BPD patients with PHT  Inclusion Criteria:  Born ≤ 28 postmenstrual weeks Respiratory support persisting at 36 weeks corrected postmenstrual age. Echocardiographic examination consistent with pulmonary hypertension  BPD patients without PHT  Inclusion Criteria:  Born ≤ 28 postmenstrual weeks Respiratory support persisting at 36 weeks corrected postmenstrual age. Normal echocardiographic examination  Healty preterms  Born ≤ 28 postmenstrual weeks Do not need respiratory support persisting at 36 weeks corrected postmenstrual age Normal echocardiographic examination  Exclusion Criteria:  Congenital heart disease (with the exception of patent ductus arteriosus or patent foramen ovale) Structural lung and airway malformations (including tracheobrochomalacia) Congenital diaphragmatic hernia Chromosome abnormality