In this study, we will set out to confirm the reliability of epidural waveform analysis (EWA) as an adjunct to loss of resistance for obstetrical epidural blocks.

After loss of resistance, 5 mL of normal saline will be injected through the epidural needle; the latter will then be connected to a pressure transducer (leveled with the heart) using a rigid tubing. A pressure recording of the epidural waveform will be saved for future assessment by a blinded observer.

After the recording of the waveform, the rigid tubing will be disconnected from the epidural needle. A 4 mL bolus of lidocaine 2% with epinephrine 5 µg/mL will be injected through the latter. An epidural catheter will be advanced 3-4 cm past the tip of the needle inside the epidural space. The needle will then be removed and the catheter secured to the skin in the usual fashion.

Fifteen minutes after the injection of the local anesthetic bolus, a blinded observer will apply ice to the T1-L5 dermatomes and assess the epidural block.

Inclusion Criteria:

-American Society of Anesthesiologists classification 1-3

Exclusion Criteria:

• adults who are unable to give their own consent
• coagulopathy (assessed by history and physical examination and, if deemed clinically necessary, by blood work up ie platelets ≤ 100, International Normalized Ratio ≥ 1.4 or prothrombin time ≥ 50)
• renal failure (assessed by history and physical examination and, if deemed clinically necessary, by blood work up ie creatinine ≥ 100)
• hepatic failure (assessed by history and physical examination and, if deemed clinically necessary, by blood work up ie transaminases ≥ 100)
• allergy to local anesthetic
• prior surgery in the lumbar spine