**4. In vitro analyses examining dialysate flow dependence of solute clearance**

A dialysis machine (FMC 4008) was used to perform in vitro measurements of clearance rates in accordance with standard ISO 8637. A batch of 7.0 L stirred and thermostatically warmed dialysis liquid served as "blood" with dissolved urea and vitamin B12 as test substances. Measurements were carried out at different dialysate and blood flow rates, with each dialysate flow rate (QD=300, 500, 800 ml/min) being measured at blood flow rate settings of QB=100, 200, 300, 400 ml/min. These measurements were repeated for the 3 different types of dialysers.

Clearance data were obtained for one small molecule (urea, relative molecular weight=60) and one larger molecule (vitamin B12, relative molecular weight=1357). KoA values were then calculated according to GI. 1a, b and compared with the values provided by the manufacturers.


**Table 1.** List of dialysers used for in vitro testing

Although a number of manufacturers at one point utilised flat-plate arrangements of inter‐ weaved hollow fibres in their dialysers (e.g. HFD 1.0 by MLW), this method has since been abandoned-probably due to cost reasons. Almost all manufacturers prefer the Moiré structure of fibres in order to obtain adequate spacing. In some instances-and sometimes in addition to the Moiré structure-spacing filaments (spacer yarns) are added (e.g. Asahi PAN650SF, MTP VitaPES). Although these measures ensure improved dialysate flow distribution within the cross-sectional area of the dialyser, in vitro testing using CT imaging has revealed that some preferential channelling through peripheral areas of the dialysate compartment remains [3, 4, 5]. With its FX series, Fresenius Medical Care followed a different path. Via a pinnacle structure in the inflow and outflow tracts, the dialysate is forced into even distribution across the entire surface area [6]. In the early days, dialyser technology included a number of devices whose inlet and outlet headers were fitted diagonally to improve the distribution of dialysate across the fibre bundle (e.g. EMC TriEx). For reasons unknown, this very simple solution did not

**Figure 2.** Enhanced ultrasound image of inflowing dialysate at QD=500 ml/min (Dialyser Altair 12G, US-machine Logic

**4. In vitro analyses examining dialysate flow dependence of solute**

A dialysis machine (FMC 4008) was used to perform in vitro measurements of clearance rates in accordance with standard ISO 8637. A batch of 7.0 L stirred and thermostatically warmed dialysis liquid served as "blood" with dissolved urea and vitamin B12 as test substances. Measurements were carried out at different dialysate and blood flow rates, with each dialysate flow rate (QD=300, 500, 800 ml/min) being measured at blood flow rate settings of QB=100, 200, 300, 400 ml/min. These measurements were repeated for the 3 different types of dialysers.

Clearance data were obtained for one small molecule (urea, relative molecular weight=60) and one larger molecule (vitamin B12, relative molecular weight=1357). KoA values were then calculated according to GI. 1a, b and compared with the values provided by the manufacturers.

prevail.

7, GE Medical Systems, contrast agent: Optison®)

192 Updates in Hemodialysis

**clearance**

Fig. 3 shows that the measured characteristics generally followed the theoretical model (refer to Fig. 1). However, the clearance rates calculated using the catalogue KOA values were not achieved at low dialysate flow rates. It was not until QD=800 ml/min that the values obtained managed to improve slightly on the ones provided. Similar curves were obtained for the other types of dialysers. The KoA values calculated from clearance, QB and QD were dependent upon both blood flow rate of blood and dialysate flow rate. In an ideal scenario, this kind of relationship should not exist. The overall mass transfer coefficient, Ko, is a physical charac‐ teristic of the fibre bundle and does not vary with the flow rates that might exist on the bloodside or the dialysate-side. Different KoA values in the presence of different flow rates can be easily explained provided if the effective surface area A, which is involved in solute transport, fails to remain constant. Values that may be theoretically possible will of course not be achieved in situations where a low dialysate flow rate results in sections of the fibre bundle not being immersed in dialysate. Fig. 4 shows that the KoA value calculated for the dialyser FDY150GW depends upon the dialysate flow rate.

**Figure 3.** Comparison of measured urea clearance values and urea clearance values derived from KoA data provided by the manufacturers.

**Figure 4.** KoA values calculated based on clearance, QD and QB, using the dialyser FDY150GW as an example


**Table 2.** In vitro KoA values [ml/min] for urea at different dialysate flow rates QD (mean values for different blood flow rates QB)

The dialyser FX60 showed the lowest degree of dialysate flow rate dependence of the KoA value. Although this was likely to be due to improved dialysate flow, the device still failed to achieve the manufacturer's catalogue value.

In summary, it has to be concluded that even modern high flux dialysers providing structural changes to optimise dialysate flow do not manage to negate the underlying relationship between the flow rates of blood and dialysate. Furthermore, full use of the fibre bundle's effective surface area A can only be guaranteed at high dialysate flow rates of around 800ml/ min. Whilst this mirrors the results obtained by Leypolt [7], it contradicts the assumptions made by Golper and Ward, who concluded that structural improvements have made high dialysate flow rates unnecessary.
