In patients under mechanical ventilation, Usefulness of dynamic indices based on the heart-lung interactions for guiding volume resuscitation has been well validated. Recently, validity of pulse pressure variation (PPV) has been also reported to predict fluid responsiveness. Oesophageal Doppler (OED) allows continuous monitoring of several important hemodynamic variables. Of the OED variables, corrected flow time (FTc) has been evaluated and used as a preload index, and has been reported to predict fluid responsiveness. Prone position caused physiologic and hemodynamic changes and influenced the predictability and cut-off values of SVV and PPV for fluid responsiveness. In this prospective controlled study, the investigators evaluated the validity of PPV and FTc as predictors for fluid responsiveness in supine and prone position in patients undergoing posterior lumbar spinal fusion

Inclusion Criteria:

• patients undergoing elective spine surgery
• age : > 20 and < 70, 3. Patients expecting intraoperative blood loss more than 10% of estimated blood volume

Exclusion Criteria:

• arrhythmias
• BMI > 30 kg/m2
• valvular heart disease
• left ventricular ejection fraction < 50%
• history of lung disease
• pregnancy
• foreigner
• contraindications to esophageal doppler monitoring probe insertion (i.e. oesophageal stent, carcinoma of the oesophagus or pharynx, previous oesophageal surgery, oesophageal stricture, oesophageal varices, pharyngeal pouch, intra-aortic balloon pump, coarctation of the aorta, and severe coagulopathy)