**3. Technological principles of haemodiafiltration**

Ultrafiltrate volume, derived from QUF times treatment time (**Figure 3**), is removed by the dialysis machine through increased transmembrane pressure (TMP), whereas the replacement solution is infused intravenously at equal volume minus the desired fluid volume removal to preserve extracellular fluid balance and isovolemic state. The replaced solution represents *substitution volume*, whereas convective volume represents the sum of substitution volume and desired fluid volume removal during the dialysis session.

Filtration fraction (FF) (proportion of ultrafiltration volume obtained compared with total blood volume processed during the HD session) was traditionally supposed to be 25% (**Figure 3**).

Principles of Haemodiafiltration: Rationale for Improved Patients' Survival http://dx.doi.org/10.5772/63067 9

**Figure 3.** Filtration fraction. QB, blood flow rate; QUF, ultrafiltration flow rate.

#### **3.1. Dilution modes**

**Figure 2.** The ratio between treatment techniques, processes and molecular weights of solutes. HD, haemodialysis; HDF, haemodiafiltration; HF, haemofiltration; KoA, mass transfer area coefficient; QB, blood flow; tHD, duration of HD

The utilization of convective therapies has been variable. Between 1998 and 2001, about 12% of patients were on HDF in the European countries participating in the Dialysis Outcome and

There have not been too many studies to compare the patients' survival between convective modalities and high-flux HD. One of them was Italian MAMHEBI study, which was a randomised controlled trial (RCT). Significantly higher three-year survival was noted with HF (68% versus 52%) [15]. Dialysis Outcome and Practice Patterns Study (DOPPS) was prospective observational study which showed better survival with HDF, but it compared high-efficiency HDF with composite predictor of high- and low-flux haemodialysis [14]. Vilar et al. conducted retrospective cohort study and showed better survival with on-line HDF than with high-flux haemodialysis [16], as well as Imamović et al. in incident patients' population [17], but the latter three were epidemiological studies and RCT to show survival benefit of HDF over

Ultrafiltrate volume, derived from QUF times treatment time (**Figure 3**), is removed by the dialysis machine through increased transmembrane pressure (TMP), whereas the replacement solution is infused intravenously at equal volume minus the desired fluid volume removal to preserve extracellular fluid balance and isovolemic state. The replaced solution represents *substitution volume*, whereas convective volume represents the sum of substitution volume and

Filtration fraction (FF) (proportion of ultrafiltration volume obtained compared with total blood volume processed during the HD session) was traditionally supposed to be 25%

session; SC, sieving coefficient (the proportion of a substance to be removed for a particular filter).

Practice Patterns Study (DOPPS) study [14].

8 Advances in Hemodiafiltration

conventional HD was still missing.

(**Figure 3**).

**3. Technological principles of haemodiafiltration**

desired fluid volume removal during the dialysis session.

The fluid can be substituted either after the dialyser as the reference mode (post-dilution mode) or before the dialyser (pre-dilution mode) (**Figure 4**), or both (mixed dilution mode).

**Figure 4.** Dilution modes in haemodiafiltration; HDF, haemodiafiltration.

Choice on which dilution mode to apply depends on patient haemorheology and clinical performance (**Table 1**).



**Table 1.** Pros and cons of dilution modes.
