Plasmapheresis is an effective form of definitive or adjunctive therapy for a number of conditions which the intensive care specialist may encounter. Most conditions for which the effect has been shown are associated with circulating antibodies or immune complexes. However, there is evidence to support the concept of immunomodulation through other processes, for example, an increase in splenic clearance of heat-inactivated red blood cells, suggesting improved endogenous clearance of antibodies. Other substances are removed, including intact complement, activated complement products, fibrinogen, and various inflammatory mediators.
Substances removed are distributed primarily in the extravascular space. Therefore plasmapheresis kinetics involve those forces which govern intravascular depletion, catabolism, and synthesis as well as shifting substances from the extravascular to the intravascular compartment. Substance removal from the intravascular space follows the exponential relationship
where X0 is the initial concentration of substance X, Xt is its concentration at time t, and r is the fractional rate of exchange defined by volume exchanged per hour
If there is no new synthesis or redistribution of substance X during plasmapheresis, removal of the first plasma volume will produce an initial 63 per cent reduction in plasma concentration of substance X. Additional removal of a second plasma volume produces a further 25 per cent reduction, while removal of a third plasma volume results in only a further 9 per cent reduction.
Endogenous synthesis and shifting of substance X from extravascular to intravascular space are the predominant modes of reaccumulation of the macromolecule between treatments. The magnitude of this reaccumulation is both variable and unpredictable, depending on the substance being considered and individual patient characteristics. Nevertheless, most authors consider four or five single plasma volume exchanges over a period of 7 to 10 days to be adequate short-term therapy, as this would result in a plasma concentration of a given substance of less than 10 per cent of its prepheresis concentration. Serial clinical assessment and measurement of the offending substance (when known) allow decisions to be made regarding the frequency and volume of exchanges, so that treatment protocols can be tailored to specific conditions and patients.
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