Studies on dialysate mixing in the Genius® single-pass batch system for hemodialysis therapy

Background. The Genius(R) single-pass batch system for hemodialysis contains a closed reservoir and dialysate circuit of 75 L dialysate. The unused dialysate is withdrawn at the top of the reservoir and the spent fluid is reintroduced into the container at the bottom. Although it has been claimed that both fractions remain unmixed during the dialysis session, no direct proof of this assumption has yet been provided. In the present study, we investigated whether contamination of the unused dialysate with uremic solutes occurred and at which time point it began. Two different dialysate temperatures were compared. Methods. Ten chronic hemodialysis patients were dialyzed twice with the Genius(R) system, with dialysate prepared at 37degreesC and 38.5degreesC, respectively. The sessions lasted 270 minutes with blood/dialysate flow set at 300 mL/min. Dialysate was sampled at 5, 60, 180, 210, 225, 230, 235, 240, 255, and 270 minutes both from the inlet and outlet dialysate line and blood was sampled from the arterial line predialysis, after 4 hours, and postdialysis. All samples were tested for osmolality, urea, creatinine, p-cresol, hippuric acid, and indoxyl sulfate. Results. Uremic solutes appeared in the inlet dialysate line between 3 hours 50 minutes and 4 hours 10 minutes after the start of dialysis, corresponding to 68.6 and 74.7 L spent dialysate, respectively (37degreesC vs. 38.5degreesC; P = NS). No difference in the amount of removed solutes and in the serum levels was observed between 37degreesC and 38.5degreesC. A Kt/V of 1.17 +/- 0.20 and 1.18 +/- 0.26, respectively, was reached with the 37degreesC and 38.5degreesC dialysate temperature (P = NS). Conclusion. Contamination with uremic solutes occurred at the dialysate inlet only near the end of the session when small quantities of fresh dialysate were left in the container. Differences in dialysate temperature did not result in a different separation between used and unused dialysate, or in differences in removal of toxins or Kt/V.