1st Stage of Labor
Pain is dull and poorly localized. Uterine contractions release bradykinin, histamine, and serotonin that act on chemoreceptors. This is thought to be the result of myometrial ischemia. Mechanoreceptors are also stimulated from the distension and stretching of the lower uterine segment and cervix which causes impulses to enter the spinal cord at T10, T11, T12, and L1 spinal segments from the lumbar sympathetic chain, with signals originating from the paracervical region and hypogastric plexus.
2nd Stage of Labor
Stretching of the perineum causes somatic afferent nerve fibers to act at S2, S3, and S4 levels of the spinal cord via impulses from the pudendal nerve.
Provides continuous T10-L1 sensory block via infusion of low doses of local anesthetic and/or opioid. It is a very effective block during the first stage of labor. Benefits include effective pain relief without much motor block, a means for surgical anesthesia if required, and decrease in maternal catecholamines. With supplementation, it is possible to achieve sacral block for second stage of labor. Contraindications include: patient refusal, infection at access site, overt coagulopathy, and hemodynamic instability.
Typically a single shot spinal with opioids or local anesthetic which provides good labor analgesia – especially early in labor, in distressed parturients, or as an analgesic for instrumental deliveries. The disadvantage is the inability to titrate or provide continuous analgesia. In the 1980s, microcatheters were used for continuous spinal analgesia. There were several reported cases of cauda equina syndrome when used for cesarean section, which was attributed to high concentrated local anesthetics, such as 5% lidocaine and/or inadequate mixing of local anesthetics, within the intrathecal space. Nonetheless, spinal anesthesia provides a reasonable option of providing analgesia, especially in high risk parturients and in cases of accidental dural puncture, as it can reduce the incidence of post-dural puncture headaches. It is imperative to properly dose with this technique.
Combined Spinal-Epidural Analgesia
Advantages include rapid onset and the ablility to titrate analgesia throughout labor. Theoretically it is advantageous as it may increase mobility from reduced motor block, although this isn’t a consistent finding in clinical practice. Also some evidence suggests that the duration of the first stage of labor is significantly reduced in first time parturients. Techniques include: different levels; side by side at the same interspace with specially designed needles; or most commonly using a fine-bore spinal needle which is inserted through the epidural needle to pierce the dura.
No pain relief during the 2nd stage of labor. Side effects such as profound fetal bradycardia, systemic local anesthetic toxicity, infection, and postpartum neuropathy. Local anesthetic is injected submucosally into fornix of the vagina lateral to the cervix, blocking the paracervical ganglion and providing analgesia to the cervix and uterus, but not the sensory fibers of the perineum. This is a technique used commonly for gynecologic surgical procedures, but its use is limited in obstetrics for the aforementioned reasons.
Local anesthetic is injected behind each sacrospinous ligament transvaginally, which provides good analgesia for vaginal delivery, but not labor analgesia. The pudendal nerves are derived from S2-S4 and provide sensory innervation for the perineum, vulva and lower part of vagina. Maternal complications are low but include bleeding, infection, and local anesthetic toxicity.