Acute viral bronchiolitis is the first cause of respiratory distress in infant. Airway inflammation increases the respiratory system resistances and dynamic hyperinflation. This leads to an increase in the work of breathing. In Chronic obstructive pulmonary disease patients as in neonates, prone position (PP) improves lung function and decreases the end expiratory lung volume. The investigators hypothesized that in infants with severe bronchiolitis, prone position reduces the intrinsic Positive End Expiratory Pressure (PEEPi) and the work of breathing (WOB). The investigator designed a prospective randomized crossover study with 16 infants younger than six months who need ventilatory support by nasal continuous positive airway pressure (nCPAP) for severe acute viral bronchiolitis. Work of breathing (product time pressure) and PEEPi will be estimated using an esophageal pressure probe in prone and supine position.

Inclusion Criteria:

• Infant < 6 months
• Admitted in the pediatric intensive care unit of the Femme-Mère-Enfant hospital, Hospices Civils de Lyon, France
• With severe acute viral bronchiolitis requiring ventilatory support (mWCAS > 4 and/or and or FiO2> 40% and/or hypercapnic acidosis (pH<7.30 and/or pCO2>8 kPa))
• Signed informed consent by the two parents or the owner of parental authority

Exclusion Criteria:

• Chronic respiratory, neuromuscular, ENT or cardiac disease
• Contraindication for placement of esophageal probe (esophageal surgery, varices, …)
• Children requiring invasive ventilation (more than 3 significant apneas in 1 hour, decreased consciousness, …)
• Children not affiliated to a social security scheme