Poisoning Viva

Long Case Viva Defence

Candidate's opening statement (model answer)

"Mr David Chen is a 28-year-old software engineer who presents with a deliberate mixed overdose of amitriptyline and paracetamol with alcohol, an uncertain 4 to 6 hours ago. He is drowsy with a GCS of 12, tachycardic at 124, hypotensive at 96 over 58, with dry flushed skin, dilated pupils and absent bowel sounds — an anticholinergic picture consistent with the tricyclic. His ECG shows a QRS of 120 milliseconds with a terminal R wave in aVR, signalling early sodium-channel blockade. His four-hour paracetamol level is 220 milligrams per litre, well above the Rumack-Matthew treatment line." [1]

"His main problems are:

1. Tricyclic antidepressant overdose with early cardiotoxicity — the widened QRS is the prognostic marker and mandates intravenous sodium bicarbonate.
2. Paracetamol overdose at a hepatotoxic dose and level — N-acetylcysteine is indicated.
3. A falling GCS with an unprotected airway — intubation is required before any decontamination.
4. Alcohol co-ingestion with a risk of hypoglycaemia.
5. An underlying psychiatric crisis." [1]

"My immediate priorities are to secure the airway, give sodium bicarbonate for the QRS widening, start N-acetylcysteine for the paracetamol, and give activated charcoal once intubated." [1]

Examiner probing questions and model answers

Q1: "Walk me through your decision to give charcoal — or not." [1]

"Charcoal binds both amitriptyline and paracetamol, and the ingestion was within the last four to six hours, so it is the correct decontamination modality. But his GCS is 12 and falling, and his airway is not protected. The single most dangerous contraindication to charcoal is an unprotected airway — aspiration of charcoal causes a catastrophic chemical pneumonitis. My plan is to perform rapid sequence intubation first, then give 50 grams of activated charcoal via nasogastric tube. I would never give charcoal before the airway is secured. If his GCS were 15 and his airway intact, I would give charcoal without intubation." [1]

Q2: "His QRS is 120 ms. How does that change your management, and what is your threshold?" [1]

"A QRS over 100 milliseconds after a tricyclic overdose warrants intravenous sodium bicarbonate. Over 160 milliseconds carries a 50 per cent seizure risk. I would give 1 to 2 millimoles per kilogram of sodium bicarbonate as an intravenous bolus, repeated to narrow the QRS, then an infusion to maintain a serum pH of 7.50 to 7.55. Sodium bicarbonate works by two mechanisms — the sodium load overcomes the channel blockade, and the alkalinisation shifts the drug off the sodium channel into its neutral form. I would avoid class Ia and Ic antiarrhythmics entirely, because they worsen the blockade. If he develops refractory ventricular dysrhythmia, the next step is 20 per cent lipid emulsion." [1]

Q3: "When will you start the N-acetylcysteine, and what regimen?" [1]

"I start it now — his four-hour paracetamol level is 220 milligrams per litre, well above the treatment line. The regimen is 150 milligrams per kilogram over one hour, then 50 milligrams per kilogram over four hours, then 100 milligrams per kilogram over 16 hours. N-acetylcysteine replenishes glutathione and is a sulphate donor for the toxic NAPQI metabolite. I send baseline liver function and coagulation, and repeat them after the infusion and at 24 hours. The alcohol co-ingestion does not change the NAC indication. If his level had been below the line, I would not give NAC." [1]

Q4: "He becomes more drowsy. Would you give flumazenil to reverse the sedation?" [1]

"No. Flumazenil is contraindicated in a mixed overdose with a pro-convulsant such as a tricyclic — it precipitates seizures that are refractory to standard treatment. I would reserve flumazenil for isolated, iatrogenic benzodiazepine over-sedation in a benzodiazepine-naive patient. For his sedation, the correct response is to protect the airway by intubation, not to reverse the sedation pharmacologically." [1]

Q5: "His paracetamol ingestion was staggered over several hours. Can you still use the nomogram?" [1]

"No. The Rumack-Matthew nomogram applies only to a single acute ingestion with a known time. For a staggered ingestion or an unknown time, the nomogram cannot be plotted, and the standard practice is to treat with N-acetylcysteine on clinical grounds. I would give NAC and monitor the liver function and coagulation, continuing beyond the standard course if there is any hepatotoxicity." [1]