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In the CONVINCE trial, the use of high-volume hemodiafiltration (HDF) resulted in a lower risk of death from any cause than conventional high-flux hemodialysis (HD) (Blankestijn, NEJM, 2023). However, concerns have been raised that HDF would require more water than HD, calling its sustainability into question (Shroff, NDT, 2023). We used mathematical modeling to explore this question.
We modeled (Werynski, Artificial Organs, 1995) the spKt/V of urea in post-dilution HDF using the parameters published by the CONVINCE consortium. The urea distribution volume was calculated using BMI and BSA.
A dialysate-to-blood flow ratio of 1.4 was assumed, which yields a total dialysate volume of 153 L (incl. 25.5 L convection volume) in the HDF group and 126.6 L in the HD group. The k0Aurea was set to 759 mL/min, as this resulted in a spKt/V for HDF that was identical to the one published for the CONVINCE trial.
The urea Kt/V was modeled for different dialysate volumes with a constant exchange volume of 25.2 L in post-HDF. Compared were:
1. The Kt/V achieved with HDF vs. HD when both use the same amount of dialysate;
2. The water utilization required for HDF vs. HD when aiming for the same Kt/V.
For more generalized results, we then expanded the same comparisons to different blood flow rates (300, 400, 500 mL/min), using a filtration fraction of 30% and a dialysate-to-blood flow ratio of 1.5.
1) HDF and HD with equal dialysate volumes HDF yields a spKt/V which is 0.06 higher than that in HD.
2) HDF and HD with equal spKt/V HDF requires about 27 L of dialysate volume less than HD
Figure 1: spKt/V in HDF as function of total spent dialysate with Vsub = 25.2 L.
In the simulations of different blood flows, we considered two scenarios (Figure 2):
1) HDF and HD with equal dialysate volumes HDF yields a higher spKt/V than HD at any blood flow rate.
2) HDF and HD with equal spKt/V. HDF requires less dialysate volume than HD at any blood flow rate.
Figure 2: Total spent dialysate and spKt/V in HD and in HDF for three blood flow rates
· With equal water consumption, post-dilution HDF yields higher spKt/V than HD.
· For equal spKt/V, HDF consumes less water than HD.
Contrary to the concern that HDF consumes more water than HD, HDF saves water at the same small solute clearance.
These results are valid for any blood flow rate and dialyzer in post-dilution HDF.
Convection has a higher removal efficiency than diffusion for middle molecules and for small solutes like urea.
Current dialysis machines support the automatic setting of the same dialysate volumes in HDF and HD.