Inclusion Criteria:  Gestational age 240 to 336. At least 6 hours of stabilized NIPPV settings, i.e. minor changes in settings (FiO2 ≤0.10, PIP≤ 2.0 cmH2O, RR≤ 5 brm). At least 6 hours of stabilized tcCO2, i.e. ≤5 mmHg variation. At least 6 hours from surfactant administration. Parental consent  Exclusion Criteria:  If NIPPV set rate is 10 bpm and tcCO2 related pCO2 is<40mmHg. If NIPPV set rate is 30 bpm and tcCO2 related pCO2 is>60mmHg. Unstable infants due to acute conditions (sepsis. IVH), or congenital malformations.