Page 402 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

Membranes, Synthetic, Applications 337

FIGURE 48 Schematic diagram of a hemodialysis system.

Hemofiltration (also referred to as hemodiafiltration) plishes this by separating the blood cells from the plasma
operates by ultrafiltering the blood to remove water and with a membrane; the plasma is discarded and the blood
small molecules from plasma. As the process operates cells are returned to the patient. With this approach, signif-
by forced convective flow across the membrane instead icantly lower doses of immunosuppressants are sufficient
of diffusion of the solutes alone, a dialysate solution is to manage the condition effectively.
not provided. Instead, the volume lost by filtration is re- Plasmapheresis typically employs a membrane mod-
placed with a substitution fluid. Because of its ability to ule of similar configuration as a high-flux hemodialyzer.
remove waste materials rapidly, hemofiltration has been Alternatively, a rotating membrane separation element is
prescribed to treat acute kidney failures associated with in- used in which the tendency of the blood cells to deposit on
toxication by poison or drugs. It also holds promise as the the membrane surface is counteracted with hydrodynamic
basic process by which wearable or implantable replace- lift forces created by the rotation. The membrane element
ments for the kidney may be operated. As routine therapy, and the associated plasmapheresis circuitry are shown in
however, hemofiltration may have lost its rate advantage Fig. 49. Worldwide, about 6 million plasmapheresis pro-
over dialysis after high-flux membranes for that procedure cedures are performed annually using this system, making
appeared. Also, the need to supply sterile solutions for this one of the largest biomedical membrane applications
plasma replenishment in hemofiltration adds significantly after hemodialysis.
to the treatment cost. On the other hand, hemofiltration Although proven effective in years of clinical prac-
has found increasing use as an adjunct to surgery, where tice, membrane plasmapheresis faces competition from
blood volume is increased prior to surgery by infusion of improved centrifuges capable of continuous cell separa-
fluids, then later restored by ultrafiltering excess fluid after tion. Both approaches are likely to continue playing im-
the procedure is completed. portant roles in the area of extracorporeal therapy.

B. Plasmapheresis
C. Blood Oxygenation
Plasmapheresis, also known as plasma exchange, is a pro-
cess developed in the 1970s and now used routinely to Blood oxygenation is performed mostly during cardiopul-
treat autoimmune disorders. In such diseases the patient’s monary bypass surgery to supplement the reduced capac-
immune system mistakenly targets tissues in the body for ity of the lung to exchange oxygen and carbon dioxide.
attack. Often this occurs through the generation of au- Membrane oxygenators containing highly gas-permeable
toantibodies by abnormal cells, which accumulate in the membranes such as silicone rubber are available commer-
bloodstream. As an adjunct to using immunosuppressants, cially as an alternative to bubble oxygenators. Oxygen per-
which minimizes formation of autoantibodies but have meating from the gas phase dissolves in the blood stream;
many side effects if employed alone over long periods, device integrity is especially important in this application
plasma containing the autoantibodies may be removed since leakage of gaseous oxygen into the bloodstream can
and replaced by a substitute fluid. Plasmapheresis accom- lead to serious complications.