Optimal Depth for Esophageal Stethoscope

Inclusion Criteria: * The patients with ASA class 1-2 (20-65 years old) who are undergoing elective surgery with supine or lithotomy position, without position change scheduled to last more than 90 min. Exclusion Criteria: * anatomical abnormality with the upper airway * risks of a difficult airway * abnormal central anatomical structures such as airway, diaphragm, or spine on the chest X-ray * history of disease or surgery on the stomach or esophagus * risks of bleeding or coagulopathy on the preoperative laboratory results * obese patients with BMI over 30 * contraindication to insertion of ES for the surgery * who did not take a standing chest PA X-ray before surgery * position change during the surgery * open chest surgery