One lung ventilation (OLV) is essential during thoracic surgery. During OLV, intrapulmonary shunt can be increased resulting hypoxemia. Although OLV technique had been advanced so far, hypoxemia during OLV reaches about 10% in spite of inspired oxygen fraction 100%. Iloprost is a prostaglandin analogue used for pulmonary hypertension, which can decrease pulmonary artery resistance by selectively dilating pulmonary artery. In this prospective, randomized, double blind study, the investigator is planning to investigate the effects of iloprost on respiratory physiology (intrapulmonary shunt, deadspace, oxygenation, etc) in obese patients.

Inclusion Criteria:

1. scheduled for VATS lobectomy
2. 40 < age < 80
3. American Society of Anaesthesiologists(ASA) physical status classification I~III
4. BMI > 30 kg /m2

Exclusion Criteria:

1. American Society of Anaesthesiologists(ASA) physical status classification IV or more
2. NYHA class III~IV
3. Severe obstructive lung disease and/or restrictive lung disease patients
4. patients with end-organ diseases (i.e. heart failure, respiratory failure, hepatic failure, renal failure)
5. arrhythmia
6. pregnant women
7. pulmonary edema or pulmonary arterial hypertension
8. cerebrovascular disease
9. unstable angina or myocardial infarction within 6 months
10. patients with allergic reaction at iloprost