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, investigators are planning to investigate the effects of iloprost on respiratory physiology (intrapulmonary shunt, deadspace, oxygenation, etc).