Fig. 1

Flow diagram for the studies included in this review. *Ichushi is a database of Japanese research papers. Abbreviations: CENTRAL (Cochrane Central Register of Controlled Trials); CPAP (continuous positive airway pressure); HFNO (high-flow nasal oxygen); IMV (invasive mechanical ventilation); NIV (noninvasive ventilation); RCT (randomized controlled trial); SOT (standard oxygen therapy)