Oxygen, high-flow and non-invasive respiratory support — viva

Viva structure

This is a branching structured oral examination. The examiner presents escalating clinical scenarios and probes your reasoning, device selection, parameter setting, recognition of failure, and evidence base. Each answer should show the physiological rationale, not just a protocol. [7]

Opening question

Question: A 6-month-old infant with bronchiolitis is grunting and retracting with SpO2 of 88 per cent in room air. Standard low-flow nasal cannula oxygen at 2 L/min has produced no improvement after 30 minutes. Walk me through your next steps. Answer: First, I clear the threat gate — confirm the infant is not in extremis. There is no apnoea, bradycardia, silent chest or altered conscious state, so this infant is failing standard oxygen but is stable enough for a trial of escalated non-invasive support. The problem is moderate bronchiolitis with high work of breathing and hypoxaemia, so I would start high-flow nasal cannula at 2 L/kg/min with FiO2 titrated to a target saturation of 92 to 95 per cent. I would reassess within 30 to 60 minutes and define failure criteria upfront — worsening hypoxaemia, rising CO2, exhaustion or haemodynamic instability — that would trigger intubation. [1] [7]

Probe 1 — Mechanism

Question: What does high-flow nasal cannula actually do that standard low-flow oxygen does not? Answer: HFNC delivers heated, humidified gas at a flow matched to the child's inspiratory demand, and it does four things standard oxygen cannot. First, it washes out CO2-rich dead-space gas from the nasopharynx so each breath draws fresh gas. Second, by meeting inspiratory demand it reduces the work of breathing — the respiratory muscles do less per breath. Third, the high flow generates a modest positive end-expiratory pressure that splints small airways open. Fourth, the conditioned gas — warmed to body temperature and fully humidified — preserves mucociliary clearance and keeps secretions thin. Standard low-flow oxygen only raises FiO2 imprecisely and provides none of these pressure or conditioning effects. [1]

Probe 2 — Escalation and failure

Question: The infant has now been on HFNC for 90 minutes. The nurse reports the respiratory rate has fallen from 64 to 40, the recession looks less, but SpO2 is 88 per cent on FiO2 40 per cent and the infant is drowsy. What do you think is happening? Answer: This is the pre-arrest exhaustion pattern, not improvement. A falling respiratory rate with lessening recession but persistent hypoxaemia and new drowsiness means the infant is tiring — the muscles can no longer sustain the work. The quieting chest is the loudest warning. The correct action is immediate escalation to intubation. This is a failing non-invasive trial and the cardinal error would be to mistake the falling rate for recovery or to increase HFNC flow and wait. I would call for senior and anaesthetic support, prepare for rapid sequence intubation, and continue HFNC as a bridge while preparing. [7]

Probe 3 — Device matching

Question: Now consider a different child: a 4-year-old with spinal muscular atrophy type 2 who presents with shallow, rapid breathing, morning headache and drowsiness. Capillary gas shows pH 7.28 and PCO2 68 mmHg. SpO2 is 95 per cent. What support would you use? Answer: This child has hypercapnic type 2 respiratory failure from neuromuscular weakness — the pump is too weak to ventilate, so CO2 is retained, while oxygenation is preserved. I would use BiPAP, not CPAP. The problem is ventilation, so I need active ventilatory support: BiPAP provides a pressure boost during inspiration (IPAP minus EPAP) that augments tidal volume and clears CO2. CPAP alone would be a wrong-device error here — it holds the airway open but does not actively move air. I would start BiPAP with IPAP 10 to 15 and EPAP 4 to 6 cmH2O, titrating to achieve adequate tidal volume and a falling PCO2, and I would check whether the child already uses home BiPAP and optimise their existing settings. [9]

Probe 4 — Evidence

Question: What is the evidence base for using HFNC first-line rather than CPAP in children? Answer: The key evidence comes from three sources. The Franklin 2018 NEJM trial showed HFNC halved treatment failure compared with standard oxygen in moderate bronchiolitis — 12 per cent versus 23 per cent. [1] The TRAMONTANE trial showed HFNC was non-inferior to nCPAP as initial therapy for bronchiolitis in young infants. [3] The Ramnarayan 2022 JAMA FIRST-HFNC trial compared HFNC as first-line to CPAP across a range of diagnoses in PICU and found HFNC was non-inferior for liberation from respiratory support, with the added advantage of significantly less nasal trauma. [4] Taken together, these support HFNC as an acceptable and effective first-line non-invasive modality, though the decision must always be individualised to the child's physiology and the disease. [7]

Probe 5 — Complications

Question: What complications should you watch for when a child is on CPAP or HFNC? Answer: First, nasal trauma and pressure injury — from prongs or masks that are too tight or the wrong size. I check the skin regularly and ensure correct sizing. Second, air leak and pneumothorax — positive pressure can cause or worsen a pneumothorax, particularly in conditions with high airway pressures. Sudden deterioration with asymmetrical air entry demands immediate consideration of tension pneumothorax. Third, aspiration and feed intolerance — children on support may have impaired swallowing or gastric distension, so I manage with nil-by-mouth during acute escalation and nasogastric decompression. And critically, the complication of delayed intubation — persisting with a failing non-invasive trial while the child deteriorates toward arrest. [7]

Closing question

Question: Summarise the mechanism-matching rule for the whole ladder. Answer: Low-flow oxygen treats mild hypoxaemia and adds FiO2 only. HFNC is for high work of breathing where you want flow, dead-space washout and a modest PEEP without a sealed mask. CPAP is for hypoxaemic failure from collapsed, recruitable alveoli where you need reliable set pressure. BiPAP is for hypercapnic failure or neuromuscular weakness where you need active ventilatory support to move air and clear CO2. Match the device to the mechanism, set it correctly, reassess continuously, define failure early, and intubate the moment support is not working. [9]