Anesthesia for Labor & Vaginal Delivery
The pain of labor arises from contraction of the myometrium against the resistance of the cervix and perineum, progressive dilation of the cervix and lower uterine segment, and stretching and compres-sion of pelvic and perineal structures.
Pain during the first stage of labor is primar-ily visceral pain resulting from uterine contractions and cervical dilation. It is usually initially confined to the T11–T12 dermatomes during the latent phase, but eventually involves the T10–L1 dermatomes as labor enters the active phase. The visceral afferent fibers responsible for labor pain travel with sympa-thetic nerve fibers first to the uterine and cervical plexuses, then through the hypogastric and aortic plexuses, before entering the spinal cord with the T10–L1 nerve roots. The pain is initially perceived in the lower abdomen but may increasingly be referred to the lumbosacral area, gluteal region, and thighs as labor progresses. Pain intensity also increases with progressive cervical dilation and with increas-ing intensity and frequency of uterine contractions. Nulliparous women and those with a history of dys-menorrhea appear to experience greater pain during the first stage of labor.
The onset of perineal pain at the end of the first stage signals the beginning of fetal descent and the second stage of labor. Stretching and compression of pelvic and perineal structures intensifies the pain. Sensory innervation of the perineum is provided by the pudendal nerve (S2–4) so pain during the sec-ond stage of labor involves the T10–S4 dermatomes.
Psychological and nonpharmacological techniques are based on the premise that the pain of labor can be suppressed by reorganizing one’s thoughts. Patient education and positive conditioning about the birthing process are central to such techniques. Pain during labor tends to be accentuated by fear of the unknown or previous unpleasant experiences. Techniques include those of Bradley, Dick-Read, Lamaze, and LeBoyer. The Lamaze technique, one of the most popular, coaches the parturient to take a deep breath at the beginning of each contraction followed by rapid, shallow breathing for the duration of the contraction. The parturient also concentrates on an object in the room and attempts to focus her thoughts away from the pain. Less common non-pharmacological techniques include hypnosis, trans-cutaneous electrical nerve stimulation, biofeedback, and acupuncture. The success of all these techniques varies considerably from patient to patient, and many patients require additional forms of analgesia.
Nearly all parenteral opioid analgesics and sedatives readily cross the placenta and canaffect the fetus. Concern over fetal depression limits the use of these agents to the early stages of labor or to situations in which regional anesthetic techniques are not available or appropriate. Central nervous sys-tem depression in the neonate may be manifested by a prolonged time to sustain respirations, respiratory acidosis, or an abnormal neurobehavioral exami-nation. Moreover, loss of beat-to-beat variability in the fetal heart rate (seen with most central nervous system depressants) and decreased fetal movements (due to sedation of the fetus) complicate the evalua-tion of fetal well-being during labor. Long-term fetal heart rate variability is affected more than short-term variability. The degree and significance of these effects depend on the specific agent, the dose, the time elapsed between its administration and deliv-ery, and fetal maturity. Premature neonates exhibit the greatest sensitivity. In addition to maternal respi-ratory depression, opioids can also induce maternal nausea and vomiting and delay gastric emptying. Some clinicians have advocated use of opioids via patient-controlled analgesia (PCA) devices early in labor because this technique appears to reduce total opioid requirements.
Meperidine, a commonly used opioid, can be given in doses of 10–25 mg intravenously or 25–50 mg intramuscularly, usually up to a total of 100 mg. Maximal maternal and fetal respiratory depression is seen in 10–20 min following intrave-nous administration and in 1–3 h following intra-muscular administration. Consequently, meperidine is usually administered early in labor when delivery is not expected for at least 4 h. Intravenous fentanyl, 25–100 mcg/h, has also been used for labor. Fentanyl in 25–100 mcg doses has a 3- to 10-min analgesic onset that initially lasts about 60 min, and lasts longer following multiple doses. However, maternal respira-tory depression outlasts the analgesia. Lower doses of fentanyl may be associated with little or no neonatal respiratory depression and are reported to have no effect on Apgar scores. Morphine is not used because in equianalgesic doses it appears to cause greater respiratory depression in the fetus than meperidine and fentanyl. Agents with mixed agonist–antagonist activity (butorphanol, 1–2 mg, and nalbuphine, 10–20 mg intravenously or intramuscularly) are effective and are associated with little or no cumu-lative respiratory depression, but excessive sedation with repeat doses can be problematic.
Promethazine (25–50 mg intramuscularly) and hydroxyzine (50–100 mg intramuscularly) can be useful alone or in combination with meperidine. Both drugs reduce anxiety, opioid requirements, and the incidence of nausea, but do not add appreciably to neonatal depression. A significant disadvantage of hydroxyzine is pain at the injection site following intramuscular administration. Nonsteroidal antiin-flammatory agents, such as ketorolac, are not recom-mended because they suppress uterine contractions and promote closure of the fetal ductus arteriosus.
Small doses (up to 2 mg) of midazolam (Versed) may be administered in combination with a small dose of fentanyl (up to 100 mcg) in healthy partu-rients at term to facilitate neuraxial blockade. At this dose, maternal amnesia has not been observed. Chronic administration of the longer-acting benzo-diazepine diazepam (Valium) has been associated with fetal depression.
Low-dose intravenous ketamine is a powerful analgesic. In doses of 10–15 mg intravenously, good analgesia can be obtained in 2–5 min without loss of consciousness. Unfortunately, fetal depression with low Apgar scores is associated with doses greater than 1 mg/kg. Large boluses of ketamine (>1 mg/kg) can be associated with hypertonic uterine contractions. Low-dose ketamine is most useful just prior to deliv-ery or as an adjuvant to regional anesthesia. Some cli-nicians avoid use of ketamine because it may produce unpleasant psychotomimetic effects .
In the past, reduced concentrations of volatile anesthetic agents (eg, methoxyflurane) in oxygen were sometimes used for relief of milder labor pain. Inhalation of nitrous oxide–oxygen remains in com-mon use for relief of mild labor pain in many coun-tries. As previously noted, nitrous oxide has minimal effects on uterine blood flow or uterine contractions.
Pudendal nerve blocks are often combined with perineal infiltration of local anesthetic to provide perineal anesthesia during the second stage of labor when other forms of anesthesia are not employed or prove to be inadequate. Paracervical plexus blocks are no longer used because of their association with a relatively high rate of fetal bradycardia; the close proximity of the injection site to the uterine artery may result in uterine arterial vasoconstriction, uteroplacental insufficiency, and increased levels of the local anesthetic in the fetal blood.
During a pudendal nerve block, a special needle (Koback) or guide (Iowa trumpet) is used to place the needle transvaginally underneath the ischial spine on each side ; the needle is advanced 1–1.5 cm through the sacrospinous liga-ment, and 10 mL of 1% lidocaine or 2% chloropro-caine is injected following aspiration. The needle guide is used to limit the depth of injection and protect the fetus and vagina from the needle. Other potential complications include intravascular injec-tion, retroperitoneal hematoma, and retropsoas or subgluteal abscess.
Epidural or intrathecal techniques, alone or in com-bination, are currently the most popular methods of pain relief during labor and delivery. They can pro-vide excellent analgesia while allowing the mother to be awake and cooperative during labor. Although spinal opioids or local anesthetics alone can provide satisfactory analgesia, techniques that combine the two have proved to be the most satisfactory in mos parturients. Moreover, the synergy between opioids and local anesthetics decreases doserequirements and provides excellent analgesia with few maternal side effects and little or no neonatal depression.