Hemodialysis is generally reserved for specific toxins that must be both potentially life threatening and amenable to removal by this method (see T§b.Je.151-4). A semipermiable membrane is used to create a concentration gradient to filter out toxins. The benefits include the ability to remove toxins that are already absorbed from the gut lumen, removal of substances that do not adhere to activated charcoal, and the ability to remove both the parent compound and the active toxic metabolites. Hemodialysis is much less effective where the toxin ingested has a large volume of distribution (>1 L/kg), has a large molecular weight (more than 500 Da), or is highly protein bound.
Hemodialysis is rarely absolutely contraindicated, but relative contraindications include hemodynamic instability, very small children, patients with poor vascular access or profound bleeding diatheses. Risks of hemodialysis are typically minimal in experienced centers, but they include large fluid shifts, electrolyte imbalances, infection and bleeding at the catheter site, and intracranial hemorrhage.
Hemoperfusion, which is also used for decontamination of a patient's systemic circulation, involves placing a filter filled with activated charcoal into the circuit of the hemodialysis machine. This filtration alleviates the constraints of protein binding and molecular size, both of which limit the utility of hemodialysis. Toxins that can be removed by this method must adsorb well to activated charcoal and have a small volume of distribution. In practice, it is only commonly recommended for theophylline overdoses.
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