Patients were randomized to be treated for six hours with oxygen through a Venturi mask at a FiO2 of 0.5 (control) or with oxygen at a FiO2 of 0.5 plus a CPAP of 7.5 cm H2O (CPAP). At the end of the 6-hour period, patients passed a one-hour screening test breathing oxygen through a Venturi mask at a FiO2of 0.3. Patients returned to the assigned treatment if the PaO2/FiO2 ratio was less than or equal to 300; treatment was interrupted if the PaO2/FiO2 ratio was higher than 300. Nasal oxygen (8-10 liter/minute) was given if the treatment was not tolerated (Figure 1).