Fig. 1.

The predicted effect of increasing dialyzer size and dialysate flow on the plasma concentration of urea (left panel) as compared with a solute, which is 95% bound to plasma proteins (right panel). In the left panel, the red line depicts urea levels obtained with a conventional dialysis prescription designed to achieve single pool Kt/V_urea of 1.4 with a blood flow (Q_b) of 360 ml/minute and a dialysate flow (Q_d) of 500 ml/minute during a dialysis session lasting 3.5 hours. As depicted by the blue line, the predicted effect on urea levels of doubling both Q_d and the mass transfer area coefficient KoA would be to reduce the time-averaged urea concentration by only about 15%. The reduction in time-averaged concentration (broken lines) is small because the urea clearance is a large fraction of the blood flow with the conventional prescription and the urea reduction ratio is already about 70%. The right panel depicts the effect of the same two prescriptions on plasma levels of a solute, which is 95% bound to plasma proteins. With the conventional prescription (red line), the clearance of the bound solute is less than a tenth that of urea and the solute reduction ratio is only about 20%. Doubling Q_d and KoA would nearly double the clearance of the bound solute. If solute production stayed constant and the solute was cleared by no other route, this would reduce time-averaged concentration by almost 50% (blue line). Solute concentration profiles were obtained with a previously described computer program and using a distribution volume for the bound solute of 0.2 l/kg body weight, similar to the value observed by Martinez et al. (67). for p-cresol sulfate (68). The peak predialysis concentration has been set to 100 arbitrary units for both solutes.