The most common and effective method for controlling labor pains is a local anesthetic (LA) infusion through a lumbar epidural. To achieve adequate pain control during the first stage of labor - onset of contractions to complete cervical dilation - nerve fibers up to the T10 dermatome must be anesthetized. When a patient is in active labor and an epidural catheter is placed, the anesthesiologist must activate the epidural by administering LA through the epidural to promote spread of the LA in the epidural space to anesthetize the nerve fibers involved in the conduction of labor pains. The ideal LA to achieve this goal is one that would allow for the fastest onset to achieve quick pain relief with the fewest side effects.~Two commonly used LA to provide labor analgesia are bupivacaine and lidocaine. When low concentrations - 0.25% bupivacaine and 1% lidocaine - are used for labor analgesia, both of these LA can be administered safely with very little concern of major adverse effects associated with LA toxicity. Given that there is limited and conflicting evidence for the usefulness of the bupivacaine and lidocaine mixture especially as it relates to labor epidural activation, we hope to readdress these questions in an effort to determine whether or not the LA combination offers any distinct advantage over the individual LA. The investigators intend to determine the time it takes to achieve an adequate level (T10) for labor analgesia, the total spread of local anesthetic, and the degree of motor block as these factors will be important in determining the most optimal LA solution to activate a labor epidural. With the results from this study, the investigators hope to recommend a LA solution that will allow for the fastest pain relief in the laboring mother with the fewest side effects.
This study will compare and determine the most ideal local anesthetic (LA) solution to activate a labor epidural: lidocaine, bupivacaine, or a combination of bupivacaine plus lidocaine.