This is a randomized controlled trial compared two methods to drainage subglottic secretion in prolonged intubated patients.After enrolled, patients are randomized assigned to intermittent subglottic secretion drainage group and air-impingement manipulation group.

Ventilator-associated pneumonia (VAP) is the most frequent infection occurring in patients who are admitted to the ICU. The accumulation of respiratory secretions in the subglottic space is a well-proven cause of VAP. Therefore, prevention should include the aspiration of secretions from the subglottic space. Investigators invented a manual method with high-flow air produced by resuscitator to impinge secretion from the subglottic space to oral cavity. This method has been granted as a patent by Chinese national intellectual property patent office, it has been used in investigators' daily work for more than 10 years and more than 20 Chinese ICUs have used it everyday. Investigators want to compare it with conventional method which uses a special intubation tube with an independent dorsal lumen to suction subglottic secretion.

Inclusion Criteria:

Should meet all the criteria:

1. Intubation less than 24 hours before admitting respiratory intensive care unit;
2. Anticipated intubation for more than 72 hours;
3. Anticipated survival time is more than 2 weeks

Exclusion Criteria:

Meet any of these criteria:

1. PEEP≥10cmH2O or FiO2≥0.8;
2. unstable hemodynamics;
3. severe bulla and pneumothorax;
4. enrolled in other study;
5. cuff leak test is positive.