Programmed intermittent epidural bolus (PIEB) is a technique of epidural analgesia in which boluses of local anesthetic solutions are injected into the epidural space at a fixed time interval. Despite the increasingly popular use of PIEB for labor analgesia, the optimum regimen of drug delivery has yet to be determined. The outcomes of a chosen regimen will depend on the local anesthetic solution used (drug, concentration and mass) and the parameters established for the PIEB, typically associated with patient controlled epidural analgesia (PCEA). Also, the optimum regimen will depend on the anesthetic and obstetric outcomes of interest.

The investigators have conducted several studies aiming at establishing the optimum PIEB regimen for the patient population at Mount Sinai Hospital. High sensory block levels obtained in some of the previous studies conducted at Mount Sinai Hospital and in other studies in the literature, in spite of not determining adverse effects, suggest an imperfect use of the technique, with an exaggerated and unnecessary spread of the epidural mixture. It is possible that by limiting the spread of the local anesthetic mixture, better analgesia can be provided with less overall consumption of local anesthetic. The investigators wanted to conduct a study using boluses of 2.5 mL of bupivacaine 0.25% with fentanyl 8 mcg/mL. This would maintain the same dose of local anesthetic used in previous studies, but in a much smaller volume. This concentration and volume of bupivacaine has not been tried before as a PIEB regimen.

The hypothesis of this study is that the optimum interval time between PIEB boluses of 2.5 mL of 0.25% bupivacaine plus fentanyl 8 mcg/ml will be between 30 and 60 minutes.

The investigators have conducted several studies aimed at establishing the optimum PIEB regimen for the patient population at Mount Sinai Hospital. The first study was to determine the effective interval of PIEB in 90% of women during first stage of labor (EI90), while using a fixed bolus of 10 mL of bupivacaine 0.0625% with fentanyl 2 μg/mL. This study showed that the PIEB time interval to provide effective analgesia is approximately 40 minutes, which corresponds to an hourly consumption of 9.4 mg of bupivacaine. In that study, however, 44% of the women experienced sensory blocks to ice above the T6 level (although not associated with motor block or hypotension).

The investigators subsequently conducted another study with the same anesthetic solution and the same PIEB interval of 40 minutes, to determine the effective volume (dose) of local anesthetic to produce the same outcome of effective analgesia without breakthrough pain. The conclusion was that the volume (dose) could not be reduced without compromising efficacy of the technique, and not surprisingly, the sensory block distribution was very similar to that in the first study. The investigators believe that the high sensory block levels obtained the previous studies and in other studies in the literature suggest an imperfect use of the technique, with an exaggerated and unnecessary spread of the epidural mixture. The investigators then conducted a third study to determine the effective time interval between boluses of more concentrated bupivacaine 0.125% 5ml plus fentanyl 2 mcg/ml. The EI90 for boluses of 5ml of bupivacaine 0.125% with fentanyl 2 mcg/mL was found to be approximately 35 minutes. Similar to the two previous studies, the incidence of women exhibiting sensory block to ice >T6 was still high, approximately 58.4%.

This study will further pursue the effect of volume reduction while maintaining the dose of local anesthetic. It may be true that a bolus of 5mL was still too large of a volume to limit the epidural spread. A more limited spread could lead to better usage of the local anesthetic, even reducing its hourly consumption.

Inclusion Criteria:

• American Society of Anesthesiologists Physical Status II or III
• Full term (greater than or equal to 37 weeks gestation)
• Nulliparous
• Singleton pregnancy, vertex presentation
• Active labor defined as regular painful contractions occurring at 3-5 minutes and with progressive cervical ripening
• Verbal Numerical Pain Score (VNPS) greater than 5 at requesting epidural analgesia (VNPS 0-10)
• Cervical dilatation between 2 and 5 cm

Exclusion Criteria:

• Any contraindication to epidural anesthesia
• Accidental dural puncture
• Allergy or hypersensitivity to bupivacaine or fentanyl
• Use of pharmacological analgesics within the last 4 hours
• Patient refusal to participate in the trial