ANI and Remifentanil in Skull Pin Fixation.

First start with remifentanil 6.0 ng/ml, and use the up and down method as adjust 0.5 ng/ml of concentration of remifentanil each time. An ANI lower than 30 and hyperdynamics (the increased heart rate and mean blood pressure up 20% of baseline or heart rate\>100bpm and arterial blood pressure\>180/100mmHg) indicates insufficient analgesia, therefore increase it by 0.5 ng/ml; an ANI higher than 30 and stable hemodynamics (the changes of HR and MBP within 20% of baseline or HR\<50bpm and ABP\<90/50mmHg) indicates that analgesia is acceptable, and the next patient will decrease it by 0.5 ng/ml during skull pin fixation.