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 dominant threat, the contraindications, 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 household chemical exposure assessment. State the product you need to identify, the contraindications you will apply, the pathway you will run and what you will reassess. This is one continuous case that moves from a hydrocarbon pathway into a caustic comparator. Each escalation branch leads to the next update. [1]

Question 1 — The first assessment and the decontamination decision

Stimulus update. A two-year-old is brought in forty minutes after swallowing a mouthful of lamp oil. He coughed at the scene but is now alert, with a respiratory rate of 28 and oxygen saturation 98 per cent on air. A nurse asks whether to give ipecac. Question: What do you say and do now? [8]

Consultant-level model answer. "I secure the airway, breathing and circulation, and check the oxygen saturation and work of breathing. I identify the product from the container: this is lamp oil, a very low-viscosity petroleum distillate. I would not give ipecac, would not perform gastric lavage, and would not give activated charcoal, because the dominant threat is aspiration, not systemic absorption. Lamp oil's low viscosity and low surface tension let a small aspirated volume spread into the alveoli and cause a chemical pneumonitis, and inducing emesis or lavaging would provoke just such an aspiration. I begin six-hour observation with saturation monitoring and call the poisons information centre with the product in hand." [8]

Probing follow-up. "Why does the physical property of the oil matter more than the volume he swallowed?" A strong answer is: "Because aspiration of even a small volume of a low-surface-tension hydrocarbon spreads across the alveoli and causes pneumonitis, whereas a larger volume of a high-viscosity agent may not. The aspiration hazard is a property of the agent, not just the dose." [9]

Common weak answer. "I will give ipecac to empty the stomach." This converts a contained small-volume exposure into a large-volume aspiration and is contraindicated. [8]

Escalation branch. If the candidate applies the contraindications and starts the six-hour observation, release the four-hour deterioration in Question 2. If they propose emesis or lavage, ask why hydrocarbons are managed by aspiration risk rather than absorption. [8]

Question 2 — Deterioration at four hours and the respiratory pathway

Stimulus update. At four hours the child is tachypnoeic with a respiratory rate of 44 and an oxygen saturation of 92 per cent on air. Auscultation reveals bilateral crackles. Question: Interpret this and outline the management, including the position of corticosteroids and prophylactic antibiotics. [8]

Consultant-level model answer. "The tachypnoea and hypoxaemia with bilateral crackles are evolving hydrocarbon pneumonitis. I admit him and begin supportive respiratory care with supplemental oxygen, escalating to high-flow nasal therapy or non-invasive ventilation as his work of breathing and saturation demand, with intensive care involvement for respiratory failure. I do not give corticosteroids or prophylactic antibiotics as routine, because the early illness is a chemical rather than an infective pneumonitis and these agents do not improve outcome; I treat proven secondary bacterial infection on its merits. For refractory acute respiratory distress syndrome I would arrange specialist retrieval and consider surfactant and extracorporeal membrane oxygenation." [8] [9]

Probing follow-up. "Would a normal chest X-ray at two hours have been reassuring?" A strong answer is: "No. Radiographic changes can lag behind symptoms by hours in hydrocarbon pneumonitis, so a normal early film does not exclude evolving lung injury. The decision to admit and escalate rests on the oxygen saturation, the work of breathing and the clinical course." [8]

Common weak answer. "Start corticosteroids and antibiotics now." These are not recommended as routine therapy in hydrocarbon pneumonitis. [8]

Escalation branch. If the candidate manages the deterioration correctly, pivot the viva to a caustic comparator in Question 3. [8]

Question 3 — Caustic comparator: drain cleaner and neutralisation

Stimulus update. The examiner now introduces a second case to contrast. A four-year-old presents one hour after swallowing an unknown amount of drain cleaner (sodium hydroxide), drooling with an oral burn. The registrar asks whether to give a weak acid to neutralise the alkali. Question: What is your response, and what is your mechanism-based justification? [1]

Consultant-level model answer. "I would not neutralise the alkali, I would not induce emesis, and I would not give activated charcoal. I secure the airway, keep the child nil by mouth, establish intravenous access and give fluids and analgesia. Neutralisation is contraindicated because the acid-alkali reaction is exothermic and produces heat that worsens the burn, and the gas produced can distend and rupture an already damaged stomach. The mechanism of injury is liquefaction necrosis: alkalis saponify fats and dissolve proteins, allowing deep penetration through the oesophageal wall, which is why oesophageal injury is usually severe." [1] [3]

Probing follow-up. "How would the mechanism differ if the agent had been a strong acid?" A strong answer is: "Acids denature protein into a firm coagulum, an eschar, which tends to limit the depth of penetration, so acids characteristically injure the stomach rather than the oesophagus. The pattern of injury, not just the symptoms, differs between alkali and acid." [1]

Common weak answer. "Give diluted vinegar to neutralise it." Neutralisation worsens the burn through heat and gas. [1]

Escalation branch. If the candidate applies the caustic contraindications correctly, release the endoscopy decision in Question 4. [1]

Question 4 — Endoscopy timing and the Zargar grade

Stimulus update. The drain-cleaner child remains stable. Endoscopy is booked. Question: Why is endoscopy timed between six and twenty-four hours, and what does a grade three-a finding imply for the plan? [1]

Consultant-level model answer. "I time endoscopy between six and twenty-four hours after ingestion. Before six hours the full depth of injury may not be apparent and the grade can be underestimated, leaving the child under-treated; after twenty-four hours the developing oedema and wall weakening raise the risk of iatrogenic perforation from the instrument. The endoscopy grades the burn by the Zargar classification and guides the feeding and surgical plan. A grade three-a finding (transmural ulcers with focal necrosis) carries a high risk of stricture and perforation, so I keep the child nil by mouth with nasoenteric feeding if needed, involve the paediatric surgical team early, give antibiotics only for proven infection or perforation, and plan long-term stricture surveillance and counselling about the increased lifetime risk of oesophageal malignancy." [1] [4]

Probing follow-up. "The child has no oral burns now; can you be reassured about the oesophagus?" A strong answer is: "No. The presence of oral burns predicts severe injury, but their absence does not exclude oesophageal injury. The oesophagus can be deeply burned while the mouth looks normal, so a significant caustic ingestion is presumed to have injured the gut until endoscopy shows otherwise." [1]

Common weak answer. "Endoscope immediately, before six hours." The grade can be underestimated before six hours and the procedure is timed within the six-to-twenty-four-hour window. [1]

Escalation branch. If the candidate times the endoscopy and applies the grade correctly, close with the synthesis in Question 5. [1]

Question 5 — Synthesis and the one principle to carry forward

Stimulus update. The examiner asks you to summarise. Question: What is the single most important principle in managing paediatric household chemical exposure, and how does it shape your decisions across both pathways? [1]

Consultant-level model answer. "Manage the child by the product, not by the symptom. I identify the exact product from the container, classify it as a hydrocarbon, an alkali caustic, an acid caustic or a low-concentration household product, and apply the contraindications first: never induce emesis, never lavage a hydrocarbon, never neutralise a caustic. For a hydrocarbon I observe for six hours with saturation monitoring, treat evolving pneumonitis with supportive respiratory care, and avoid routine corticosteroids and antibiotics. For a caustic I keep the child nil by mouth and endoscope between six and twenty-four hours, using the Zargar grade to drive the feeding, surgical and surveillance plan. I call the poisons information centre early and confirm every rule against the product and the local protocol." [1] [4]

Probing follow-up. "What will you never forget at the door?" A strong answer is: "Never induce emesis in a hydrocarbon or caustic ingestion, never lavage a hydrocarbon, never neutralise a caustic, and never discharge a hydrocarbon-exposed child before the six-hour observation." [8]

Common weak answer. "Give charcoal and observe." Activated charcoal is not indicated for hydrocarbons or caustics, and observation alone is insufficient for a significant caustic ingestion that needs timed endoscopy. [1]