Regional anesthesia is a common clinical anesthesia method, and regional anesthesia-assisted sedation play a central role in promoting patient comfort and relieving anxiety. With increasing age, elderly people experience changes in pharmacokinetics and pharmacodynamics and increased drug sensitivity. Ciprofol has advantages in adverse events such as hypotension and respiratory depression. There are fewer studies on the recommended dosage of ciprofol-assisted sedation for intrathecal anesthesia in elderly patients. The aim of study is to identify the 95% effective dose of ciprofol-assisted sedation in elderly patients undergoing spinal anesthesia using a biased coin design up-and-down sequential method(BCD-UDM)

Select the patients who will receive knee arthroplasty in the Second Hospital of Shanxi Medical University. The patients will receive a loading dosage of 0.05mg/kg of ciprofol to begin sedation following the completion of intravertebral anesthesia, and the maintenance dose is calculated using a biased coin design up-and-down sequential method. Based on the relevant literature and pilot study, 0.2 mg·kg-1·h-1 is selected as initial test dose with a dose gradient of 0.02 mg·kg-1·h-1. The Modified Alertness Sedation Score (MOAA/S ) scores and the bispectral index (BIS) were recorded for patients before administration (T0), 5 min after administration (T1), 10 min after administration (T2), 20 min after administration (T3), 30 min after administration (T4), 45 min after administration (T5), and 60min after administration (T6). If the first patient had ≥ 4 time points with MOAA/A score ≤3 from T0-T6, the sedation was regarded as satisfactory, the dose of next patient is randomly assigned to the next lower dose with 5% probability, or to the same dose with 95% probability; If the previous patient was not satisfactorily sedated, the next patient's infusion dose was increased by 0.02 mg·kg-1·h-1. When the number of satisfied cases of sedation reaches 45, the trial is terminated. At the same time, the change of vital signs and adverse reactions before and after administration were recorded

Inclusion Criteria:

• Age ≥ 65 years
• ASA grade I~III
• BMI 18~28 kg/m2

Exclusion Criteria:

• Contraindications to intrathecal anesthesia
• Suffering from severe psychiatric or neurologic disorders or taking sedative drugs within the last 3 months
• Difficult airway or having respiratory diseases such as upper airway obstruction
• Severe cardiovascular system diseases
• Severe hepatic or renal dysfunction
• Allergy or contraindication to propofol and its drug-related components
• Patient Prefusal