Aims: The key purpose of this study is to determine the impact of two approaches "volumetric and pressure ventilation" on intra operative blood loss and posterior spine fixation. Design: In This clinical trial, 78 patients in two groups of 39 people are studied. Conduct: With no prior knowledge, one group started to use volumetric ventilation with current volume 7ml/kg/IBW, 12 cycles per minute inspiration rate and inhale to exhale ratio 1:2 without peep, during the operation. The inspiration rate is altered to achieve end exhale carbon dioxide of 30-45 mmHg. The other group receives pressure ventilation with initial pressure 15 H2O/cm and 12 cycles per minute inspiration rate and inhale to exhale ratio 1:2 without peep. Then the current volume is adjusted to 7ml/kg/IBW by changing the pressure to achieve the end exhale carbon dioxide 35-40 mmHg by changing the inspiration rate. Participations: Among patients aged 18-75 in class1, 2 ASA, nominated for posterior spine fixation operation surgery were justified for the study and written consent derived from them. In this study patients with cardiopulmonary diseases history, uncontrolled hypertension or diabetes, SCI, coagulopathies, or treated by aspirin, heparin, and warfarin, and BMI more than 35 are omitted. Interventions: Blood pressure and heart beat of the studied patients are checked every 15 minutes after anesthesia. Then satisfaction of surgeon asked. Main outcome: Duration and intensity of bleeding are compared until the end of operations between the two groups.