Renal replacement therapy: Physical properties of hollow fibers influence efficiency

Introduction: Physical properties of filters for continous renal replacement therapy have a great impact on biocompatibility. According to Poiseuille's law a filter with more and shorter hollow fibers should offer a decreased pressure drop and, therefore, lower transmembrane pressure (TMP). The aim of this study was to study the effect of a new filter configuration in terms of TMP and clotting compared with the standard configuration. Methods: In a prospective randomized cross-over study 2 polysulphone hollow fiber hemofilters, one handmade, which differed only in length and number of hollow fibers were compared. In each group 12 filters were investigated during continous venovenous hemofiltration in patients with acute renal failure due to septic shock. Pressures were measured every 3 hours and running time until filter clotting was documented. Mediators before and after the filter, at the end of treatment and in filtrate were assessed. Results: The standard filter with longer hollow fibers had significantly lower TMPs (106 vs. 194 mmHg, p=0.02) and longer running times (1276 vs. 851 min, p=0,04). There were no differences in hematocrit, total protein, cellular and plasmatic coagulation or blood temperature. No significant elimination of mediators was shown. Conclusion: In contrast to our expectations, the filter with the longer hollow fibers had a better performance, as it ran longer and had lower TMP. This may be due to slower blood flow leading to an increase in blood viscosity in a filter with a larger cross section.