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ipecacuanha but may be used after successful emesis if this method has been deemed necessary; methionine, used orally for paracetamol poisoning, is also adsorbed.

Other oral adsorbents have specific uses. Fuller's earth and bentonite (both natural forms of aluminium silicate) bind and inactivate the herbicides, paraquat (activated charcoal is superior) and diquat; cholestyramine and colestipol will adsorb warfarin.

Gastric lavage incurs dangers as well as benefits; it is best confined to the hospitalised adult who is believed to have taken a potentially life-threatening amount of a poison within 1 h (or longer in the case of drugs that delay gastric emptying, e.g. aspirin, tricyclic antidepressants, sympathomimetics, theophylline, opioids). Lavage is probably worth undertaking in any unconscious patient who is believed to have ingested poison, and provided the airways are protected by a cuffed endotracheal tube. Paradoxically, lavage may wash an ingested substance into the small intestine, enhancing its absorption. Leaving activated charcoal in the stomach after lavage is appropriate to lessen this risk. Nevertheless, patients who have ingested tricyclic antidepressants or centrally depressant drugs must be subject to continued monitoring after the lavage.

The passing of a gastric tube, naturally, takes second place to emergency resuscitative measures, institution of controlled respiration or suppression of convulsions. Nothing is gained by aspirating the stomach of a corpse.

Emesis has been used for children and also for adults who refuse activated charcoal or gastric lavage, or if the poison is not absorbed by activated charcoal. Its routine use in emergency departments has been abandoned, as there is no clinical trial evidence that the procedure improves outcome for poisoned patients. Emesis is induced, in fully conscious patients only, by Ipecacuanha Emetic Mixture, Pediatric (BNF), 10 ml for a child 6-18 months, 15 ml for an older child and 30 ml for an adult, i.e. all ages may receive the same preparation but in a different dose, which is followed by a tumblerful of water (250 ml). The active constituent of ipecacuanha is emetine; it can cause prolonged vomiting, diarrhoea and drowsiness that may be confused with effects of the ingested poison. Even fully conscious patients may develop aspiration pneumonia after ipecacuanha.

Both emesis and lavage are contraindicated for corrosive poisons, because there is a risk of perforation of the gut, and for petroleum distillates, as the danger of causing inhalational chemical pneumonia outweighs that of leaving the substance in the stomach.

Cathartics or whole-bowel irrigation4 have been used for the removal of sustained-release formulations, e.g. theophylline, iron, aspirin. Evidence of benefit is conflicting. Activated charcoal in repeated (10 g) doses is generally preferred. Sustained-release formulations are now common, and patients have died from failure to recognise the danger of continued release of drug from such products, after apparently successful gastric lavage.

SPECIFIC ANTIDOTESs

Specific antidotes reduce or abolish the effects of poisons through a variety of mechanisms, which may be categorised as follows:

• receptors, which may be activated, blocked or bypassed

• enzymes, which may be inhibited or reactivated

• displacement from tissue binding sites

• exchanging with the poison

• replenishment of an essential substance

• binding to the poison (including chelation).

4 Irrigation with large volumes of a polyethylene glycol-electrolyte solution, e.g. Klean-Prep, by mouth causes minimal fluid and electrolyte disturbance (it was developed for preparation for colonoscopy). Magnesium sulphate may also be used.

5 Mithridates the Great (7132-63 BC) king of Pontus (in Asia Minor) was noted for 'ambition, cruelty and artifice'. 'He murdered his own mother ... and fortified his constitution by drinking antidotes' to the poisons with which his domestic enemies sought to kill him (Lempriere). When his son also sought to kill him, Mithridates was so disappointed that he compelled his wife to poison herself. He then tried to poison himself, but in vain; the frequent antidotes which he had taken in the early part of his life had so strengthened his constitution that he was immune. He was obliged to stab himself, but had to seek the help of a slave to complete his task. Modern physicians have to be content with less comprehensively effective antidotes, some of which are listed in Table 9.1.

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