The effect of Body-mass index (BMI) on requirements of local anesthetics used in spinal anesthesia has not been well established. The investigators tried to quantify the effect of BMI on spinal anesthesia by comparing the median effective dose of intrathecal bupivacaine between two different BMI groups.

The effect of BMI on requirements of local anesthetics used in spinal anesthesia has not been well established. The investigators tried to quantify the effect of BMI on spinal anesthesia by comparing the median effective dose (ED50)of intrathecal bupivacaine between two different BMI groups. The investigators determined the ED50 and ED 95 of intrathecal hyperbaric bupivacaine of subjects with BMI below 27.5 kg/m2, and subjects with BMI over 27.5 kg/m2, respectively. By comparing the difference of ED50 and ED95 between the groups, the investigators tried to quantify the effect of BMI on the bupivacaine dose requirement.

Inclusion Criteria:

• Patients undergoing elective total knee replacement arthroplasty

Exclusion Criteria:

• Patients with previous spine surgery, diabetic and other neuropathy, skin infection at the site of injection, allergy to bupivacaine and other common contraindications for spinal anesthesia were excluded from this study.
• The patients with height less than 140 cm or more than 160 cm, body-mass index (BMI) less than 20 or greater than 35 were also excluded.