Liberal fluid administration is one of risk factors of developing acute lung injury (ALI) in thoracic surgery. Therefore, the investigators try to restrain fluid administration, and in the case of intraoperative hypotension, the investigators often administer vasoactive agents or inotropes. One lung ventilation (OLV) which is routinely employed for thoracic surgery decrease arterial oxygenation and oxygen delivery to brain can be also decreased. In this study, the investigators compared dopamine and phenylephrine in respect to maintaining cerebral oxygen saturation in major thoracic surgery. The investigators hypothesis is that dopamine is better than phenylephrine to maintain cerebral oxygen saturation in thoracic surgery.