This arm aims to explore the 95% effective dose (ED95) of fospropofol disodium for sedation in elderly patients undergoing lower extremity surgery with spinal anesthesia.

The initial induction dose is 6.0mg/kg, with a dose gradient of 0.5mg/kg (range: 4.0-8.0mg/kg). Sedation success is defined as a Modified Observer's Assessment of Alertness/Sedation (MOAA/S) score ≤3 within 3 minutes after drug administration. Dose adjustment follows the k=3 sequential design: ① 3 consecutive successes → next patient receives a 0.5mg/kg lower dose; ② 1 failure → next patient receives a 0.5mg/kg higher dose; ③ 2 consecutive failures → skip the next gradient.

Immediately after successful induction (MOAA/S ≤3 within 3 minutes), continuous infusion of fospropofol disodium for sedation maintenance will be initiated. The initial maintenance rate is 6.0mg/kg/h, with a gradient of 0.5mg/kg/h (range: 3.0-7.5mg/kg/h). Maintenance success is defined as MOAA/S score ≤3 in ≥4 of 6 assessments (once every 10 minutes). Dose adjustment follows the k=3 sequential design: ① 3 consecutive successes → next patient receives a 0.5mg/kg/h lower rate; ② 1 failure → next patient receives a 0.

The primary outcome is the 95% effective dose (ED95) of fospropofol disodium for adjuvant sedation during spinal anesthesia in elderly patients (≥65 years), including two key components: ① Induction ED95: The intravenous bolus dose at which 95% of patients achieve a Modified Observer's Assessment of Alertness/Sedation (MOAA/S) score ≤3 within 3 minutes of administration (dose range: 4.0-8.0mg/kg, gradient: 0.5mg/kg); ② Maintenance ED95: The continuous intravenous infusion rate at which 95% of patients maintain an MOAA/S score ≤3 in ≥4 out of 6 intraoperative assessments (performed every 10 minutes) after successful induction (rate range: 3.0-7.5mg/kg/h, gradient: 0.5mg/kg/h). Both ED95 values (and their 95% confidence intervals [CI]) will be calculated using Probit regression analysis, with sedation success (effective=1, ineffective=0) as the dependent variable and dose/rate as the independent variable.

The time from the start of fospropofol disodium bolus injection (sedation induction phase) to the first achievement of a Modified Observer's Assessment of Alertness/Sedation (MOAA/S) score ≤3.