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

Inclusion Criteria: * Written consent obtained from patient or next of kin a day before operation date * Patient age between 18-65 years old undergoing syrgery under general anesthesia * Superior vena cava central venous catheter anticipated for CVP mesurement, innotropic or vasopressor administration,pulmonary artery catheter insertion route * Anticipated right external jugular venous sampling time less than 5 minutes * The site of blood sample collection easily access during operation Exclusion Criteria: * No jugular central venous catheter installation or other route selected * Duration of paired sample collection more than 10 minutes * Can not collected blood sample from right external jugular vein within 3 attemps * The site of blood sample collection difficulty access during operation * Pateint or next of kin refuse to participate