The goal of this clinical trial is to evaluate the safety and potential efficacy of PNEUMOSTEM® for improving respiratory outcomes in very premature infants diagnosed with Early Pulmonary Arterial Hypertension. The main questions it aims to answer are:
* In very premature infants diagnosed with early pulmonary arterial hypertension, will a single intratracheal administration of PNEUMOSTEM®(Allogeneic umbilical cord blood-derived mesenchymal stem cells) result in improvement of pulmonary arterial hypertension based on echocardiographic assessment?
* In very premature infants diagnosed with early pulmonary arterial hypertension who show improvement of pulmonary arterial hypertension based on echocardiographic assessment following a single intratracheal administration of PNEUMOSTEM®(Allogeneic umbilical cord blood-derived mesenchymal stem cells), at what time point does this improvement occur?
Participants will:
* Single intratracheal dose of PNEUMOSTEM® at 2.0 x 10,000,000 cells/kg
* Acute adverse event monitoring: 24 hours post-administration for safety assessment
* Follow- up time points: Day 1(Baseline, PNEUMOSTEM® administration), Day 2, Week 1, Week 2, Postnatal Day 28, PMA 36\~40 weeks