**5. In vivo analyses examining the dialysate flow rate dependence of solute clearance**

Naturally, in vitro investigations involving dialysers have to be treated with caution, with results not directly transferable to the clinical dialysis setting. Due to its viscosity and non-Newtonian flow properties, blood is a much more complex substance than an aqueous test solution. Also, unlike dialyser clearance rates, in vivo clearance rates depend upon physio‐ logical factors such as compartmentalisation of the blood and the uraemic toxins contained therein, fistula recirculation, cardiopulmonary recirculation, individual differences in haema‐ tocrit, as well as differences in protein and lipid concentrations. In spite of this, when looking at a sufficiently large number of patients and dialyses treatments, the basic principles discussed above should be mirrored in the clinical dialysis setting.

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

*QD/dialyser type FDY150GW FX60 VitaPES150HF*

QD = 300 ml/min 740 ± 21 728 ± 26 792 ± 64 QD = 500 ml/min 753 ± 49 751 ± 42 936 ± 34 QD = 800 ml/min 817 ± 104 756 ± 70 1063 ± 106 Catalogue value 874 937 1167

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

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

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

flow rates QB)

194 Updates in Hemodialysis

achieve the manufacturer's catalogue value.

dialysate flow rates unnecessary.


**Figure 5.** (a): Dialysate flow rate dependence of the spKt/V value in dialyser FDY150GW (b): Dialysate flow rate de‐ pendence of the spKt/V value in dialyser FX60 (c): Dialysate flow rate dependence of the spKt/V value in dialyser Vi‐ taPES150HF

At a constant blood flow rate (QB=300 ml/min), a group of 6 patients received three treatments each at QD=300, 500 und 800 ml/min, with each set repeated for each of the 3 types of dialysers tested in vitro. In order to assess the effectiveness of treatment, the urea reduction ratio, single pool Kt/V (spKt/V) and equilibrated Kt/V (eqKt/V) were obtained (Daugirdas formula). In addition, OCM data were obtained for the dialysis machine FMC 5008 (Eff.Kt, Eff.Kt/V).

The figures show (as do the urea reduction ratios and eqKt/V results) that each step increase in dialysate flow can significantly improve the effectiveness of haemodialysis.

The interesting question for dialysis patients is of course the exact impact this may have on treatment time. A sample calculation using results from the FX60 dialyser will demonstrate this (see Table 3).


(Vcalc from OCM EffKt and measured Kt/V; tcalc=Kt/V(QD300)\*V/K)

**Table 3.** Time saved by using a dialysate flow rate of QD=500 ml/min or QD=800 ml/min instead of a dialysate flow rate of QD=300 ml/min

The same Kt/V value achieved at a dialysate flow rate QD=300 ml/min can be achieved at a dialysate flow rate of QD=500 ml/min, whilst treatment time can be reduced by between 13 and 25 minutes, depending on the individual patient's body mass and clearance. If a dialysate flow rate of QD=800 ml/min is used, treatment time will be reduced accordingly, namely by 26-36 min. Treatment time reduction of this order is certainly of interest to patients and should be able to outweigh the slight increase in costs incurred by higher dialysate flow rates. Naturally, if treatment time remains unchanged, a higher dialysate flow rate can also be used to improve the Kt/V value-as depicted in Fig. 5.
