ANESTHESIA FOR SPONTANEOUS VAGINAL DELIVERY
Before deciding on one of the various analgesic options available
to the parturient, it is important to understand the mechanisms and pain pathways
of labor.
Pain Pathways
The International Association for the Study of Pain defines pain
as "an unpleasant sensory and emotional experience associated with actual or potential
tissue damage, or described in terms of such damage."[61]
Pain perception by the parturient is a dynamic process that involves both peripheral
and central mechanisms.
Many factors have an effect on the degree of pain experienced
by a woman during labor, including psychological preparation, emotional support during
labor, past experiences, the patient's expectations of the birthing process, and
augmentation of labor with oxytocin. An abnormal presentation (such as occiput-posterior)
may also cause early labor pain to be more intense. There is, however, no doubt
that for most women, childbirth is associated with very severe pain, and it often
exceeds all expectations.[62]
In fact, as reported
on the McGill pain questionnaire, labor pain is one of the most intense pains that
a woman can experience, and it is typically worse than a toothache, back pain, and
pain associated with a deep laceration ( Fig.
58-5
). A study of women in the first stage of labor reported that 60%
of primiparous women described the pain of uterine contractions as being "unbearable,
intolerable, extremely severe, or excruciating."[63]
During the first stage of labor, pain impulses arise primarily
from the uterus. Uterine contractions may result in myometrial ischemia, which ultimately
causes the release of bradykinin, histamine, and serotonin. In addition, stretching
and distention of the lower uterine segment and cervix may stimulate mechanoreceptors.
These noxious impulses follow the sensory nerve fibers that accompany sympathetic
nerve endings; they travel through the paracervical region and the hypogastric plexus
to enter the lumbar sympathetic chain.[64]
These
stimuli enter the spinal cord at the T10, T11, T12, and L1 spinal segments. Parturients
describe this pain as dull in
Figure 58-5
McGill pain questionnaire comparing pain scores for women
in labor and those of patients in a general hospital pain clinic and emergency department
for 20 conditions. (Redrawn from Melzack R: The myth of painless childbirth.
The John J. Bonica Lecture. Pain 19:321–337, 1984.)
nature and often poorly localized. With onset of the second stage of labor and stretching
of the perineum, somatic afferent nerve fibers transmit impulses through the pudendal
nerve to the spinal cord at the S2, S3, and S4 levels ( Fig.
58-6
and Fig. 58-7
).
