Protocol
Patients received high-flow nasal oxygen for 1 h in the supine semirecumbent position. Gas flow was set at 60 L/min, the temperature of the humidification chamber (MR860 or ARIVO2, Fisher and Paykel healthcare) was set according to patient's comfort, FiOFor safety reasons, enteral feeding was interrupted 1 h before prone positioning and re-established after the study ended.