Applications
Influence on active therapy
While supportive therapy remains the corner stone of the management of acute poisoning, specific antidotes are available for metals (chelation agents), anticholinesterase inhibitors (atropine, pralidoxime, obidoxime), methanol and ethylene glycol (ethyl alcohol, 4-methyl pyrazole), paracetamol (N-acetylcysteine) and opioids (naloxone). Given a clear diagnosis, a clinician usually administers the antidote without waiting for laboratory confirmation, but subsequent analyses may help to decide whether to continue with the therapy. For example, both parenteral and oral therapy with desferriox amine in cases of iron poisoning is indicated if patients deteriorate and the serum iron concentration is extremely high. Measurements of cholinesterase activity in serum or red cells are useful in a situation of high-dose infusions of atropine into patients exposed to organophosphorus insecticides or thiocarbamates. Measures designed to reduce the absorption of poisons from the gut, such as the use of emetics, purgatives, gastric lavage and irrigation, are now considered to be of limited value and unwarranted in most cases of poisoning. The efficacy of whole-bowel irrigation is also questionable, although some advocate its use to remove sustained-release or enteric-coated preparations of, for example, iron salts and other potentially lethal poisons that have passed into the small bowel, and in the decontamination of body packers. A single oral dose of activated charcoal has largely replaced other means of reducing absorption, although it is generally useful only when given within 1 hour of ingestion and fails to absorb inorganic ions, alcohols, strong acids or alkalis, or organic solvents. Techniques to increase the rate of elimination of poisons, such as diuresis, adjusting the urinary pH, haemodialysis and peritoneal dialysis, venous–venous haemofiltration and charcoal haemoperfusion, are now hardly used. Forced diuresis is now frowned upon; it is probably beneficial only in cases of poisoning with thallium and, when coupled with alkalisation of urine, chlorophenoxy herbicides. Alkalisation of urine effectively increases the elimination of salicylates, phenobarbital and chlorophenoxy herbicides. Acidification of the urine has little merit in increasing the elimination of weakly basic substances, such as amfetamines and phencyclidine. Theoretically the elimination of tricyclic antidepressants might be increased by acidification. However, the dangerous effect of this poisoning is the enlarging of the PQ complex, which toxic cardiac effect requires alkalisation by sodium bicarbonate. Haemodialysis can enhance the elimination of hydrophilic toxins with a small volume of distribution and is useful in treating severe poisoning with salicylates, lithium, methanol, ethylene glycol and chlorophenoxy herbicides. Peritoneal dialysis is by no means as efficient as haemodialysis but is more accessible in remote regions and in devel oping countries. Haemofiltration also has a role in this context. ‘Gut dialysis’, or the use of multiple oral doses of activated charcoal, is thought to operate by creating a drug concentration gradient across the gut wall that leads to movement of the drug from the blood in the superficial vessels of the gut mucosa into the lumen. So far, its efficacy has been demonstrated for carbamazepine, dapsone, phenobarbital, quinine and theophylline, and there is evidence for its application in poisoning with calcium antagonists. Most of these procedures carry inherent risks to the patient and, as pointed out above, their applications are limited only to a handful of poisons. Toxicological analyses to identify and quantify the poison should be used to ensure that such interventions are used appropriately and at the same time to prevent overtreatment of patients who would recover without them. Clinical toxicologists have to know the toxic effects of the particular drug. For instance, verapamil, diltiazem, amlodipine and nefidrine are all calcium blockers. However, each has its own required treatment and prognosis.
