inclusion criteria: age ≥18 years presence of acute respiratory distress syndrome for ≤24 hours of all of the following conditions, within one week of a clinical insult or new or worsening respiratory symptoms: pao2/fio2 <150 mmhg with positive end-expiratory pressure (peep) ≥8 cmh2o (or, if arterial blood gas not available, spo2/fio2 that is equivalent to a pao2/fio2 <150 mmhg with peep ≥8 cmh2o and a confirmatory spo2/fio2 between 1-6 hours after the initial spo2/fio2 determination) bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules respiratory failure not fully explained by cardiac failure or fluid overload; need objective assessment (e.g., echocardiography) to exclude hydrostatic edema if no risk factor present pao2/fio2 <200 mmhg after meeting inclusion criteria and before randomization

inclusion criteria: age ≥18 years presence of acute respiratory distress syndrome for ≤24 hours of all of the following conditions, within one week of a clinical insult or new or worsening respiratory symptoms: pao2/fio2 <150 mmhg with positive end-expiratory pressure (peep) ≥8 cmh2o (or, if arterial blood gas not available, spo2/fio2 that is equivalent to a pao2/fio2 <150 mmhg with peep ≥8 cmh2o and a confirmatory spo2/fio2 between 1-6 hours after the initial spo2/fio2 determination) bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules respiratory failure not fully explained by cardiac failure or fluid overload; need objective assessment (e.g., echocardiography) to exclude hydrostatic edema if no risk factor present pao2/fio2 <200 mmhg after meeting inclusion criteria and before randomization