Modern anesthesia practice has an excellent record of safety for the parturient. The anesthesia-related maternal mortality rate in the United States is estimated at 1 per 1 million live births (Hawkins, 2011). Indeed, in the latest report from the Centers for Disease Control and Prevention, Creanga and colleagues (2015) cited anesthesia as the cause of pregnancy-related death in only 0.7 percent of maternal deaths in the United States from 2006 to 2010. Also, the 2010 to 2012 triennial report from the United Kingdom and Ireland described a direct anesthetic mortality rate of 0.17 per 100,000 maternities (Knight, 2014). Finally, the Serious Complication Repository (SCORE) Project of the Society for Obstetric Anesthesia and Perinatology (SOAP) captured data from 257,000 parturients receiving an anesthetic between 2009 and 2014 (D’Angelo, 2014). No deaths were reported, and serious anesthesia-related complications occurred in 1 of 3000 patients. The most frequent was high neuraxial block.

For safe anesthesia administration, the obstetric anesthesiologist should understand the unique characteristics of the gravida. These include alterations in maternal physiology, maintenance of uterine perfusion, and fetal response to anesthetic interventions. So too must the obstetrician be familiar with the effects of anesthesia on these parameters during surgery.

Changes in maternal physiology affect several aspects of anesthetic management (Gaiser, 2014). Cardiovascular changes include increases in cardiac output and blood volume that begin in the first trimester. By 28 weeks’ gestation, these measure 30 to 40 percent above baseline (Table 19-1). Dilutional anemia caused by plasma expansion reduces the hematocrit.

Despite the increase in blood volume and cardiac output, the parturient is susceptible to hypotension from aortocaval compression in the supine position. This is especially true after loss of sympathetic tone associated with regional anesthesia. If the uterus occludes the vena cava in the supine position, preload to the heart is obstructed (Lee, 2012b). Only about 10 percent of pregnant patients at term develop symptoms of shock in the supine position. However, fetal compromise from lowered uterine perfusion can develop even in an asymptomatic mother. For this reason, uterine displacement is an encouraged practice after midpregnancy.

The most important respiratory change during pregnancy is the decrease in functional residual capacity (FRC) (Table 19-2). In the second trimester, the FRC declines by 20 percent and causes a decreased supply of oxygen. This is coupled with a 20-percent increase in oxygen demand as the maternal metabolic rate increases. These are compounded by airway closure during normal tidal ventilation, which develops in a third of parturients while in supine position. This effect is even more likely in smokers and older women. The result is rapid oxygen desaturation during periods of apnea or airway obstruction.

Minute ventilation also changes with pregnancy. This parameter increases at term by 50 percent due to an increase in tidal volume. As a result, normal pco₂ falls about 10 torr, with a compensatory fall in bicarbonate. This new normal for arterial pco₂ should be taken into account when interpreting arterial blood gases during pregnancy (Table 19-3).

Throughout the respiratory tract, capillary engorgement raises the likelihood of trauma during placement of airways and gastric tubes. Thus, a smaller endotracheal tube—6.0 or 7.0 mm—is recommended. Nasal intubations or nasogastric tubes are avoided.

The parturient may be at increased risk of aspirating gastric contents. Gastric volume, pH, and emptying are probably not altered during pregnancy, but gastroesophageal sphincter tone is usually reduced. For example, gravidas often describe symptoms of gastroesophageal reflux disease (GERD). Heartburn indicates that the pressure gradient across the gastroesophageal junction is diminished. Concomitantly, the patient who receives opioids will have delayed gastric emptying. Thus preventively, prior to anesthesia administration, all pregnant women are given preoperative aspiration prophylaxis with a nonparticulate antacid to neutralize gastric contents. Parturients at increased risk include those with obesity or with concern for a difficult airway. These women may additionally receive an H₂-receptor antagonist to reduce acid production and metoclopramide to improve motility and raise gastroesophageal sphincter tone (American Society of Anesthesiologists Task Force on Obstetric Anesthesia, 2016).

During pregnancy, requirements for inhalational anesthetics decline 25 to 40 percent, and loss of consciousness may occur even with “sedative” doses of agents (Lee, 2014). Beginning early in pregnancy, dosage requirements for local anesthetics in the epidural and subarachnoid spaces also drop by 30 percent. This is probably secondary to progesterone action. Drug doses are altered accordingly.

Failed tracheal intubation remains a problem in obstetric patients because of many factors, including normal fluid retention made even worse by preeclampsia. Thus, difficult intubation is more common—1 in 533 gravidas—compared with approximately 1 in 2200 nonpregnant surgical patients (D’Angelo, 2014).

Of fetal effects, fetal oxygenation depends on maternal oxygenation and uterine blood flow. Maternal hyperventilation, either spontaneous or during mechanical ventilation, that leads to alkalosis causes a leftward shift of the oxyhemoglobin dissociation curve. This increases maternal affinity for oxygen and decreases its release to the fetus. In addition, the mechanical effects of positive-pressure ventilation may cause a 25-percent fall in uterine blood flow by decreasing venous return to the heart. Maternal hypotension can produce fetal hypoxia as well.

In summary, the physiologic changes of pregnancy most relevant to the anesthesiologist are decreased pulmonary functional residual capacity, aortocaval compression if supine, decreased lower esophageal sphincter tone, and reduced anesthetic drug requirements. Finally, difficult intubation is more common in pregnant women.

Forceps and Vacuum-Assisted Delivery

Vaginal delivery by forceps or vacuum extraction requires analgesia, muscle relaxation, and patient cooperation. Suitable anesthesia for operative vaginal delivery and perineal repair can include local infiltration or pudendal block, intravenous or inhalational analgesia, spinal (subarachnoid) block, or lumbar epidural block. Of these, pudendal block is often inadequate for operative vaginal delivery other than outlet procedures. General anesthesia is rarely necessary and is not practical or safe when delivery takes place in a labor-delivery-recovery (LDR) room, which typically lacks anesthesia equipment.

Intravenous analgesia is used when neuraxial block is contraindicated or urgency such as acute fetal distress does not allow time for spinal block placement. Narcotic analgesia just prior to delivery is ill advised because of the risk of neonatal depression. In contrast, ketamine is a potent amnesic and analgesic that supports cardiovascular and respiratory functions and produces minimal depression of airway reflexes (Joselyn, 2010). Doses of 0.5 mg/kg intravenously will produce profound analgesia within 1 minute of administration. Its effects last approximately 15 minutes. The drug does cross the placenta but has few or no neonatal effects. Moreover, it does not cause uterine relaxation.

One disadvantage of ketamine is its potent amnesic effects, such that the mother will have little recall of the birth. Another is emergence delirium. Because it is a phencyclidine (“angel dust”) derivative, dreams are common, but unpleasant or frightening hallucinations may also occur. These effects can be attenuated by informing the patient she may have dreams and suggesting they will be pleasant, and by administering a benzodiazepine such as midazolam after delivery, although this will prolong the amnesic effects. Notably, ketamine has sympathomimetic effects and can increase blood pressure and heart rate substantively. Thus, it should be used with caution in women with significant hypertension or preeclampsia.

Inhalational analgesia has rarely been used during labor and delivery in the United States. It requires an anesthesia machine for administration, and regulations require trace anesthetic gas scavenging (National Institute for Occupational Safety and Health, 1994). This changed in 2014 when the Food and Drug Administration approved a mobile device capable of administering 50-percent nitrous oxide by mask that can be used in an LDR room (Barbieri, 2014). This device allows a woman to self-administer nitrous oxide on demand and is scavenged to protect personnel and family in the LDR room from exposure. In the operating room (OR) setting, nitrous oxide is readily available.

Disadvantages of nitrous oxide analgesia are its amnesic effects and the potential for sedation to progress to unconsciousness with risk of aspiration (Likis, 2014). The mobile device for nitrous oxide administration has a one-way valve that can only be triggered when the woman keeps a tight seal between the mask and her face. In the OR, many anesthesiologists have the woman hold the mask herself so that if she becomes too sedated, it will fall away from her face. Use of nitrous oxide requires monitoring of the level of consciousness and using pulse oximetry to document adequate oxygenation. Both intravenous and inhalational anesthesia options are most effective when supplemented with local anesthesia or a pudendal block. When given alone, they often are insufficient for performing episiotomies or repairing perineal lacerations.

Pudendal nerve block is a minor regional block that is reasonably effective and very safe. It involves injection of 5 to 10 mL of local anesthetic just below the ischial spine (Fig. 19-1). Either 1-percent lidocaine or 2-percent 2-chloroprocaine can be used. Pudendal block is generally satisfactory for spontaneous vaginal deliveries and episiotomies, and for some outlet or low operative vaginal deliveries. However, it may be insufficient for deliveries requiring additional manipulation. The potential for local anesthetic toxicity is higher with pudendal block compared with perineal infiltration because of large vessels proximal to the injection site. Therefore, aspiration of the needle before injection and intermittently during injection is particularly important. When perineal and labial infiltration is required in addition to pudendal block, it is important to closely monitor the total amount of local anesthetic given. Specific calculation of a maximum safe dose for each patient before injection is recommended (Dorian, 2015). The toxic dose of lidocaine approximates 4.5 mg/kg. For a 50-kg woman, this would equal 225 mg. Of note, for any drug solution, 1-percent = 10 mg/mL. Thus, if a 1-percent lidocaine solution is used, the calculated allowed amount would be: 225 mg ÷ 10 mg/mL = 22.5 mL.

A spinal block, which is also called an intrathecal or subarachnoid block, provides excellent anesthesia and muscle relaxation. It is fast and relatively simple to perform, and in hyperbaric preparations provides focused perineal anesthesia—the “saddle block.” Spinal anesthesia for delivery only requires a sensory level of T10, so hypotension is less likely than during cesarean delivery. The ability to push may be compromised by diminished motor strength and significant sensory block. Another disadvantage is that it is time-limited when given as a single injection. However, long-acting local anesthetics such as bupivacaine can provide 2 hours of anesthesia for extensive repairs. Also, preservative-free morphine can be added to the local anesthetic for prolonged postoperative analgesia if the repair is extensive.

If an operative vaginal delivery is anticipated, a lumbar epidural catheter can be placed for labor analgesia and then intensified for delivery with higher concentrations of local anesthetic. Of note, epidural anesthetics are segmental blocks, that is, a confined band of analgesia. As a result, there occasionally is sacral nerve sparing, and perineal anesthesia may be incomplete. Epidural blocks have an upper and lower sensory level. Thus, if the lower sacral nerves are not completely blocked, the obstetrician may need to supplement with local anesthesia or a pudendal block.

Vaginal Breech Delivery

The American Society of Anesthesiologists Task Force on Obstetric Anesthesia (2016) has guidelines that support early insertion of a spinal or epidural catheter for obstetric indications such as preeclampsia or vaginal breech delivery or for anesthetic indications such as a difficult airway or obesity. This practice is considered to reduce the need for general anesthesia if an emergent procedure becomes necessary. In these cases, the insertion of a spinal or epidural catheter may precede the onset of labor or a patient’s request for labor analgesia.

In providing analgesia for a vaginal breech delivery, the anesthesiologist has several goals:

1. 
   

   Provide sacral analgesia to prevent early pushing in the first stage of labor. Women with a breech presentation often have earlier complaints of rectal pressure than are seen with a vertex presentation. Early pushing might result in a prolapsed cord or delivery of a fetal lower extremity through an undilated cervix. Although combined spinal-epidural or continuous lumbar epidural analgesia can provide ideal conditions, frequent reassessments of the block are required during labor and delivery (Benhamou, 2002).

   
   
2. 
   

   The anesthesiologist must ensure that the woman is able to push adequately during second-stage labor. This means effective analgesia without excessive motor blockade.

   
   
3. 
   

   Adequate perineal anesthesia is provided for delivery of the aftercoming head and for possible Piper forceps placement to the head. A dilute solution of local anesthetic usually has been administered as regional anesthesia during the first stage of labor. Thus, it is often necessary to administer a more concentrated solution of anesthetic such as 3-percent 2-chloroprocaine or 2-percent lidocaine at delivery. Although epidural analgesia does not relax the cervix at delivery, it provides effective pain relief and skeletal muscle relaxation. A relaxed pelvic floor and perineum aids placement of forceps and delivery of the aftercoming head. Of greatest concern is the risk of fetal head entrapment. To relieve this, epidural anesthesia offers suitable relaxation and patient comfort for needed manipulations.

   
   
4. 
   

   There must be an ability to provide anesthesia for an emergency cesarean delivery at any time during management of a breech delivery. This includes emergency administration of general anesthesia if neuraxial analgesia is insufficient for delivery. Thus, delivery should take place in an OR, and a nonparticulate antacid is given at the time of transfer to that room.

   
   
5. 
   

   To satisfy these goals, at the time of delivery, an agent that provides uterine relaxation should be immediately available. Intravenous nitroglycerin boluses have the advantage of acting rapidly, lasting briefly, and creating minimal side effects (Caponas, 2001). Administration of nitroglycerin helps mediate smooth muscle relaxation. Although well-designed clinical trials are lacking, nitroglycerin appears safe for both the mother and fetus/neonate. Reports describe intravenous doses ranging from 50 to 1500 μg. Both the sublingual and intravenous routes provide rapid onset of uterine relaxation, and the effect lasts only minutes. The parturient should be warned about acute onset of headache, and hypotension is treated with a pressor such as phenylephrine.

   
   
6. 
   

   Of other agents, terbutaline provides excellent uterine relaxation but has numerous maternal side effects. It also has a longer duration of action, which may lead to uterine atony after delivery (Kulier, 2000). General anesthesia using a high concentration of an inhaled agent can provide good uterine relaxation. Of disadvantages, it requires emergency airway management with tracheal intubation and may be associated with postpartum hemorrhage.

   
   
7. 
   

   Newborns delivered from a breech presentation tend to be more depressed than those with a vertex presentation. Accordingly, an individual skilled in neonatal resuscitation should be immediately available.

External Cephalic Version

Manually turning a malpositioned fetus to a vertex position is described in Chapter 21 (p. 348). External cephalic version of a breech presentation may be assisted by neuraxial anesthesia, namely, spinal or epidural, to decrease procedural pain (American College of Obstetricians and Gynecologists, 2016; Lavoie, 2010). Using combined spinal-epidural or epidural analgesia for the version procedure also has the advantage of providing an epidural catheter for later use. It can be employed in the event of a cesarean delivery if the version is unsuccessful or for labor analgesia if the version is successful.