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PRINCIPLES OF TOXICOLOGY

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Toxicology is the study of the toxic or harmful effects of chemicals. It is also concerned with the symptoms and treatment of poisoning and the identification of the poison.

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The variety of potential adverse effects and the diversity of chemicals in the environment make toxicology a very broad science. There are several fields of toxicology, including environmental (e.g., air and water pollution), economic (e.g., food additives, pesticides), legal (e.g., forensics, regulation of emissions, and additives), laboratory (e.g., analytical testing for chemicals), and biomedical (e.g., toxicities of drugs used to treat disease in humans and animals).

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The general principles of the toxic effects of chemicals are, for the most part, the same as the principles of the therapeutic effects of drugs.

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GENERAL PRINCIPLES IN THE TREATMENT OF POISONING

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Intentional and accidental poisonings are major medical problems. Every natural or synthetic chemical can cause injury if the dose is high enough.

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The single most important treatment of poisoned patients is supportive care.

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This is so important. You must treat the patient and not the poison. Provide airway support and ventilation and support blood pressure if needed. Toxicology screens of blood or urine take time and rarely change your therapy. If you know the poison, great; if not, treat the patient.

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To reduce absorption in an alert, relatively asymptomatic patient, use activated charcoal.

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Three procedures are widely used to reduce the absorption of poisons from the GI tract: inducing emesis, gastric lavage, and activated charcoal. To be effective, emesis must be induced within 1 hour of ingestion, and it works best if induced within 5 minutes of ingestion. Gastric lavage needs to be carried out within 1 hour of ingestion. Emesis induced with ipecac and gastric lavage only empties the stomach. Any poison that has moved into the small intestine is not removed. Activated charcoal remains in the GI tract, absorbing poison throughout.

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To enhance elimination, a number of techniques can be used. Multiple doses of charcoal reduce the half-life and increase clearance. Increasing the pH of the urine enhances elimination of weak acids. Hemodialysis and hemoperfusion can be used to help remove specific agents from the blood.

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SPECIFIC ANTIDOTES

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For some overdoses and poisons, specific antidotes are available. These are prime exam material and are relatively easy to learn (some you already know).

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Toxin Antidote
acetaminophen N-acetylcysteine
arsenic, mercury, gold BAL (dimercaprol)
β-blocker glucagon
benzodiazepines flumazenil
carbon monoxide oxygen, hyperbaric oxygen
coumarin vitamin K
cyanide nitrites
digoxin digoxin-specific Fab fragments
ethylene glycol or methanol fomepizole
heparin protamine
iron deferoxamine
isoniazid pyridoxine
lead dimercaprol, penicillamine, or succimer
narcotics naloxone
nitrites methylene blue
organophosphates atropine, pralidoxime

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