Weaning from mechanical ventilation is a critical issue and the diaphragmatic disfunction has been demonstrated to play an important role in extubation failure. the aim of present investigation is to evaluate diaphragmatic excursion velocity during in patients undergoing spontaneous breathing trial through tissue Doppler analysis in both inspiration and expiration.

Inclusion Criteria:

• premature infants with gestational age <32 weeks and supported by invasive mechanical ventilation ≥48 hours
• All the indications for extubation are met：cough or vomiting due to spontaneous breathing and sputum aspiration; ventilator parameters in recent 24 hours: MAP < 8cmH2O, RR < 30 times/min, FiO2 < 30%; Arterial blood gas: pH > 7.25, (PaCO2< 60mmHg, BE < 8mmol/L, oxygen saturation > 90%
• a successful SBT
• parents or legal guardians sign informed consent to attend this study

Exclusion Criteria:

• accidental extubation
• chest CT or bronchoscopy during hospitalization indicated congenital airway dysplasia
• complex congenital heart disease
• congenital metabolic diseases, neuromuscular diseases
• severe brain injury
• surgical mechanical ventilation
• give up before extubation;
• pneumothorax or pleural effusion
• no consent is signed

premature infants with gestational age <32 weeks and supported by invasive mechanical ventilation ≥48 hours