Conventionally, a relatively high level of fraction of inspired oxygen (FiO2) has been used for secure a margin of safety in patients undergoing cardiac surgery including off-pump coronary artery bypass grafting (OPCAB). Since the potential adverse effects of hyperoxemia (via reactive oxygen species, vasocontriction, perfusion heterogeneity, myocardiac injury, etc.), various studies on this topic has been performed. However, the results are conflicting and inconsistent, and the consensus about whether the use of additional oxygen supply in cardiac surgery has not been reached among practitioners yet. This study is a multicenter study (Seoul National University, Asan Medical Center, Severance Hospital) where the effect of different level of FiO2 on postoperative clinical outcomes is evaluated.

A sub-study will also be conducted in part of patients of the present study. In 40 out of 500 (expected) patients, two different levels of FiO2 will be sequentially set with a 5-minute interval after the induction of general anesthesia. Mixed venous and regional cerebral oxygen saturation will then be recorded according to the change of FiO2.

Inclusion Criteria:

• All patients scheduled for elective off-pump coronary artery bypass grafting

Exclusion Criteria:

• emergency
• robot-assisted surgery
• minimally invasive cardiac surgery requiring thoracotomy/one-lung anesthesia
• preoperative pulmonary morbidity requiring oxygen therapy
• preoperative use of ECMO or IABP
• preoperative tracheal intubation or mechanical ventilation