Inclusion Criteria  age between 16 and 85 years use of dedicated noninvasive ventilator PaCO2 ≤50 mmHg PaO2/FiO2 ≤300mmHg expectation of noninvasive ventilation >12 hours consciousness (Kelly score ≤3 or GCS≥13)  Exclusion Criteria  use of noninvasive ventilation before randomization > 24 hours use of noninvasive ventilation due to heart failure, asthma, acute excerbation of COPD presence of contraindication of noninvasive ventilation such as malformation, rencent pulmonary or esophageal surgery et al. end stage disease (expectation of life < 6 months) pneumothorax noninvasive ventilation intolerance refusal of paticipation pregant woman requirement of emergency intubation