Monitoring airway pressure is essential for patients with mechanical ventilation. However, static airway pressure does not reflect alveolar pressure at all. Airway pressure is supposed to completely interrupt the communication between proximal airway opening and the distal alveolar and/or small airway structures. In this condition, some alveoli may still be inflated but do not communicate with proximal airways and auto-PEEP will give a biased estimated of mean alveolar pressure. To be note, distinguishing the airway closure and alveolar collapse can be challenging at times. The quasi-static PV curve is a useful bedside tool to set mechanical ventilation, which may help us to identify the airway closure and alveolar collapse. Meanwhile, the quasi-static PV curve can only reflects a global behaviour of the lung, while EIT may be a useful tool to assess the regional information on airway closure and alveolar collapse.

Inclusion Criteria:

• Adult patients undergoing controlled mechanical ventilation
• The duration of endotracheal intubation < 48 hrs

Exclusion Criteria:

• Severe hemodynamic instability
• Severe chronic lung disease requiring long-term home oxygen therapy
• Patients without analgesic sedation
• Decline to participate in the study
• Refusal to sign informed consent