All patients age between 18-65 year old who undergoing surgery under general anesthesia and superior vena cava central venous catheter installation anticipated are eligible. The tips of superior vena cava central venous catheter was confirmed by Chest x-ray. Next 1 hour after skin incision, 1 ml of venous blood sample is taken from superior vena cava central venous catheter of right external jugular vein then send to central laboratory for venous blood gas analysis. The demographic data will be recorded. Correlation between superior vena cava central venous blood and right external jugular venous blood oxygen saturation will be determined the primary outcome and partial pressure of carbon dioxide will be determined as a secondary outcome.
After installation of central venous catheter in patients under general anesthesia, paired sample of superior vena cava central venous blood and right external jugular venous blood will be collected and send to central laboratory for venous blood gas analysis. Correlation between superior vena cava central venous blood and right external jugular venous blood oxygen saturation will be determined as primary outcome.