Internal Hemodiafiltration (iHDF): A Possible Option to Expand Hemodiafiltration Therapy

Disruptive technologies for hemodialysis: medium and high cutoff membranes. Is the future now?

In the past decade, a new class of hemodialysis (HD) membranes (high retention onset class) became available for clinical use. The high cutoff (HCO) and the medium cutoff (MCO) membranes have wider pores and more uniformity in pore size, allowing an increased clearance of uremic toxins. Owing to the mechanism of backfiltration/internal filtration, middle molecules are dragged by the convective forces, and no substitution solution is needed. The HCO dialyzer is applied in septic patients with acute kidney injury requiring continuous kidney replacement therapy. The immune response is modulated thanks to the removal of inflammatory mediators. Another current application for the HCO dialyzer is in hematology, for patients on HD secondary to myeloma-kidney, since free light chains are more efficiently removed with the HCO membrane, reducing their deleterious effect on the renal tubules. In its turn, the MCO dialyzer is used for maintenance HD patients. A myriad of clinical trials published in the last three years consistently demonstrates the ability of this membrane to remove uremic toxins more efficiently than the high-flux membrane, an evolutionary disruption in the HD standard of care. Safety concerns regarding albumin loss as well as blood contamination from pyrogens in the dialysate have been overcome. In this update article, we explore the rise of new dialysis membranes in the light of the scientific evidence that supports their use in clinical practice.

Continuous renal replacement therapy and extended indications

Extracorporeal blood purification (EBP) techniques provide support for critically ill patients with single or multiple organ dysfunction. Continuous renal replacement therapy (CRRT) is the modality of choice for kidney support for those patients and orchestrates the interactions between the different artificial organ support systems. Intensive care teams should be familiar with the concept of sequential extracorporeal therapy and plan on how to incorporate new treatment modalities into their daily practices. Importantly, scientific evidence should guide the decision-making process at the bedside and provide robust arguments to justify the costs of implementing new EBP treatments. In this narrative review, we explore the extended indications for CRRT as an adjunctive treatment to provide support for the heart, lung, liver, and immune system. We detail practicalities on how to run the treatments and how to tackle the most frequent complications regarding each of the therapies, whether applied alone or integrated. The physicochemical processes and technologies involved at the molecular level encompassing the interactions between the molecules, membranes, and resins are spotlighted. A clinical case will illustrate the timing for the initiation, maintenance, and discontinuation of EBP techniques.

Internal Hemodiafiltration versus Low-Flux Bicarbonate Dialysis: Results from a Long-Term Prospective Study

Introduction

About ten years ago it was discovered that changes in filter design which increase passive filtration improved dialysis efficiency. Later, these modified membranes showed similar intradialytic efficiency when used in on-line hemodiafiltration or in bicarbonate dialysis, called internal hemodiafiltration.

Aim and Methods

On the basis of these previous results, we studied the long-term effects of internal hemodiafiltration, in comparison with low-flux bicarbonate dialysis. The pre-dialysis beta2-microglobulin level was chosen as the primary outcome variable. A prospective multicenter study with a cross-over scheme, 2 treatments and 3 periods, was designed. Twenty-four patients, followed in two dialysis centers, were enrolled. Many other parameters were measured every month at the first dialysis session of the week. The intra-dialytic removal of urea, beta2-microglobulin and homocysteine was also calculated.

Results

The removal of uremic toxins was significantly higher in internal hemodiafiltration than in low-flux bicarbonate dialysis. The pre-dialysis value of urea, phosphorus, beta2-microglobulin and homocysteine was lower during internal hemodiafiltration as compared with low-flux bicarbonate dialysis. The mean pre-dialysis value of hemoglobin was significantly higher during internal hemodiafiltration than low-flux bicarbonate dialysis, with a trend towards a significantly lower consumption of erythropoiesis stimulating agents during internal hemodiafiltration as compared with low-flux bicarbonate dialysis.

Conclusions

Long-term treatment with internal hemodiafiltration improves the removal of small molecules and stops the continuous increase of middle molecules as seen in low-flux bicarbonate dialysis. Internal hemodiafiltration may substitute low-flux bicarbonate dialysis, but we need new prospective studies about long-term hard end-points.

Effect of blood flow rate on internal filtration in a high-flux dialyzer with polysulfone membrane

Internal filtration/backfiltration (IF/BF) of a dialyzer depends on several parameters. This study evaluated the effect of the blood flow rate (Q (B)) on the internal filtration flow rate (Q (IF)) measured using Doppler ultrasonography for a high-flux dialyzer with a polysulfone membrane, APS-15E. In an in vitro study, bovine blood was circulated through the dialyzer, at a Q (B) of 100-350 mL/min. The clearances (CL) of creatinine, β(2)-microglobulin, and α(1)-microglobulin were then investigated. Q (IF) increased with the Q (B) value. A good correlation was obtained between Q (IF) and the pressure difference between the pressures at the inlet of the blood compartment and the pressure at the outlet of the dialysate compartment. The creatinine CL values strongly depended on Q (B) because molecular diffusion was dominant. The β(2)-microglobulin CL also depended on Q (B), because its removal rate seemed to be affected by both diffusive and convective transport caused by the IF/BF. An extremely low CL value was obtained for α(1)-microglobulin because of its low diffusivity and membrane fouling induced by proteins plugging the membrane. In conclusion, the IF/BF in the dialyzer strongly depends on Q (B). Furthermore, the dependence of the solute clearance on Q (B) decreased with increasing molecular size of the solute because of the decrease in diffusivity through the membrane.

A dual-chambered hemodialyzer for convection-enhanced hemodialysis

Convective clearance during hemodialysis (HD) improves dialysis outcomes in kidney failure patients, and, thus, trials have been undertaken to increase convective mass transfer, which is directly related to internal filtration rates. The authors designed a new hemodialyzer to increase the internal filtration rates, and here describe the hemodialytic efficacy of the devised unit. The developed dual-chambered hemodialyzer (DCH) contains two separate chambers for dialysate flow within a single housing. By placing a flow restrictor on the dialysate stream between these two chambers, dialysate pressures are regulated independently. Dialysate is maintained at a higher pressure than blood pressure in one chamber, and at a lower pressure in the other chamber. The dialysis performance of the DCH was investigated using an acute canine renal failure model. Urea and creatinine reductions and albumin loss were monitored, and forward and backward filtration rates were measured. No procedurally related malfunction was encountered, and animals remained stable without any complications. Urea and creatinine reductions after 4-h dialysis treatments were 75.2 ± 6.5% and 67.7 ± 8.9%, respectively. Post-treatment total protein and albumin levels remained at pretreatment values. Total filtration volume was 4.98 ± 0.5 L over 4 h, whereas the corresponding backfiltration (BF) volume was 4.77 ± 0.6 L. The developed dual-chamber dialyzer has the benefit of providing independent control of forward filtration and BF rates. HD using this dialyzer provides a straightforward means of increasing the internal filtration and convective dose.

Middle-molecule clearance in CRRT: in vitro convection, diffusion and dialyzer area

Several studies have attempted to compare different doses and modes of therapy in continuous renal replacement therapies in critically ill patients. It is commonly asserted in the literature that convective therapies can achieve higher clearance of middle molecules than achieved by dialysis alone. However, regardless of the actual prescription, most therapies will actually contain a mixture of both diffusive and convective clearance. Molecular transport in purely convective prescriptions may be hindered by clotting and protein interactions with the dialyser. We measured middle molecule clearance using a tracer molecule, Ficoll, in citrated bovine blood. Using a 2 x 2 factorial design, we examined the impact of prescription [postdilution continuous venovenous hemofiltration (CVVH) vs. continuous venovenous hemodialysis (CVVHD)] and membrane area (0.4 m2 vs. 2.0 m2) on blood-side and dialysate-side middle-molecule clearance. In large dialysers, convective and diffusive prescriptions resulted in nearly identical middle molecule clearance from 10 to 100 kDa molecular weight. In the smaller dialyser, middle molecule clearance was higher when a diffusive therapy (CVVHD) was prescribed versus a convective therapy (postdilution CVVH). We hypothesized that high ultrafiltration rates in the smaller dialyzer resulted in a concentration polarization at the membrane that formed a prefilter, limiting middle-molecule clearance. This effect has implications for design and analysis of clinical trials of continuous renal replacement therapy (CRRT).