Dynamic filtration of blood: a new concept for enhancing plasma filtration

We have shown previously that blood flow pulsations created by intermittent squeezing of the inlet blood line significantly increased the plasma filtration rate in membrane plasmapheresis. However, in order to avoid hemolysis, the filtration increase had to be limited to about 50%. We have now devised a more efficient pulsation generator. By properly matching the tubing compliance and the pulsation amplitude, it is possible to extract 50 ml/min of plasma from 90 ml/min of blood at 36% hematocrit with a 1000 cm2 polypropylene hollow fiber filter without hemolysis. Simultaneous recording of the time course of plasma filtration rate measured by an electromagnetic flow meter and transmembrane showed that the increase in mean plasma flow rate was due to a dynamic filtration process which prevents the establishment of concentration polarization. The transmembrane pressure (Ptm) increases over a 0.5-second interval when the tube is squeezed. The membrane responds with an increase in filtration since the concentration polarization layer takes a few seconds to build up. The Ptm then drops when the tube is released before the polarization layer has time to build up appreciably and a sudden acceleration of the blood flow (velocity spike) helps clean the membrane, reducing the polarization. Tests with bovine show that the system is very efficient in reducing membrane plugging with small area filters.