Drug Concentration and Volume on Adequate Labor Analgesia With PIEB

Neuraxial labor analgesia is performed by the administration of a local anesthetic/opioid
mixture in the epidural space. The delivery method is a combination of continuous infusion,
provider-administered boluses and patient-administered boluses (patient controlled epidural
analgesia [PCEA]) via epidural catheter. Pain during the first stage of labor is primarily
due to lower uterine segment and cervical stretching. The pain signals enter the spinal cord
through sensory nerves at the T10 through L1 dermatomes. Pain signals from the late first
state and second stage of labor also arise from the vagina and perineum. These signals travel
with the pudendal nerve and enter the spinal cord at S2-S4. Thus, the anesthetic solution
administered through the lumbar epidural catheter must spread cephalad in the epidural space
to reach the T10 nerve roots and spinal cord, and must spread caudad to reach the caudal
nerve roots in the epidural space.

The optimal method for maintaining labor analgesia is unknown. Several studies have
demonstrated that PIEB, in combination with PCEA, provide superior maintenance of labor
analgesia (less need and longer time to provider intervention, lower local anesthetic
consumption, less motor block, and improved patient satisfaction scores) than maintenance
with a continuous infusion with PCEA.The mechanism for this difference is not known; however,
one suggested mechanism is improved spread of the local anesthetic within the epidural space.
Kaynar et al. injected methylene blue dye through a multi-orifice catheter using either a
continuous infusion or intermittent bolus and the area of diffusion was measured on a piece
of paper. The authors found that intermittent boluses were associated with a greater surface
area of diffusion than continuous infusion. In a cadaver study, dye was injected into the
lumbar epidural space, and cryomicrotome sections were taken.Dye flowed in rivulets through
small channels in the epidural space, as opposed to moving as a unified front. The authors
found dye injected at high pressures had more uniform spread through the epidural space,
supporting the concept of intermittent epidural injection providing superior analgesia.

Epidural infusion pumps capable of delivering PIEB of local anesthetic with PCEA have become
commercially available and many studies have attempted to assess the optimal parameter
settings (including volume of programmed bolus, bolus interval, rate of bolus administration)
to provide superior labor analgesia. Recently we completed a double-blinded randomized
control trial evaluating two bolus delivery rates, hypothesizing that those patients
randomized to receive higher bolus delivery rates would have improved labor analgesia.
However, no difference was found between groups.What is more, both groups had a mean cephalad
sensory level to cold of T6. This sensory level is higher than the traditional goal of T10
(upper dermatome level of uterine innervation), but despite the adequate sensory level, a
large number of patients required supplemental physician-delivered boluses of local
anesthetic during labor. Forty percent of the women in the high-rate group and 36% of the
low-rate group required a manual re-dose during labor. This suggests that either a higher
volume, or higher concentration of local anesthetic (i.e., higher dose) is needed to maintain
adequate labor analgesia.

Traditionally higher concentration local anesthetic solutions have been associated with
increased motor blockade leading to a higher incidence of instrumental vaginal delivery.
Several local anesthetic solutions with varying drug concentrations are available for labor
analgesia and are used clinically in the United States. We plan to perform a randomized,
controlled, double-blind study to test the hypothesis that patients whose labor analgesia is
maintained using PIEB with low-volume bolus (6.25 mL) of a higher local anesthetic
concentration solution (0.1% bupivacaine with fentanyl 1.95 mcg/mL) will require less
supplemental analgesia (manual provider re-doses) than patients whose PIEB is delivered with
a high-volume bolus (10 mL) of lower density local anesthetic solution (0.0625% bupivacaine
with fentanyl 1.95 mcg/mL).

The aim of this study is to evaluate the association between bolus volume and concentration
of local anesthetic during maintenance of labor analgesia with programmed intermittent
epidural bolus (PIEB) analgesia.

The hypothesis of this study is: patients whose labor analgesia is maintained using PIEB with
low-volume bolus (6.25 mL) of higher local anesthetic concentration solution (0.1%
bupivacaine with fentanyl 1.95 mcg/mL) will have a longer duration of adequate analgesia
(time to first manual re-dose request) than patients whose PIEB is delivered with a
high-volume bolus (10 mL) of lower concentration local anesthetic solution (0.0625%
bupivacaine with fentanyl 1.95 mcg/mL).