During weaning from mechanical ventilation process, the energy expenditure in T tube is greater than pressure support ventilation.

The energy expenditure (EE) was measured during the two periods of 30-minutes, with the 10 first minutes being discarded for analysis purposes. The other variables were recorded and monitored during the 10th and the 30th minute, in both pressure support (PS) and t-Tube (TT) modes. Patients were randomized in terms of sequence selection: PS - TT or TT - PS. In the baseline period, patients were receiving mechanical ventilation (Servo 900C and Servo 300; Siemens-Elema, Solna, Sweden), with PS ranging from 10 to 15cmH2O. During the PS mode, patients received assisted pressure ventilation of 10 cmH2O, 5 cmH2O of final positive expiratory pressure, the sensitivity of -1cmH2O and FiO2 of 0.4. During TT mode, oxygen flow was delivered in order to keep the same FiO2 that was provided in PS (0.4) as controlled by monitor screen (Datex Ohmeda S/5 - Compact Airway Module, M-COVX model, Finland). In this mode, an extensor was employed, one end of which was connected to the oxygen-enriched and the other end was secured to a 3-output ("T") connector, which was attached to the patient's ventilatory prosthesis. During the rest period (30 minutes), patients returned to the mechanical ventilation parameters utilized in the baseline period.

Inclusion Criteria:

• Patients with acute respiratory failure receiving MV for over 24 hours, in the ICU of the Hospital de Clínicas de Porto Alegre

Exclusion Criteria:

• Patients with chest drainage, hemodynamic instability (characterized by the use of vasoactive drugs), renal failure, mental status change, agitation, sudoresis, tachycardia, axillary temperature above 38°C and inspiratory oxygen fraction (FiO2) over 0.6.

Patients with respiratory failure receiving MV fore more than 24 hours, who had been admitted to the Intensive Care Unit (ICU) of the Hospital de Clínicas de Porto Alegre and who met the criteria for weaning from MV according to the parameters previously defined were selected to participate in the study.