Postoperative pulmonary complications are main cause of overall perioperative morbidity and mortality in the patients following general anesthesia. A protective ventilation strategy refers to the use of low VT (in the range of 4-8 ml/kg of the predicted body weight) with positive end-expiratory pressure (PEEP), with or without recruitment maneuver. Protective ventilation has been considered the optimal practice in patients suffering from the acute respiratory distress syndrome (ARDS). However, few human studies have assessed how to ventilate healthy lungs in patients undergoing general anesthesia, especially in prone position. Prior studies reported that in the patients undergoing major abdominal surgery in supine position, intraoperative lung protective ventilator settings had the potential to protect against pulmonary complications. Therefore, the investigators planned this study to better specify the effect of intraoperative protective ventilation in surgical patients in the prone position.

Inclusion Criteria:

• the patient undergoing elective lumbar spine surgery of two hours or more in the prone position
• the age: 19 yrs and older
• the patients had a preoperative risk index for pulmonary complications of more than 2.

Exclusion Criteria:

• Patients with altered mental status or increased intracranial pressure
• Body mass index more than 35 kg/m2
• Patients with persistent hemodynamic instability or intractable shock
• Severe cardiac disease defined as New York Heart Association class III or IV or acute coronary syndrome or persistent ventricular tachyarrhythmias
• Recent history of invasive ventilation (within two weeks)
• Recent history of pneumonia, ALI/ARDS or sepsis (within two weeks)
• History of pulmonary resection, emphysema or chronic obstructive pulmonary disease (COPD)
• Repeated systemic corticosteroid therapy for acute exacerbations of COPD or asthma
• Recent immunosuppressive medication defined as need of chemotherapy or radiation therapy (within two months)
• History of neuromuscular disease
• Emergency operation
• Patient refusal
• Pregnancy