Different medications can be used as analgesics in labor epidurals. Bupivacaine is one of the most commonly used drugs for that purpose. The efficacy of a certain medication injected epidurally depends on the dose that is given. A certain dose can be administered in different concentrations, which will consequently mean different volumes. Studies have been done to determine volumes for different concentrations of bupivacaine. However, those studies fall short in the sense that this information cannot be readily applicable in clinical practice, as we don't use bupivacaine plain. The combination of bupivacaine and fentanyl or sufentanil is common practice, as it is well established that the opioids will reduce the concentration of local anesthetic required to produce optimal effect with the least side effects.

This study is being planned to determine and compare the EV90 of the bupivacaine 0.125%-fentanyl mixture for initiation of labor epidural analgesia, following the administration of the 2 different test doses (3cc of lidocaine 2% or 3cc of the bupivacaine 0.125%-fentanyl mixture) used in our Department.

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 use of adjuncts(in our case: Fentanyl). 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 in combination with 5mcgr/ml fentanyl that will provide effective analgesia for 90% 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 1, the volume of the bupivacaine/fentanyl mixture was considered inadequate and additional volume of the mixture will be given.

After the randomised test dose of either 3cc lidocaine 2% or 3cc of the bupivacaine/fentanyl mixture is given, the loading volume of the bupivacaine/fentanyl mixture will be given. The volume of the bupivacaine/fentanyl mixture for the first patient was chosen at 10 ml (This is the volume with which is generally started in standard practice), and the dose for each subsequent patient depends on the outcome of the previous injection. Doses will be increased or decreased in increments of 2 ml. If the previous response was ineffective, the next patient will receive 2 ml more than the last patient. If the response of the previous patient is effective, the biased coin up-and-down allocation design, which is used to cluster doses around EV90, will be applied to determine if the dose remains the same or decreases.

40 patients will be included in each group within the study. The target volume, EV90, which is defined as the volume at which adequate response would occur in 90% of patients, will be estimated for each of the two test dose groups using Dixon and Mood method. The 95% confidence interval of EV90s will be estimated and based in comparing the EV90 between two groups. Isotonic Regression method will also be conducted to assess the EV90s. The side effects will also be summarized by the volume levels descriptively.

Inclusion Criteria:

• ASA I or II
• Full term (minimum 37 weeks gestation)
• Singleton pregnancy, vertex presentation
• Regular painful contractions occurring at least every 5 minutes
• VAS Pain score at requesting analgesia > 5 (VNPS 0-10)
• Cervical dilatation < 5 cm

Exclusion Criteria:

• Refusal to provide written informed consent.
• Any contraindication to epidural anesthesia
• Accidental dural puncture
• Allergy or hypersensitivity to bupivacaine or fentanyl
• Use of opioids or sedatives within the last 4 hours.