Ventilator mode change was associated with decrease in blood loss during posterior lumbar interbody fusion (PLIF) due to decrease in the peak inspiratory pressure (PIP). The purpose of this study was to determine the effect of equal ratio ventilation (ERV), which sets the I:E ratio of the ventilator to 1:1 during volume controlled ventilaiton, on surgical blood loss during PLIF. Investigators hypothesized that ERV would decrease surgical blood loss due to decrease in the PIP.

After anesthesia, patients will receive ventilator settings according to theri group. The CVP will be measured before and after the prone position to ensure the proper positioning. Airway pressures including peak inspiratory pressure, mean/plateau airway pressure, arterial blood gas analysis data, hct, lactate, body temperature, mean arterial pressure, cardiac index. Recodings will be done 5min afte the induction, 5 min after the prone positioning, sikin suture, and 5min after supine position. Amount of bleeding, coagulation profile and Hct will be recorded 72 hrs after the surgery.

Inclusion Criteria:

• 1) Age : 20~75 yr-old 2) Surgery : Posterior lumbar interbody fusion(PLIF) 2~3 levels

Exclusion Criteria:

• 1) Urgent or Emergency case 2) Previous lnstrumentation of lumbar spine (Exclusion : Discectomy, Partial hemilaminiectomy, etc) 3) Concurrent other operation 4) Patients who cannot understand informed consent (ex. Illiterate, Foreigner) 5) Obesity (BIM>30kg/m2) 6) Recent myocardial infarction (within 3 months) 7) Reduced left and right ventricular function (Ejection fraction<40%) or Congestive heart failure 8) Preoperative dysrhythmia 9) Aspirin or Plavix during perioperative periods 10) Lung disease 11) Chronic kidney disease or Dialysis 12) Severe hepatic disease