Clinical respiratory parameters:
Respiratory data were collected for all infants during the first 7 days of life. Data included mean airway pressure (MAP), fraction of inspired oxygen (FiO2), arterial blood gas measurements, total mechanical ventilation days, total oxygen days, and success/failure of extubation events. Oxygen saturation index (OSI) was calculated on postnatal days 3 and 7; OSI = MAP x FiO2 x100 / arterial oxygen saturation 10. We stratified respiratory severity into mild, moderate, and severe based on OSI of ≤ 2.9, 3.0-6.4, and ≥6.5, respectively 11.