1 Fresenius Medical Care, Bad Homburg, Germany and 2 Nephrology Center Stuttgart, Stuttgart, Germany
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Abstract |
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Methods. This study aims at investigating the in vitro and in vivo performance of the new FX60 dialyzer with the focus on dialysate flow distribution and dialysate consumption. A new method to analyse dialysate flow distribution, based on local clearances, is suggested. The effect of reducing dialysate flow from 500 to 300 ml/min is investigated in vivo.
Results. K0Aurea, a common measure to quantify device performance, is found to approach 1000 ml/min for the FX60 with a surface of only 1.4 m2. Local clearance measurement shows equal performance of the Helixone® fibre bundle over the entire cross-section. A dialysate flow reduction by 20% (in vivo) only results in a minor loss in clearance.
Conclusions. The newly developed high-flux dialyzer (FX60) shows a remarkable performance (K0Aurea). The excellent utilization of dialysate could be proven in vivo and is attributed to the superior dialysate flow distribution. A reduction of dialysate flow by 20% could lead to substantial economic savings.
Keywords: dialysate consumption; dialysate flow distribution; high-flux dialyzer; local clearance; polysulfone membrane
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Introduction |
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A recent development has overcome this limitation. The FX dialyzer series includes an advanced Fresenius Polysulfone® membrane, called Helixone® [4,5] and compared with conventional dialyzers, almost all components of the FX class of dialyzers have undergone significant modifications to improve the overall device performance.
A very light, environmentally friendly materialpolypropylenewas chosen to manufacture the FX-class dialyzer housing. For a given effective surface area, the housing diameter was reduced without altering the length; this improves the dialysate flow distribution because of a higher average flow velocity.
In the conventional dialyzer design, dialysate enters straight through the dialysate port (and only from one side) into the actual fibre compartment. This often leads to an inappropriate dialysate distribution, with the dialysate flowing directly from the inlet to the outlet leading to poor coverage of large parts of the fibre bundle. In this new design, the special pinnacle structure of the housing acts as a dialysate distributor and ensures that dialysate enters from all sides into the fibre bundle at similar flow rates. This contributes to a more homogeneous dialysate flow distribution.
It is well known that dialysate flow distribution in standard dialyzers is sub-optimal, especially when fibre bundles with a larger diameter are used. Usage of spacer yarns in different variations is one approach used for the optimization of dialysate flow distribution. In the FX class a different approach has been taken: a special ondulation, the micro-crimp geometry, of the fibres ensures that high-fibre bundle-packing densities can be realized whilst still being sure that single fibres do not come too close to one another. This allows an increase of fibre-packing density from 45 to 60% of the theoretical maximum.
While in a standard dialyzer, the pressure drop on the dialysate side is almost negligible, a denser fibre bundle increases the pressure drop, thereby enhancing internal filtration and leading to an improved middle-molecule clearance during a standard haemodialysis treatment [6].
In addition to the fibre bundle as a whole, the geometry of a single fibre yields improvement potential. Unlike cellulosic membranes, synthetic dialysis membranes have a larger wall thickness, which results in a larger diffusive resistance, leading to a reduction in the diffusive clearance especially for the smaller molecules. The reduction of wall thickness by more than 10% down to 35 µm (from 40 µm) improves small molecular clearance without any loss of mechanical stability.
Internal filtration is dependent on fibre bundle and the single-fibre geometry. By reducing the fibre diameter from 200 to 185 µm, the pressure drop increases by approximately 25%, resulting in a higher internal filtration and an improved middle-molecular clearance [7].
It is the aim of this paper to demonstrate that the new dialyzer design described above was successful in improving overall dialyzer performance. Clearances are reported in vitro and in vivo. A new clearance-based method to analyse dialysate flow distribution is suggested and applied. The influence of a dialysate flow reduction on clearances was investigated in vivo.
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Subjects and methods |
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Clearance evaluation
In vitro clearance data were determined according to the European Norm (EN 1283) as follows. The blood flow, QB, was varied within the recommended blood flow range. The dialysate flow, QD, was set to 500 ml/min, and the net ultrafiltration, QF, was carefully adjusted to zero. The temperature was stabilized at 37°C. The dialysate and the aqueous test solution used on the blood side were used in single-pass mode and not re-circulated.
Samples of the test solution were taken from the inlet and the outlet of the dialyzer and the concentration of the different analytes was determined. The clearance was calculated according to the following formula:
| (001) |
K0A calculation
To specify dialyzer performance for a given solute it is necessary to give a clearance for each blood flow. The parameter K0A (mass transfer area coefficient) [8], which is finding increasing usage in particular in the USA, overcomes this difficulty. To a very good approximation, a single K0A value is sufficient to describe dialyzer performance for a given solute. For any given K0A, clearances under varying blood and dialysate flow conditions can be calculated according to the following formula:
| (002) |
Measurement of local clearances
Different methods for determining dialysate flow distributions are available, reaching from complex approaches like nuclear magnetic resonance measurements to very simple methods like dye injection. Our approach was different. If certain areas in a dialyzer are not sufficiently perfused with dialysate, the clearance in these areas is poor. Therefore, the most direct way to identify flow distribution non-uniformity is to check local clearances. To do so, a standard experiment for the in vitro determination of clearances was set up. Both dialysate and blood reservoir were used, single-pass and net ultrafiltration was adjusted to zero. The only modification was the use of a different end cap on the blood outlet side. This end cap is divided into 25 areas with a separate outflow thus allowing the collection of samples from different areas separately. The flow in each segment is determined individually as well and the local clearance calculated according to:
| (003) |
In vivo clearance datadependency on dialysate flow
A group of 10 stable outpatients undergoing chronic haemodialysis treatment for an average of 57.5 months, mainly using high-flux dialyzers (F60 S), was switched to the new FX60 capillary dialyzers and was monitored for more than 6 months. Clinical visits and standard laboratory results were documented at the beginning and after 6 months, and were then compared. Treatment modalities were kept stable as far as the clinical situation permitted. Dialysate flow rates were 400 ml/min, blood flow rates increased from 315±30 ml/min at the beginning to 365±47 ml/min more than 6 months later. Treatment times were kept constant with 13.65±1.31 h/week on a three-dialysis session per week basis. Furthermore, clearance values of urea, creatinine, uric acid, phosphate, and ß2-m were obtained using blood flow rates of 300 ml/min and dialysate flow rates of 500, 400, and 300 ml/min for a period of 2 weeks, respectively. Plasma samples were obtained at the inflow and outflow sites of the FX60 dialyzer 60 min after starting the midweek dialysis. Filtrate rates were reduced to zero, 2 min prior to sampling in order to avoid additional convection. Unchanged amounts of heparin were given continuously during the dialysis sessions.
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Results |
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Measurement of local clearances
To check the dialysate flow distribution, clearances were determined for individual cross-sectional areas of the FX60 and compared with a standard dialyzer. The results obtained for a standard fibre bundle and the new Helixone® fibre bundle are shown in Figure 2. In a standard bundle design, clearance is better in the outer areas and drops by approximately 20% towards the centre. In contrast, the Helixone® fibre bundle leads almost to constant clearance distribution over the whole cross-section.
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In vivo clearance datadependency on dialysate flow
Urea clearance values with 300 ml/min blood flow and 500 ml/min dialysate flow rates were found to be 242±19 ml/min and reduced to 221±28 ml/min (-8.5%) at a 400 ml/min dialysate flow. As expected, a further reduction to 211±34 ml/min (-12.6%) was demonstrated at a 300 ml/min dialysate flow. Creatinine clearance values were determined: 172±17 ml/min at a dialysate flow of 500 ml/min, with a reduction to 168±17 (-1.9%) and 164±25 ml/min at 400 and 300 ml/min dialysate flow. For phosphate, we found a plasma clearance of 182±28 ml/min at 500 ml/min, 180±29 ml/min (-1.1%) at 400 ml/min, and 175±27 ml/min at 300 ml/min dialysate flow rate. Uric acid clearances were 191±19 ml/min at 500 ml/min, 190±15 ml/min (-0.5%) at 400 ml/min, and 184±25 ml/min at 300 ml/min dialysate flow. ß2-m clearance values measured 78±12 ml/min at 500 ml/min, 78±16 ml/min (0%) at 400 ml/min, and 69±13 (-10.5%) at 300 ml/min dialysate flow (see Figure 3).
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Discussion |
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This study investigated in vitro and in vivo performance of the FX60 dialyzer with a special focus on dialysate flow distribution. The in vitro and in vivo data given show that the design efforts resulted in excellent clearance values approaching a K0A value for urea of 1000 ml/min (30% above F60 S performance) with only 1.4 m2 of membrane surface. Optimization of dialysate flow distribution lead to almost constant local clearances over the entire cross-section of the dialyzer.
The excellent utilization of dialysate could be proven in vivo as well. Clearances for different marker molecules were determined in vivo at dialysate flow rates of 300, 400, and 500 ml/min. Theoretically, the decrement of clearances with reduction of dialysate flow rates is most impressive regarding small molecular weight substances and subsequently diminishes with increasing molecular weight shedding information about the importance of convective transport properties for the elimination of high molecular weight substances. The in vivo clearance properties of the FX60 dialyzer shown in Figure 3 are very close to the data, which have been expected by computer simulation.
If dialysate flow rates are lowered from 500 to 400 ml/min (a 20% reduction), for clearance values of small molecular weight substances a decrease of only between 1 and 9% occurs. This small decrement was achieved by the new fibre-bundle design leading to an optimized dialysate flow distribution. In view of the fact that in most dialysis facilities in Europe, blood flow rates are well below maximal values (with respect to fistula performance and surface of the dialyzer used), it may be recommended to increase blood flow rates by 10%. This increase would easily abolish the loss of clearance performance through a dialysate flow reduction of 20% bringing with it substantial economic savings.
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Notes |
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References |
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