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.

The allocation sequence is generated by computer random number generation, and the allocation is placed in sequentially numbered opaque sealed envelopes by a non-investigator. Enrolment and data collection are performed by trained research staff who are not involved in the care of the patients. The treating clinicians are not possible to be blinded to the assignment group, but all other staff involved in both the collection and collation of data, and administration of neurocognitive testing, are blinded to group allocation.

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.