To Explore the Optimal Dose of Alfentanil for Skull Pin Fixation in Intracranial Surgery

We first start with alfentanil 100 ng/ml, and use the up and down method as adjust 25 ng/ml of concentration of alfentanil each time. An SPI higher than 80 and hyperdynamics (the increased HR and MBP up 20% of baseline or HR\>100bpm and ABP\>180/100mmHg) indicates insufficient analgesia, therefore increase it by 25 ng/ml at the next patient; an SPI lower than 80 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 25 ng/ml during skull pin fixation. If the analgesia is inadequate, increase alfentanil concentration will be prescribed. On the other hand, if analgesia is adequate, however hyperdynamics was noted, beta blocker or calcium channel blocker will be prescribed. If hypotension (ABP\<90/50 mmHg) or bradycardia (HR\<50 bpm) occur, we first stop alfentanil infusion and treated with ephedrine or atropine, respectively.