Inclusion Criteria:  Patients hospitalized at Pediatric Intensive Care Unit (PICU) or Neonatal Intensive Care Unit (NICU) of the Medical University Vienna. Patients aged >36 weeks (corrected gestational age) and <24 months. Patient intubated and mechanically ventilated for at least 6 hours, with an expected requirement of invasive ventilatory support for at least 12 hours. Clinical picture strongly suggestive for acute bronchiolitis or pneumonia (fever, fine crackles, prolonged expiration, lung hyperinflation and/or findings of new infiltrates consistent with acute pulmonary parenchymal disease on chest X-ray). Severe pediatric acute respiratory distress syndrome (ARDS), defined by OSI ≥12.3 (wean FIO2 to maintain SpO2 ≤ 97% to calculate oxygen saturation index). Written informed consent obtained from parents.  Exclusion Criteria:  Clinical context  Need for O2 supplementation to maintain SpO2>94% in the 4 weeks preceding hospitalization in the PICU/NICU Cyanotic congenital heart disease Cardiogenic pulmonary edema Severe pulmonary hypertension Untreated pneumothorax Severe neurological abnormalities Other severe congenital anomalies such as congenital diaphragmatic hernia Ongoing cardiopulmonary resuscitation or limitation of life support  Contradictions for prone positioning (adapted from Guerin, C., et al., Prone positioning in severe acute respiratory distress syndrome. N Engl J Med, 2013. 368(23): p. 2159-68):  Intracranial pressure >30 millimeters of mercury (mmHg) in supine position or cerebral perfusion pressure <60 mmHg Massive hemoptysis requiring an immediate surgical or interventional radiology procedure Tracheal surgery or sternotomy during the previous 15 days Serious facial trauma or facial surgery during the previous 15 days Deep venous thrombosis treated for less than 2 days Cardiac pacemaker inserted in the last 2 days Unstable spine, femur, or pelvic fractures Use of extracorporeal membrane oxygenation (ECMO) before inclusion Lung transplantation Burns on more than 20% of the body surface  Other non-inclusion criteria  Indication not to attempt resuscitation Patient already recruited for other clinical studies Patients who already received surfactant in the last 4 weeks Thoracic skin lesions or wounds on the thorax, where the EIT-electrode-belt would be placed