Sedation will be given to the patients intraoperatively. Riker Sedation-Agitation Scale (SAS) will be used to assess the level of sedation during intraoperative period. Titrated dose of midazolam will be given to the patient with the target of SAS score of 3 (difficult to arouse but awakens to verbal stimuli or gentle shaking, follow simple commands but drifts off again). During intraoperative period, oxygen will be administered at 2 and 4 liters per minutes for a period of five minutes. ETCO2, PaCO2 and PaO2 level will be recorded during each level of oxygen administration.