nanotechnology to hemodialysis will probably contribute to further developments in hemodialyzer manufacturing.

Increases in mass transfer-area coefficients and urea Kt/V with increasing dialysate flow rate are greater for high-flux dialyzers

The hemodialyzer mass transfer-area coefficient (K(o)A) for urea increases with increasing dialysate flow rate (Q(d)). The magnitude of the increase in K(o)A varies depending on the particular dialyzer under consideration; however, dialyzer properties that govern this phenomenon have not been established. We hypothesized that Q(d)-dependent increases in K(o)As are influenced by the water permeability of the dialysis membrane. We evaluated in vitro the effect of blood flow rate (Q(b)) and Q(d) on urea and creatinine K(o)As for two low-flux (Polyflux 6L and 8L) and two high-flux (Polyflux 14S and 17S) dialyzers containing Polyamide S membranes with similar membrane surface areas. Additional experiments were also performed on high-flux dialyzers containing Polyamide S membranes with very large surface areas (Polyflux 21S and 24S). K(o)As, calculated from the mean of blood- and dialysate-side clearances, were determined at zero net ultrafiltration for three different Q(b) and Q(d) combinations: Q(b) of 300 mL/min and Q(d) of 500 mL/min; Q(b) of 450 mL/min and Q(d) of 500 mL/min; and Q(b) of 450 mL/min and Q(d) of 800 mL/min. Urea and creatinine K(o)As were independent of the Q(b) but increased when Q(d) was increased from 500 to 800 mL/min. These increases in both urea and creatinine K(o)As were greater for high-flux than low-flux dialyzers (P < 0.0001). As expected, urea and creatinine K(o)As also increased with increasing membrane surface area. We conclude that dialysis membrane water permeability (or flux) is a dialyzer property that influences the dependence of small-solute K(o)As and clearance on Q(d). Whether this phenomenon is caused by enhanced internal filtration for dialyzers containing high-flux membranes requires further study.

Quantitative characterization of hemodialyzer solute and water transport

Clinicians are frequently faced with the task of selecting a hemodialyzer for a dialysis-dependent patient. Several quantitative dialyzer parameters, such as clearance, sieving coefficients, and ultrafiltration coefficient, are routinely used in this selection process. However, the quantitative basis and exact meaning of these indices are often unclear or misinterpreted. The purpose of this article is to provide a detailed description of several of these parameters with the hope that this information will enable clinicians to make dialyzer selection from a more quantitative perspective.

A comparison of on-line hemodiafiltration and high-flux hemodialysis: a prospective clinical study

Some of the morbidity associated with chronic hemodialysis is thought to result from retention of large molecular weight solutes that are poorly removed by diffusion in conventional hemodialysis. Hemodiafiltration combines convective and diffusive solute removal in a single therapy. The hypothesis that hemodiafiltration provides better solute removal than high-flux hemodialysis was tested in a prospective, randomized clinical trial. Patients were randomized to either on-line postdilution hemodiafiltration or high-flux hemodialysis. The groups did not differ in body size, treatment time, blood flow rate, or net fluid removal. The filtration volume in hemodiafiltration was 21 +/-1 L. Therapy prescriptions were unchanged for a 12-mo study period. Removal of both small (urea and creatinine) and large (ss(2)-microglobulin and complement factor D) solutes was significantly greater for hemodiafiltration than for high-flux hemodialysis. The increased urea and creatinine removal did not result in lower pretreatment serum concentrations in the hemodiafiltration group. Pretreatment plasma beta(2)-microglobulin concentrations decreased with time (P< 0.001); however, the decrease was similar for both therapies (P = 0.317). Pretreatment plasma complement factor D concentrations also decreased with time (P<0.001), and the decrease was significantly greater with hemodiafiltration than with high-flux hemodialysis (P = 0.010). The conclusion is that on-line hemodiafiltration provides superior solute removal to high-flux hemodialysis over a wide molecular weight range. The improved removal may not result in lower pretreatment plasma concentrations, however, possibly because of limitations in mass transfer rates within the body.

Effects of a reduced inner diameter of hollow fibers in hemodialyzers

BACKGROUND

The clearance of middle molecules in high-flux hemodialyzers is due to the higher contribution of convection in the overall solute transport. Although net filtration can be maintained low by the machine control, internal filtration in the proximal part of the dialyzer remains high. The final fluid balance is achieved by significant amounts of backfiltration in the distal part of the dialyzer. To increase further middle molecule clearance (MMK), hemodiafiltration has been used. This technique, however, requires complex machines and large amounts of substitution fluid. We present a novel solution to increase the convective transport of middle molecules in high flux dialyzers without the need for substitution fluids. In particular, high-flux dialyzers with a reduced hollow fiber diameter are compared with standard dialyzers in terms of internal filtration and solute clearances.

METHODS

Hemodialyzers with 175 micro inner diameter polysulfone fibers were compared with standard 200 micro polysulfone hollow fiber dialyzers. The study was carried out in vitro using a previously published method to measure internal filtration and backfiltration rates. The method is based on the detection by a gamma camera of segmental variations in concentration along the length of the dialyzer of a nondiffusable Tc99-labeled marker molecule injected in the blood in vitro circuit. At the same time, pressures were detected in the blood and dialysate compartment. The system was operated at zero net filtration maintaining volumetrically constant both dialysate and blood circuits. In vivo clearances were also measured for solutes with different molecular weight.

RESULTS

The pressure drop in the blood compartment at 300 mL/min of blood flow passed from 112 to 159 mm Hg. At the same blood flow, the internal filtration-backfiltration rates increased from 23. 1 to 48.2 mL/min. This resulted in a significant increase of in vivo in clearances of vitamin B12 and inulin of more than 30%. Urea, creatinine, and phosphate clearance did not display any change.

CONCLUSIONS

A reduction of the inner diameter of the hollow fibers in high-flux dialyzers may result in a significant increase of the blood compartment resistance. In turn, this results in increased rates of internal filtration and backfiltration. The practical effect in clinical dialysis is demonstrated on middle molecules. While, in fact, the clearances for small solutes such as urea and creatinine are not affected, the clearances of larger solutes such as vitamin B12 or inulin increase significantly (P < 0.01).

Potential of dual-skinned, high-flux membranes to reduce backtransport in hemodialysis

BACKGROUND

Potential backfiltration of cytokine-inducing material is a clinical concern during hemodialysis conducted with high-flux membranes. Novel hollow-fiber membranes were developed that had asymmetric convective solute transport properties, aimed at reducing the passage of potentially harmful molecules from dialysate to blood, while maintaining the desired fluid and solute movement from blood to dialysate.

METHODS

Sieving coefficient as a function of molecular weight was measured in vitro using polydisperse dextrans. Measurements were conducted using two different flat-sheet membranes in series or using hollow fiber membranes having two integrally formed skin layers. Based on measured experimental parameters, model calculations simulated the performance of a clinical-scale dialyzer containing these new membranes versus that of a commercially available high-flux dialyzer.

RESULTS

Asymmetric convective solute transport was demonstrated using both commercial flat-sheet and newly developed hollow-fiber membranes. For two flat-sheet membranes in series, the extent of asymmetric transport was dependent on the order in which the solution was filtered through the membranes. For the hollow-fiber membranes, the nominal molecular weight cut-off was 20 kD in the blood-to-dialysate direction and 13 kD in the dialysate-to-blood direction. For this membrane, model calculations predict that clearance of a beta2-microglobulin-sized molecule (11,800 D) would be significantly greater from blood to dialysate than in the reverse direction, even under conditions of zero net ultrafiltration.

CONCLUSION

A novel hollow-fiber dialysis membrane was developed that allows greater convective solute transport from blood to dialysate than from dialysate to blood.