Kinetics and dosing predictions for daily haemofiltration.

BACKGROUND: Thrice-weekly haemofiltration affords excellent outcome when it is used to treat chronic renal failure patients. Daily haemofiltration (DHF) has recently been proposed as a more intensive therapy option, but the total ultrafiltration or exchange volume (replacement volume plus net ultrafiltration volume) requirements for adequate solute clearances during this novel therapy are unknown.
METHODS: We calculated theoretical solute kinetic profiles during six times per week DHF for comparison with those during thrice-weekly haemodialysis using a high-flux dialyser (HFHD) or during continuous ambulatory peritoneal dialysis (CAPD). HFHD and CAPD were chosen for comparison because K/DOQI guidelines have defined adequate treatment doses for these therapies. Steady-state concentrations were calculated using a two-compartment model of an anuric patient with 35 l of total body water for five solutes: urea, creatinine, vitamin B(12), inulin and beta(2)-microglobulin. Solute distribution volumes and generation rates were taken from the literature, and excess fluid (1 l/day) was assumed to accumulate in and be removed from the extracellular fluid compartment. Theoretical predictions of solute clearance were compared for a 15-l exchange volume/session during DHF, urea Kt/V of 3.6/week during HFHD and urea Kt/V of 2.0/week during CAPD as solute-specific values of the equivalent renal clearance (EKR) and standard Kt/V (stdKt/V) recently defined by Gotch. Additional comparisons of solute clearances were performed between DHF and other daily therapies including six times per week short daily haemodialysis (SDHD) and six times per week nocturnal haemodialysis (NHD).
RESULTS: The calculated results predict that: (i) urea clearance during DHF with an exchange volume of 90 l/week (6x15 l) is equivalent to those during HFHD and CAPD based on urea stdKt/V; and (ii) middle molecule clearances during DHF exceed those achieved during HFHD and CAPD based on either EKR or stdKt/V. As expected, DHF therapy was inferior regarding the clearance of urea and other small solutes to SDHD and NHD; however, DHF therapy was superior to SDHD regarding the clearance of larger middle molecules, approaching the clearances achieved by NHD.
CONCLUSIONS: We predict that an exchange volume of approximately 40% of total body water (15/35 l=43%) per session will provide adequate clearance of small solutes and substantial clearance of middle molecules during six times per week DHF therapy. These theoretical predictions require clinical validation.