Programmed Intermittent Epidural Bolus for Labor Analgesia During First Stage of Labor: A Sequential Allocation Trial to Determine the Optimum Interval Time Between Boluses of a Fixed Volume of 2.5ml of Bupivacaine 0.25% Plus Fentanyl 8 mcg/ml

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.