Study population
This study was approved by the ethics committees of the First People's Hospital in Hangzhou. Informed consent was obtained from the parents of the neonates in the study according to institutional guidelines. Between June 2011 and July 2013, all pre-term neonates with a birth weight (BW) of 600–1,500 g and GA of 25–32 weeks admitted to the neonatal intensive care unit were prospectively enrolled into this study. Neonates with congenital heart diseases, multiple malformations, or documented chromosomal abnormalities were excluded. Also excluded from the final analysis were infants who died of non-respiratory causes, or for whom blood samples were not obtained 7 days post-partum.