The effect of driving pressure (DP)-guided positive end expiratory pressure (PEEP) on early postoperative pulmonary ventilation is to be determined for patients undergoing neurosurgery. Patients are recruited to receive volume controlled ventilation with either a fixed PEEP (5cmH2O) or DP titrated PEEP. Early postoperative regional distribution of lung ventilation, expressed as global inhomogeneity (GI) is evaluated by electrical impedance tomography (EIT), a noninvasive, radiation free modality. Center of ventilation (COV) by EIT, as well as the lung ultrasonography（LUS), perioperative ventilatory parameters, arterial oxygenation index (PaO2/FiO2) , serum indicators and postoperative pulmonary complications are secondary outcome variables.

After screened for preoperative risk factors, 44 patients undergoing elective neurosurgery and planned to be extubated in the operating room are randomly assigned to two groups: (1) PEEP = 5cmH2O; (2) driving pressure (DP)-guided PEEP. The ventilation protocol consists of volume-controlled mechanical ventilation (Datex Ohmeda S/5 Advance; General Electric Company Healthcare, Helsinki, Finland) at a tidal volume (VT) of 8 ml/kg predicted body weight (PBW), fresh gas of 2 litre /min, inspiratory to expiratory ratio of 1:2, and a respiratory rate adjusted to normocapnia (arterial carbon dioxide partial pressure between 35 and 45 mmHg). For group 2, trial for the lowest DP was started 10 min after position adjustment by increasing PEEP from 2 to 10 cm H2O incrementally. DP was calculated as "plateau pressure - PEEP". Each PEEP level was maintained for 10 respiratory cycles, with DP in the last cycle recorded. Then the PEEP level producing the lowest DP was identified and maintained intraoperatively. A plateau pressure of no more than 30 cmH2O is targeted in each group. Early postoperative distribution of regional ventilation global inhomogeneity (GI) is the primary endpoint evaluated by EIT (electrical impedance tomography), which was performed by a trained technician who is blinded to randomization. Center of ventilation (COV) by EIT, lung ultrasonography（LUS) , and arterial blood gas are evaluated, with inflammatory and oxidative mediators tested from venous sample. Brain relaxation will be scored by the neurosurgeon. Postoperative pulmonary complications within 3 days are also recorded.

Inclusion Criteria:

• Informed consent has been obtained
• Elective neurosurgery
• Expected ventilation duration > 2 hours
• Scheduled to be extubated in the operation
• American Society of Anesthesiologists (ASA) physical status >2

Exclusion Criteria:

• Mechanical ventilation of > 1 hour within the last 2 weeks before surgery
• Dysphagia resulting from preoperative cranial nerve damage
• Body mass index ≥ 35 kg/m2
• Acute respiratory failure (pneumonia, acute lung injury or acute respiratory distress syndrome)
• Emergency surgery
• Severe cardiac disease
• Progressive neuromuscular illness
• Pregnancy
• Refusal to participate
• Contradicted to EIT scan or lung ultrasound scan