Part 1: Preterm newborns (>1 week postnatal age, gestational age> 23 weeks, birth weight <1000g), intubated and mechanically ventilated, and breathing spontaneously. Infants should be breathing on a triggered mode of ventilation (SIMV, assist control, or pressure support) with the following ventilator parameters: assist control/pressure support < 20 cm H2O, PEEP4-7 cm H2O, SIMV rate< 30, FIO2<40%/.