OSCE — Poisoning Overview & Toxidromes

Candidate brief

You are the emergency department doctor called to assess a young man with reduced consciousness after a suspected overdose. You have 8 minutes to resuscitate and assess him in parallel, identify the likely toxidrome, decide on decontamination and antidotes with doses, and state your investigations, monitoring and disposition. Ask the examiner for any history, examination findings or results you need.

Scenario

This station is entirely fictional. The patient, the history, the timings and every numerical value are invented for assessment purposes and do not describe a real person.

A 24-year-old man is brought to the emergency department by ambulance. His flatmate found him asleep on the sofa about 45 minutes ago and could not fully rouse him. On the coffee table were an empty blister strip labelled alprazolam 0.5 mg and a second, unlabelled strip of white tablets, about half empty. The flatmate says the patient had been prescribed “pain medicines” for back pain. There was no trauma, no alcohol, and no injecting paraphernalia. Ingestion is estimated at well under one hour ago. In the department he rouses briefly to voice, mumbles a few words and drifts off.

Examiner information

All values are fictional. Give them only when asked.

If time remains, ask: “If this had been a tricyclic antidepressant overdose with a QRS of 130 ms, how would you manage the wide complexes?”

Assessed actions

Safety-critical failures

Any one of the following fails the station:

• Gives empiric flumazenil for the drowsiness of an unknown, mixed overdose: across randomised trials, adverse events were more common with flumazenil than placebo (138/498 versus 47/492; risk ratio 2.85) ^[38].
• Delays glucose for thiamine — treating the hypoglycaemia must not wait for thiamine, which is itself low risk and potentially beneficial ^[12].
• Gives activated charcoal routinely, or to this drowsy patient, without assessing airway protection first: single-dose charcoal is not routine, and decontamination decisions follow initial resuscitation ^{[3] [2]}.
• Manages a tricyclic-wide QRS with a class I antiarrhythmic instead of sodium bicarbonate: tricyclic poisoning responds well to sodium bicarbonate, with a 1–2 mmol/kg bolus to assess response ^[31].

Worked answer

Open with resuscitation and risk assessment running together: the overall approach to the poisoned patient is initial acute resuscitation combined with risk assessment, weighing the exposure and its anticipated severity against the patient's status and the treatments that may be required ^[2]. Position the airway, give oxygen for SpO2 88%, and take the scene history while doing so.

Check point-of-care glucose immediately in any obtunded overdose. At 3.2 mmol/L this is hypoglycaemia in a patient who cannot swallow: give intravenous glucose now. Thiamine is low risk and potentially beneficial, but hypoglycaemia treatment should not be delayed for it ^[12]; a national veterans' study found no cases of Wernicke's encephalopathy among patients who received dextrose before thiamine ^[12].

The examination pattern — 2 mm poorly reactive pupils, warm non-diaphoretic skin, normal temperature, hypoventilation and reduced consciousness — fits the opioid toxidrome. The well-defined toxidromes are the opioid, anticholinergic, cholinergic and sympathomimetic, with a GABAergic syndrome also described ^[1], and knowing them allows rapid pattern recognition of specific poisonings ^[1]. Serotonin toxicity is not suggested here: it combines neuromuscular excitation (clonus, hyperreflexia, myoclonus, rigidity), autonomic stimulation (hyperthermia, tachycardia, diaphoresis, tremor, flushing) and altered mental state ^[46], and this patient has none of those features.

Give naloxone titrated to ventilation. The aim is a balance between restoring respiration — not consciousness — and avoiding withdrawal, achieved by titration after initial doses of 0.4–2 mg ^[36]. Low-dose naloxone, 0.04 mg with appropriate titration, has been recommended to reverse ventilatory depression ^[37]; in a case series a median dose of 0.08 mg (range 0.04 to 0.12 mg) reversed opioid-induced ventilatory and CNS depression ^[45]. Naloxone is rapidly eliminated with a half-life of 60–120 minutes, so expect re-sedation and admit to a monitored area ^[36].

Obtain the ECG early. In this patient the QRS is 88 ms. In tricyclic overdose the dividing line is 0.10 second: patients at or above it had a 34 per cent incidence of seizures and a 14 per cent incidence of ventricular arrhythmias, while neither occurred below that duration ^[40]. Tricyclic cardiotoxicity responds well to sodium bicarbonate ^[31], and a bolus of 1–2 mmol/kg can be used to assess the response to alkalinisation ^[31].

Decontamination is selective. Single-dose activated charcoal should not be administered routinely ^[3]; it may be considered when a potentially toxic amount of a poison known to adsorb to charcoal was ingested up to one hour previously ^[3] — this patient is inside that window but has GCS 11, so assess and protect the airway before any charcoal, because resuscitation precedes decontamination ^[2]. Its effect decays with time — mean absorption reductions of 47.3 per cent at 30 minutes, 40.07 per cent at 60 minutes, 16.5 per cent at 120 minutes and 21.13 per cent at 180 minutes ^[3] — and the latest recommendations note that the useful window differs by poison and formulation, with administration beyond 6 hours acceptable when ongoing absorption is suspected, for example with modified-release preparations or pharmacobezoar ^[49]; a systematic review reported benefit beyond one hour in many clinical scenarios ^[4].

Investigations: ECG, point-of-care glucose, paracetamol and salicylate concentrations — paracetamol treatment decisions rest on the 150 mg/L at 4 hour line of the Rumack–Matthew nomogram ^[13] — plus urea, creatinine, liver tests and a venous gas. Phone the poison information centre: clinical resources such as Toxbase exist precisely to provide detailed guidance on the management of poisoning ^[1]. Disposition is a monitored bed for re-sedation risk, with psychiatric assessment once he has recovered.

Teaching debrief

• Titrate naloxone to ventilation, not wakefulness — 0.4–2 mg in titrated aliquots, or 0.04 mg increments — and remember the 60–120 minute half-life when planning observation ^{[36] [37]}.
• Flumazenil is not a waking drug for the unknown overdose: adverse events were 2.85 times more common than with placebo in randomised trials of suspected benzodiazepine intoxication ^[38].
• Glucose before thiamine: no Wernicke cases were identified when dextrose preceded thiamine in a national cohort, and hypoglycaemia treatment should not be delayed ^[12].
• Not every wide QRS is one lesion: tricyclic widening responds to sodium bicarbonate ^[31], but bupropion widens QRS by blocking cardiac gap junctions and does not respond well to alkalinisation — median QRS change with bicarbonate was −2 ms (not significant) ^{[22] [31]}.
• In India, aluminum phosphide is the leading cause of poisoning-related mortality, with about 54 per cent pooled mortality and no definite antidote — one more reason to involve the poison centre early for unfamiliar agents ^{[19] [1]}.

References16ShowHide

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