Plasma filtration plasmapheresis

Equation 8.7 [6] was obtained to correlate the experimental data on membrane plasmapheresis, which is the MF of blood to separate the blood cells from the plasma. The filtrate flux is affected by the blood velocity along the membrane. Since, in plasmapheresis, all of the protein molecules and other solutes will pass into the filtrate, the concentration polarization of protein molecules is inconceivable. In fact, the hydraulic pressure difference in plasmapheresis is smaller than that in the UF of plasma. Thus, the concentration polarization of red blood cells was assumed in deriving Equation 8.7. The shape of the red blood cell is approximately discoid, with a concave area at the central portion, the cells being approximately 1-2.5 pm thick and 7-8.5 pm in diameter. Thus, a value of r (= 0.000257 cm), the radius of the sphere with a volume equal to that of a red blood cell, was used in Equation 8.7. 

Membrane plasmapheresis is also the first step for treatment of pathological plasma in the case of autoimmune diseases, as the patient retains his own red blood cells while his plasma is replaced by an albumin solution or fresh frozen plasma obtained from donors (plasma exchange therapy). Other more selective plasma purification techniques consist in eliminating pathologic immunoglobulins or LDL cholesterol familial hypercholesterolemia, either by a secondary filtration, chemical adsorption or immunoadsorption. A description of various applications of plasmapheresis can be found in the book edited by Smit Sibinga and Rater [15]. 

The following processes can be described as selective therapeutic plasmapheresis. In a first step, blood is withdrawn from the patient and separated by crossflow filtration in a hollow-fiber membrane cartridge water and some plasma solutes are transferred through a semipermeable membrane under a convection process. The transmembrane pressure applied from blood to filtrate compartment ensures flow and mass transfers. Then, the filtrate perfuses the adsorption columns where toxins are retained and is finally mixed with blood cells and other plasma components before returning to the patient (Figure 18.11). 

In recent years, the fractionation and purification of blood and blood products has emerged as a significant enterprise. Separation of blood and plasma into various cellular and protein fractions has become more of a necessity, given the specific requirements of newer therapies. The first step, the separation of plasma from whole blood (a procedure known as plasmapheresis), is now carried out with a filtration process using synthetic microporous membranes. Chemical engineers pioneered the development of this process and have provided the understanding of what determines its performance in terms of fundamental transport principles. 

Therapeutic plasma exchange (TPE), or plasmapheresis (PP), is an extracorporeal therapy most frequently used in the treatment of hematologic disorders, and autoimmune neuropathies and vasculitides [37]. This modality occasionally is also employed in the treatment of poisoning. The apparatus involves central venous access and a blood circuit between the patient and a pheresis machine. Cytopheresis by centrifugation or filtration then separates the formed elements of blood from plasma. The cells are returned to the patient while the plasma (with the poison) is discarded. Fluid volume is typically replaced with crystalloid, colloid, or fresh frozen plasma (FFP) if clotting factor repletion is necessary. 

Plasmapheresis. The separation of plasma from whole blood by continuous membrane filtration represents an improvement over conventional centrifugation techniques in terms of efficiency, safety and cost. In the past, plasmapheresis was carried out with blood donors by collecting their whole blood in plastic bags which were then centrifuged to separate the red cells from the plasma. The supernatant plasma was then decanted and the red cells returned to the donorenabling plasma to be drawn from the same person as frequently as three times per week. Most of this plasma is then processed to yield purified components such as albumin or anti-hemophilic factor (Factor VIII). 

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