Esophageal pressure (PES), which has been used as a surrogate for pleural pressure. The volume of esophageal balloon can influence the accuracy of monitoring esophageal pressure. The optimal balloon volume is directly dependent on surrounding pressure. In the present study,the investigators will observe the optimal volume of esophageal balloon during the different PEEP in bench and clinical study.

The esophageal pressure (Pes) is used as a surrogate for pleural pressure to obtain transpulmonary pressure. Catheter with air balloon is the most commonly used method to measure the Pes. The optimal injected volume of the balloon is the key factor in accurate measurement of Pes. The recoil pressure of the balloon turns up while the balloon is over-filled, resulting in over-estimation of the PES; on the other hand, an under-filled balloon also cannot properly transmit the surrounding pressure of balloon. However, the researchers showed the optimal balloon volumes is related to the surrounding pressure and even is not correspond with manufacturer's recommendations. Theoretically, when balloon transmural pressure(PTM) is zero, representing the balloon in a condition with equivalent pressure inside and outside of the balloon, it was defined as optimal volume. However, in clinical settings, it is difficult to determine the balloon PTM, and therefore the optimal volume cannot be obtained, because the surrounding pressure of the balloon cannot be conveniently measured.

In the present study, the investigators will develop a simple method to obtain the optimal balloon volume and observe the effect of positive end-expiratory pressure on optimal balloon volume during esophageal pressure monitoring. The investigators want to validate the accuracy of method in the bench study and clinical feasibility in mechanical ventilated patients.

Inclusion Criteria:

• postoperative patients with delayed emergence from general anesthesia admitted to the ICU for mechanical ventilation.

Exclusion Criteria:

• age under 18 years;
• diagnosed or suspected esophageal varices;
• history of chronic obstructive pulmonary diseases or asthma;
• history of esophageal, gastric or lung surgery;
• evidence of active air leak from the lung, including bronchopleural fistula, pneumothorax, pneumomediastinum, or an existing chest tube;
• evidence of severe coagulopathy.

postoperative patients receiving mechanical ventilation