Anesthesia

Receiving an anesthetic during pregnancy or delivery is certainly not an uncommon event. Each year about 2 percent of pregnant women will undergo a surgical procedure involving approximately 75,000 anesthetics (Mazze and Källén, 1989). In addition, almost 90 percent of women will have some type of analgesia during their labor and delivery in larger hospitals (Hawkins and colleagues, 1997a). The most common indications for surgery during pregnancy are trauma, ovarian cysts, appendicitis, breast tumors, and cervical incompetence. However, craniotomy with induced hypotension, cardiac surgery requiring cardiopulmonary bypass, and even liver transplantation have been performed successfully (Gibbs and Hawkins, 1996). Despite this record of safety, the lay public, as well as many health care professionals, have strong biases against the use of anesthetics (or any other drugs) during pregnancy. For example, during the congressional debate on late-term abortions the erroneous statement was made that, “The fetus usually dies from the anesthesia administered to the mother before the procedure begins.” The American Society of Anesthesiologists (ASA) issued a corrective statement and several members testified before Congress asserting that “many pregnant women are safely anesthetized every day without ill effects to the fetus” (St. Louis Post-Dispatch, 1995).

Modern anesthesia practices have an excellent record of safety for the parturient. The United States’ rate of anesthesia-related maternal mortality in 1990 was estimated at 1.7 per 1 million live births (Hawkins and colleagues, 1997b). The most recent triennial report from Great Britain covering 1994-1996 found only one direct anesthetic death for a rate of 0.5 per 1 million maternities (HMSO, 1998). When maternal mortality data from Israel were reviewed for the period 1990-1995, there were no deaths due to anesthesia (Yoles and colleagues, 1998). Despite the safety of anesthesia for the pregnant patient, the anesthesiologist must be cognizant of several problems unique to the pregnant woman, including:

(a) alterations in maternal physiology;

(b) possible teratogenic effects of anesthetic agents in early gestation;

(c) maintenance of uterine perfusion and fetal effects of anesthetic interventions, and

(d) prevention of premature labor, the greatest cause of fetal loss.

PHYSIOLOGIC CHANGES OF PREGNANCY

Numerous changes in maternal physiology may affect anesthetic management (Conklin and Backus, 1999). Increases in cardiac output and blood volume begin early in the first trimester and are 30–40 percent above the nonpregnant state by 28 weeks. Despite the increase in blood volume and cardiac output, the parturient is susceptible to hypotension from aortocaval compression in the supine position, especially after sympathectomy associated with regional anesthesia. Although only about 10 percent of pregnant patients at term develop symptoms of shock in the supine position, fetal compromise can occur even in the asymptomatic mother. Uterine displacement must be maintained at all times.

The most important respiratory change during pregnancy is the decrease in functional residual capacity (FRC). Beginning in the second trimester there is a 20 percent decrease in FRC coupled with a 20 percent increase in oxygen consumption. In addition, 30 percent of parturients have airway closure during normal tidal ventilation in the supine position, especially smokers and older parturients. These changes predispose to rapid falls in Po₂ during periods of apnea or airway obstruction. Minute ventilation increases at term by 50 percent due to an increase in tidal volume, so normal Pco₂ falls about 10 torr with a compensatory fall in bicarbonate. There is capillary engorgement throughout the respiratory tract, which increases the likelihood of trauma during placement of airways and gastric tubes. A smaller-size endotracheal tube (6 or 7) is recommended, and nasal intubations or nasogastric tubes should be avoided.

The increased risk of aspiration in the pregnant patient is due to both hormonal and mechanical factors. Beginning early in pregnancy, elevated levels of gastrin increase secretion of acid, while progesterone decreases intestinal motility. The enlarged uterus displaces the pylorus, which decreases gastric emptying and alters the gastroesophageal junction. Symptoms of heartburn indicate there is a lower pressure gradient across the gastroesophageal junction. All pregnant patients should have preoperative aspiration prophylaxis with a nonparticulate antacid, and perhaps an H₂ blocker and metoclopramide as well.

During pregnancy there is a 25–40 percent decrease in requirements for inhalational anesthetics, and loss of consciousness may occur even with “sedative” doses of agents (Chan and colleagues, 1996). There is also a 30 percent decrease in dosage requirement for local anesthetics in the epidural and subarachnoid spaces as early as the first trimester of pregnancy, probably due to progesterone affecting the sensitivity of the nerve (Datta and colleagues, 1986).

Fetal oxygenation depends on maternal oxygenation and uterine blood flow. Maternal hyperventilation with alkalosis, either spontaneous or during mechanical ventilation, causes a leftward shift of the oxyhemoglobin dissociation curve increasing maternal affinity for oxygen and decreasing 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, regardless of etiology, can produce fetal hypoxia as well.

ANESTHESIA FOR OPERATIVE VAGINAL DELIVERY

Application of forceps or vacuum extraction requires analgesia, muscle relaxation, and patient cooperation. Anesthesia for instrument vaginal delivery and perineal repair might include local or pudendal block, intravenous or inhalational analgesia, spinal (subarachnoid) block, or lumbar epidural block. It should be noted that pudendal block is often inadequate for operative vaginal delivery other than outlet procedures. General anesthesia is rarely necessary or practical, especially when delivery occurs in a labor-delivery-recovery (LDR) room (Hawkins and colleagues, 1998).

Intravenous analgesia is used when regional block is contraindicated or urgency (eg, acute fetal distress) does not allow time for placement of a block. Narcotic analgesia just prior to delivery is ill advised because of the risk of neonatal depression. Ketamine is a potent amnesic and analgesic that supports cardiovascular and respiratory functions and produces minimal depression of airway reflexes (Kohrs and Durieux, 1998). Doses of 0.25–0.5 mg/kg intravenously will produce profound analgesia within 1 minute of administration. Although placental passage occurs, there is little or no neonatal effect and no depression of uterine tone. The disadvantages of this drug are its potent amnesic effects (ie, the mother will have little recall of the birth) and occasional emergence delirium. Because it is a phencyclidine derivative, dreams are common, but unpleasant or frightening hallucinations might also occur. These effects can be attenuated by administration of a benzodiazepine such as midazolam after delivery, although this will prolong the amnesic effects. Ketamine should be used with caution in women with significant hypertension or preeclampsia because of its sympathomimetic effects.

Inhalational analgesia is rarely used during labor and delivery in the United States because it requires an anesthesia machine for administration, which is rarely available in the LDR setting, and because OSHA regulations on trace anesthetic gases require scavenging (NIOSH, 1977). However, in the operating room setting, nitrous oxide at concentrations of 30–50 percent can provide good analgesia. Its disadvantages are its amnesic effects and the potential for its sedative effects progressing to unconsciousness with risk of aspiration. Many anesthesiologists have the patient hold the mask so that if she becomes too sedated it will fall away from her face. Use of nitrous oxide requires monitoring the patient’s level of consciousness and using pulse oximetry to document adequate oxygenation. Both intravenous and inhalational anesthesia are most effective when supplemented by local anesthesia or a pudendal block by the obstetrician.

Alone, they often are insufficient for performing episiotomies or repairing perineal lacerations.

Spinal 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. Its main disadvantage is that it is time-limited when given as a single injection, but long-acting local anesthetics such as bupivacaine can provide 2 hours of anesthesia.

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

ANESTHESIA FOR COMPLICATED VAGINAL DELIVERY (BREECH, TWINS)

The ASA Practice Guidelines for Obstetrical Anesthesia state that, “Either monitored or stand-by anesthesia care, determined on a case-by-case basis for complicated vaginal delivery (eg, breech presentation, twins, and trial of instrumental delivery), should be made available when requested by the obstetrician. The anesthesiologist should be notified as soon as practical when a difficult vaginal delivery is anticipated” (American Society of Anesthesiologists Task Force on Obstetrical Anesthesia, 1999).

BREECH PRESENTATION

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

• Provide sacral analgesia to prevent early pushing in the first stage of labor.

• Ensure the patient is able to push adequately during the second stage of labor to allow spontaneous delivery of the lower half of the body.

• At delivery provide profound perineal anesthesia for delivery of the after-coming head and possible placement of forceps.

• Be able to provide anesthesia for emergency cesarean delivery at any time.

Although continuous epidural analgesia can provide ideal conditions, frequent reassessments of the block are required during labor and delivery. At the time of delivery, an agent that provides uterine relaxation should be immediately available if needed for delivery of the after-coming head. Nitroglycerin has the advantages of being rapid in onset and short in duration with minimal side effects (Wessén and colleagues, 1995). Terbutaline has numerous maternal side effects and a longer duration of action, which may lead to uterine atony (Ravindran and colleagues, 1980). General anesthesia with a high concentration of an inhaled agent can provide good uterine relaxation but requires emergency airway management with endotracheal intubation and may also be associated with postpartum hemorrhage due to uterine atony.

Several studies have shown that external version of a breech presentation is more successful when regional anesthesia is used for analgesia (Schorr and colleagues, 1997; Neiger and colleagues, 1998). This option also allows an epidural catheter to be placed for later use in the event of a cesarean delivery if the version is unsuccessful, or for labor analgesia if the version is successful.

TWIN DELIVERY

Twin pregnancies produce exaggerated physiologic changes of pregnancy (eg, aortocaval compression when supine) and rapidly changing conditions during delivery. If twin A is in a position other than vertex, a cesarean delivery is often chosen and the anesthesia plan made accordingly. If twin A is vertex, a trial of labor and vaginal delivery may be attempted, but even a successful vaginal delivery may be followed by intrauterine manipulation, breech extraction or cesarean delivery for twin B. The anesthesia plan must be flexible to accommodate all these needs (Koffel, 1999). Analgesia/anesthesia for labor, version or breech extraction, or cesarean delivery can be accomplished with continuous epidural anesthesia, but the local anesthetic choice and concentration will vary depending on circumstances. For example, labor analgesia might require only 0.125 percent bupivacaine, whereas intrauterine manipulations or instrumented delivery might need a denser sensory block with 0.5 percent bupivacaine or 1.5 percent lidocaine, and cesarean delivery might require a surgical block using 2 percent lidocaine or 3 percent 2-chloroprocaine.

To rapidly respond to these changing conditions, the anesthesiologist must be present in the delivery suite. The epidural catheter should be tested and functioning well. All monitors should be in place in case urgent cesarean delivery is required. Aspiration prophylaxis should have been given en route to the operating room or before, and the mother should be receiving continuous oxygen by nasal cannula or mask. A uterine relaxant such as nitroglycerin should be immediately available (Dufour and colleagues, 1998). Above all, the obstetricians must communicate their thoughts and plans at all times so that the anesthesiologist can be prepared.

ANESTHESIA FOR CESAREAN DELIVERY

There are three types of anesthesia used for cesarean delivery: general endotracheal, epidural, and spinal. As noted in the ASA Practice Guidelines: “The anesthetic technique should be individualized based on several factors. These include anesthetic, obstetric and/or fetal risk factors (eg, elective versus emergency), and the preferences of the patient and anesthesiologist. Resources for the treatment of potential complications (eg, airway management, inadequate analgesia, hypotension, pruritus, nausea) should be available” (American Society of Anesthesiologists Task Force on Obstetric Anesthesia, 1999).

In the United States, regional anesthesia is preferred over general anesthesia for cesarean delivery (Hawkins and colleagues, 1997). In 1997 in hospitals with more than 1500 deliveries per year, 8 percent of cesarean deliveries were performed using general anesthesia, 47 percent were performed using spinal anesthesia, and 45 percent were performed using epidural anesthesia. The literature suggests that a greater number of maternal deaths occur during general anesthesia than during spinal or epidural anesthesia, and that problems with the airway (including aspiration) are the leading anesthetic causes of maternal mortality (Hawkins and colleagues, 1997). In addition, the literature indicates that a larger proportion of neonates in the general anesthesia groups are assigned Apgar scores of less than 7, although these studies rarely utilized randomized comparisons (American Society of Anesthesiologists Task Force on Obstetric Anesthesia, 1999).

The mother often prefers to be awake to see her newborn and to have a family member with her. She is more likely to be more alert and to have better pain control after regional anesthesia, which may make initial bonding and attempts at breast-feeding more successful. As regional analgesia for labor becomes more popular, a working epidural catheter may already be in place and can provide an excellent anesthetic for cesarean delivery without having to initiate another technique.

However, there are situations in which regional anesthesia may be contraindicated: patient refusal, hemodynamic instability due to hemorrhage or sepsis, clinical coagulopathy, infection at the site, or lack of personnel trained in regional anesthesia. There may also be technical difficulties for the anesthesiologist, such as the presence of Harrington rods. The choice of regional versus general anesthesia must be made quickly in the setting of an emergent cesarean delivery. The obstetrician and anesthesiologist must communicate each other’s concerns effectively. For example, the obstetrician may feel there is little time to initiate regional anesthesia, while the anesthesiologist may feel there is a real potential for airway difficulties (eg, in the morbidly obese patient). Maternal safety must always come first.

Choosing between spinal or epidural anesthesia is often provider-dependent, but unless there is already a well-functioning epidural catheter in place, a spinal anesthetic may be faster and easier to initiate. Although time-limited, a spinal anesthetic with bupivacaine will provide at least 2 hours of surgical anesthesia. Because a much lower dose of local anesthetic is used, local anesthetic toxicity is not a concern with spinal anesthesia. Preservative-free morphine can be added to either spinal or epidural anesthesia to provide excellent long-lasting pain relief postoperatively.

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