Biomedical Engineering Reference
In-Depth Information
pressure (pushing air into the trachea). More elaborate techniques employ
pressure curves during each cycle and have their rhythm triggered by the natural
breathing of the patient. However, the problem remains that when a lung needs
such support, it is already highly stressed and needs time to heal and rest. All
artificial addition of oxygen or gas into the lungs necessarily means stress to the
lungs, so that the condition is bound to become aggravated. Relief of the stressed
lung is possible only with extracorporeal carbon dioxide removal and oxygena-
tion. This again means stress to the whole body due to the exposition of blood to
a large foreign surface, the anticoagulation regime and the unphysiological flow.
It is supposed, that extracorporeal lung support could yield better clinical results,
if it were instituted earlier.
1.3 Membranes for plasma separation
1.3.1
Introduction
In the literature, the terms `plasmapheresis', `plasma separation' and `apheresis'
are used. `Apheresis' is the broadest term, in that it describes any process that
removes/eliminates (Greek: aphairesis) harmful substances from blood. In this
sense, also (haemo)dialysis falls under the term `apheresis'. The terms
`plasmapheresis' and `plasma separation' are synonymous and will both be
used in this chapter to describe the same process: to separate blood plasma from
blood cells.
The idea behind membrane plasma separation is to eliminate harmful
substances from a patient's blood. Smaller molecules, i.e. water-soluble toxins
such as urea or creatinine or low molecular weight protein toxins such as beta-2-
microglobulin can effectively be removed by dialysis. For larger molecules
plasma separation is the method of choice. Blood plasma together with the
toxins is separated from the blood cells (white cells, red cells, platelets), which
are returned to the patient. The plasma can then be treated to remove the toxins
or be discarded and replaced by healthy donor plasma. The treatment of the
plasma can either be a fractionation (by cascade filtration with a plasma
fractionation membrane) or adsorption of the toxins to be removed.
Over the years, a great number of treatment variants that include plasma
separation have been developed. Those aim at removing harmful substances that
cannot be removed by haemodialysis, such as immunoglobulins (IgM, IgG),
immune complexes, low density lipoproteins (LDL), fibrinogen, light and heavy
chains, etc. Those cannot be removed by haemodialysis, as these molecules are
larger than the smallest essential blood components that are to be preserved and
not removed, namely the smaller proteins, the prominent example of which is
albumin. As long as albumin is to be retained, it is not possible to remove the
larger toxins by a one stage membrane separation process such as haemodialysis,
which functions on the basis of size exclusion.
