SPI and Remifentanil in Skull Pin Fixation

first start with remifentanil 5.0 ng/ml, and use the up and down method as adjust 0.5 ng/ml of concentration of remifentanil each time. Inadequate antinociception (Surgical Pleth Index(SPI) \> 80 and hyperdynamics (the increased heart rate (HR) and mean blood pressure (MBP) above 20% of baseline or HR\>100bpm and arterial blood pressure(ABP)\>180/100mmHg)) is defined as failure, therefore we increase remifentanil by 0.5 ng/ml; otherwise, the setting is considered successful without abovementioned situations.