- The subject has participated in other clinical trials at 4 weeks before the beginning of the present study；The subject is taking or has taken β-blockers (such as metoprolol) within 2 weeks before the beginning of the present study；The subject has undergone craniotomy twice or more；The subject has a history of sinus bradycardia (preoperative electrocardiogram of HR <= 50 beats/min), sick sinus syndrome, myocardial infarction, Ⅱ grade and grade Ⅱ cardiac function, and severe hypertension (systolic blood pressure (SBP)> = 180 mmHg or diastolic blood pressure (DBP)> = 110 mmHg)；The subject has a history of lung disease (blood oxygen content <95% when breathing air at an awake state)；The subject has a history of liver dysfunction (1.5 times or more that of the normal value)；The subject has a history of kidney dysfunction (levels of serum creatinine and blood urea nitrogen are out of the normal range)；Hypersensitivity to dexmedetomidine；Tolerance to dexmedetomidine；The subject has a history of drugs or alcohol abuse；The weight exceeds ±15% of standard weight range, and the standard weight is calculated as the following formula: standard weight=body height (cm)-100；Pregnant or lactating women；The subject is considered unsuitable for this trial.