Correlation Between External Jugular Venous and Superior Vena Cava Central Venous Blood Oxygen Saturation in Patient Under General Anesthesia

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.