Reverse mid-dilution: new way to remove small and middle molecules as well as phosphate with high intrafilter convective clearance

Background. The removal of small and middle molecules has a relevant impact on haemodialysis (HD) patient survival. Mid-dilution (MD) is a technique combining ease of use with high diffusive-convective clearances. However, MD may increase the intrafilter blood pressure due to the high filtration fraction. We devised a new filter configuration, reverse MD, with an inverted blood inlet and outlet. We compared biochemical and technical performances of reverse MD vs standard MD. Methods. Eight HD patients underwent one standard MD treatment and one reverse MD. Samples for instantaneous clearance and total mass removed from dialysate spilling (urea, phosphate, β2-microglobulin, angiogenin) were obtained. Dialysate and blood pressures in the circuit were monitored every 15 min. The reinfusion rate was set at 6 l/h for both treatments. Results. Absolute removals were very high and statistically comparable in both the configurations. Pressures were significantly lower with the reverse compared with the standard MD: inlet blood pressure was 731 ± 222 and 595 ± 119 mmHg in the standard and in the reverse MD, respectively. The transmembrane pressures were lower in the reverse compared with the standard MD (422 ± 90 and 611 ± 136 mmHg for 1st stage; 188 ± 54 and 307 ± 56 mmHg for 2nd stage). Conclusions. Reverse MD could be an ideal technique for high ultrafiltration routine treatments without any external fluid reinfusion. It allows a very high removal of small and middle molecules, with relatively lower intrafilter pressures.