Backfiltration in Clinical Dialysis: Nature of the Phenomenon, Mechanisms and Possible Solutions

However this representation only describes an average phenomenon and does not define the actual pressure profile inside the filter. Although the average TMP is positive, the local P is not necessarily positive at every point along the length of the dialyzer. Equation 1 also assumes that the pressure drop inside the dialyzer fibres is linear with distance which, according to the Hagen-Poiseuille law, is only true under specific conditions. As depicted in Figure 1, the pressure drop is linear only when blood viscosity remains constant along the fibres, and this only occurs when no ultrafiltration takes place. When water is removed by ultrafiltration, increases in hematocrit and plasma protein concentration cause the blood viscosity to increase along the length of the device, which results in a non-linear pressure drop. The overall water flux in a single fibre of the dialyzer is described in detail in Figure 2 where dS represents a single surface unit and dl a single unit of the fibre length. Expanding this concept to the whole dialyzer, the overall water flux in a given dialyzer will be expressed by the formula: