Bronchiolitis (RSV and Viral)

Bronchiolitis starts like a cold and then moves into the lower airways. It is a viral infection, and respiratory syncytial virus (RSV) is the most frequent cause. It is seasonal and usually shows up in winter. In Australia and Aotearoa New Zealand it is one of the most common reasons infants get admitted. It usually hits infants under 12 months of age.

The cold comes first: rhinorrhea, nasal congestion, cough. Then the lower airways get involved, and you see distress with diffuse crackles or wheeze. You make the diagnosis at the bedside. For typical cases, routine laboratory tests, viral testing and chest X-rays are not recommended.

Why Kids Are Different

• Obligate nose breathers: infants breathe mainly through their noses. Secretions in there can cause distress and get in the way of feeding (NASEMSO guideline 2022).
• Feeding and breathing are linked: feeding trouble, vomiting and dehydration are part of this illness. A child who can't feed is sicker. Not just more uncomfortable.
• Younger and preterm infants carry more risk: younger chronological age and gestational age under 37 weeks are both listed as risk factors for more serious illness.
• Apnea is uncommon but real: the 2025 Australasian guideline says apnea is rare, under 2% of infants with bronchiolitis.

Kids aged 12 to 24 months can get bronchiolitis too. The older the child, the less certain the diagnosis. The Australasian guideline covers infants under 12 months only. In older children, consider other causes.

Don't judge severity on one number. Look at work of breathing, oxygenation, feeding, hydration and mental state together. Is the child on low-flow oxygen, with a falling respiratory rate and heart rate? That child is responding. Is the effort rising, or is the child tiring? That child needs a senior clinician now.

Age-Specific Presentation

• Under 6 weeks: the oxygen threshold is 92%, not 90%. Watch apnea and feeding closely.
• 6 weeks to 12 months: this is the typical age group. Without an underlying condition, the oxygen threshold is 90%.
• Infants under 12 months with an underlying condition: the oxygen threshold is 92%.
• 12 to 24 months: diagnostic uncertainty grows here. Consider other causes.

Consider Other Diagnoses

• Exclusions in the NASEMSO guideline: anaphylaxis, croup, epiglottitis, foreign body aspiration, drowning and asthma are not bronchiolitis. Each one needs its own pathway.
• Serious bacterial illness or a complication: sepsis, pneumonic consolidation, pneumothorax, empyema, pleural effusion, immunodeficiency or significant heart disease. Think about these when the course is unexpected or the child gets worse without warning.

1. Suction what you can see: bulb or catheter suction of the nose and mouth when secretions cause distress or get in the way of feeding. Superficial suction is the step to use (Australasian guideline 2025: consider superficial suction for distress or feeding difficulty; NASEMSO: suctioning is very effective for distress).
2. Keep the infant feeding: check the feed and wet-nappy history at every reassessment. When feeding drops off, nasal saline drops at feeds can be tried. They are not routine (Australasian guideline 2025).
3. Give oxygen to target: consider starting low-flow oxygen when SpO₂ is persistently below 90% (6 weeks and older) or 92% (under 6 weeks, or under 12 months with an underlying condition). These are thresholds for starting oxygen, not a field target. In the field, the NASEMSO goal is 94% to 98%. Don't wait for a number if the infant is tiring or in severe distress. Oxygen is not given for work of breathing alone (Australasian guideline 2025; NASEMSO guideline 2022).
4. Hydrate when needed: IV or enteral hydration when the infant cannot keep fluids up. In the field, IV access is only for dehydration or IV medication (NASEMSO guideline 2022).
5. Limit fluids to avoid overload: consider 50% to 75% of maintenance because of the risk of syndrome of inappropriate antidiuretic hormone (SIADH) secretion and hyponatremia. Monitor for overhydration (Australasian guideline 2025, consensus).
6. Hand hygiene: use hand hygiene for every infant with bronchiolitis (Australasian guideline 2025, strong recommendation).

Not Routinely Recommended

• Chest physiotherapy (Australasian guideline 2025).
• Deep nasal suction (Australasian guideline 2025).
• Routine chest X-ray, blood or urine tests, and viral testing, including testing only for cohorting (Australasian guideline 2025).
• Heliox (NASEMSO guideline 2022).
• Inhaled steam or nebulized saline in the field, because there is insufficient data (NASEMSO guideline 2022).

Escalate on how the child responds, not on the clock. Low-flow oxygen comes first. When SpO₂ stays below target or the work of breathing climbs, step up. The pathway below is the hospital logic of the 2025 Australasian guideline, simplified.

• High-flow nasal cannula (HFNC): use for hypoxemic infants who fail low-flow oxygen. Don't use HFNC routinely as first line in moderate hypoxemic disease, and don't use it in non-hypoxemic mild or moderate disease (Australasian guideline 2025). Flow rate and titration follow the unit protocol. In a 2018 randomized trial of infants outside intensive care, HFNC cut escalation of care for treatment failure: 12% versus 23%.
• CPAP: consider nasal CPAP for impending or severe respiratory failure. In severe disease, consider HFNC before CPAP (Australasian guideline 2025).
• Bag-valve-mask ventilation: use it for respiratory failure or impending failure. Reach for a supraglottic device or intubation only when bag-valve-mask ventilation fails. Use the least invasive airway that works (NASEMSO guideline 2022).
• Reassess 4 to 5 hours after starting support: a falling respiratory rate, heart rate or pediatric early warning score (PEWS) shows the child is responding (Australasian guideline 2025).

Few drugs earn a place here. Most of the ones people reach for are not recommended. Doses vary by protocol; follow local protocols and medical direction. Nirsevimab and clesrovimab are prevention, and neither one treats an active infection.

• Capnography (EtCO₂): NASEMSO uses pulse oximetry and end-tidal CO₂ routinely, as adjuncts to other respiratory monitoring (Treatment item 1). Watch for a rising EtCO₂ with tiring, or a falling level of consciousness. That suggests ventilation is failing. For respiratory failure or impending failure, NASEMSO calls for bag-valve-mask ventilation. For severe distress, add HFNC or CPAP where available.
• Labs: check glucose and sodium when feeding is poor, the child is dehydrated or mental state has changed. Routine labs are not recommended (Australasian guideline 2025).
• Discharge criteria (Australasian guideline 2025): the child is clinically stable; SpO₂ is at or above the threshold after observation, if oxygen was given; feeding and output are adequate; the caregiver is confident and has written return advice; the social situation and transport are suitable; and local follow-up is arranged where needed.
• Admission risk: the guideline says there is insufficient evidence for absolute discharge criteria. Younger infants (under 8 weeks), lack of social support and poor feeding are risks for return. Severe distress that fails support means escalating to ICU-level care.

Neonates and Young Infants

• Oxygen threshold: 92%, not 90%, for infants under 6 weeks (Australasian guideline 2025).
• Risk factors: gestational age under 37 weeks, younger chronological age, prenatal or postnatal tobacco smoke and reduced breastfeeding. Each one raises the risk of more serious illness (Australasian guideline 2025).
• Apnea: the 2025 guideline calls apnea rare. It recommends continuous oximetry when apnea or apnea risk is present. It does not define apnea risk factors. So treat any pause in breathing as serious.

Infants With Underlying Conditions

• Conditions that raise risk: chronic lung disease, congenital heart disease, chronic neurological condition, congenital diaphragmatic hernia, trisomy 21 and other genetic disorders (Australasian guideline 2025).
• Oxygen threshold: 92% for infants under 12 months with an underlying condition (Australasian guideline 2025).

RSV Prevention

• Nirsevimab and clesrovimab: both are monoclonal antibodies. One intramuscular dose covers neonates and infants born during or entering their first RSV season, from birth when they arrive in season (DailyMed labels). Find the dosing in the medications table.
• Maternal vaccine: Abrysvo goes to pregnant people at 32 through 36 weeks gestation. It protects the infant against lower respiratory disease from birth through 6 months (DailyMed label).
• One product, not all three: most infants need one or the other, not both. An infant born within 14 days of maternal vaccination still needs the infant antibody (CDC). Clesrovimab is labeled for the first season only.
• Guideline position: the guideline says to consider monoclonal antibodies for infants at increased risk. It also says to consider universal nirsevimab as a population approach, and maternal immunization. Routine universal infant RSV vaccination is not recommended (Australasian guideline 2025).

Children 12 to 24 Months

• Second season, label: nirsevimab 200 mg IM (two 100 mg injections) for children up to 24 months who remain vulnerable to severe RSV disease through their second RSV season (DailyMed label).
• Second season, CDC: nirsevimab is recommended for children aged 8 to 19 months entering their second season who have chronic lung disease of prematurity that needed medical support (corticosteroids, diuretics or supplemental oxygen) in the 6 months before the season, severe immunocompromise, cystic fibrosis with severe lung disease or weight-for-length below the 10th percentile, or American Indian or Alaska Native ancestry. From 20 months, CDC does not recommend nirsevimab. Clesrovimab has no second-season use.
• Diagnosis: the Australasian guideline stops at infants under 12 months, so the evidence is less certain for older children (Australasian guideline 2025).

• Oxygen target in the field: NASEMSO sets the field goal at SpO₂ 94% to 98%. The AU 2025 hospital thresholds of 90% or 92% are the points to consider starting oxygen. They are not a field target. If the infant is tiring, don't wait for a number. Use the local protocol and state the target on handoff. This conflict needs a medical director decision.
• Transport: lights and sirens are not routine for transport (NASEMSO guideline 2022). Hand the caregiver written return advice (Australasian guideline 2025).
• Infection control: use hand hygiene (Australasian guideline 2025). Follow the agency's infection-control policy.
• Handoff: SpO₂ trend, oxygen device and target, feeding and wet-nappy history, last nebulized treatment, and the response to suction and oxygen.