Many techniques have been developed to decrease drug absorption and increase drug elimination, but most of these manoeuvres are only helpful with a few drugs. These, as well as antidotes to specific drugs, are described below.
Decreasing drug absorption
Vigorous attempts to empty the gastrointestinal tract are indicated when drugs that cause potentially fatal complications other than coma or respiratory depression have been ingested. Examples of such drugs are aspirin, paracetamol, colchicine, organophosphates, iron salts and tricyclic antidepressants. It is important to remember that the risk of aspiration into the lungs associated with the use of gastric lavage may well cause more problems than the effects of the drugs themselves. Gastric lavage does not remove stomach contents completely.
Induced emesis may be useful in small children as they are more difficult to lavage. It is rarely used in adults, as only small amounts of drug are recovered. Paediatric ipecacuanha emetic mixture (not the undiluted fluid extract) is the emetic of choice. The dose is 10 ml for a child and is given to children over 6 months old. Other emetics such as apomorphine, saline, copper sulphate and mustard are dangerous and should not be used.
Basicdrugs such as quinidine or tricyclic antidepressants,where gastric emptying is delayed Paracetamol
This procedure is of use only when a large quantity of drug has been taken. The earlier gastric lavage is performed, the greater the amount of drug that is retrieved. It is of little value after 4 hours except for the drugs. Lavage is contraindicated for some poisons, e.g. corrosives, petrol or paraffin.
If administered promptly (within 4 hours) and in sufficient quantity, activated charcoal significantly reduces the gastrointestinal absorption of many drugs. The ratio of charcoal to the amount of poison to be absorbed is about 10: 1 and hence is most useful when relatively small doses of drugs are toxic, as with tricyclic antidepressants, and when emesis, gastric lavage and aspiration are contraindicated. It can be administered by mouth or down a nasogastric tube.
Absorption should be minimized for those poisons which are absorbed through the skin by removal of contaminated clothing and careful washing of the skin with soapy water.
Should be performed by an experienced nurse and doctor
The main danger is from pulmonary aspiration, and it is vital that the tracheobronchial tree is protected either by an intact cough reflex or by a cuffed endotracheal tube
The patient should be positioned lying on the left side, with the head over the end or side of the bed so that the mouth and throat are at a lower level than the larynx and trachea
A wide-bore tube (Jacques’gauge 30) is lubricated with glycerine or Vaseline and passedinto the stomach. Aspiration is performed first, and then followed by lavage using 300 ml of water at body temperature for the first washing
This processshould be repeated at least three or four times, using up to 500ml of water on each occasion An aliquot of the washing should be saved in caseit is needed for drug analysis
Increasing drug elimination
Forced alkaline diuresis
This potentially lethal technique is rarely necessary, as few drugs are excreted in their unchanged form. Its benefits have been shown to be outweighed by its serious complications unless monitoring of fluid balance and urine pH is scrupulous. It is mainly used in salicylate poisoning, which is discussed below.
The use of this technique is limited by its low efficacy but it may be indicated for patients severely poisoned by ethylene glycol.
This technique may be useful for patients with severe poisoning by lithium salts or methyl or ethyl alcohols. Rarely, patients with severe salicylate poisoning (blood salicylate level >900 mg litre-lor 6.5 mmol litre “) refractory to forced alkaline diuresis may be helped by haemodialysis.
This involves the passage of heparinized blood through devices containing absorbent particles, such as activated charcoal or resins, to which drugs are adsorbed. Its use should be considered in patients severely poisoned with certain drugs (e.g. theophylline, short- and mediumacting barbiturates and glutethimide) who fail to improve despite the use of adequate supportive measures. Antagonizing the effects of poisons These techniques will be considered under the individual drugs. Specific antidotes are available for a small number of drugs. Antidotes act in a number of ways:
INTERACTION WITH POISON TO FORM AN INERT COMPLEX THAT IS THEN EXCRETED, e.g. desferrioxamine in iron overdose
ACCELERATION OR DETOXIFICATION OF A POISON, e.g. methionine, N-acetylcysteine in paracetamol poisoning
PREVENTION OF THE FORMATION OF A MORE TOXIC COMPOUND, e.g. ethanol used as a competitive substrate for the metabolizing enzyme to prevent formation of toxic metabolites in methanol poisoning
COMPETITION WITH THE POISON FOR ESSENTIAL RECEPTORS, e.g. naloxone in opiate poisoning
BLOCKADE OF RECEPTORS THROUGH WHICH THE TOXIC EFFECTS ARE MEDIATED, e.g. atropine used to block cholinergic receptors in organophosphate poisoning