Dexmedetomidine(DEX) could provide dose-dependent sedation , analgesia , anti-anxiety and inhibition of sympathetic nerves and other effects. Because of its minimal impact on the respiratory , currently it was more and more widely used to sedate patients undergoing regional anesthesia.Many anesthetic pharmacokinetics and pharmacodynamics are often affected by age, and current studies with regard to the effects of age on dexmedetomidine pharmacodynamic are rare. This study was designed to explore the right DEX dose of different female Age of patients to produce suitable sedation.Dexmedetomidine be used in patients with combined spinal and epidural anesthesia for sedation,which is monitored by the Narcotrend,during the operation.The relation between Narcotrend index (NTI) and the depth of sedation for patients is also investigated.

90 patients scheduled for combined spinal epidural spinal anesthesia were included in one of three groups. In each group,determination of median effective (ED50) doses was performed by the Dixon up-and-down method, an initial dose of 1.0 μg/kg dexmedetomidine, with dose adjustment intervals of 0.1 μg/kg in first three turning points and 0.05μg/kg in the last three turning points.Initial doses was 1.4μg/kg, .Sedative efficacy was defined as an OAA/S of ≤3,30 min after the beginning of drug administration.

Inclusion Criteria:

1. ASA Ⅰ ~ Ⅱ patient undergoing lower extremity surgery
2. Written informed consent from the patient or the relatives of the participating patient.
3. BMI:18.0～25.0kg/m2

Exclusion Criteria:

1. Mental illness can not match

2. epidural anesthesia contraindicated

3. People who have Slow-type arrhythmias

4. People who were language or hearing impaired

5. Sensory block reached to T8 or higher.

6. People who had lung infection or sleep apnea syndrome.

7. Pregnancy

8. Chronic renal failure

9. Alcohol or drug abuse

10. Already taking gabapentin, pregabalin, benzodiazepin or antidepression drug

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