Inclusion Criteria:  premature infants with gestational age <30 weeks postmenstral age ≥32weeks a history of caffeine therapy no current positive pressure respiratory support, and free of apnea for at least five consecutive days, but still oxygen dependent parents or legal guardians sign informed consent to attend this study  Exclusion Criteria:  congenital cardiorespiratory malformation, or chromosomal abnormalities Grade III/IV intraventricular hemorrhage, or probable brain injury attributable to confirmed central nervous system infection, severe periventricular leukomalacia or other entities; underwent tracheostomy currently on sedatives, opioids, or other medication related to depressed breath