The current "gold standard" epidural analgesia for labor is associated with unwanted side effects, such as motor block (affecting ambulation and the ability to push) and sympathetic blockade (causing maternal hypotension). Here, using direct clinical application of preclinical findings, the investigator assess a novel method for prolonged pain-selective anesthesia during labor avoiding motor block, sympathetic block or other neural side effects. The approach is based on the pain-selective anesthesia the investigator developed in preclinical models, where the exclusive presence of TRPV1 channels on pain fibers (nociceptors) was exploited. The investigator used polarized, membrane-impermeable local anesthetics, together with the co-administration of direct TRPV1 agonists to open TRPV1 channels, so allowing these impermeable local anesthetics to gain selective access to nociceptors. Early preclinical studies used capsaicin as the TRPV1 agonist and QX-314 as the polarized local anesthetic, but neither of these drugs are appropriate for clinical use. Here the investigators use lidocaine as the TRPV1 activator and chloroprocaine as the polarized local anesthetic and hypothesize that this co-administration will elicit pain-selective (nociceptor-specific) anesthesia for labor without side effects. Positive findings in this study will be the first evidence in humans of pain-selective local anesthesia and will be the harbinger for future studies of systemic and regional administration of these drugs for pain-selective local anesthesia for chronic and other pain states.
Nulliparity
Early active labor, cervical dilatation less than 5 cm,
Age between 18 to 40,
American Society of Anesthesiologists physical status class II
Narcotic administration in the previous 3 hours
Previous uterine surgery
Pre-eclampsia
Inability to adequately understand the consent
Single group
Chlorprocaine
DrugStudy participants will receive chloroprocaine alone, together with standard epidural fentanyl (2mcg/mL in the epidural mixture).
Lidocaine
DrugStudy participants will receive lidocaine alone - together with standard epidural fentanyl (2mcg/mL in the epidural mixture).
Lidocaine-chlorprocaine combination
Drugstudy participants randomized to this study arm will receive a a 50:50 mixture of lidocaine and chloroprocaine (based on the ED50s of each drug from Stage 1A and on the synergistic interactions from Stage 1B) - together with standard epidural fentanyl (2mcg/mL in the epidural mixture).
VAS pain score / modified Bromage motor block score
Pain (VAS 1-100) where the anchors of the VAS score are 1=no pain, 100=worst imaginable pain, until the first epidural top-up. We use 1 as the lowest anchor for no pain because 0 is not divisible mathematically. Modified Bromage motor block score where the scores are determined as follows: 1 = unable to move legs or feet; 2 = unable to flex knees, free movement of feet; 3 = just able to flex knees with free movement of feet; 4 = free movement of legs and feet; 5 = able to perform partial knee-bend movements while standing. VAS and Bromage will also be reported individually.
Segmental sympathectomy
Thermal imaging of the feet and hands using a FlirC3 thermal camera. Measuring the proximal and distal temperatures.
Sensory assessment to cold sensation
Using an ice cube to test the patient perception of cold sensation on the L2 and T10. Both right and left side perception will be recorded. Recorded from three options - perception of cold, perception of touch, or no perception.
Anesthesia requirement.
From the epidural PCEA pump we will record the frequency of epidural PCA self- administered doses, the need for any rescue supplemental boluses, the numbers of failed attempted self-administered boluses (requests were not allowed because the button was pressed within the 10-minute lockout time), the total dose administered, and the total dose / time of labor. For the first 30 minutes after placing the epidural catheter, up until the first epidural top-up
Pushing ability in labor.
This will be assessed by the midwife, who is blinded to the study group. As occasionally the delivery of the fetus is impaired by the large size or malrotation of the fetus, this score is assessed not on the successful vaginal delivery, but rather on the strength of pushing and the ability to sense the need to push. * In the second stage of labor: rate the ability to sense the urge to push: 0-100 (0= complete inability and 100=best imaginable). * In the second stage of labor: rate the ability push (strength): 0-100 (0= complete inability and 100=best imaginable). For the first 30 minutes after placing the epidural catheter, up until the first epidural top-up
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