In preterm infants with neonatal respiratory distress syndrome (RDS), exogenous pulmonary surfactant(PS) replacement therapy is one of the most important therapeutic breakthrough to reduce neonatal incidences of bronchopulmonary dysplasia(BPD) and/or death. But not all preterm infants with RDS can be beneficial. Otherwise, the international neonatal acute RDS (NARDS) collaborative group provides the first consensus definition for NARDS in 2017. And whether or not PS being beneficial in preterm infants with NARDS remains unknown.

To date, the optimal dose of PS is inconsistent, although the recommended dose of PS is given in the 2019 update guideline. PS is not recommended to adult and pediatric ARDS. Systematic review indicates that PS does not reduce the incidences of BPD and death in infants with meconium aspiration syndrome(MAS, a subtype of NARDS). A reasonable speculation is that preterm infants with NARDS do not benefit from PS. And the speculation can explain why not all preterm infants with RDS can be beneficial from PS. In the era of pre-NARDS, the preterm infants fulfilling the definition of NARDS may have been considered as RDS in the first three days after birth.

Meantime, the dose of PS given to infants with NARDS remains unknown. the aim of the present study is to assess the optimal dose of PS. the control group: PS is given according to the European RDS management guideline in 2019 edition. The study group: PS is stopped when the pressure is equal between before patent ductus arteriosus(bPDA) and after PDA(aPDA). the primary outcomes are the closure rate of PDA within 7 days, the incidence of BPD and/or death.

Inclusion Criteria:

• gestation age less than or equal to 32 weeks
• PS is needed

Exclusion Criteria:

• main congenital abnormalities
• parents' refusal or quit