Toxicology — General Approach

General principles for the poisoned or overdosed patient. See also the specific toxidrome pages: anticholinergic, serotonin syndrome, neuroleptic malignant syndrome, opiate overdose, and paracetamol overdose.

Risk assessment

• Substance — dose, route of exposure
• Time since exposure
• Patient vulnerability — age, weight, comorbidities
• Clinical effects evident — sedation, ECG changes

Poisons Information Centre: 13 11 26 (24/7) — call with the above information for any concerning exposure.

Common calls to a poisons information centre (AJGP 2023)

A 2022 Queensland PIC audit of GP calls found paracetamol-containing products (7.7%) and household cleaning products (9.4%) among the most common, with ingestion (54%) and ocular exposure (14%) the leading routes.

• Repeated supratherapeutic paracetamol dosing — check paracetamol concentration and ALT; treat with NAC if concentration >20 mg/L or ALT >50 U/L.
• Paediatric accidental paracetamol ingestion — doses <200 mg/kg in an asymptomatic child can be managed with parental reassurance and avoiding further dosing for 24 hours (no testing needed). Intentional self-poisoning always warrants hospital referral for serum concentration measurement regardless of the stated dose.
• Household cleaning products — most accidental ingestions are small with mild symptoms; an asymptomatic child tolerating oral intake can be observed at home after a PIC risk assessment. High-risk products (oven/drain cleaners, concentrated disinfectants) warrant hospital referral if ingested in quantity.
• Ocular exposure (common: pool chemicals, Euphorbiaceae plant sap, hand sanitiser) — irrigate immediately for 30 minutes before examining; evert eyelids to remove particles; check pH post-irrigation (normal 6.5–7.4, continue irrigating if abnormal); fluorescein staining/slit-lamp for corrosive exposures; ophthalmology referral if injury is found.

Key investigations

In general practice while awaiting ambulance/transfer, the only tests typically useful are an ECG (QRS widening, QT prolongation) and blood glucose. Electrolytes and paracetamol level would generally only be done in a rural ED setting.

ECG: QRS widening

QRS >120 ms is abnormal. Drug classes associated with QRS widening (sodium channel blockade):

• Antidepressants — TCAs, venlafaxine
• Antiepileptics — carbamazepine
• Antihistamines — diphenhydramine
• Antipsychotics — chlorpromazine (antipsychotics as a class are not strongly associated with QRS widening but are commonly taken in overdose)
• Cardiovascular drugs — flecainide, propranolol
• Local anaesthetics — bupivacaine, ropivacaine
• Others — bupropion, chloroquine/hydroxychloroquine/quinine, cocaine, dextropropoxyphene, orphenadrine

ECG: QT prolongation and torsades risk

Seen with TCAs, some SSRIs (citalopram, escitalopram, fluoxetine), antipsychotics, and amiodarone. Use a QT nomogram — plot the median QT (measured across 6 leads) against heart rate; a point above the line implies torsades risk. QT prolongation is usually secondary to QRS widening rather than true JT-interval lengthening.

Drug classes associated with QT prolongation/torsades: antiarrhythmics (amiodarone, disopyramide, sotalol), other cardiovascular drugs (propranolol), antidepressants (citalopram, escitalopram, fluoxetine, moclobemide, TCAs), antihistamines (loratadine), antimicrobials (ciprofloxacin, clarithromycin, erythromycin, fluconazole, moxifloxacin, pentamidine, voriconazole), antipsychotics (amisulpride, chlorpromazine, haloperidol, ziprasidone), arsenic, chloroquine, cisapride, cocaine, methadone. A continuously-updated full list is at CredibleMeds.

Decontamination

• GI — activated charcoal: single dose, only if <1 hour since ingestion and no aspiration risk.
• Whole bowel irrigation: for slow-release preparations (iron, lead, lithium, metformin XR, slow-release potassium).
• Gastric lavage and induced emesis are not recommended.
• Skin: remove all clothing, soapy shower.
• Eyes: irrigate with water/normal saline for 15–20 minutes (longer for corrosive agents).

Complications

• Aspiration pneumonitis — chemical lung injury from aspirated gastric contents in sedated patients; rarely progresses to bacterial colonisation/sepsis. Distinct from aspiration pneumonia, which follows bacterial infection from oral flora in patients with swallowing difficulty.
• Rhabdomyolysis — seen with a long lie or hyperthermia. CK rises by 12 hours, peaks day 1–3. Features: AKI, hyperkalaemia, dark urine.

From https://tgldcdp.tg.org.au/viewTopic?topicfile=toxicology-general-approach (eTG Toxicology general approach).