Branching cross-examination

This is a MedVellum formative viva. It is not an official RACP, MRCPCH, ABP, ACGME or RCPSC station, mark scheme, duration or pass standard. Release each update only after the candidate states the failing system or the decision rule, the immediate action and the reassessment endpoint. [1]

Candidate brief

You are the senior paediatric clinician in the emergency department. Speak as you would during a poisoning assessment. State the ingestion history you need, the nomogram decision, the antidote you will start and what you will reassess. This is one continuous case. Each escalation branch leads to the next update. [1]

Question 1 — The ingestion history and the first sixty minutes

Stimulus update. A parent carries an eight-year-old who may have swallowed paracetamol tablets about two hours ago. The child is alert and mildly nauseated. Question: What do you say and do now, and what history must you establish? [1]

Consultant-level model answer. "I begin with the airway, breathing and circulation and check bedside glucose. This is a suspected paracetamol ingestion, so the decisive history is the ingestion pattern: the formulation and amount, the time of ingestion as precisely as possible, whether it was a single dose or staggered, and any co-ingestants. I establish that this was a single acute ingestion at a known time of about two hours ago. Because the child presents within the charcoal window after a significant ingestion, I give activated charcoal at 1 gram per kilogram now, weighing benefit against vomiting. I draw the paracetamol level at four hours and plot it on the Rumack-Matthew nomogram; I will not interpret a level drawn before four hours." [1] [4]

Probing follow-up. "Why will you not interpret a level drawn now?" A strong answer is: "Because absorption is still ongoing before four hours and a level drawn now underestimates the eventual peak. The nomogram is only valid from four hours after a single acute ingestion." [1]

Common weak answer. "I will draw a level now and treat if it is high." A level drawn at two hours cannot be interpreted on the nomogram and may falsely reassure. [1]

Escalation branch. If the candidate establishes the single-time history and plans the four-hour level with charcoal, release the four-hour level in Question 2. If they plot a level before four hours, ask why the nomogram requires four hours. [1]

Question 2 — The four-hour level and the nomogram decision

Stimulus update. The four-hour paracetamol level returns at 180 milligrams per litre. The ALT and INR are normal. Question: What is your decision, and what do you start? [1] [4]

Consultant-level model answer. "For a single acute ingestion with a known time, I plot the four-hour level on the Rumack-Matthew nomogram using the 150 milligrams per litre treatment line. A level of 180 milligrams per litre is above the line, so I start intravenous N-acetylcysteine on the standard regimen: 150 milligrams per kilogram over one hour, then 50 over four hours, then 100 over sixteen hours, a total of 300 milligrams per kilogram over twenty-one hours. I confirm the exact regimen with the poisons information centre because it is region-specific." [1]

Probing follow-up. "What if the level had been below the line?" A strong answer is: "If the level is clearly below the 150 milligrams per litre treatment line for a single acute ingestion with a known time, and the liver function is normal, NAC is not indicated. I would observe, address the safeguarding concern, and ensure the carer understands the safety net." [1]

Common weak answer. "I will treat everyone just in case." The nomogram exists to guide the decision; treating well below the line without reason is not standard, though empirical treatment is correct when the ingestion is staggered, unknown-time or already hepatotoxic. [1]

Escalation branch. If the candidate starts the NAC regimen correctly, reveal in Question 3 a reaction during the loading dose. [1]

Question 3 — An anaphylactoid reaction to NAC

Stimulus update. During the loading dose the child develops flushing, an itchy urticarial rash and mild wheeze. Question: Interpret this and act. [9]

Consultant-level model answer. "This is an anaphylactoid (non-IgE) reaction to the N-acetylcysteine loading dose, driven by the high peak concentration: flushing, rash, itch and mild bronchospasm are typical. I slow or briefly pause the infusion and give an antihistamine, then resume the regimen once the reaction settles. It is not a reason to abandon the antidote. If bronchospasm or hypotension is severe I treat along the standard anaphylaxis pathway, but most reactions are rate-related and settle with slowing." [9]

Probing follow-up. "Why is this not a true allergy?" A strong answer is: "Because it is mediated by direct histamine release from the high peak NAC concentration rather than IgE, and it is reproducibly reduced by lowering the peak (two-bag regimens) or by antihistamine pretreatment. Resuming the infusion, sometimes more slowly, is expected to be tolerated." [9]

Common weak answer. "Stop the NAC and give adrenaline." This abandons the antidote unnecessarily for a typical rate-related reaction. [9]

Escalation branch. If the candidate manages the reaction and resumes the infusion, reveal in Question 4 that the ALT and INR begin to rise. [1]

Question 4 — Rising hepatotoxicity and the prothrombin time trend

Stimulus update. At twenty-one hours, at the end of the standard course, the ALT is 1200 units per litre and the INR is 3.2. The paracetamol level is now negligible. Question: Interpret this and decide on the course. [10]

Consultant-level model answer. "The falling paracetamol level alongside a rising ALT and INR means established hepatotoxicity, not recovery. The INR is significantly deranged, so I extend the N-acetylcysteine, typically continuing the third-bag rate, and I monitor the ALT and the prothrombin time trend closely because the prothrombin time trend is the key prognostic marker in paracetamol-induced liver failure. I escalate the level of care and involve the toxicology and liver services." [10]

Probing follow-up. "Could this simply be recovery because the paracetamol level is now negligible?" A strong answer is: "No. After the peak serum concentration the paracetamol level declines while hepatocyte injury unfolds. A falling level with a rising ALT and INR is the hallmark of evolving injury. I track the ALT and INR, not the paracetamol level, from here." [10]

Common weak answer. "Stop the NAC because the level is now normal." The level is uninformative once the nomogram phase is past; the ALT and INR trend drive the decision. [10]

Escalation branch. If the candidate extends the NAC and watches the trend, reveal in Question 5 that the INR climbs and the child becomes encephalopathic. [10]

Question 5 — King's College criteria and escalation

Stimulus update. Twenty-four hours later the INR is 7.5, the creatinine is 340 micromoles per litre, the arterial pH is 7.25 after fluid resuscitation, and the child is drowsy with grade three encephalopathy. Question: What are the King's College criteria, and what do you do? [10]

Consultant-level model answer. "For paracetamol-induced acute liver failure the King's College criteria are an arterial pH below 7.3 after adequate fluid resuscitation, regardless of encephalopathy grade, or all three of an INR above 6.5, a creatinine above 300 micromoles per litre, and grade three to four encephalopathy. This child meets both limbs. Meeting the criteria predicts a poor outcome without transplant, so I contact the regional liver transplant centre immediately, before the NAC course finishes, continue NAC and supportive care, and arrange transfer. I run a structured handover of the ingestion, the levels, the trend, the treatment given and the current physiology." [10]

Probing follow-up. "What is the one principle you most want the team to carry forward?" A strong answer is: "Take the precise ingestion history and time, apply the nomogram only when valid, give charcoal and NAC early, track the ALT and the prothrombin time trend, and escalate to the liver transplant centre the moment King's College criteria appear." [1]

Common weak answer. "I will finish the NAC course and reassess tomorrow." A child meeting King's College criteria needs the transplant centre now, not after the course. [10]