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)