Permissive hypercapnia increased the survival rate in patients with acute respiratory distress syndrome (ARDS) who required mechanical ventilation in critical care medicine. This has been explained by its association with ventilator induced lung injury. Since then, a protective lung ventilation strategy has been very important, with a low tidal volume of 4-6 ml/kg. Patients undergoing surgery will inevitably require mechanical ventilation. In particular, patients undergoing one lung ventilation for thoracic surgery may have increased airway pressure and a greater chance of ventilator induced lung injury. Recently, protective lung ventilation has been applied to patients undergoing one ung ventilation during thoracic surgery. The purpose of this study is to evaluate the difference in the degree of pulmonary oxygenation and the incidence of postoperative pulmonary complications in hypercapnia induced by controlling the respiratory rate with a constant tidal volume.

Inclusion Criteria:

1. Adult patients aged 40-80 years who are planning to have thoracoscopic single lobectomy or segmentectomy with one lung ventilation during surgery.
2. American Society of Anesthesiologists (ASA) classification 1~3

Exclusion Criteria:

1. patients with heart failure (NYHA class III~IV)
2. patients who are having moderate obstructive lung disease or restrictive lung disease
3. Low DLCO (< 75%)
4. patients with brain disease history or increased ICP
5. patients with pulmonary hypertension (mean PAP>25mmHg)
6. patients with liver disease (AST level ≥100 IU/mL or ALT ≥ level 50 IU/L) or kidney disease (Creatine level ≥ 1.5 mg/dL)
7. patients with pre-existing hypercapnia or metabolic acidosis
8. body mass index (BMI) > 30 kg/m2
9. patients who have had contralateral lung surgery
10. patients who cannot read explanation and consent form
11. patients who are pregnant

Patients, care givers and outcomes assessors are blinded. The investigator should not be included in the blind because they need to adjust the ventilator settings.