Prone position in spinal surgery can increase airway pressure and decrease dynamic lung compliance by a frame used for postural stabilization. Also, it can increase hemodynamic instability such as reduced blood pressure by aggravating cardiac index.

Former study shows pressure controlled ventilation on the pulmonary mechanics can improve alveolar use and oxygenation than volume controlled ventilation. The latter one means controlling a patient's breathing completely through tidal volume and set breathing rate. This could be useful for the patients not possible to breath by themselves because it guarantees the perfect breathing.

The pressure controlled ventilation is used when there is a possibility to change a patient's environment. The pressure will be maintained steadily, but volume and flux will be changed. That means through the pressure already set, the whole breathing will be maintained from the beginning to end. If a patient's resistance is increased, the volume will be decreased even though the way of blood pressure control is same. However, the tidal volume per minute can be controlled somewhat and barotrauma can be decreased by controlling respiratory rate. Through this study, we are expecting the pressure controlled ventilation in prone position can improve lung mechanics and oxygenation.

Inclusion Criteria:

• scheduled for lumbar spine surgery under general anaesthesia

Exclusion Criteria:

• Patients with coronary artery occlusive disease
• morbid obesity (body mass index ≥ 30 kg/m2)
• cerebrovascular disease and major obstructive or restrictive pulmonary disease