We anticipated that we could not complete this study.

Acute hypoxaemic respiratory failure (AHRF) refers to pathological states in which arterial blood oxygenation is severely impaired，and which need invasive positive pressure ventilation (IPPV) as respiratory support technique in most cases.However，IPPV carries well-known risks of complications such as ventilator induced lung injury (VILI) or ventilator associated pneumonia (VAP)，and the incidence of which is increased as the prolongation of IPPV so as to lead to higher mortality rate. Consequently，early extubation is extraordinarily necessary.

More recently, NPPV has shown to shorten the duration of IPPV，reduce the mortality and morbidity rates in patients with chronic obstructive pulmonary disease (COPD). Despite this evidence, the efficacy of NPPV in patients with AHRF has not been evidenced. However，NPPV has been shown to provide adequate ventilation and oxygenation，and reduce inspiratory muscle effort，neuromuscular drive，and dyspnea scores. Moreover，to some patints，NPPV is similar with IPPV in providing oxygenation.

The duration of weaning is from the first day a patient met standard criteria for weaning readiness to the time of successful extubation (lasting at least 48 h)，which represents 40-50% of the total duration of IPPV.As a result，duration of IPPV would be shortened if that of weaning was shortened.

The investigators hypothesized that in mechanical ventilated patients with AHRF who met standard criteria for weaning readiness and suffer failure of spontaneous breathing trial, use of NPPV for early extubation, providing adequate ventilation and oxygenation, would shorten the duration of IPPV as the primary end-point variable, thereby reduce the incidence of complication and mortality rates. Accordingly, the investigators conducted a prospective，randomized clinical trial to assess the efficacy of this strategy compared with the conventional-weaning approach.

Inclusion Criteria:

• Orotracheal intubation
• Arterial oxygen tension (PaO2)<60mmHg（venturi mask，FiO2=0.5），and arterial carbon dioxide tension(PaCO2)≤45mmHg；
• Meeting standard criteria for weaning readiness
• Suffering failure of spontaneous breathing trial.

Exclusion Criteria:

• Age<18
• Duration of invasive positive pressure ventilation<48h
• Tracheotomy
• Percentage of cuff leak volume in tidal volume<15.5%
• Unable to spontaneously clear secretions from their airway
• Recent oral，nasal，facial or cranial trauma or surgery
• Recent gastric or esophageal surgery
• Active upper gastro-intestinal bleeding
• Severe abdominal distension
• Lack of co-operation
• Chronic respiratory disease such as chronic obstructive pulmonary disease，asthma，interstitial lung disease,etc.