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]