A reliable method to assess the water permeability of a dialysis system: the global ultrafiltration coefficient

Background

Recent randomized controlled trials suggest that sufficiently high convection post-dilutional haemodiafiltration (HC-HDF) improves survival in dialysis patients, consequently this technique is increasingly being adopted. However, when performing HC-HDF, rigorous control systems of the ultrafiltration setting are required. Assessing the global ultrafiltration coefficient of the dialysis system [GKD-UF; defined as ultrafiltration rate (QUF)/transmembrane pressure] or water permeability may be adapted to the present dialysis settings and be of value in clinics.

Methods

GKD-UF was determined and its reproducibility, variability and influencing factors were specifically assessed in 15 stable patients routinely treated by high-flux haemodialysis or HC-HDF in a single unit.

Results

GKD-UF invariably followed a parabolic function with increasing QUF in dialysis and both pre- and post-dilution HC-HDF (R2 constantly >0.96). The vertex of the parabola, GKD-UF-max and related QUF were very reproducible per patient (coefficient of variation 3.9 ± 0.6 and 3.3 ± 0.3%, respectively) and they greatly varied across patients (31–42 mL/h−1/mmHg and 82–100 mL/min, respectively). GKD-UF-max and its associated QUF decreased during dialysis treatment (P < 0.01). The GKD-UF-max decrease was related to weight loss (R2 = 0.66; P = 0.0015).

Conclusions

GKD-UF is a reliable and accurate method to assess the water permeability of a system in vivo. It varies according to dialysis setting and patient-related factors. It is an objective parameter evaluating the forces driving convection and identifies any diversion of the system during the treatment procedure. It is applicable to low- or high-flux dialysis as well as pre- or post-dilution HDF. Thus, it may be used to describe the characteristics of a dialysis system, is suitable for clinical use and may be of help for personalized prescription.

Influence of convection on small molecule clearances in online hemodiafiltration

BACKGROUND

Dialysis efficacy is mostly influenced by dialyzer clearance. Urea clearance may be estimated in vitro by total ion clearance, which can be obtained by conductivity measurements. We have previously used this approach to assess in vitro clearances in a system mimicking predilutional and postdilutional online hemodiafiltration with a wide range of QD, QB, and ultrafiltration rates. Our current study elaborates on a formula that allows the prediction of the influence of ultrafiltration on small molecule clearances, and validates the mathematical approach both experimentally in vitro and clinically in vivo data.

METHODS

Two conductivimeters in the dialysate side of an E-2008 Fresenius machine were used. HF80 and HF40 polysulfone dialyzers were used; reverse osmosis water and dialysate were used for blood and dialysate compartments, respectively. Study conditions included QB of 300 and 400 mL/min and QD of 500 and 590 mL/min, with a range of ultrafiltration rate from 0 to 400 mL/min in postdilutional hemodiafiltration and to 590 mL/min in predilutional hemodiafiltration. Urea clearances were determined in the in vivo studies, which included 0, 50, 100, and 150 mL/min ultrafiltration rates.

RESULTS

The ultrafiltration rate and clearance were significantly correlated (R > 0.9, P < 0.001) and fitted a linear model (P < 0.001) in all of the experimental conditions. The following formula fitted the experimental points with an error <2% for both postdilutional and predilutional online diafiltration in vitro, respectively. K = K0 + [(QB - K0)/(QB)] x ultrafiltration rateK = K0 + [((QD x QB)/(QB + QD) - K0)/QD] x ultrafiltration rate where K is the clearance; K0 is the clearance with nil ultrafiltration rate; QD is the total dialysate produced (in commercial HDF, QD = QDi + Qinf). Since weight loss was maintained at 0, ultrafiltration rate = infusion flow. QB is the "blood" line flow. The formula was also verified in vivo in clinical postdilutional hemodiafiltration with a QB taking into account the cellular and water compartments.

DISCUSSION

In vitro, by simply determining the clearance in conventional dialysis, the total clearance for any ultrafiltration rate may be estimated in both predilutional and postdilutional online diafiltration with an error of less than 2%. The same applies to in vivo postdilutional hemodiafiltration when the formula takes into account the cellular and water composition of blood.

Analysis of the influence of the infusion site on dialyser clearances measured in an in vitro system mimicking haemodialysis and haemodiafiltration

BACKGROUND

Blood flow (QB), dialysate flow (QD), and dialyser characteristics are the three major factors driving dialysis efficacy. Haemodiafiltration has added an increased convective volume to increase efficacy. We aimed to assess the influence of the infusion site of the replacement fluid in an in vitro system emulating haemodiafiltration.

METHODS

An in vitro system allowing us to control the dialysate temperature, concentration gradient, the flow of both dialyser sides over a range wider than that compatible with clinic, was set to evaluate the influence of the different parameters on dialysis efficacy. The total ion clearance was used as an accepted method for small molecule clearance assessment. Cellulose triacetate (CT190C, Baxter; FB170U, Nipro) and polysulfone (HF80, Fresenius) dialysers were included in the study. Dialysis as well as on-line diafiltration both with pre- and postdilutional infusion were assessed. The experimental conditions presented in this study included QD 620 and 970 ml/min. The convective flows ranged from 50 to 200 ml/min.

RESULTS

For a QD = 620 ml/min and a QB = 350 ml/min the total ion clearance ranged from 269 to 274 for HF80, from 291 to 294 for FB170 and from 294 to 302 for CT190. The variability of the measurements was very low (SD < 1%). Total ion clearance increased by 17-21% when QB was raised from 300 to 400 ml/min. Increasing QD from 420 to 970 ml/min (for QB = 350 ml/min), resulted in an increase in total ion clearance which was more marked at lower QD (from 420 to 620 ml/min) and plateaued thereafter (from 620 to 970 ml/min). Postdilutional on-line diafiltration with 100 ml/min of infusate resulted in an additional increase in total ion clearance of 5.4-8.6%. This increase was proportional to the infused volume. On the contrary, predilutional on-line diafiltration resulted in a decrease in total ion clearance which was also proportional to the infused volume (between -5.1 and -6.9% at 100 ml/min infusion volume and -9.7 to -12.9% at 200 ml/min).

CONCLUSIONS

The present in vitro system provided accurate and reproducible results on dialyser clearances. Our experiments confirmed previous studies on the influence of QB and QD on dialyser efficacy. Further, they show that the proportional increase in postdilutional on-line diafiltration is lesser than that previously reported. More importantly, they also show that pre-dilution infusion in high efficiency systems results in a drop in dialyser clearance compared to dialysis alone, again proportional to the infusion rate. Thus, increasing the convective flow may increase dialysis efficacy even more than increasing QD alone. However, the choice of infusion site is crucial to obtaining this benefit in small molecule clearances.

Precise quantification of dialysis using continuous sampling of spent dialysate and total dialysate volume measurement

The "gold standard" method to evaluate the mass balances achieved during dialysis for a given solute remains total dialysate collection (TDC). However, since handling over 100 liter volumes is unfeasible in our current dialysis units, alternative methods have been proposed, including urea kinetic modeling, partial dialysate collection (PDC) and more recently, monitoring of dialysate urea by on-line devices. Concerned by the complexity and costs generated by these devices, we aimed to adapt the simple "gold standard" TDC method to clinical practice by diminishing the total volumes to be handled. We describe a new system based on partial dialysate collection, the continuous spent sampling of dialysate (CSSD), and present its technical validation. Further, and for the first time, we report a long-term assessment of dialysis dosage in a dialysis clinic using both the classical PDC and the new CSSD system in a group of six stable dialysis patients who were followed for a period of three years. For the CSSD technique, spent dialysate was continuously sampled by a reversed automatic infusion pump at a rate of 10 ml/hr. The piston was automatically driven by the dialysis machine: switched on when dialysis started, off when dialysis terminated and held during the by pass periods. At the same time the number of production cycles of dialysate was monitored and the total volume of dialysate was calculated by multiplying the volume of the production chamber by the number of cycles. Urea and creatinine concentrations were measured in the syringe and the masses were obtained by multiplying this concentration by the total volume. CSSD and TDC were simultaneously performed in 20 dialysis sessions. The total mass of urea removed was calculated as 58038 and 60442 mmol/session (CSSD and TDC respectively; 3.1 +/- 1.2% variation; r = 0.99; y = 0.92x -28.9; P < 0.001). The total mass of creatinine removed was 146,941,143 and 150,071,195 mumol/session (2.2 +/- 0.9% variation; r = 0.99; y = 0.99x + 263; P < 0.001). To determine the long-term clinical use of PDC and CSSD, all the dialysis sessions monitored during three consecutive summers with PDC (during 1993 and 1994) and with CSSD (1995) in six stable dialysis patients were included. The clinical study comparing PDC and CSSD showed similar urea removal: 510 +/- 59 during the first year with PDC and 516 +/- 46 mmol/dialysis session during the third year, using CSSD. Protein catabolic rate (PCR) could be calculated from total urea removal and was 1.05 +/- 0.11 and 1.05 +/- 0.09 g/kg/day with PDC and CSSD for the same periods. PCR values were clearly more stable when calculated from the daily dialysate collections than when obtained with urea kinetic modeling performed once monthly. We found that CSSD is a simple and accurate method to monitor mass balances of urea or any other solute of clinical interest. With CSSD, dialysis efficacy can be monitored at every dialysis session without the need for bleeding a patient. As it is external to the dialysis machine, it can be attached to any type of machine with a very low cost. The sample of dialysate is easy to handle, since it is already taken in a syringe that is sent directly to the laboratory. The CSSD system is currently in routine use in our unit and has demonstrated its feasibility, low cost and high clinical interest in monitoring dialysis patients.