 Interventions: Patients were randomized to receive mechanical ventilation either with tidal volume VT of 10-12 mlkg predicted body weight high VT group or with VT of 5-7 mlkg predicted body weight low VT group with an O2 inspiratory fraction FIO2 enough to keep arterial oxygen saturation 90 with positive end-expiratory pressure PEEP of 5 cmH2O during 12 hours after admission to the study. 