Study population After approval by the institutional ethics committee and written informed consent, we prospectively enrolled adult patients who were scheduled for elective non-cardiac surgery. During the preoperative anesthesia visit, we selected patients with low-risk as indicated by the revised cardiac risk index of 0 and normal LV function (ejection fraction ≥ 60% and septal e' ≥ 8 cm/s. We excluded patients with an arrhythmia, poor lateral axis in supine position, regional wall motion abnormality, or other structural heart diseases. Patients with any of the following conditions were also excluded: unfavorable airway or facemask fit, intractable coughing, hiccups, or hypotension (mean BP < 60 mmHg) requiring IV positive inotropes or vasoconstrictors during the study period.