Patients were in supine position, intubated with an uncuffed endotracheal tube size 3.5 or 4.0 mm, and put on a timecycled, pressure-limited ventilation mode (Pressure Control, AVEA ventilator, Cardinal Health, Yorba Linda, CA, USA). Aims of ventilation were transcutaneously measured oxygen saturation (SpO 2 ) 88 to 92%, and partial pressure of arterial carbon dioxide (PaCO 2 ) 45 to 65 mmHg (if pH >7.25). Inspir-atory times were fixed at 0.5 seconds, positive end-expiratory pressure (PEEP) was set 1 to 2 cmH 2 O below total PEEP (i.e. extrinsic PEEP + intrinsic PEEP). The flow-time curve was observed thoroughly throughout the study period in each patient to examine if expiration was complete in order to prevent dynamic hyperinflation. Patients were sedated with midazolam and morphine, paralysis was achieved using intravenous rocuronium. Endotracheal suctioning was performed 30 minutes prior to the start of, but not during, the experimental protocol. Bronchodilators (either nebulized or intravenous) or ketamine were not used before or during the study period.