Assuming a variability of the paCO2 as previously reported for difficult-to-wean preterm infants in our unit (Czernik C, J Matern Fetal Neonatal Med 2012) and a treatment failure rate of 22% within 72 hours after extubation, we calculated a sample size of 34 patients in each study arm to detect a difference in the paCO2 of 7 mmHg, using a two-sided significance of 0.05 and a power of 0.8.