Newly developed economical and efficient push/pull hemodiafiltration

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.

An experimental and numerical study of the flow and mass transfer in a model of the wearable artificial kidney dialyzer

BACKGROUND

Published studies of the past decades have established that mass transfer across the dialyzer membrane is governed by diffusion, convection and osmosis. While the former is independent of the pressure in the liquids, the latter two are pressure dependent and are enhanced when the pressure difference across the membrane is increased. The goal of the present study is to examine the impact of pulsatile flow on the transport phenomena across the membrane of a high-flux dialyzer in a wearable artificial kidney (WAK) with a novel single small battery-operated pulsatile pump that drives both the blood and dialysate in a counter-phased manner, maximizing the trans-membrane pressure.

METHODS

Both in-vitro experimental and numerical tools are employed to compare the performance of the pulsatile WAK dialyzer with a traditional design of a single-channel roller blood pump together with a centrifugal pump that drives the dialysate flow. The numerical methods utilize the axisymmetric Navier-Stokes and mass transfer equations to model the flow in the fibers of the dialyzer.

RESULTS

While diffusion is still the dominating transport regime, the WAK pump enhances substantially the trans-membrane pressure and thus increases mass convection that might be as high as 30% of the overall transfer. This increase is obtained due to the design of the pulsatile WAK pump that increases ultrafiltration by increasing the trans-membrane pressure.

CONCLUSIONS

The experimental and numerical results revealed that when pumping at similar flow rates, a small battery-operated pulsatile pump provides clearances of urea and creatinine similar as or better than a large heavy AC-powered roller pump.

Convection-enhanced High-Flux Hemodialysis

Internal filtration contributes to convective clearance in high-flux hemodialysis but its contribution is limited by low pressure gradients. Therefore, a modification using a conventional dialyzer was conceived to enhance internal filtration and backfiltration (BF) rates. The modified dialyzer includes two longitudinal independent regions for blood flow, which were created by redesigning dialyzer caps. Blood pressures remained higher than dialysate pressures in one region and lower in the other region, allowing continuous internal filtration and BF in these respective regions. Modified and conventional dialyzers were compared in terms of pressure gradients and solute clearances. Thus, our experiments involved two groups: the modified dialyzer group and the conventional dialyzer group. A renal failure model was established using a dog weighing 25-30 kg by renal artery and vein ligation. With the exception of the dialyzers, experimental conditions were identical in the two groups. The pressure gradients between blood and dialysate were much higher for the modified dialyzer than for the conventional dialyzer. No significant differences were observed with respect to small solute clearances between the two groups, but mid-range solute clearances were significantly higher in the modified group. More optimization is required before the devised unit can be used clinically. However, the devised unit offers a straightforward means of regulating internal filtration and BF rates.

Future developments in the treatment of end-stage renal disease: a North American perspective

New technology for the treatment of end-stage renal disease will need to be pharmacoeconomically persuasive in reducing the life-cost of treatment to obtain entry into the market. Increased automation, with closed-loop sensing technology, will occur in the near term. Clearance-based terminology for quantifying performance of equipment will give way to direct quantification of toxin removal. Experiments on the frequency and duration of treatment will redefine what is considered to be adequate therapy in terms other than simple urea removal. Near-term changes in current vascular access technology will be driven by the current cost of access failure. Automated peritoneal dialysis will displace continuous ambulatory peritoneal dialysis, and online compounding of solution for use is likely for both hemodialysis and peritoneal dialysis. In a longer time frame these technologies will merge. Xenografting from the pig will be a reality, and gene therapy of the mesothelium will provide a more user-friendly therapy for end-stage renal disease.

Push/pull hemodiafiltration: technical aspects and clinical effectiveness

Push/pull hemodiafiltration (HDF) is characterized by alternate repetition of filtration and backfiltration during hemodialysis with high-flux membrane. In the pressure-controlled push/pull (PC P/P) HDF system, which is the newest push/pull HDF system, there are about 25 repetitions of dilution and concentration of the blood while it passes through the hemodiafilter. Hence, the PC P/P is functionally close to the predilution mode of on-line HDF. In the PC P/P, body fluid is replaced usually by more than 120 L of dialysate during the 4 h treatment. In selecting a hemodiafilter for PC P/P, one must be certain that the blood flow channels in the hemodiafilter do not collapse by the positive pressure on the dialysate side in the backfiltration phase. Thus, the polyacrylonitrile hollow-fiber hemodiafilter and polysulfon hollow-fiber hemodiafilter are suitable for PC P/P. In the short term, PC P/P has been reported to be effective against joint pain, itchiness, insomnia, irritability, and restless leg syndrome experienced by hemodialysis patients. Midterm clinical effectiveness of PC P/P includes the requisite lowering of the erythropoietin dose and improvement in skin pigmentation. The albumin loss per treatment with the PC P/P was significantly lower than that with the conventional HDF approach when a protein-permeable membrane is used. In terms of the removal rate of prolactin, no significant difference was found between PC P/P and conventional HDF. On the other hand, the removal rates of myoglobin and beta2M, where molecular size was smaller than prolactin, was significantly greater with the PC P/P than with conventional HDF.

Dialysis: a 5-year perspective

The view of dialysis in the next 5 years will, by requirement, deal with ideas presently in the laboratories of industry and academics. The healthcare environment is heavily cost constrained and will likely only yield to more expensive therapies if one measures them in pharmacoeconomic terms and shows that they reduce the life costs for a patient with end-stage renal disease. The technologies of hemodialysis and peritoneal dialysis appear to be moving toward one another in terms of generating sterile pyrogen-free dialysis fluid online and moving toward customization of therapy for the individual patient. Sensors will provide closed loop feedback to modulate therapy prescription intradialytically for hemodialysis and on an exchange-to-exchange basis for continuous ambulatory peritoneal dialysis. Hemodialysis will, in part, move back into the home as new, smart, and smaller equipment prospectively designed for that purpose becomes available. Renewed interest in vascular access will provide alternatives to the present shunt and fistula. The recognition of middle molecule toxicity may require selective removal of identified toxins by immunoadsorption. Patient, rather than physician, quality of life will largely dictate the acceptance of technical innovation over the next 5 years.

Alternate repetition of short fore- and backfiltrations reduces convective albumin loss

An effective therapeutic means to remove relatively large polypeptide uremic toxins seems to be a hemofiltration (HF) or hemodiafiltration (HDF) employing a larger-pore membrane, that is, a protein-permeable membrane. With either method, however, a significant amount of albumin will be lost into the ultrafiltrate or dialysate. Now, repetition of alternate short fore- and backfiltrations may prevent the development of the ultrafiltration-induced higher albumin concentration on the membrane surface (protein concentration polarization), where a single forefiltration time is shorter than the time needed for completion of protein concentration polarization. Since the albumin concentration on the protein-permeable membrane surface will be one of the determinants of albumin loss by convection, such HDF treatment may reduce protein loss into the dialysate. To examine this assumption, we alternately repeated short and rapid fore- and backfiltrations (push/pull HDF) through a protein-permeable membrane, each less than 1 second in duration and at each filtration volume of 15 ml, where a pyrogen-free dialysate was supplied. The present results indicated that the albumin amount lost by push/pull HDF was approximately one-third of that by conventional HDF. Nevertheless, the reduction rates of beta 2-microglobulin and myoglobin were significantly greater by push/pull HDF than by conventional HDF.