Discontinuation from MV will be attempted when the primary physician judges that the patient is ready to be weaned, according to the standard criteria. Mechanical ventilation will be reinstituted in the following circumstances: respiratory rate above 35 breaths /min, P02 below 65 mm Hg with FIO2less than 0.6, ph 7.34 or less, heart rate equal to or above 1 10 beats min or Increased by 20 % or more, or if arrhythmias appear, systolic blood pressure without inotropes <90 mmHg or above 20% of usual blood pressure, uncoordinated thoraco-abdominal movements, activation of accessory muscles, agitation or depressed mental status, or excessive sweating.