Acute viral bronchiolitis is the leading cause of community-acquired acute respiratory failure in developed countries (20 000 to 30 000 hospitalizations each year in France). Between 5% and 22% of these children are hospitalized in a critical care unit to benefit from a respiratory support.

Non-invasive ventilation, in particular the nasal Continuous Positive Airway Pressure (nCPAP), reduces the work of breathing in children with bronchiolitis and is associated with decreased morbidity and hospitalization costs compared with invasive ventilation. Nowadays, this technique is considered as the gold standard in the pediatric intensive care units (PICU) in France. High Flow Nasal Cannula (HFNC) has been proposed as an alternative to the nCPAP because of its better tolerance and simplicity of implementation. However, the proportion of failure remains high (35 to 50%), providing only a partial response to the care of these children, especially prior to the PICU.

In a physiological study (NCT02602678, article published), it has been demonstrated that prone position (PP) decrease, by almost 50%, the respiratory work of breathing and improve the respiratory mechanics in infants hospitalized in intensive care units for bronchiolitis.

Investigators hypothesize that prone position, during High Flow Nasal Cannula (HFNC), would significantly reduce the use of non-invasive ventilation (nCPAP and others) or invasive ventilation, as compared to supine position during HFNC, in infants with moderate to severe viral bronchiolitis.

Inclusion Criteria:

• Infant under 6 months
• Hospitalized in critical care unit (continuous monitoring unit or intensive care unit)
• With a clinical diagnosis of acute viral bronchiolitis (criterion of the American Academy of Pediatrics 2014)
• m-WCAS score ≥ 3 and / or hypercapnic acidosis with pH <7.35 and pCO2> 50mmHg (6.7 kPa)
• Informed consent signed by at least one of the parents with oral consent of the other parent (and / or legal guardian) recorded in the medical file (with deferred written consent).

Exclusion Criteria:

• Infant admitted with criteria for invasive or non-invasive ventilation (hypercapnic acidosis with pH <7.25 without ventilatory support and/or hypoxia with impossibility of maintaining SpO2> 92% whatever the FiO2 and/or more than 3 significant apneas per hour and/or severe consciousness disorder)
• Patient already positioned in the prone position before randomization for more than 3 hours
• Significant comorbidities with a history of respiratory pathology (bronchodysplasia with ventilatory support), Ear Nose and Throat pathology (pharyngolaryngomalacia) or neuromuscular and / or hemodynamically significant congenital heart disease.
• Contraindication to Prone position : recent abdominal surgery (laparoschisis or omphalocele) or recent sternotomy
• Patient who is not affiliated (or does not benefit from) to a national social security system