Plasma protein adsorption to highly permeable hemodialysis membranes

Blood-incompatibility in haemodialysis: alleviating inflammation and effects of coagulation

Blood-incompatibility is an inevitability of all blood-contacting device applications and therapies, including haemodialysis (HD). Blood leaving the environment of blood vessels and the protection of the endothelium is confronted with several stimuli of the extracorporeal circuit (ECC), triggering the activation of blood cells and various biochemical pathways of plasma. Prevention of blood coagulation, a major obstacle that needed to be overcome to make HD possible, remains an issue to contend with. While anticoagulation (mainly with heparin) successfully prevents clotting within the ECC to allow removal of uraemic toxins across the dialysis membrane wall, it is far from ideal, triggering heparin-induced thrombocytopenia in some instances. Soluble fibrin can form even in the presence of heparin and depending on the constitution of the patient and activation of platelets, could result in physical clots within the ECC (e.g. bubble trap chamber) and, together with other plasma and coagulation proteins, result in increased adsorption of proteins on the membrane surface. The buildup of this secondary membrane layer impairs the transport properties of the membrane to reduce the clearance of uraemic toxins. Activation of complement system-dependent immune response pathways leads to leukopenia, formation of platelet–neutrophil complexes and expression of tissue factor contributing to thrombotic processes and a procoagulant state, respectively. Complement activation also promotes recruitment and activation of leukocytes resulting in oxidative burst and release of pro-inflammatory cytokines and chemokines, thereby worsening the elevated underlying inflammation and oxidative stress condition of chronic kidney disease patients. Restricting all forms of blood-incompatibility, including potential contamination of dialysis fluid with endotoxins leading to inflammation, during HD therapies is thus still a major target towards more blood-compatible and safer dialysis to improve patient outcomes. We describe the mechanisms of various activation pathways during the interaction between blood and components of the ECC and describe approaches to mitigate the effects of these adverse interactions. The opportunities to develop improved dialysis membranes as well as implementation strategies with less potential for undesired biological reactions are discussed.

A porcine model of hemodialyzer reactions: roles of complement activation and rinsing back of extracorporeal blood

Hemodialysis reactions (HDRs) resemble complement-activation-related pseudoallergy (CARPA) to certain i.v. drugs, for which pigs provide a sensitive model. On this basis, to better understand the mechanism of human HDRs, we subjected pigs to hemodialysis using polysulfone (FX CorDiax 40, Fresenius) or cellulose triacetate (SureFlux-15UX, Nipro) dialyzers, or Dialysis exchange-set without membranes, as control. Experimental endpoints included typical biomarkers of porcine CARPA; pulmonary arterial pressure (PAP), blood cell counts, plasma sC5b-9 and thromboxane-B2 levels. Hemodialysis (60 min) was followed by reinfusion of extracorporeal blood into the circulation, and finally, an intravenous bolus injection of the complement activator zymosan. The data indicated low-extent steady rise of sC5b-9 along with transient leukopenia, secondary leukocytosis and thrombocytopenia in the two dialyzer groups, consistent with moderate complement activation. Surprisingly, small changes in baseline PAP and plasma thromboxane-B2 levels during hemodialysis switched into 30%-70% sharp rises in all three groups resulting in synchronous spikes within minutes after blood reinfusion. These observations suggest limited complement activation by dialyzer membranes, on which a membrane-independent second immune stimulus was superimposed, and caused pathophysiological changes also characteristic of HDRs. Thus, the porcine CARPA model raises the hypothesis that a second "hit" on anaphylatoxin-sensitized immune cells may be a key contributor to HDRs.

Intraoperative Renal Replacement Therapy: Practical Information for Anesthesiologists

Previous publications regarding perioperative renal replacement therapy (RRT) have focused on the general care of the RRT-dependent patient and provided a broad overview of the various RRT modalities. The goal of this review article is to provide anesthesiologists with specific practical information regarding the possible intraoperative advantages and limitations of each modality, mandatory equipment to institute intraoperative therapy, and background knowledge necessary to communicate effectively with nephrologists and/or support staff regarding the intraoperative RRT goals.

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.

Amino Acid Loss during Continuous Venovenous Hemofiltration in Critically Ill Patients

BACKGROUND/OBJECTIVES

During continuous venovenous hemofiltration (CVVH), there is unwanted loss of amino acids (AA) in the ultrafiltrate (UF). Solutes may also be removed by adsorption to the filter membrane. The aim was to quantify the total loss of AA via the CVVH circuit using a high-flux polysulfone membrane and to differentiate between the loss by ultrafiltration and adsorption.

METHODS

Prospective observational study in ten critically ill patients, receiving predilution CVVH with a new filter, blood flow 180 mL/min, and predilution flow 2,400 mL/h. Arterial blood, postfilter blood, and UF samples were taken at baseline, and 1, 8, and 24-h after CVVH initiation, to determine AA concentrations and hematocrit. Mass transfer calculations were used to determine AA loss in the filter and by UF, and the difference between these 2.

RESULTS

The median AA loss in the filter was 10.4 g/day, the median AA loss by UF was 13.4 g/day, and the median difference was -2.9 g/day (IQR -5.9 to -1.4 g/day). For the individual AA, the difference ranged from -1 g/day to +0.4 g/day, suggesting that some AA were consumed or adsorbed and others were generated. AA losses did not significantly change over the 24-h study period.

CONCLUSION

During CVVH with a modern polysulfone membrane, the estimated AA loss was 13.4 g/day, which corresponds to a loss of about 11.2 g of protein per day. Adsorption did not play a major role. However, individual AA behaved differently, suggesting complex interactions and processes at the filter membrane or peripheral AA production.

Ex vivo Rezafungin Adsorption and Clearance During Continuous Renal Replacement Therapy

BACKGROUND/AIMS

To determine adsorption and transmembrane clearances (CLTM) of rezafungin, a novel long-acting echinocandin, in continuous venovenous hemofiltration (CVVH).

METHODS

A validated ex vivo bovine blood CVVH model using polysulfone and AN69 hemodiafilters was used to evaluate urea and rezafungin CLTM at 3 different ultrafiltrate flow rates. Rezafungin adsorption to the CRRT apparatus was determined for each hemodiafilter.

RESULTS

The sieving coefficient (SC) from CVVH with 3 different ultrafiltrate flow rates was 0 for both HF1400 and Multiflow-150 hemodiafilters, while urea SC was approximately 1 at all flow rates. Hemodiafilter type and ultrafiltrate flow rate did not influence CLTM. Rezafungin adsorption to the CVVH apparatus was not observed for either hemodiafilter.

CONCLUSION

Rezafungin is not removed by CVVH by membrane adsorption or via CLTM. Ultrafiltrate flow rates and hemodiafilter types are unlikely to influence rezafungin CLTM. No dosage adjustment of rezafungin is likely required for critically ill patients receiving CVVH.

Ultrafiltration Rate as a Dose Surrogate in Pre-Dilution Hemofiltration

For critically ill patients treated with continuous hemofiltration (HF), doses recently shown to improve survival can usually be achieved only in the pre-dilution mode. However, use of the pre-dilution mode results in reduced treatment efficiency, relative to post-dilution at the same ultrafiltration rate (Qf) and blood flow rate (Qb). The objective of this study is to determine the effect of Qf on removal parameters for solutes over a wide molecular weight spectrum in pre-dilution HF. Experiments were performed in an isovolemic, plasma-based pre-dilution system with Qb=200 ml/min. Removal parameters were measured for a 1.2 m2 polysulfone hemofilter (HF1200, Minntech) at Qf values of 20, 40, and 60 ml/min, corresponding to 17, 34 and 51 ml/h/kg for a 70 kg patient (N=3 hemofilters for each Qf). Clearance of urea and creatinine (small solute surrogates) was derived from plasma and ultrafiltrate concentrations at 30, 60, 120, 180, and 240 min while clearance of vancomycin and inulin (middle molecule surrogates) was estimated from changes in plasma concentrations over time. In addition, the sieving coefficient (SC) of vancomycin and inulin was measured at the same time points and at baseline (T=0 min). Our findings indicate pre-dilution had a predictable effect on clearance for each solute, as clearance increased linearly with Qf. Sieving coefficient values were not significantly influenced by either Qf or time and the equivalence of SC values in the middle molecule range suggest attenuation of secondary membrane effects. These data indicate filter performance can largely be preserved despite high Qf values by use of pre-dilution. Moreover, Qf appears to be a reasonable dose surrogate in pre-dilution HF.

Enhancing dialyser clearance-from target to development

Products of metabolism accumulate in kidney failure and potentially have toxic effects. Traditionally these uraemic toxins are classified as small, middle-sized and protein-bound toxins, and clearance during dialysis is affected by diffusion, convection and adsorption. As current dialysis practice effectively clears small solutes, increasing evidence supports a toxic effect for middle-sized and protein-bound toxins. Therefore, newer approaches to standard dialysis practice are required to look beyond urea clearance. Current dialysers have been developed to effectively clear small solutes and secondly to increase middle-sized toxin clearances. However, there is no ideal dialyser which can effectively clear all uraemic toxins. Advances in nanotechnology have led to improvements in manufacturing, with the production of smoother membrane surfaces and uniformity of pore size. The introduction of haemodiafiltration has led to changes in dialyser design to improve convective clearances. Both diffusional and convectional clearances can be increased by changing dialyser designs to alter blood and dialysate flows, and novel dialyser designs using microfluidics offer more efficient solute clearances. Adjusting surface hydrophilicity and charge alter adsorptive properties, and greater clearance of protein-bound toxins can be achieved by adding carbon or other absorptive monoliths into the circuit or by developing composite dialyser membranes. Other strategies to increase protein-bound toxins clearances have centred on disrupting binding and so displacing toxins from proteins. Just as the hollow fibre design replaced the flat plate dialyser, we are now entering a new era of dialyser designs aimed to increase the spectrum of uraemic toxins which can be cleared by dialysis.

Clinical Validation of Therapeutic Drug Monitoring of Imipenem in Spent Effluent in Critically Ill Patients Receiving Continuous Renal Replacement Therapy: A Pilot Study

Objectives

The primary objective of this pilot study was to investigate whether the therapeutic drug monitoring of imipenem could be performed with spent effluent instead of blood sampling collected from critically ill patients under continuous renal replacement therapy.

Methods

A prospective open-label study was conducted in a real clinical setting. Both blood and effluent samples were collected pairwise before imipenem administration and 0.5, 1, 1.5, 2, 3, 4, 6, and 8 h after imipenem administration. Plasma and effluent imipenem concentrations were determined by reversed-phase high-performance liquid chromatography with ultraviolet detection. Pharmacokinetic and pharmacodynamic parameters of blood and effluent samples were calculated.

Results

Eighty-three paired plasma and effluent samples were obtained from 10 patients. The Pearson correlation coefficient of the imipenem concentrations in plasma and effluent was 0.950 (P<0.0001). The average plasma-to-effluent imipenem concentration ratio was 1.044 (95% confidence interval, 0.975 to 1.114) with Bland-Altman analysis. No statistically significant difference was found in the pharmacokinetic and pharmacodynamic parameters tested in paired plasma and effluent samples with Wilcoxon test.

Conclusion

Spent effluent of continuous renal replacement therapy could be used for therapeutic drug monitoring of imipenem instead of blood sampling in critically ill patients.

In vitro glucose kinetics during continuous renal replacement therapy: implications for caloric balance in critically ill patients

PURPOSE

To examine the impact of continuous renal replacement therapy (CRRT) on glucose kinetics and therefore caloric balance.

METHODS

In vitro experiments were conducted to characterize glucose kinetics in a variety of CRRT modalities and prescriptions. Additional experiments evaluated the impact of citrate anticoagulation using anti-coagulant dextrose solution A (ACD-A) on CRRT glucose movement. A formula was developed to predict the glucose delivery to/from the patient per day of CRRT, and this data was extrapolated to determine the net caloric impact of CRRT.


RESULTS

A total of 104 experiments were conducted with an overall glucose extraction coefficient of 1.04 (95% CI 1.03-1.05). CRRT-related glucose removal was directly related to effluent (dialysate and/or hemofiltration) rate and pre-filter blood glucose concentration, and inversely related to dialysis solution glucose concentration. In all modalities tested, CRRT resulted in a net negative glucose balance, with estimated caloric losses ranging between 20 kcal and 550 kcal depending on the conditions tested. The addition of ACD-A resulted in net glucose delivery in some conditions and a positive caloric balance of up to 470 kcal per day.

CONCLUSIONS

CRRT can have a significant effect on glucose balance and result in either significant daily caloric gains or losses, and this effect can be predicted based on CRRT prescription and patient characteristics. Clinicians should be aware of this potential impact when prescribing nutritional therapy to patients undergoing CRRT, as an imbalance in caloric feeding can adversely affect outcomes in critically ill patients.

Proteomic investigations on the effect of different membrane materials on blood protein adsorption during haemodialysis

BACKGROUND

During haemodialysis procedure, the contact of blood with the membrane material contained in the hemodialyser results in protein deposition and adsorption, and surface-adsorbed proteins may trigger a variety of biological pathways with potential pathophysiologic consequences. The present work was undertaken to examine for protein adsorption capacity of two membranes used for clinical haemodialysis, namely cellulose triacetate (a derivatized cellulosic membrane) and the synthetic polymer polysulfone-based helixone.

MATERIALS AND METHODS

We performed a prospective cross-over study in chronic haemodialysis patients, routinely treated with a cellulose triacetate dialyser (n=3) or with a helixone dialyser (n=3). Dialysers from each patient were obtained after dialysis session, and flushed with a litre of saline to remove residual blood. Adsorbed proteins were then eluted by a strong chaotropic buffer. Patients were next switched to the other membrane dialyser for four weeks, at the end of this period protein adsorption being evaluated again. After silver staining, expression profile protein of the two groups was analyzed by 2-DE gels, analyzed and identified by Peptide Mass-finger printing and MALDI-TOF-MS/MS sequency. Moreover nanoLC-MS/MS shotgun profiling was pursued using a semi-quantitative label free approach by emPAI data analysis.

RESULTS

A total of 54 differentially expressed proteins were identified: 22 proteins more concentrated in helixone membrane (predominantly low abundant plasma proteins) and 32 in cellulose triacetate (most represented by high abundant plasma proteins). The difference proved to be related to membrane material and not to patient's characteristics.

DISCUSSION

Proteomic techniques represent a useful approach for the investigation of proteins surface-adsorbed onto a haemodialysis membrane, and can also be applied for critical assessment to compare efficiencies of different dialyser membrane materials in the adsorption of plasma proteins.

Effect of hemodialysis and hemofiltration on plasma C.E.R.A. concentrations

C.E.R.A., a continuous erythropoietin receptor activator, is a long-acting erythropoiesis-stimulating agent approved for the treatment of anemia in patients with chronic kidney disease. Although the large molecular weight and the carbohydrate chain make it unlikely that C.E.R.A. could be removed during hemodialysis or hemofiltration, no such data have been published. In vitro studies were performed to assess the removal of C.E.R.A. during hemodialysis and hemofiltration, using both low-flux and high-flux membranes and parameters very similar to those used in clinical practice. Clinical pharmacokinetic studies of plasma C.E.R.A. concentrations in patients undergoing hemodialysis were also performed following subcutaneous injection of C.E.R.A. In the in vitro studies, plasma C.E.R.A. concentrations were not significantly different from baseline values in the primed blood reservoir over a 4-hour period during hemodialysis (P = 0.12). C.E.R.A. concentrations in the plasma obtained from the venous end of the hemofilter increased proportionally with the plasma total protein concentrations, reflecting the consequence of hemoconcentration and suggesting that C.E.R.A and plasma total proteins were retained by hemofiltration membranes to a similar degree. These in vitro studies showed that C.E.R.A. was not removed by simulated hemodialysis or hemofiltration either via transmembrane transport or adsorption to the membrane. The results were corroborated by the clinical pharmacokinetic data, which showed no detectable changes in plasma C.E.R.A. concentrations during hemodialysis using either low-flux or high-flux dialyzers. These results suggest that C.E.R.A. can be administered to patients at any time during hemodialysis or hemofiltration without appreciable loss in the extracorporeal circuit.

Longitudinal hemodiafilter performance in modeled continuous renal replacement therapy

BACKGROUND/AIMS

With advanced anticoagulation, many institutions operate continuous renal replacement therapy (CRRT) circuits longer than manufacturers' recommendations. This extended use may change hemodiafilter performance and clearance properties. However, hemodiafilter performance over time has not been assessed. We investigated solute clearance over time in modeled CRRT.

METHODS

In vitro continuous hemofiltration (CH) and continuous hemodialysis (CD) were operated for 48 h using AN69 polyacrylonitrile, cellulose triacetate, F70 polysulfone, and Optiflux F160NR polysulfone hemodiafilters with citrated bovine blood. Urea, creatinine, gentamicin, vancomycin, and albumin clearances were assessed in CH (ultrafiltration rates = 1 and 3 l/h). Clearances of urea, creatinine, gentamicin, and albumin, were assessed in CD with dialysate flow rate of 2 l/h.

RESULTS

Solute CH clearances were significantly higher at 3 l/h. Only creatinine and gentamicin clearances were affected by time. Creatinine CD clearance significantly declined at 48 h for all hemodiafilters, especially polysulfone hemodiafilters.

CONCLUSIONS

CRRT duration affects solute transmembrane clearance. Clinicians should consider hemodiafilter age when assessing hemodialysis dose or drug clearance.

A novel alginate hollow fiber bioreactor process for cellular therapy applications

Gel-matrix culture environments provide tissue engineering scaffolds and cues that guide cell differentiation. For many cellular therapy applications such as for the production of islet-like clusters to treat Type 1 diabetes, the need for large-scale production can be anticipated. The throughput of the commonly used nozzle-based devices for cell encapsulation is limited by the rate of droplet formation to approximately 0.5 L/h. This work describes a novel process for larger-scale batch immobilization of mammalian cells in alginate-filled hollow fiber bioreactors (AHFBRs). A methodology was developed whereby (1) alginate obstruction of the intra-capillary space medium flow was negligible, (2) extra-capillary alginate gelling was complete and (3) 83 +/- 4% of the cells seeded and immobilized were recovered from the bioreactor. Chinese hamster ovary (CHO) cells were used as a model aggregate-forming cell line that grew from mostly single cells to pancreatic islet-sized spheroids in 8 days of AHFBR culture. CHO cell growth and metabolic rates in the AHFBR were comparable to small-scale alginate slab controls. Then, the process was applied successfully to the culture of primary neonatal pancreatic porcine cells, without significant differences in cell viability compared with slab controls. As expected, alginate-immobilized culture in the AHFBR increased the insulin content of these cells compared with suspension culture. The AHFBR process could be refined by adding matrix components or adapted to other reversible gels and cell types, providing a practical means for gel-matrix assisted cultures for cellular therapy.

Etanercept clearance during an in vitro model of continuous venovenous hemofiltration

BACKGROUND/AIMS

Etanercept is a tumor necrosis factor-alpha antagonist used in inflammation-mediated conditions. Continuous venovenous hemofiltration (CVVH) has also been used in patients with inflammatory conditions. This study evaluated etanercept clearance using an in vitro CVVH model.

METHODS

Etanercept clearance was assessed in vitro in bovine blood at 1-3 mg/l final serum concentration, and urea control at 750 mg/l. CVVH was performed using polyacrylonitrile, polysulfone, and polymethylmethacrylate filters at 3 l/h ultrafiltrate and 200 ml/min blood flow rates. Transmembrane clearance was estimated using sieving coefficient calculations, and adsorptive removal rate was approximated using a mass balance calculation.

RESULTS

Urea sieving coefficient remained constant (1.04 +/- 0.01). Ultrafiltrate etanercept concentrations were undetectable (sieving coefficient < 0.02) and transmembrane and adsorptive clearances were negligible.

CONCLUSION

Etanercept is not cleared appreciably by transmembrane or adsorptive mechanisms in CVVH using polyacrylonitrile, polysulfone, or polymethylmethacrylate hemofilters.

Specific adsorption of some complement activation proteins to polysulfone dialysis membranes during hemodialysis

Dialyser bioincompatibility is an important factor contributing to complications of hemodialysis with well known systemic consequences. Here we studied the local processes that occur on dialysis membranes by eluting proteins adsorbed to the polysulfone dialyser membranes of 5 patients after 3 consecutive routine maintenance hemodialysis sessions. At the end of each procedure, a plasma sample was also collected. These eluates and their accompanying plasma samples were separated by 2-dimensional gel electrophoresis; all proteins that were present in all patients were analyzed by tandem mass spectrometry; and a ratio of the relative spot intensity of the eluate to plasma was calculated. Of 153 proteins detected, 84 were found in all patients, 57 of which were successfully identified by mass spectrometry as 38 components of 23 unique proteins. In 10 spots the relative eluate intensity differed significantly from that in the plasma, implying preferential adsorption. These proteins included ficolin-2, clusterin, complement C3c fragment, and apolipoprotein A1. Our finding of a selective binding of ficolin-2 to polysulfone membranes suggests a possible role of the lectin complement pathway in blood-dialyser interactions.

Impact of hemodialysis therapy on hepatitis C virus infection: a deeper insight

Hepatitis C Virus (HCV) infection remains prevalent in patients receiving regular dialysis all over the world. The adverse impact of anti-HCV serologic status on mortality in the dialysis population has been documented. Antiviral therapy for hepatitis C in chronic kidney disease (CKD) patients, including the dialysis population, is still unsatisfactory. Several findings support a different course of HCV in dialysis patients versus the non-uremic population. The HCV viral load appears lower in hemodialysis patients with HCV despite the immune compromise caused by chronic uremia; the histologic abnormalities seem milder, and a severe clinical course of chronic hepatitis C is unusual in most hemodialysis (HD) patients. It appears that the HD procedure per se can preserve patients from an aggressive course of HCV by reducing the viral load (HCV RNA). The mechanisms by which the HD procedure lowers HCV viremia remain largely speculative: the passage of viral particles into the dialysate, the trapping of the virus on the surface of the dialyzer membrane, and an indirect host-mediated mechanism have been cited. The latter hypothesis implicates the production of interferon-alpha, hepatocyte growth factor, or other cytokines provided with antiviral activities during the hemodialysis sessions. Clinical trials aimed at clarifying this issue are under way.

Operational characteristics of continuous renal replacement modalities used for critically ill patients with acute kidney injury

Renal replacement therapy (RRT) is required in a significant percentage of patients developing acute kidney injury (AKI) in an intensive care unit (ICU) setting. One of the foremost objectives of continuous renal replacement therapy (CRRT) is the removal of excess fluid and blood solutes that are retained as a consequence of decreased or absent glomerular filtration. Because prescription of CRRT requires goals to be set with regard to the rate and extent of both solute and fluid removal, a thorough understanding of the mechanisms by which solute and fluid removal occurs during CRRT is necessary. The following provides an overview of solute and water transfer during CRRT and this information is placed in the appropriate clinical context with a discussion of recent clinical trials assessing the relationship between CRRT dose and patient survival. Moreover, the differences between solute removal in CRRT and other dialysis modalities, especially sustained low-efficiency dialysis (SLED) and extended daily dialysis (EDD), along with the potential clinical implications are discussed.

Resolving controversies regarding hemodiafiltration versus hemodialysis: the Dutch Convective Transport Study

Hemodialysis patients suffer from a high incidence of cardiovascular disease. Among the many predisposing factors, such as high blood pressure, dyslipidemia, and fluid overload, the accumulation of high molecular weight uremic toxins, the so-called middle molecules, may play an important role. Since convective therapies such as online hemodiafiltration have a better clearance profile for these compounds than standard hemodialysis, it has been suggested that these dialysis strategies may reduce cardiovascular morbidity and mortality. As reliable data on these issues are not available, the Dutch Convective Transport Study (CONTRAST) was recently initiated. This prospective randomized trial was designed to compare online hemodiafiltration with low-flux hemodialysis with respect to cardiovascular morbidity and mortality.

Intravenous iron-gluconate during haemodialysis modifies plasma β2-microglobulin properties and levels

BACKGROUND

Intravenous iron replacement therapy is routinely used for correction of anaemia in patients with end-stage renal failure. Free or labile iron, present both in parenteral iron formulations and in blood of haemodialysis (HD) patients, has the potential to induce severe oxidative processes. This study evaluated the acute in vivo effect of intravenous iron administration on the oxidation of plasma beta2-microglobulin (beta2m) and on its plasma levels after HD.

METHODS

Iron-gluconate was administered intravenously to 14 patients receiving HD with low-flux cellulose-triacetate membranes during the first hour of the 4 h HD treatment. Each patient underwent three different dialysis treatments, during which an infusion of 62.5, 125 or 0 mg (control) of iron-gluconate was administered in random order. Plasma beta2m levels and iron parameters were monitored immediately before and after each HD treatment. The molecular isoforms of beta2m were studied by two-dimensional gel electrophoresis and western analysis. Levels of oxidized beta2m were evaluated by reaction with 2,4-dinitrophenylhydrazine and western analysis.

RESULTS

Both doses of iron-gluconate caused remarkable changes in the molecular properties of beta2m, including shift in isoelectric point, molecular mass and degree of oxidation. Iron administration also limited the decline in plasma beta2m levels to <7.5%, compared with 27.9+/-2.7% during HD without iron.

CONCLUSIONS

Intravenous iron-gluconate led to a characteristic increase in molecular weight and in negative charge of beta2m, both of which can be assumed to be consistent with reduced membrane sieving coefficients and membrane adsorption, and thus with reduced clearance of beta2m.

Removal of the protein-bound solute p-cresol by convective transport: a randomized crossover study

BACKGROUND

Middle molecules (MMs) and protein-bound solutes are poorly removed by conventional hemodialysis (HD). Hemodiafiltration (HDF) combines convection and diffusion in a single therapy and has been shown to be superior for the elimination of several small and larger retention molecules. Information on the elimination of protein-bound solutes during convective strategies is scarce. The primary aim of this randomized crossover study is to evaluate the influence of internal filtration, postdilution HDF, and predilution HDF on removal of the protein-bound solute p-cresol.

METHODS

Fourteen stable patients on regular thrice-weekly medium-flux HD therapy were assigned to 5 treatment periods of 2 weeks each for: HD with HF80(S); HD with FX80; postdilution HDF, 20 L, with FX80; predilution HDF, 20 L, with FX80; and predilution HDF, 60 L, with FX80 (all dialyzers from Fresenius Medical Care, Bad Homburg, Germany).

RESULTS

As for the water-soluble solutes, elimination of p-cresol was better during HDF and increased with greater filtration volumes. Removal of beta2-microglobulin (beta2m) also was enhanced during HDF. However, the positive effect of convection was offset when high predilution substitution volumes were applied, probably as a result of dilution. Within each dialytic approach, removal of the MM beta2m and the protein-bound solute p-cresol was significantly less than that of the water-soluble molecules urea nitrogen and creatinine.

CONCLUSION

Our data indicate that convection can provide superior protein-bound solute removal compared with high-flux HD. Our findings also suggest that better elimination of the unbound fraction is the most plausible underlying mechanism.

Polypropylene hollow fiber for cells isolation: methods for evaluation of diffusive transport and quality of cells encapsulation

Formulation of membrane properties is important prior the successful implantation of encapsulated cells producing therapeutically relevant compounds. The purpose of our study was to specify the methods allowing preliminary evaluation of hollow fibers (HF) chosen for immunoisolation. We have selected as estimates (1) diffusive permeability for small and large solutes, and HF cut off (in vitro), (2) histological evaluation of tissue overgrowth after sc. implantation into mice. It was found that diffusive coefficients were linearly dependent on the particle diameter except that of albumin (2-3 times higher than theoretically estimated). This discrepancy imply that for certain particles the interaction with membrane material may be significant. The histological evaluation showed that siliconized HF implanted for 105 days were accepted (there was thin fibrotic layer on the external surface of the HF, no surrounding haemopoietic cells were found). It is concluded that proposed methods for preliminary evaluation of hollow fibers chosen for immunoisolation seems to be reliable and suitable for testing diffusive permeability of each relevant cell product.

The Acute Dialysis Quality Initiative--part IV: membranes for CRRT

The extracorporeal membrane used in a continuous renal replacement therapy (CRRT) for the treatment of a critically ill patient with acute renal failure (ARF) is vitally important for several reasons, including its influence on biocompatibility and filter performance. The clinical relevance of membrane-related biocompatibility markers traditionally used in chronic hemodialysis remains unclear in CRRT. Numerous approaches may be used to assess membrane and filter performance in CRRT, but no specific methodology is accepted widely at present. Although a potential benefit of certain membranes used for CRRT is adsorptive removal of inflammatory mediators, this issue has not been assessed carefully in well-designed clinical trials. These and other issues should be the subject of future clinical research efforts.

Properties of membranes used for hemodialysis therapy

Recent trends show a progressive increase in the use of modified cellulosic and synthetic dialyzers and a corresponding decrease in the utilization rate of unmodified cellulosic dialyzers. The purpose of this article is to describe current membrane and dialyzer technology, with the focus almost entirely on modified cellulosic and synthetic membranes. A general overview of membrane-related determinants of dialyzer performance is first presented, followed by a discussion of specific characteristics of some of the more commonly used membranes and dialyzers.

Albumin loss in on-line hemodiafiltration

BACKGROUND

Based on the increased hydraulic permeability of the new high permeability polyethersulfone membrane, DIAPES HF-800, we investigated the kinetics and handling of albumin in high volume on-line hemodiafiltration (HDF).

METHODS

Seven patients on predilutional HDF were studied in two consecutive sessions. Blood flow rate and transmembrane pressure were continuously monitored. Spent dialysate was spilled at 20 ml/h every hour. Albumin was measured in blood and dialysate by immunonephelometry. Albumin and proteins adsorbed onto the dialyzer membrane were eluted after treatment with Triton X. Ultrafiltrates collected at 1 and 2 hours of treatment were pooled from different patients and incubated for 24 hours at 37 degrees C with bovine serum albumin (BSA). Total sulphydryl groups were evaluated using Ellmann's reagent [5, 5'-dithio-bis(2-nitrobenzoic acid)].

RESULTS

In all 7 patients, the total loss of albumin was 3.99 +/- 1.81 g, ranging between 1.09 and 6.82 g/session. Most albumin loss occurred in the first 60 min of pre-dilutional hemodiafiltration (1.92+0.83 g). There was no correlation between transmembrane pressure, urea clearance and the loss of albumin. Plasma water urea clearance values were stable over the treatment (234 +/- 14.3 ml/min). Plasma albumin concentration did not decrease during HDF sessions. Albumin adsorbed onto the dialyzers was 0.7 +/- 1.6 mg but the total amount of adsorbed proteins was much higher (130 + 90 mg). In addition, the ultrafiltrate collected during HDF sessions was able to induce oxidation of bovine serum albumin as measured by total protein sulfhydryl groups: bovine serum albumin incubated in the presence of ultrafiltrate collected at 1 hour had a sulfhydryl loss of 56.3 +/- 5.7% (p < 0.0001 vs control), and bovine serum albumin incubated with ultrafiltrate collected at 2 hours had a loss of 67.5 +/- 3.8% (p < 0.003 vs control).

CONCLUSION

The present study shows the high inter- and intra-patient variability of transmembrane passage of albumin in chronically uremic patients undergoing pre-dilutional HDF. Factors involved do not seem to be correlated to transmembrane pressure but rather to an interaction with the polymer surface. Albumin adsorption was minimal and was significantly lower than that of other plasma proteins. Albumin loss during HDF seemed to have no acute impact on plasma albumin. In addition, we demonstrated the presence of prooxidative compounds able to oxidize albumin, of which extracorporeal removal by HDF procedure could be beneficial for HD patients.

Influence of the charge of low molecular weight proteins on their efficacy of filtration and/or adsorption on dialysis membranes with different intrinsic properties

Hemodialysis membranes eliminate by filtration low-molecular-weight toxic metabolites (urea and creatinine) with minimum interactions between blood components and the membrane itself. However, the ability of a membrane to adsorb specific proteins could be beneficial if the accumulation of these same proteins is implicated in the genesis of a pathological condition. Beta-amyloidosis which accompanies the elevation of beta2-microglobulin (11.8 kDa) in the plasma of dialysed patients is one such condition (Biochem. Biophys. Res. Commun. 129 (3) (1985) 701-706: Lancet 1 (1986) 1240-1311). To determine whether increases in plasma beta2-microglobulin levels were due to differences in filtration efficacy of the membrane used and/or to certain characteristics of this protein, e.g. its charge (pI 5.7) the adsorption and filtration of [3H] beta2-microglobulin and [3H] lysozyme of similar MW 14.5 kDa, but pI: 10.8 were compared on different membranes. It was found that, neither [3H] beta2-microglobulin nor [3H] lysozyme are removed by cuprophan, whereas over 75% of beta2-microglobulin is removed by filtration on polyacrylonitrile, polyacrylonitrile-polyethyleneimine, polysulfone and >95% by adsorption to polymethylmethacrylate-BK. For lysozyme, removal by adsorption is >95% on polyacrylonitrile and polyacrylonitrile-polyethyleneimine, 72% on polymethylmethacrylate-BK and by filtration is 95% on polysulfone. Hemodialysis membranes must therefore not simply be considered as filters of low-molecular-weight metabolites but should be equally assessed for their capacity to eliminate potentially deleterious low-molecular-weight plasma proteins.

Low-Molecular Weight Proteins in End-Stage Renal Disease: Potential Toxicity and Dialytic Removal Mechanisms

ABSTRACT. Low-molecular-weight proteins (LMWP) are now recognized as a distinct class of uremic toxins, and numerous compounds in this category have been identified. Dr. Henderson has spent much of his career investigating ways to enhance the removal of intermediate- and large-sized uremic retention molecules. As LMWP clearly fall under this category, it is fitting to provide a review of several aspects of this molecular class. Normal renal metabolism of LMWP is discussed along with the changes that occur during chronic renal insufficiency. The effect of end-stage renal disease on plasma LMWP concentrations is assessed. As examples of the potential uremic toxicity of this molecular class, leptin, adrenomedullin, and the compounds associated with increased susceptibility to infection are highlighted. Finally, an overview of LMWP removal mechanisms for both hemodialysis and the convective therapies is provided.

Quantification of middle molecular weight solute removal in dialysis

The pioneering work of Gotch emphasized the critical need to be quantitative with respect to treatment prescription. Through his meticulous derivations and analyses regarding Kt/V(urea), he has provided powerful insight into the standard therapy prescriptions that we now employ clinically. However, time has seen the proliferation of treatment techniques, most of which are too "young" to have been characterized with respect to clinical outcomes. Further, the relationship between removal of urea and removal of middle molecular size solutes associated with these newer techniques deviates from that associated with conventional, clinically qualified techniques. In this article we examine the solute clearance profile of some of these new methodologies and their relationship to current criteria for treatment adequacy. Our approach is to discuss components of the overall transport process and then utilize modeling of surrogate molecules over the size range of interest whose transport characteristics are known. Alteration in the solute clearance profile of these surrogate markers in response to changes in prescription variables will thus offer insight into the spectrum of toxic middle molecules that are removed when size, space of distribution, and generation rate are known.

Intradialytic removal of protein-bound uraemic toxins: role of solute characteristics and of dialyser membrane

BACKGROUND

The efficiency of dialysis membranes is generally evaluated by assessing their capacity to remove small, water-soluble and non-protein-bound reference markers such as urea or creatinine. However, recent data suggest that protein-bound and/or lipophilic substances might be responsible for biochemical alterations characterizing the uraemic syndrome.

METHODS

In the present study, the total concentrations of four uraemic retention compounds (indoxyl sulphate, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF) and p-cresol) and of tryptophan, the only protein-bound amino acid and a precursor of indoxyl sulphate, were compared with those of urea and creatinine in pre- and post-dialysis serum and in dialysate of 10 patients; two high-flux (HF) membranes (cellulose triacetate (CTA) and polysulphone (PS)) and a low-flux polysulphone (LFPS) membrane were compared in a crossover design, using HPLC.

RESULTS

Except for hippuric acid (67.3+/-17.5% decrease), major differences were found in the percentage removal of the classical uraemic markers on one hand (creatinine 66.6+/-7.0% and urea 75.5+/-5.8% decrease) and the studied protein-bound and/or lipophilic substances on the other (indoxyl sulphate, 35.4+/-15.3% and p-cresol 29.0+/-14.2% decrease; tryptophan, 27.5+/-40.3%, and CMPF, 22.4+/-17.5% increase; P<0.01 vs urea and creatinine in all cases). Hippuric acid removal was more pronounced than that of the remaining protein-bound compounds (P<0. 01). After correction for haemoconcentration, per cent increase of tryptophan and CMPF was less substantial, while per cent negative changes for the remaining compounds became more important. There was a correlation between creatinine and urea per cent removal at min 240 (r=0.51, P<0.01), but all the other compounds showed no significant correlation with either of these two. The three membranes were similar regarding the changes of total solute concentrations from the start to the end of dialysis.

CONCLUSIONS

Urea and creatinine are far more efficiently removed than the other compounds under study, except for hippuric acid. There are no striking differences between the HF membranes. Moreover, compared with the LF membrane these HF membranes do not appear to be superior in removing the studied compounds.

Effect of membrane composition and structure on solute removal and biocompatibility in hemodialysis

Effect of membrane composition and structure on solute removal and biocompatibility in hemodialysis. Significant changes in extracorporeal membranes have occurred over the past five decades in which hemodialysis (HD) has been available as a therapy for both acute renal failure (ARF) and end-stage renal disease (ESRD). For cellulosic membranes, these changes have included a reduction in thickness, hydroxyl group substitution, and an increase in pore size. These modifications have resulted in enhanced efficiency of small solute removal, a broader spectrum of overall solute removal, and an attenuation of complement activation in comparison to the thick, unsubstituted cellulosic membranes of low permeability used in the early days of HD therapy. Synthetic membranes, originally developed specifically for use in high-flux HD and hemofiltration, have also evolved during this same time period. In fact, the initially clear distinction between low-flux regenerated cellulosic and high-flux synthetic membranes has become blurred, as membrane formulators have developed products designed to appeal to enthusiasts for both membrane formats. The purpose of this review is to characterize both the solute removal and biocompatibility characteristics of dialysis membranes according to their composition (that is, polymeric makeup) and structure. In this regard, the manner in which membrane biocompatibility interacts with flux is highlighted.

Diffusive and convective solute clearances during continuous renal replacement therapy at various dialysate and ultrafiltration flow rates

Clearances of several solutes (urea, creatinine, phosphate, urates, beta(2)-microglobulin [beta(2)-M]) were measured during venovenous continuous renal replacement therapy (CRRT) at various ultrafiltration (Q(UF); 0 to 2 L/h) and dialysate flow rates (Q(D); 0 to 2.5 L/h). Preset Multiflow-60 and Multiflow-100 hollow-fiber dialysers (M-60 and M-100; Hospal-Gambro, St-Leonard, Canada) were compared (five patients for each type). First, we evaluated the impact of predilution on convective clearances: a progressive decrease in patient clearances, similar for both filters, was observed, reaching a maximum of 15%, 18%, and 19% for urea, urates, and creatinine, respectively, with predilution at a Q(UF) of 2 L/h. Second, we compared convective and diffusive clearances. Because effluent to plasma ratio (E/P) remained at 1 for small solutes (urea, creatinine, phosphate, urates) during convection, clearances were equal to the effluent rate for both dialyzers. However, we observed greater diffusive clearances for small molecules with M-100 than with M-60 at a Q(D) of 1.5 to 2.5 L/h, the difference being more significant as molecular weight increased. For beta(2)-M, diffusive clearance was very low and rapidly reached a plateau of 8 and 12 mL/min for M-60 and M-100, respectively, at a Q(D) greater than 1.5 L/h. Convective clearances for beta(2)-M increased nonlinearly up to 20 +/- 2 mL/min at a progressively greater Q(UF) (from 0.5 to 2 L/h) for both M-60 and M-100. This nonlinear increase was attributed to an increase of almost 40% in E/P for beta(2)-M from a Q(UF) of 0.5 to 2 L/h. Third, the interaction between convection and diffusion was assessed by measuring solute clearances at a fixed Q(UF) (1 and 2 L/h) and variable Q(D) (0.5 to 2.5 L/h). For small molecules, no significant interaction between convection and diffusion was noticed with M-100, whereas only a small interaction was noticed with M-60. However, for beta(2)-M, the addition of diffusion (Q(D), 0.5 to 2.5 L/h) did not result in any significant increase in total clearances over convective clearances for M-60 and M-100. This observation suggests that the diffusive clearances for beta(2)-M observed with M-60 and M-100 at a Q(UF) of 0 L/h and at various Q(D) probably occurs by convective fluxes across the membrane. These results demonstrate that convection is more efficient than diffusion in removing mixed-molecular-weight solutes during CRRT.

Dialyzer-dependent changes in solute and water permeability with bleach reprocessing

The effects of bleach reprocessing on the performance of high-flux dialyzers have not been comprehensively characterized. We compared the effects of automated bleach/formaldehyde reprocessing on solute and hydraulic permeability for cellulose triacetate (CT190) and polysulfone (F80B) dialyzers using an in vitro model. Dialyzers were studied after initial blood exposure (R0) and after 1 (R1), 5 (R5), 10 (R10), and 15 (R15) reuse cycles. Ultrafiltration coefficient (K(uf)), serial clearances, and/or sieving coefficients (SCs) of urea, creatinine, vancomycin, inulin, myoglobin, and albumin were determined. Urea, creatinine, and vancomycin clearances and SCs did not significantly differ from R0 to R15 with either dialyzer. Inulin clearances and SC also did not significantly change from R0 to R15 for the CT190. However, these same values for the F80B significantly increased (P < 0.05). The inulin clearance and SC values for the CT190 dialyzer were significantly higher than those for the F80B at all stages except R15. Myoglobin clearances significantly increased over 15 reuses for both dialyzers (P < 0.01). However, CT190 myoglobin clearances were significantly higher at all stages (R0 = 37.7 +/- 9.7; R15 = 52.5 +/- 8.8 mL/min) than the F80B (R0 = negligible; R15 = 41.3 +/- 16.5 mL/min; P < 0.01). Albumin pre- and postdialysis SCs significantly increased for both dialyzers (P < 0.01). K(uf) for R0 and R15 were 52.3 +/- 3.3 and 52.6 +/- 7.6 mL/h/mm Hg for CT190 (P = not significant) and 48.8 +/- 4.4 and 87.3 +/- 7.0 mL/h/mm Hg for F80B (P < 0.0001). We conclude that bleach reprocessing significantly increases larger solute and hydraulic permeability of high-flux cellulosic and polysulfone dialyzers. This effect is more pronounced for the polysulfone membrane. Until 10 reuses or greater, the removal of solutes greater than 1,500 d is significantly compromised with the polysulfone dialyzer used in this study.

Hemodialysis Systems for Intermittent, Semi-Continuous, and Continuous Therapies in Acute Renal Failure

As is the case in end-stage renal disease (ESRD), both intermittent and continuous renal replacement therapies (RRTs) are employed in acute renal failure (ARF). In fact, a continuum of treatment options is available in ARF. At one end of the ARF RRT spectrum is conventional intermittent hemodialysis (IHD), in which relatively high blood and dialysate flow rates are used (typically ≥250 and 500 mL/min, respectively). Continuous renal replacement therapies (CRRTs), which employ much lower flow rates, comprise the other end of the spectrum. Finally, hybrid therapies, which combine characteristics of both IHD and CRRT, have recently been described. These therapies' removal mechanisms for solutes over a broad molecular weight range are discussed. An understanding of these mechanisms is important when determining the amount of therapy that can be provided by any RRTs. Additional studies are required to improve the understanding of solute removal by the various RRT used in ARF.

Dialysis membrane adsorption during CRRT

Experimental and clinical studies have suggested that dialysis membrane biocompatibility may influence the morbidity and mortality of patients with acute renal failure. Complement activation by dialysis membranes may also prolong the recovery from acute renal failure. In this article, we review the concept of dialysis membrane adsorption, with particular attention to adsorption/inhibition of factor D, a highly specific serine protease of the alternative pathway of complement. The adsorptive properties of some dialysis membranes may be useful during continuous renal replacement therapies (CRRT) in critically ill patients.

Acute renal failure: role of dialysis membrane biocompatibility

Recent clinical studies of acute renal failure in adults have focused attention on the biocompatibility of the dialysis membrane as a possible factor influencing patient morbidity and mortality. In this article, we review the concept of dialysis membrane biocompatibility and highlight the difficulty of finding an ideal definition. We then expand on the possible roles of complement and neutrophil activation by dialysis membranes, which may prolong the recovery from acute renal failure. The results of several clinical studies analyzing the impact of dialysis membranes on the course and outcome of acute renal failure are discussed. Finally, the possible relevance of biocompatibility in continuous renal replacement therapies is emphasized.