Inclusion Criteria:  ≥ 18 years old； scheduled for elective abdominal surgery under general anesthesia； ASAⅠ-Ⅲ  Exclusion Criteria:  Patients and their family members refused to be enrolled； patients were unable to communicate because of language disorders such as dementia and hearing impairment； patients were admitted to ICU 48 hours before operation； preoperative diagnosis of ARDS, aspiration pneumonia, pulmonary embolism or pulmonary edema or other acute respiratory diseases； mechanical ventilation within one week before operation； death or discharge within 24 hours after operation； second operation due to postoperative complications