• Perioperative change of Observer's Assessment of Alertness/Sedation scale (OAA/S) [ Time Frame: OAA/S score will be recorded from patient arrival in the operating room to about 30 minutes after surgery at following time points: before sedative administration, before skin incision, every 5 minutes intraoperatively and every 10 minutes in PACU. ]
  Conventional subjective sedation scales with 1 to 5. Successful sedation was defined as an OAA/S scale score between 1 and 3, while failure was defined as an OAA/S score of more than 3.
• Numerical rating scale (NRS) [ Time Frame: Immediately after surgery, the surgeons report NRS only once ]
  The surgeons rated anesthesia management on a numerical rating scale (NRS), ranging from 0 to 10, with 10 defined as the best condition and 0 as the worst.

• Intraocular pressure [ Time Frame: Intraocular pressure will be measured at two time points: before sedative medication administration and before skin incision. ]
  Measured with Tono-Pen tonometer
• Bispectral index (BIS) [ Time Frame: Bis will be continuously recorded with BIS monitor from arrival in the operating room to discharge to ward (about 30 minutes after surgery). ]
  Anesthesia depth index
• Visual analogue scale (VAS) [ Time Frame: VAS will be recorded at the following time points until 1h postoperatively: before sedative medication administration, before skin incision, every 10 minutes intraoperatively and 1h postoperatively ]
  Patient self-report pain score ranging from 0 to 10, with 0 defined as no pain at all and 10 as the intolerable pain.

Due to the delicacy and long duration of the procedure, ophthalmic surgery put forward higher requirements to anesthesia management:

1. The patient with local anesthesia without sedation is in a state of awareness, which will cause intense stress resulting in increased blood pressure, heart rate, muscle tension and even sense of pain;
2. Traction of extraocular muscles and eyeballs can induce oculocardiac reflex, followed by bradycardia, atrial or ventricular arrhythmia, conduction block, and even more worsen, life-threatening cardiac arrest may occur;
3. With the increase in average life expectancy and the improvement of quality of life requirements, the number of elderly patients in ophthalmic surgery is also increasing. These elderly patients are often combined with hypertension, diabetes, etc. and decreased tolerance of with general anesthesia;
4. For some retinal detachment surgery with difficulty in resetting, the patients will be required to change to the prone position immediately after surgery to improve the success rate of resetting. Conventional general anesthesia management are cumbersome and costly, which may not be likely to achieve the swift emergence.

Nowadays, monitoring Anesthesia Care (MAC) has developed into a flexible and unique anesthesia technology combining intravenous anesthesia with regional block anesthesia. MAC provides reasonable balance between economy, comfort and safety, efficiency by continuous monitoring of changes in respiratory and circulatory system, during process of sedation and analgesia. Airway management will be another issue as surgeons operate on the side of head and face. Studies have shown that MAC can achieve adequate sedation and analgesia to decrease blood pressure, provide acceptable surgical fields and reduce adverse reactions such as perioperative stress, pain and anxiety. Meanwhile, MAC make patients comfortable enough to cooperate with the surgeons, easy to be awakened with relatively short operation duration and improvement of perioperative safety.

• Balanced Anesthesia
• Adjuvants,Anesthesia

• Drug: Dexmedetomidine
  loading dose 0.5-2.5ug/kg followed by continuous infusion 0.2-0.5ug/kg/h
• Drug: Midazolam
  loading dose 30-50ug/kg followed by continuous infusion 10-30ug/kg/h

• Experimental: Dexmedetomidine
  Dexmedetomidine will be used for perioperative sedation.
  Intervention: Drug: Dexmedetomidine
• Experimental: Midazolam
  Midazolam will be used for perioperative sedation.
  Intervention: Drug: Midazolam

Inclusion Criteria:

• Patients scheduled for ophthalmic surgery with retrobulbar nerve block

Exclusion Criteria:

• Patients with severe cardiac, pulmonary, hepatic, or renal dysfunction. Patients with a preoperative history of mental disorder, hearing disorder, II-III degree atrioventricular block, known allergy or hypersensitivity to a2 receptor agonists and refusal of (or contraindications to) PNB. Patients with anticipated difficult airway.