One lung ventilation (OLV) is essential during thoracic surgery. During OLV, intrapulmonary shunt can be increased resulting hypoxemia. Wang et al measured intrapulmonary shunt by transesophageal echocardiography. Although OLV technique had been advanced so far, hypoxemia during OLV reaches about 10% in spite of inspired oxygen fraction 100%. Applying PEEP(Positive End-expiratory Pressure) at dependent lung and CPAP at non-dependent lung can mitigate the hypoxemia. Above all, PEEP can be easily applicable. However, there are controversial studies whether it is supportive or not. In this prospective, cross-over study, the investigators are planning to investigate the effects of PEEP on intrapulmonary shunt, oxygenation and cardiac function as well.

Inclusion Criteria:

• scheduled for VATS lobectomy
• 40 ≤ age ≤ 80
• American Society of Anaesthesiologists(ASA) physical status classification I~III

Exclusion Criteria:

• American Society of Anaesthesiologists(ASA) physical status classification IV
• NYHA class III~IV
• Severe obstructive lung disease and/or restrictive lung disease patients
• those with end-organ diseases (i.e. heart failure, respiratory failure, hepatic failure, renal failure)
• arrhythmia
• esophageal varix
• pregnant women