Patients scheduled for cerebral aneurysm clipping surgery or arteriovenous malformation surgery(ASA 1to 3 over 18years) receive standard care general anesthesia including central venous line for rapid ventricular pacing technique. A contralateral Foresight sensor is placed on the forehead of the patient at induction. After the craniotomy two micro electrodes(PBtO2 and cerebral blood flow) are placed in the brain and fixated. A subdural strip electrode is placed on the surface of the brain to record the electrical activity. Effect of mild hyperventilation and oxygenation with 100% O2 is evaluated.