Effect of Optimal Positive End-Expiratory Pressure on Oxygenation and Intrapulmonary Shunt During One-lung Ventilation

Inclusion Criteria: * Aged 18 yr to 65 yr * Body mass index(BMI)18-28kg/m2 * American Society of Anesthesiologists Physical Status Classifications I to II * Normal cardiac and pulmonary function * Diagnosed with uncomplicated lung cancer and was to receive video-assist thoracoscopic lobectomy under general anesthesia. Exclusion Criteria: * Forced expiratory volume at one second/forced vital capacity \<70% * Asthma * Chronic obstructive pulmonary disease * Acute lung infection * Past history of thoracic surgery * Cardiac,hepatic,renal and endocrine diseases * Preoperative glucocorticoid medication * Preoperative chemotherapy * Peripheral oxygen saturation（SpO2） kept below 90% for more than 10 minutes during operation * Blood transfusion during operation * Duration of one-lung ventilation less than an hour * Occurrence of severe complications like allergic shock