Minimum Local Anesthetic Volume of Bupivacaine in Labour Epidurals

Epidural administration of local anesthetics during labor produces analgesia, sympathetic block and motor block. The characteristics of the block depend on the volume and concentration of the local anesthetic that is used as well as the potential use of adjuncts. Although many researchers have investigated the efficacy of different drugs at different concentrations, the volumes used have been arbitrary. There is limited data regarding the minimum volume that is necessary for a successful anesthetic for a given concentration of bupivacaine.

This study is conducted as a prospective, randomized, up-down sequential allocation trial. The aim is to determine the minimum volume of 0.125% bupivacaine that will provide effective analgesia for 95% of parturients in the first stage of labor. The verbal numeric rating scale (VNRS) is used to rate the pain felt over the first 20 minutes, where 0 is no pain and 10 is the worst pain imaginable. After 20 minutes, if the VNRS was greater than 2, the volume of bupivacaine was considered inadequate and additional medication was given.

The volume of bupivacaine for the first patient was arbitrarily chosen as 8 ml, and the dose for each subsequent patient depends on the outcome of the previous injection. Doses will be increased or decreased in increments of 1 ml. If the previous response was ineffective, the next patient will receive 1 ml more than the last patient. If the response of the previous patient is effective, the Narayana rule, which is used to cluster doses around EV95, will be applied to determine if the dose remains the same or decreases.

50 patients will be included in the study. An estimate of EV95 will be calculated based on a logistic model with non-log-transformed doses, fit using Firth's penalized maximum likelihood approach for small sample bias correction. Confidence intervals will be calculated based on the profile likelihood approach.

Inclusion criteria

• ASA I or II
• Full term (more than 37 weeks gestation)
• Singleton pregnancy, vertex presentation
• Regular painful contractions occurring at least every 5 minutes
• Cervical dilatation < 5 cm

Exclusion criteria

• Any contraindication to epidural anesthesia
• Accidental dural puncture
• Allergy or hypersensitivity to bupivacaine
• Women who have received opioids or sedative medications within the last 4 hours