Figure 65

Water permeability of a membrane and control of volumetric ultrafiltration in hemodialysis. The water permeability of a dialysis membrane can vary considerably and is a function of membrane thickness and pore size. The water permeability is indicated by its ultrafiltration coefficient (KUf). The KUf is defined as the number of milliliters of fluid per hour that will be transferred across the membrane per mm Hg pressure gradient across the membrane. A high-flux membrane is characterized by an ultrafiltration coefficient of over 20 mL/h /mm Hg. With such a high water permeability value a small error in setting the transmembrane pressure can result in excessively large amounts of fluid to be removed. As a result, use of these membranes should be restricted to dialysis machines that have volumetric ultrafiltration controls so that the amount of ultrafiltration can be precisely controlled.

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