Patients were studied shortly after rewarming (peripheral temperature over 31°C) and becoming hemodynamically stable (within 3 to 5 h of admission to the ICU). During the study, patients were kept sedated with diazepam (0.1 to 0.2 mg/kg) and paralyzed with vecuronium bromide (0.1 to 0.2 mg/kg/h). They were transorally intubated with cuffed endotracheal tubes (Portex, Portex Ltd; Hythe, Kent, England) (8.0 to 9.0 internal diameter [ID] and 31 cm long) and mechanically ventilated with a ventilator (Siemens Servo 900C, Siemens-Elema, Solna, Sweden).
The Vt was set at 12 ml/kg and frequency at 12/m in with fractional inspiratory time of 35 percent, including an end-inspiratory pause of 10 percent. Other than changing PEEP level, baseline settings were kept constant in each subject throughout the experiment. Special care was taken to avoid gas leaks in the equipments and around the tracheal cuff. To reduce the effects of the compliance and resistance of the system connecting the patients to the ventilator on the mechanics measurements, a single length of standard low-compliance adult tubing was used (2 cm ID, 110 cm long) and the humidifier was omitted from the inspiratory line.