Definitions:
Gestational age was estimated based on best obstetric estimate using early ultrasound, last menstrual period or Ballard examination [20] in that order. Acute chorioamnionitis was based on placental pathology. The primary outcome was successful extubation, defined as survival for minimum of five days on either no or non-invasive respiratory support. Extubation failure was defined as need for re-intubation within 5 days of extubation. Hemodynamically significant PDA was defined by Echocardiogram findings suggestive of a moderate to large PDA in an infant with low diastolic blood pressure, wide pulse pressure, hypotension, renal insufficiency, or signs of pulmonary edema (tachypnea, increased respiratory support and oxygen requirements), either before or within five days of elective extubation. Echocardiograms in our center were generally ordered for symptomatic patients as assessed by the primary clinical team.
Ventilation index (VI) was used as an objective measure of adequacy of ventilation in response to the respiratory support provided. VI was calculated as the product of the set rate, partial pressure of carbon dioxide in arterial or capillary blood (pCO2) and the difference between peak inspiratory and positive end expiratory pressure divided by 1000[18]. We calculated the highest respiratory severity score (RSS) in the first 6 hours of age, as an objective marker of oxygenation in relation to mean airway pressure received by the patient. RSS was calculated as the product of the mean airway pressure (MAP) and fraction of inspired oxygen (FiO2) [19].