This study aims to determine the minimum effective dose of bupivacaine 0.5% in 90% of patients undergoing lower limb surgery with ultrasound-guided spinal anesthesia using Taylor's approach. A biased coin design up-and-down sequential method is applied. The initial dose is 15 mg of bupivacaine 0.5%. After 45 successful spinal anesthesia, the ED90 of bupivacaine 0.5% will be calculated.

The minimum effective dose of local anesthetics for spinal anesthesia using Taylor's approach is not known. This study aims to determine the minimum effective dose of bupivacaine 0.5% using a biased coin design up-and-down sequential method. Patients will receive real-time ultrasound guided spinal anesthesia using Taylor's approach. The initial dose is chosen as 15 mg on the basis of clinical experience. Subsequently, if a patient has an inadequate block, the bupivacaine dose is increased by 2 mg in the next subject. If a patient has a successful block, the next subject is randomized to receive either a lower dose (with a decrement of 2 mg), with a probability of b = 0.11, or the same dose, with a probability of 1 - b = 0.89. Forty five successful blocks will be accomplished. MED90 will be calculated by isotonic regression.

Inclusion Criteria:

• American Society of Anesthesiologists physical statusⅠ- Ⅲ
• Lower limb surgery, expected operation time within 2 hours

Exclusion Criteria:

• Patient refusal
• American Society of Anesthesiologists physical status IV/V
• Peripheral neuropathy
• skin infection at the site of injection
• allergy to bupivacaine or lidocaine
• BMI > 35 kg/m2
• coagulation disorders