**3. Structural changes to allow optimisation of the dialysate compartment**

Fig. 2 shows an enhanced ultrasound image depicting an unfortunate case of channelling observed in a test dialyser during the inflow of dialysate.

In order to avoid, or at least reduce, this channelling phenomenon, different structural alterations are commonly applied:


1 / ln <sup>a</sup> (1 / ) 1 /

*B D*

*B D B*

*Q Q KQ*

1 − *K* / *QB*

where *Ko* is the "overall mass transfer coefficient" and *A* is the dialyser membrane surface area.

In dialysis log sheets, the *KoA*-value is usually recorded for urea, thus allowing the calculation

0 100 200 300 400 500 **QB [ml/min]**

Fig. 1 shows how clearance rate K, derived from the KoA value for a low molecular substance (urea), changes in relation to blood flow rates, with the relationship demonstrated at different dialysate flow rates, and for a high-flux dialyser. At low dialysate flow rates, the dialysate compartment soon becomes saturated, leading to a large reduction in the concentration gradient. This in turn means that an increase in blood flow will no longer produce a substantial improvement in the rate of clearance. It is only when the dialysate flow rate is high enough for the substance in question to be removed quickly from the dialysate chamber that a higher

**Figure 1.** Dependence of clearance K on the flow rates of blood QB and dialysate QD (KoA=1000 ml/min)

Qd = 800 ml/min Qd = 500 ml/min Qd = 300 ml/min Qd = 200 ml/min

é ù - <sup>=</sup> ê ú - - ë û

(1 / )

*K Q*

*B*

*Q KQ K A*

*O*

*O*

For QB=QD the following applies: *KO <sup>A</sup>*<sup>=</sup> *<sup>K</sup>*

190 Updates in Hemodialysis

0

50

100

150

200

**K [ml/min]**

250

300

350

400

*<sup>K</sup> K A*

of urea-clearance in relation to blood and dialysate flow rates.

<sup>=</sup> -

b

(1)

**•** Pinnacle structure in the dialysate inflow and outflow areas.

**Figure 2.** Enhanced ultrasound image of inflowing dialysate at QD=500 ml/min (Dialyser Altair 12G, US-machine Logic 7, GE Medical Systems, contrast agent: Optison®)

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 prevail.
