Toxicology: General Principles
Self-poisoning is the most common toxicological problem: serious adverse effects are rare with good basic supportive management. The difficulty of performing human toxicological research means that the evidence base is very limited. Expert advice on the management of poisoning is available through a system of national poisons information centres.
History
A straightforward,
non-judgemental tone can help establish rapport. The details of the ingestion
(e.g. drugs, alcohol, suicide note, social factors and precipitants) should be
documented.
Resuscitation: ABCDE
Airway Ensure
the airway is open and protected, give oxygen if unwell.
Breathing Ensure
adequate respiratory rate and oxygenation saturation.
Circulation Look
for signs of shock, obtain intravenous access and bloods.
Shock responds to
intravenous fluid in most cases.
Arrhythmias may need
antidotes before usual treatments are effective (Chapter 25).
Disability Pupils
and Glasgow Coma Scale, seizures, agitation.
Exclude hypoxia and
hypoglycaemia.
Toxidromes are groups of
physical findings suggesting certain drug ingestions (see opposite).
Agitation can be a medical
emergency, putting staff and patients at risk. A combination of physical
restraint and chemical sedation may be required (Chapter 27).
Many toxic seizures are
self-limiting. Benzodiazepines are first-line treatment; barbiturates are
second-line. Phenytoin should not be used in seizures related to toxic
ingestions as its mechanism of action (Na+ channel blockade) is the same as the cause of many such fits.
Exposure Check
temperature, check for trauma or evidence of intravenous drug use (IVDU).
Examination
Any patient who is comatose
or has major ABC derangement needs resuscitation first. Poisoning can be
difficult to detect clinically: patients may be asymptomatic despite having
taken a lethal dose.
It is easy to assume that
all signs and symptoms are attributed to poisoning. Impaired consciousness may
be due to a head injury.
Management
Supportive care is the
mainstay of management: ensuring the patient does not come to harm while the
drugs are eliminated. Supportive care is a continuation of the principles of
resuscitation above. The patient should be nursed in an environment close to a critical
care area where close monitoring is possible. The observation unit of an
Emergency Department is ideal.
Risk assessment
Patients should be assessed
early for their risk of coming to harm:
• Risk of deterioration: patients who have taken particularly toxic drugs
that may have delayed presentation, e.g. tricyclic antidepressants, beta
blockers, digoxin. These patients should be observed in a high acuity area
until the danger is passed.
• Risk of absconding: patients who try to leave may be intoxicated,
confused, attention-seeking, psychotic, seriously depressed and intent on
finishing the job, or sometimes just bored. An assessment of their mental
capacity is essential (Chapter 26), to allow detention of the patient against
their wishes if necessary.
Minimising systemic toxicity
Systemic effects of drugs
are minimised by:
•
Decontamination – minimisation of drug absorption.
•
Elimination – maximising drug removal.
Specific poisons and their
treatments are discussed in Chapter 25.
Decontamination
Activated charcoal
Charcoal is ‘activated’ by
superheating, increasing its surface area to volume ratio to 900m2/g,
i.e. the area of a tennis court per gram. Activated charcoal adsorbs
drugs, minimising gut absorption. Charcoal also adsorbs drugs excreted
in the bile, some of which would normally be reabsorbed by the body: the
enterohepatic circulation.
Charcoal appears helpful if
given soon after ingestion, ideally within one hour. It should be given only
if:
•
the patient is alert, and drinks it voluntarily OR
•
the patient is intubated.
Charcoal is not helpful in
poisoning due to alcohols (ethanol, methanol, ethylene glycol), hydrocarbons,
alkalis, acids or metals.
Gastric washout, Ipecac
Gastric washout is rarely
performed, as it is unlikely to offer benefit over charcoal, and carries the
risk of aspiration. Washout is appropriate in a patient presenting within a few
hours of ingestion of a highly toxic overdose who has already been intubated.
Charcoal can be instilled after the washout. Ipecac is a plant extract that
causes vomiting; it is rarely used, as it can cause serious GI side effects.
Whole bowel irrigation
Whole bowel irrigation is
performed by infusing a clear inert solution (polyethylene glycol) orally until
no drug residue is passed. It is useful for slow-release preparations not
adsorbed by charcoal,
e.g. iron, lithium. It
requires close nursing supervision and is messy and so is usually performed in
the intensive care unit (ICU).
Elimination
Repeated charcoal
Repeat doses of activated
charcoal prevent reabsorption (and therefore enhance elimination) in drugs that
undergo enterohepatic recirculation, e.g. theophylline, anti-epileptic drugs
and digoxin.
Urinary alkalinisation
enhances excretion of weak acids such as aspirin. Forced diuresis with large
fluid volumes is dangerous as circulatory overload can occur.
Haemodialysis is used in
severe poisoning by drugs that cannot be removed by other
means, e.g. aspirin, lithium.
