Dialysis is the process in which the blood is circulated through a bath in which a semipermeable membrane separates the components of the blood from the constituents of the dialysis fluid (Figure 3.1). In dialysis the various substances in blood will diffuse across into the dialysis bath provided that they are small enough (low molecular weight) to transit the membrane and their concentration is lower in the bath than in blood. Many factors relate to the potential effectiveness of dialysis. Tissue binding of the toxin, high volume of distribution for the toxin, and high molecular weight are three factors which diminish the efficacy of dialysis. Lithium, methanol, isopropanol, salicylates, theophylline, and ethylene glycol are examples
of substances which respond well to this therapy and are quite rapidly removed. A toxin which does not respond is digoxin. This is due to the fact that digoxin is largely tissue bound. Because very little of this drug is found in the blood, it makes sense that treatment of the blood to affect its removal would not be especially efficacious.
Dialysis can be conducted with an external bath, in which case it is known as hemodialysis. It can also be carried out in the patient's peritoneal cavity (peritoneal dialysis). The latter form is easier to set up and does not require as much instrumentation; however, the rate of toxin removal is much slower by peritoneal than by hemodialysis.
Hemoperfusion is a process in which the blood is pumped through an external cartridge (Figure 3.2). It has the advantage of rapidly exposing the blood to a filtering device and is theoretically faster for removal of toxins. The cartridge used may contain charcoal or some other adsorbent. Hemoperfusion can be effective for toxins that are sluggish in responding to dialysis due, for example, to their protein binding in serum or their relatively high molecular weights.
FIGURE 3.2 Hemoperfusion.
FIGURE 3.2 Hemoperfusion.
Some Drugs Removed by Dialysis
Methanol Methotrexate Methyldopa Oxalic acid Phenobarbital Potassium chloride Quinidine Theophylline
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