In all centers, CPAP was generated using a flow generator with an adjustable inspiratory oxygen fraction set to deliver a flow of up to 140 liters per minute (Whisperflow, Caradyne, Ireland) and a spring-loaded expiratory pressure valve (Vital Signs Inc, Totoma NJ) and applied using a latex-free polyvinyl chloride transparent helmet (CaStar, Starmed, Italy) (15); all centers measured the inspiratory oxygen fraction using an oxygen analyzer (Oxicheck, Caradyne, Ireland) through the Venturi mask or the helmet.