General anesthesia is associated with loss of pulmonary functional residual capacity and the consequent development of atelectasis and closure of the small airway.

Mechanical ventilation in a lung with reduced functional residual capacity and atelectasis increased the dynamic alveolar stress-strain, inducing a local inflammatory response in atelectatic lung areas known as ventilatory-induced lung injury. This phenomenon may appear even in healthy patients undergoing general anesthesia and predisposes them to hypoxemic episodes that can persist in the early postoperative period.

Lung recruitment maneuvers restore the functional residual capacity and, therefore, protect the lungs from lung injury. A key issue in this kind of treatment is detecting the lung's closing pressure in order to maintain the end-expiratory pressure above such a limit.

This is a prospective and observational study designed to measure the lung's closing pressure. The investigators will study 20 mechanically ventilated patients scheduled for mediastinoscopy surgery under general anesthesia.

Lung mechanics and capnography will be assessed during surgery. A slow pressure-volume curve will be automatically performed by the ventilator, and the small airway closing pressure will be determined as the inflection point in the alveolar slope of the pressure-CO2 and volume-CO2 curves. Then, a lung recruitment maneuver will be applied following a descending positive end-expiratory pressure trial to determine the lung's closing pressure. The investigator will see if the lung's closing pressure found with these two techniques is similar.

Inclusion Criteria:

• Age > 17 years ole
• Programmed surgery

Exclusion Criteria:

• Pregnancy
• No programmed surgery
• Hemodynamic instability
• Severe COPD
• Acute respiratory infection

Adult patients undergoing general anesthesia for mediastinoscopy.