The aim of this study is to evaluate the use of the sagittal abdominal diameter (SAD) as a potential predictor of the skin to epidural space distance (SESD) and to compare its accuracy versus that of the BMI as a previously validated predictor

Adult patients from either sex with BMI from 30-39.9 kg/m², scheduled for lower abdominal operations or surgeries on the lower extremities under epidural anesthesia will be eligible. Pregnant women will be excluded.

The sagittal abdominal diameter (SAD), which is the anteroposterior diameter of the abdomen in the supine position at L4-5 level will be measured in all subjects along with the BMI prior to epidural placement.

The epidural space will be accessed at the L3-L4 or L4-L5 interspace with a 16-G or 18-G Tuohy needle using a loss of resistance to saline technique. The proper placement of the needle in the epidural space will be confirmed by connecting the hub of the epidural needle via a rigid extension tubing to a saline filled pressure transducer to demonstrate the characteristic epidural space pressure wave changes described by Arnuntasupakul and his colleagues (2016), which is a pulsatile waveform synchronized with arterial pulsations.

The distance from skin to epidural space will be marked on the needle with a sterile marker and will be recorded.

An epidural catheter will be passed into the epidural space and the epidural block will be activated in the usual way until adequate sensory and motor block is established in both lower extremities (Bromage score 3-4).

Successful placement of the epidural needle will be considered if the following two criteria are fulfilled: (1) Detection of the characteristic epidural pressure wave form as described above and (2) Establishment of adequate sensory and motor block.

Simple linear regression will be used to deduce an equation relating the skin to epidural space distance to the sagittal abdominal diameter or to the BMI.

Inclusion Criteria:

• Age 18 years or older
• BMI 30 to 39.9
• Either sex
• Elective surgery on lower extremities performed under epidural anesthesia

Exclusion Criteria:

• Pregnant women
• Contraindications to epidural block

Adult subjects with BMI of 30 to 39.9 kg/m2 who are scheduled for lower extremity surgery under epidural anesthesia.