Introduction: Sorbents in Extracorporeal Blood Therapy

Abstract

S are compounds that selectively or non-selectively bind or remove certain chemicals from solution while not removing other chemicals. Technically, there are 2 types of sorbents: adsorbents attract and bind chemical to their outer surface, whereas absorbents bind chemicals to inner surfaces. Practically, the distinction is not very clear or important in medical applications so the general term sorbents applies well enough. Commonly, when we think of sorbents, we think of columns packed with small granules, such as charcoal tanks and water softeners of water treatment systems, hemoperfusion columns, and the Sorb® columns used with the Redy® sorbent dialysis system. However, sorbents can also be used in powdered form, such as charcoal or sodium polystyrene sulfonate taken orally to remove drugs or potassium or the sorbent suspension in the BioLogic-DT® System for treatment of hepatic failure or drug overdose. Some sorbents are actually soluble (or partially soluble) compounds. In this regard, even calcium carbonate, calcium acetate, and sevelamer are sorbents when they bind phosphate in the gut. In standard hemodialysis, selectivity of chemical removal is provided in 2 ways. The pore size of the membranes allows passage of all compounds below a certain molecular weight from plasma water into the dialysis. Addition of certain chemicals into the dialysate prevents excessive removal of vital substances from the patient, including sodium, chloride, bicarbonate, calcium, potassium, magnesium, and glucose. Except for these few vital compounds, however, everything that goes through the membranes of a hemodialyzer is discarded. Hemodialysis works in part because the liver has the capability to replenish most of the other vital substances such as essential amino acids and metabolic intermediates, and the endocrine system can replace hormones lost such as aldosterone, corticosteroids, thyroid hormone, small quantities of insulin, and the like. When used in extracorporeal blood therapy, sorbents function much differently than standard hemodialysis. Sorbent compounds provide intrinsic selectivity of chemical removal, competition between similar chemicals for binding sites, and the opportunity to load the sorbent with any substances desired. The release of bound substances may occur in exchange for binding of other substances (such as sodium and hydrogen exchange for ammonium made by urease in the Sorb column) or release may occur in response to the surrounding concentrations reflecting the concentration in the blood (as in the release of glucose loaded on charcoal, which is greater when the patient’s blood glucose is low and less when the glucose level is high). The intrinsic selectivity of sorbents allow extracorporeal devices that essentially would be impossible by standard dialysis techniques. There are many examples of sorbent applications in extracorporeal blood therapy in this