Obstetrical Anesthesia
I. Physiologic Changes of Pregnancy
During pregnancy, alterations occur in nearly every maternal organ system, with associated implications for anesthesiologists (Table 40-1) (Braveman FR, Scavone BM, Blessing M, Wong CA. Obstetrical anesthesia. In: Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Ortega R, Stock MC, eds. Clinical Anesthesia. Philadelphia: Lippincott Williams & Wilkins; 2013:1144–1177).
II. Placental Transfer and Fetal Exposure to Anesthetic Drugs
Most drugs (opioids, local anesthetics, inhaled anesthetics) readily cross the placenta (Table 40-2).
Fetus and Newborn. Several characteristics of the fetal circulation delay equilibration between fetal arterial and venous blood and thus delay the onset or magnitude of depressant effects of anesthetic drugs (Table 40-3).
Neurobehavioral studies in neonates born in the presence of epidural anesthesia may reveal subtle changes in newborn neurologic and adaptive function.
These changes are minor and transient, lasting only 24 to 48 hours.
III. Anesthesia for Labor and Vaginal Delivery
Whereas analgesia for the first stage of labor (pain caused by uterine contractions) is provided by block of T10 to L1, analgesia for the second stage of labor (pain caused by distention of the perineum) is provided by block of S2 to S4.
Nonpharmacologic methods of labor analgesia (massage, aromatherapy, hydrotherapy, biofeedback, transcutaneous electrical nerve stimulation, acupuncture, hypnosis) remain unproven for efficacy.
Other Nonpharmacologic Methods. Continuous labor support is the presence during labor of nonmedical support by a trained person (shorter labors, fewer operative deliveries, fewer analgesic interventions).
Table 40-1 Physiologic Changes of Pregnancy
Hematologic Alterations
Increased total blood volume (25%–40%)
Increased plasma volume (40%–50%): Resultant relative anemia of pregnancy plateau at 32–34 weeks of gestation
Increased fibrinogen
Decreased anticoagulant activity and impaired fibrinolysis: Pregnancy is a state of chronic compensated disseminated intravascular coagulation
Decreased platelet count (dilution and increased consumption)
Decreased cholinesterase activity (20%–30%)
Cardiovascular Changes
Increased cardiac output (30%–50%)
Aortocaval compression (supine hypotensive syndrome occurs in about 50% of parturients)
Ventilation Changes
Increased minute ventilation (50%)
Increased alveolar ventilation (70%)
Decreased functional residual capacity (20%)
Increased oxygen consumption (20%)
Decreased PaCO2 (10 mm Hg)
Increased PaO2 (10 mm Hg)
Airway edema
Gastrointestinal Changes
Delayed gastric emptying: Increased risk for aspiration
Decreased lower esophageal sphincter tone (heartburn)
Altered Drug Responses
Decreased requirements (32%–40%) for inhaled anesthetics (MAC) by 8–12 weeks (parallels increased progesterone levels)
Decreased local anesthetic requirements (engorgement of veins, resulting in decreased volume of the epidural and subarachnoid space versus progesterone-induced increased sensitivity of nerves to local anesthetics)
MAC = minimum alveolar concentration.
Systemic Medication. The time and method of administration must be chosen carefully to avoid maternal and neonatal depression.
Opioids
Meperidine appears to produce less neonatal ventilatory depression than does morphine. Meperidine administered intravenously (IV) (analgesia in 5–10 minutes) or intramuscularly (peak effect in 40–50 minutes) rapidly crosses the placenta.
Table 40-2 Determinants of Drug Passage Across the Placenta
Physical and Chemical Characteristics of the Drug
Molecular weight (<500)
Lipid solubility
Nonionized vs. ionized
Concentration Gradient
Dose administered
Timing of IV administration relative to uterine contraction
Use of vasoconstrictors
Hemodynamic Factors
Aortocaval compression
Hypotension from regional blockade
IV = intravenous.
Fentanyl (1 μg/kg IV) provides prompt pain relief (during forceps application) without severe neonatal depression. For more prolonged analgesia, fentanyl or remifentanil may be administered with patient-controlled delivery devices.
Naloxone (10 μg/kg IV) may be administered directly to newborns to reverse excessive opioid depression.
Ketamine (0.2–0.4 mg/kg IV) provides adequate analgesia without producing neonatal depression.
Regional Anesthesia. Regional techniques (central neuraxial blockade [spinal, epidural, combined spinal–epidural]) provide excellent analgesia with minimal depressant effects in the mother and fetus. Hypotension resulting from sympathectomy is the most frequent complication that occurs with central neuraxial blockade. (Maternal systemic blood
pressure is typically monitored every 2 to 5 minutes for about 15 to 20 minutes after the initiation of the block and at regular intervals thereafter.) Regional analgesia may be contraindicated in the presence of coagulopathy, acute hypovolemia, or infection at the needle insertion site (chorioamnionitis without frank sepsis is not a contraindication). There is no difference in the rate of cesarean delivery in women receiving early neuraxial analgesia compared with systemic opioid analgesia.
Table 40-3 Characteristics of Fetal Circulation that Delay Drug Equilibration
The fetal liver is the first organ perfused by the umbilical vein.
Dilution of umbilical vein blood by fetal venous blood from the gastrointestinal tract, head, and extremities. This explains why thiopental (4 mg/kg IV) administered to the mother does not produce significant depressant effects in the fetus.
Table 40-4 Tests to Rule out Intrathecal or Intravascular Placement of a Lumbar Epidural Catheter
Aspiration: May not be diagnostic
Local anesthetic: 7.5 mg of bupivacaine or 45 mg of lidocaine
Epinephrine: 15 μg; a false-positive reaction may be seen with uterine contractions; may decrease uteroplacental perfusion
Epidural analgesia may be used for pain relief during labor and vaginal delivery and may be converted to anesthesia for cesarean delivery if required.
Effective analgesia during the first stage of labor may be achieved by blocking the T10 to L1 dermatomes with dilute concentrations of local anesthetic with or without the use of opioids that have their effect at the opioid receptors in the dorsal horn of the spinal cord (Table 40-4). For the second stage of labor and delivery, because of pain from vaginal distention and perineal pressure, the block should be extended to include the S2 to S4 segments.
The first stage of labor may be slightly prolonged by epidural analgesia, but this is not clinically significant provided aortocaval compression is avoided. Epidural analgesia initiated during the latent phase of labor (2–4 cm cervical dilation) does not result in a higher incidence of dystocia or cesarean section.
Prolongation of the second stage of labor by epidural analgesia (presumably related to loss of the urge to push by the patient) may be minimized by the use of an ultra-dilute concentration of local anesthetic in combination with an opioid.
Analgesia for the first stage of labor may be achieved with 5 to 10 mL of bupivacaine, ropivacaine, or
levobupivacaine (0.125%–0.25%) followed by continuous infusion (8–12 mL/hr) of 0.0625% bupivacaine or levobupivacaine or 0.1% ropivacaine. The addition of 1 to 2 μg/mL of fentanyl (or 0.3–0.5 μg/l mL of sufentanil) permits a more dilute local anesthetic solution to be administered. During delivery, the sacral dermatomes may be blocked with 10 mL of 0.5% bupivacaine or 1% lidocaine or if a rapid effect is needed, 2% chloroprocaine may be administered in the semirecumbent position.
Patient-controlled epidural analgesia is an alternative to bolus or infusion techniques.
Spinal Analgesia
A single subarachnoid injection for labor analgesia has the advantages of a reliable and rapid onset of neuraxial blockade.
Spinal analgesia with 10 μg of fentanyl or 2 to 5 μg of sufentanil alone or in combination with 1 mL of isobaric bupivacaine 0.25% may be appropriate in multiparous patients whose anticipated course of labor does not warrant a catheter technique.
Spinal anesthesia (“saddle block”) is a safe and effective alternative to general anesthesia for instrumental delivery.
There is a risk of postdural puncture headache, and the motor block may be undesirable.
Combined Spinal–Epidural Analgesia
Combined spinal–epidural analgesia is an ideal analgesic technique for use during labor because it combines the rapid onset of profound analgesia (spinal injection) with the flexibility and longer duration of epidural techniques.
After identification of the epidural space, a long pencil-point spinal needle is advanced into the subarachnoid space through the epidural needle. After intrathecal injection (10–20 μg of fentanyl or 2.5–5 μg of sufentanil alone or in combination with 1 mL of bupivacaine 0.25% produces profound analgesia lasting 90 to 120 minutes with minimal motor block), an epidural infusion of bupivacaine 0.03% to 0.625% with added opioid is started.
Women with hemodynamic stability and preserved motor function who do not require continuous fetal monitoring may ambulate with assistance. (Walking has little effect on the course of labor.)
The most common side effects of intrathecal opioids are pruritus, nausea, vomiting, and urinary retention. The risk of postdural puncture headache does not seem to be increased. Fetal bradycardia may occur.
The incidence of emergency cesarean delivery is no greater after combined spinal–epidural analgesia than after conventional epidural analgesia.
This technique should be used with caution in women who may require an urgent cesarean section and women who are at most increased risk (morbidly obese, difficult airway).
A paracervical block interrupts transmission of nerve impulses from the uterus and cervix during the first stage of labor.
A pudendal nerve block may provide anesthesia for outlet forceps delivery and episiotomy repair.
IV. Anesthesia for Cesarean Delivery
The choice of anesthesia is often influenced by the urgency of the operative procedure and the condition of the fetus. Most patients undergoing cesarean delivery in the United States do so under spinal or epidural anesthesia.
Neuraxial Anesthesia. Blockade to the T4 dermatome is necessary to perform cesarean delivery. The most common complication is hypotension and the attendant risk of uteroplacental perfusion (left uterine displacement, intravenous fluid administration, liberal use of vasopressors). It is common to administer a nonparticulate antacid before induction of anesthesia. Blood pressure should be measured frequently for the first 20 minutes after initiation of anesthesia. Although supplemental oxygen is frequently administered, there is no evidence of benefit to the mother or fetus. Intrathecal morphine provides superior and longer lasting analgesic compared with bilateral transversus abdominis place block.
Spinal anesthesia is provided most often with 1.6 to 1.8 mL of hyperbaric bupivacaine 0.75% lasting approximately 120 to 180 minutes. Improved perioperative analgesia can be provided by addition of fentanyl (6.25 μg) or preservative-free morphine (100 μg) to the local anesthetic solution. It is probably not necessary to adjust the dose of local anesthetic based on the parturient’s height.
Despite a block extending to T4, parturients often experience visceral discomfort, particularly with
exteriorization of the uterus and traction on abdominal viscera (25 μg of fentanyl IV may be useful).
Table 40-5 Epidural Anesthesia for Cesarean Section
2-Chloroprocaine 3%
Lidocaine 2% with epinephrine 1:200,000
0.5% Bupivacaine, ropivacaine, or levobupivacaine
Oxygen should be routinely administered by face mask to optimize maternal and fetal oxygenation.
Lumbar Epidural Anesthesia. Compared with spinal anesthesia, lumbar epidural anesthesia requires more time and drug to establish an adequate sensory level, but there is a lower risk of postdural puncture headache, and the level of anesthesia can be adjusted by titration of local anesthetic solution injected through the indwelling catheter.
Adequate anesthesia is usually achieved with injection through the lumbar epidural catheter of 15 to 25 mL of local anesthetic solution (Table 40-5).
Addition of morphine (3–5 mg) to the local anesthetic solution provides postoperative analgesia.
Combined spinal–epidural anesthesia for cesarean delivery provides a rapid onset of a dense block with a low anesthetic dose and the ability to extend the duration of anesthesia and perhaps to provide continuous postoperative analgesia.
General anesthesia may be necessary when contraindications exist to regional anesthesia or when time precludes central neuraxial blockade. Situations in which uterine relaxation facilitates delivery (multiple gestations, breech position) are most often managed with general anesthesia (Table 40-6).
A newborn’s condition after cesarean section with general anesthesia is comparable to that when regional techniques are used. The uterine incision to delivery time (<180 seconds) is more important to fetal outcome than is the anesthetic technique.
The usual amount of blood loss during cesarean section is 750 to 1,000 mL, and transfusion is rarely necessary.Related posts:
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