This aim of this study is to measure the peak tidal inspiratory flow (PTIF), using spirometry, in young infants with moderate to severe acute viral bronchiolitis (AVB).

PTIF is important to consider for the management of AVB with High Flow Nasal Cannulae (HFNC). Indeed, efficiency with HFNC is optimal provided that the settled flow is equal or higher than the patient's PTIF. However, PTIF values in infants with AVB have never been determined.

HFNC allows the administration of a heated and humidified mixture of air and oxygen at high flows. This device has been widely adopted in pediatric intensive care units (PICUs) and for interhospital transport of critically ill children, as the system is easily set up and is well tolerated by patients. In the context of AVB, retrospective audits and observational studies have suggested promising outcomes on both physiological and clinical variables. However, in less than 6 months old infants with moderate to severe AVB, the failure rate with this device - defined as worsening of respiratory failure or the occurrence of discomfort or severe apnea - remains high, from 30% to 50% in key studies.

HFNC flow setting remains empiric, 2 L/kg/min being used by most teams. A potential explanation for this high failure rate is that the flow usually used is lower than the patient's inspiratory flow. The aim of this study is to determine peak tidal inspiratory flow (PTIF) values in infants up to 6 months old with moderate to severe AVB.

PTIF will be evaluated with a spirometer connected to a face mask during 20 cycles of spontaneous ventilation. The measurements will be performed within the first 24 hours of hospitalization in the department of Neonatal and Pediatric Intensive Care of Arnaud de Villeneuve University Hospital at Montpellier (France).

Statistical analysis Relation between the DIP and Silverman score, mWCAS, RR, LOS will be expressed with parametric and non-parametric correlation coefficients according to the distribution of the data.

Inclusion Criteria:

• Infants up to 6 months old, with AVB according to conventional clinical criteria;
• Admitted to the department of Neonatal and Pediatric Intensive Care at Arnaud de Villeneuve University Hospital in Montpellier (France);
• Supported with HFNC, according to the department's protocol (2 < m-WCAS < 5);
• Not requiring immediate intubation for invasive ventilation;
• Signed parental consent.

Exclusion Criteria:

• Infant with heart disease, cystic fibrosis or neuromuscular disorder;
• Infant requiring ventilatory support with nasal continuous positive pressure (nCPAP) or with HFNC and without possibility of weaning, including for a few minutes;
• Intolerance of the spirometry mask.

NC