At Mount Sinai Hospital, epidural analgesia for labor pain is delivered by programmed intermittent epidural bolus (PIEB), in combination with pushes of medication activated by the patient, a technique called patient controlled epidural analgesia (PCEA). Studies have shown that delivering analgesia in this manner can prolong the duration of analgesia, diminish motor block, lower the incidence of breakthrough pain, improve maternal satisfaction and decrease local anesthetic consumption comparing to a conventional continuous infusion. The use of this PIEB technique in routine practice has reduced the total consumption of local anesthetic and the percentage of patients requesting additional boluses (PCEA or manual rescues).

However, at the same time, sensory blocks above those targeted for labor pain relief have been reported in our institution, suggesting that the spread of the freezing medication is wider than necessary. Based on the information already available in the literature, the investigators will conduct this study to determine the best regimen of PIEB achievable with a slower delivery speed.

The hypothesis of this study is that PIEB boluses with 125 mL/h will decrease by 50% the incidence of women presenting sensory block to ice equal or higher than T6 as compared to a delivery rate of 250 mL/h.

PIEB is a novel technique, which provides better analgesia and less motor block; however, the ideal regimen has not been established. To optimize the PIEB regimen, the investigators have conducted several clinical trials using bupivacaine with fentanyl. Although PIEB provides the excellent analgesia to laboring parturients, studies completed by the instigators have shown that the incidence of sensory block to ice ≥T6 was 66%. For labor analgesia during the first stage of labor, effective sensory block requires T10 to L1; therefore, a sensory block ≥T6 seems unnecessarily high, and the investigators seek to confine epidural spread within a proper range.

Our previous studies showed that the effective volume 90% using 0.0625% bupivacaine was 10.7 mL, and incidence of epidural blockade ≥T6 was similar even using a 5-mL bolus with 0.125% bupivacaine. The investigators concluded that it is not possible to reduce volume of the PIEB, or use a higher concentration of local anesthetic without compromising the quality of analgesia. There is some evidence in the literature that supports the idea that higher injection pressures result in a wider spread of local anesthetic administered into the epidural space, therefore a slower epidural delivery flow rate may result in less spread.

Inclusion Criteria:

• ASA II or III
• Term pregnancy (gestational age ≥ 37 weeks)
• Nulliparous
• Singleton pregnancy with vertex presentation
• Active labour defined as occurring regular uterine contractions at least every 5 minutes with progressive cervical dilatation
• Verbal Numerical Rating Score (VNRS) greater than 5 at requiring epidural analgesia (VNRS 0-10)
• Cervical dilation between 2 and 5 cm

Exclusion Criteria:

• Any contraindication to epidural analgesia
• Accidental dural puncture
• Drug allergy to lidocaine, bupivacaine or fentanyl
• Use of pharmacological analgesics within the last 4 hours
• Patient refusal to participate in the trial