Mechanisms of Labor Pain

In the first stage of labor (onset of labor until full cervical dilatation, i.e., 0 to 10 cm) the distension of the cervix results in pain related to the activation of mechanoreceptors. Uterine contraction can result in myometrial ischemia, releasing potassium, histamine, serotonin, and bradykinins which stimulate chemoreceptors. This pain is experienced primarily during contractions as this is when the chemoreceptors and mechanoreceptors are stimulated.

Afferent impulses are transmitted by the nerves that accompany the sympathetic nerves and terminate in the dorsal horn of the spinal cora. These afferent nerves pass through the paracervical region; from there the visceral afferents pass through the pelvis by means of the inferior hypogastric, middle hypogastric, and superior pelvic plexes. They then enter the lumbar sympathetic chain, enter the white rami communicants at the T10, T11, T12, and L1 spinal nerves, and pass through the posterior roots to synapse in the dorsal horn.

In early labor, pain is referred primarily to the T11 and T12 dermatomes. As labor progresses, pain is also referred to T10 and L1 dermatomes. Visceral pain associated with

N. Vadivelu et al. (eds.), Essentials of Pain Management,

DOI 10.1007/978-0-387-87579-8_22, © Springer Science+Business Media, LLC 2011

Figure 22.1 Labor pain results from uterine contraction, cervical dilatation, and vaginal and peroneal distention.

contractions and cervical dilatation can be alleviated with (1) segmental epidural blockade of T10-L1; (2) bilateral paravertebral blocks at the T10, T11, T12, and L1; (3) bilateral paracer-vical blocks; (4) bilateral lumbar sympathetic blocks; or (5) spinal opioid administration.

The somatic component of labor pain results from the distension of the vagina, the pelvic floor, and the perineum. These impulses are transmitted primarily through the puden-dal nerves. The perineum also receives innervation from the ilioinguinal nerve, the genital branch of the genitofemoral nerve, and the posterior femoral cutaneous nerve. Somatic pain occurs late in the first stage of labor and into the second stage (full dilatation to delivery of the fetus). It is primarily related to the decent of the presenting part. This late first stage is

Table 22.1 Regional techniques for analgesia in labor.

Visceral pain (T10-L1) (stage 1 of labor)

• Bilateral paracervical blocks (associated with fetal bradycardia, therefore, rarely used)

• Bilateral lumbar sympathetic block

• Intrathecal opioids

Somatic pain (transition and stages 2 and 3 of labor)

• Bilateral pudendal nerve blocks

• Saddle block (spinal anesthesia)

• Low caudal epidural block (S2-S4)

• Epidural (lumbar or caudal)

• Combined spinal epidural

• Continuous spinal

"transition," during which both somatic and visceral pain may be significant. Descent of the presenting part distends, and may tear, the vagina and perineal tissue, resulting in severe pain. The unanesthetized parturient will have the uncontrollable urge to valsalva (i.e., "push") at this time. The interval between onset of fetal descent and delivery has been described as the most painful period of labor (Table 22.1).

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