there are many ways to dose an epidural. lidocaine is faster-acting but wears off quicker. it also seems to provide denser analgesia, which may not be advantageous in all settings (laboring women)
we use lidocaine in the test doses for recognition of inadvertent spinal catheter placement, which would be reliably reflected in a T7-T8 spinal with 45 mg lidocaine (or 3cc of a 1.5% test dose). if we dont see evidence of a spinal injection, then we feel more comfortable starting a bupivacaine infusion to provide analgesia, etc.