Exclusion Criteria Patients were excluded if they Hypercapnia (> 45 mm Hg with respiratory acidosis), cardiogenic pulmonary edema, COPD, pulmonary fibrosis, hypoventilation obesity syndrom, arterial pressure < 60 mm Hg or treatment by epinephrine > to 0,1 gamma/kg/minute, deterioration of awareness (Glasgow scale < or = 12), acute confusional state.