Oxygen, high-flow and non-invasive respiratory support — formative SAQs

SAQ 1 (10 marks, 15 minutes)

A 5-month-old infant presents with bronchiolitis, marked recession, grunting and SpO2 of 88 per cent in room air. After 30 minutes of standard low-flow nasal cannula oxygen at 2 L/min, there is no improvement. Outline your approach to escalating respiratory support, including device selection, starting parameters, reassessment plan and failure criteria. [7]

Model answer

Immediate assessment and threat gate (2 marks) — Assess airway patency, breathing and circulation. Clear the threat gate: confirm the infant is not in extremis — no apnoea, bradycardia, silent chest or altered conscious state — which would mandate immediate bag-mask ventilation and intubation. This infant is failing standard oxygen but still has a drive to breathe and is therefore a candidate for escalation to non-invasive support. [7]

Device selection and rationale (2 marks) — The problem is moderate bronchiolitis with high work of breathing and hypoxaemia that has failed standard low-flow oxygen. The appropriate next step is high-flow nasal cannula (HFNC), which provides precise FiO2, dead-space washout, reduced work of breathing and a modest PEEP effect. CPAP or BiPAP would be alternatives if HFNC fails or if the problem demands reliable set pressure, but HFNC is the evidence-supported first escalation step in moderate bronchiolitis. [1] [7]

Starting parameters (2 marks) — Initiate HFNC with a flow of 2 L/kg/min, titrated between 1 and 2 L/kg/min, with FiO2 set to the minimum achieving the target saturation of 92 to 95 per cent. Use correctly sized nasal prongs. Ensure the gas is heated and humidified. [1] [7]

Reassessment plan (2 marks) — Reassess within 30 to 60 minutes of starting, assessing work of breathing (rate, recession, grunting), efficacy (air entry, breath sounds) and effect (SpO2, colour, conscious state). If improving, continue and wean FiO2 first. If not improving, check the interface for leak and consider escalating to CPAP (5 to 7 cmH2O) or BiPAP (IPAP 10 to 15, EPAP 4 to 6 cmH2O). [7]

Failure criteria and escalation (2 marks) — Define failure at the outset: persistent or worsening hypoxaemia, rising PaCO2 on blood gas, worsening acidosis, exhaustion (falling rate with quieting chest), haemodynamic instability, or apnoea. If any criterion is met, proceed to intubation without delay — the cardinal error is persisting with a failing non-invasive trial. [7]

SAQ 2 (10 marks, 15 minutes)

Compare and contrast HFNC, CPAP and BiPAP in terms of their physiological mechanisms, typical clinical indications and starting settings in children. Support your answer with the key trial evidence. [9]

Model answer

Physiological mechanisms (3 marks) — HFNC delivers heated, humidified gas at high flow (2 L/kg/min), providing four mechanisms: dead-space washout of the nasopharynx, reduced work of breathing by meeting inspiratory demand, a modest flow-dependent PEEP effect, and preserved mucociliary clearance from conditioned gas. CPAP delivers a single, set continuous positive pressure (5 to 7 cmH2O) via a sealed interface, reliably recruiting collapsed alveoli and reducing intrapulmonary shunt — improving oxygenation in hypoxaemic failure. BiPAP cycles between a higher inspiratory pressure (IPAP 10 to 15 cmH2O) and a lower expiratory pressure (EPAP 4 to 6 cmH2O), providing the same recruitment as CPAP via EPAP plus active ventilatory support via the pressure support (IPAP minus EPAP), which augments tidal volume and clears CO2. [9]

Clinical indications (3 marks) — HFNC is indicated for moderate respiratory distress with hypoxaemia, particularly bronchiolitis, where work of breathing is high but the child is not in extremis. CPAP is the tool for hypoxaemic (type 1) failure from recruitable lung disease — pneumonia, PARDS, pulmonary oedema — where reliable set pressure is needed to splint alveoli. BiPAP is the tool for hypercapnic (type 2) failure from neuromuscular weakness, central hypoventilation, chest-wall deformity or the tiring child, where active ventilatory support is required. A critical point: giving CPAP alone to a child with neuromuscular weakness is a wrong-device error — it holds the airway open but does not move air. [9]

Key trial evidence (4 marks) — The Franklin 2018 NEJM trial established HFNC as effective first-line therapy for moderate bronchiolitis, with treatment failure of 12 per cent versus 23 per cent for standard oxygen. [1] The TRAMONTANE trial (Milési 2017) showed HFNC was non-inferior to nCPAP for bronchiolitis in young infants. The Ramnarayan 2022 JAMA FIRST-HFNC trial compared HFNC as first-line to CPAP in PICU and found HFNC was non-inferior for liberation from respiratory support with significantly less nasal trauma, supporting HFNC as an acceptable first-line choice across diagnoses. [4] Current PICU bronchiolitis guidelines (Milési 2023) recommend HFNC or nCPAP for moderate-to-severe disease after standard oxygen fails. [7]