Obesity is associated with difficulties in epidural space catheterization. The investigators want to prove that a pre-procedural ultrasound exam of lumbar spine increase the first-pass success rate of the epidural space identification among obese parturients.

This study was a randomized controlled open trial with two parallel groups.

Study was conducted in only one center : the maternity of University Hospital of Caen (Normandy, France)

Written informed consent was obtained from eligible subjects after the pre-anesthetic interview in the 8th month of pregnancy.

Operators were medical interns or residents in anesthesiology. Before inclusion, they all spent an individual interview in the aim to practice ultrasound exam of lumbar spine.

Eligible subjects were full-term parturients with pre-pregnancy BMI ≥ 30 kg/m2 undergoing vaginal delivery and receiving epidural labor analgesia. Exclusion criteria were usual contraindications for neuraxial anesthesia, marked spinal deformity, previous spinal surgery and emergency context.

The study subjects were randomly assigned to palpation or ultrasound using sealed opaque envelopes containing group allocation.

Standard monitors were applied and patients were positioned sitting during landmark identification and throughout the anesthetic procedure.

In control group, needle-puncture site was determinate by the conventional technique of cutaneous palpation. Conventional palpation of the anatomical landmarks was performed and the intercristal line (Tuffier) was assumed to cross the spine at L4 spinous process or L3-L4 interspace. The skin was marked with horizontal and vertical lines at the L3-L4 and L2-L3 interspaces.

In experimental group, ultrasound exam of lumbar spine allowed to determinate the puncture site. A systematic approach described by Balki [1] was performed, using a portative ultrasound device with a 5-2 MHz curved array probe (Samsung HM70A, Samsung medical France, St-Ouen, France).

The epidural space was located using a midline approach with loss of resistance (saline solution) using and 18-gauge Tuohy needle. After successful identification of the epidural space, a 20-gauge multiorifice epidural catheter was inserted through the Tuohy needle up to 5 cm into the epidural space.The epidural analgesia protocol was PCEA (Patient Controled Epidural Analgesia) using subsequent admixture : lévo-bupivacaïne (0,625 mg/ml), sufentanil (0,02 µ/ml), clonidine (37 µg/ml).

The primary outcome was the rate of successful epidural space identification at the first needle pass (one skin puncture without redirection).

Secondary outcomes are described further.

Early procedural complications and delivery modalities (C-section, forceps) were notified.

An interview was planned 48 hours after the procedure for each parturient in the aim to assess the maternal satisfaction and detect late procedure complications.

Assuming α = 0.05 and β = 0.2 (80% power) risks and using the χ2 test, 28 patients were required in each group.

Inclusion Criteria:

• Full-term parturients (≥ 37 weeks of amenorrhea)
• With pre-pregnancy BMI ≥ 30 kg/m2
• Admit for a vaginal delivery
• Receiving labour epidural analgesia

Exclusion Criteria:

• Usual contraindications for neuraxial anesthesia (thrombocytopenia, coagulopathy, uncorrected hypovolemia, infected puncture site, intracranial hypertension)
• Emergency maternal and foetal cases
• Twin pregnancy
• Marked spinal deformity ou previous spinal surgery
• Refusal to participate