Analgesia and Anesthesia for Labor and Delivery
Cynthia A. Wong
Table Of Contents
Cynthia A. Wong, MD
PHYSIOLOGIC AND ANATOMIC CHANGES DURING PREGNANCY AND LABOR-ANESTHETIC IMPLICATIONS
ANALGESIA FOR LABOR AND VAGINAL DELIVERY
ANESTHESIA FOR CESAREAN DELIVERY
ANESTHESIA FOR SPECIAL CIRCUMSTANCES (TABLE 5)
ANESTHESIA FOR OTHER PROCEDURES
COMPLICATIONS OF REGIONAL ANALGESIA/ANESTHESIA
Anesthesiologists and anesthesia play a critical role on the modern labor and delivery unit. Operative deliveries obviously require the participation of an anesthesiologist in the care of the parturient (and by extension, the fetus). In addition, the anesthesiologist plays a critical role in the care of many high-risk parturients. As part of the perinatal team, anesthesiologists contribute knowledge in the areas of invasive monitoring and the diagnosis and treatment of the extremes of hemodynamic and respiratory instability. Finally, the number of parturients choosing regional labor analgesia for uncomplicated labor and vaginal delivery has increased from 21% to 55% in 1992 to 41% to 66% in 1997.1
|PHYSIOLOGIC AND ANATOMIC CHANGES DURING PREGNANCY AND LABOR-ANESTHETIC IMPLICATIONS|
Many physiologic and anatomic changes occur during pregnancy, labor and delivery, and the postpartum period that directly impact the administration of analgesia and anesthesia (Table 1).
In general, the cardiovascular system is hyperdynamic during pregnancy. However, arterial responsiveness to vasopressors is reduced in pregnancy in both in vivo and in vitro animal models.2,3 In contrast, the venous pressor response increases in pregnancy.4 Indeed, hemodynamic stability becomes more dependent on sympathetic nervous system activity as pregnancy progresses,5 probably as a result of dependence on venous return. Sympathetic blockade in the term parturient, a consequence of regional analgesia/anesthesia, results in a marked decrease in blood pressure compared with that in nonpregnant control subjects.5
Capillary engorgement of the oral, nasal, pharyngeal, and tracheal mucosa may contribute to a markedly increased incidence of difficult endotracheal intubations in obstetric patients.6 Nasotracheal intubation should be avoided because of the risk of epistaxis.
Term parturients in the supine position experience a 10% to 20% decrease in cardiac output secondary to aortocaval compression.7 Compression of the vena cava begins as early as 13 to 16 weeks' gestation8 and is nearly complete in a majority of parturients at term. In the lateral decubitus position, there is partial caval compression,9 but collateral circulation through the azygous system maintains venous return.10 There is partial compression of the aorta in the supine position that is completely relieved by the lateral position.11
Objective parameters of fetal well-being, including fetal scalp capillary blood pH, transcutaneous pO2, and fetal heart rate pattern, deteriorate when parturients labor in the supine position.12,13 This is true whether or not regional analgesia/anesthesia-induced sympathetic blockade is present. However, the adverse effects may be more profound in the parturient with sympathetic blockade. In addition, aortocaval compression may occur in the lateral decubitus position when maximum lumbar flexion is assumed during positioning for regional analgesia/anesthesia. Cardiac output is more likely to decrease in this position compared to the sitting position.14
Metabolism and Respiration
Oxygen requirements and carbon dioxide production increase 60% during pregnancy.15 Minute ventilation increases by 45% primarily as a result of an increase in tidal volume.16 In addition to the change in minute ventilation, the most important lung volume/capacity change is a 20% decrease in functional residual capacity (FRC) (i.e., volume of the lung at the end of normal expiration).16
During labor and delivery, minute ventilation may increase up to 300%, oxygen consumption increases by 75%, and the PaCO2 decreases as low as 10 to 15 mmHg.17 Maternal aerobic oxygen requirements exceed oxygen consumption during labor and delivery, resulting in a progressive rise in maternal lactate levels.18
The increased maternal oxygen requirement and decreased FRC (decreased oxygen reserve) have profound implications for the induction of general anesthesia in the parturient. Because of concerns of pulmonary aspiration (see later), parturients are almost never ventilated by mask during the induction of anesthesia. On induction, an intravenous sedative hypnotic (e.g., thiopental) is administered, immediately followed by a neuromuscular blocking agent. Patients remain apneic until the endotracheal tube is placed. Provided there is no difficulty with intubation, the apneic period is approximately 1 minute. During this period, the pO2 in the parturient falls at more than twice the rate of nonpregnant women.19 If oxygen is administered before the induction of anesthesia, resulting in a pO2 close to 500 mmHg, the parturient becomes hypoxemic after 3 minutes of apnea compared to 7 minutes in nonpregnant women.19 Increased minute ventilation and decreased FRC also increase the rate of uptake of inhaled anesthetic agents.
Finally, high spinal or epidural anesthesia is associated with a further decrease in FRC20; therefore, the parturient is at increased risk, compared to the nonpregnant patient, for having hypoxemia develop during regional anesthesia. For this reason, supplemental oxygen usually is delivered during regional anesthesia.
Several changes in the gastrointestinal system result in the admonition that a “parturient is always a full stomach.” The gravid uterus displaces the stomach, often resulting in the displacement of the esophagus into the thorax. There is a reduction in lower esophageal sphincter tone secondary to anatomic displacement21 and relaxation of the sphincter by progesterone.22 In addition, intragastric pressure rises. Altogether, these changes result in a reduced esophageal-stomach barrier pressure and the well-known clinical complaint of heartburn.
Gastric emptying, as measured by several techniques, including acetaminophen absorption,23,24 epigastric impedence,25 and applied potential tomography,26 is unchanged in pregnancy. However, labor significantly slows gastric emptying.25,27 Furthermore, opioids administered by any route, including systemic,25 epidural,28,29 or intrathecal,30 slow gastric emptying even more. In the immediate postpartum period (2 to 48 hours), gastric emptying does not appear to be delayed unless the parturient has received opioid analgesia.23–26,31
Pulmonary aspiration and failed intubation were responsible for 48% of anesthesia-related maternal mortality in the United States between 1979 and 1990.32 Indeed, Mendelson's syndrome, or pulmonary aspiration, was first reported in parturients receiving general anesthesia.33 Ollson34 reported that the incidence of pulmonary aspiration is higher in patients undergoing cesarean delivery (1 in 661) than in the general population undergoing surgery (1 in 2131). In addition, the incidence of aspiration is more than fourfold higher for emergency surgery compared with that of elective surgery.35
Maternal mortality associated with regional anesthesia between 1988 and 1990 decreased compared to 1979 and 1981, whereas the mortality associated with general anesthesia was constant and most often related to difficult intubation or aspiration or both.32 Aspiration often is associated with difficult intubation.36 These data support the premise that regional anesthesia is safer for the parturient.
If general anesthesia is necessary, appropriate precautions are taken. The patient is most at risk for aspiration during induction and emergence from general anesthesia. Aspiration of acidic, compared to neutral, liquid results in more severe pathology.33 Histamine 2 (H2)-receptor antagonists reliably increase gastric pH by decreasing gastric acid production.37 H2-receptor antagonists do not neutralize acid already present in the stomach. Therefore, a nonparticulate antacid (e.g., sodium citrate) is always administered before the induction of general anesthesia. A nonparticulate antacid is used because aspiration of particulate antacid may cause symptoms of aspiration pneumonitis.38
Many anesthesiologists also add metoclopramide to the aspiration prophylaxis cocktail. Metoclopramide increases lower esophageal sphincter tone39 and reduces gastric volume by increasing gastric motility.40
General anesthesia in the parturient is always induced by rapid sequence (see later). Mask ventilation is avoided, as air inadvertently introduced into the stomach increases the risk of aspiration and the airway is not protected with an endotracheal tube. Similarly, deep “conscious” sedation is avoided in the parturient, as this may be associated with obtundation of protective airway reflexes.
As a final precaution, the American Society of Anesthesiologist's (ASA) Guidelines for Obstetrical Anesthesia recommend that before elective cesarean delivery, institutional presurgical fasting guidelines should be followed.41 For most institutions, this means a fasting period for solid food of 6 to 8 hours.
A related controversial issue is maternal oral intake during labor. Although laboring parturients who eat have larger gastric volumes than do parturients who fast,42 an association between eating during labor and an increased risk of aspiration has not been shown. Proponents of eating argue that oral intake improves patient comfort and satisfaction43 and prevents maternal ketoacidosis.42 Pulmonary aspiration is a rare event. Opponents of feeding argue that one cannot predict which parturient will require emergency surgery. Although the risk of aspiration is low, the morbidity and mortality are high. Mild ketosis has not been shown to adversely affect outcome, and a minority of patients find fasting stressful.43 ASA Guidelines state that the “oral intake of modest amounts of clear liquids may be allowed for uncomplicated laboring patients.”41 The American College of Obstetricians and Gynecologists (ACOG) Guidelines of Perinatal Care states that “patients in active labor should avoid oral ingestion of anything except sips of clear liquids.”44
Progesterone concentration rises throughout pregnancy and declines abruptly after delivery of the placenta.45 The increase in plasma progesterone may influence a number of physiologic processes, including response to pain (see later).
Central and Peripheral Nervous System
Anatomic and physiologic changes in the nervous system alter responses to pain and susceptibility to both general and regional anesthesia. Pregnancy-induced neurohumoral changes may alter responses to pain. In a rat model, the pregnancy-induced increased concentration of plasma β-endorphin was associated with an increased tolerance to visceral stimulation, and this effect was reversed by naloxone.46 Pregnancy is associated with lower plasma substance P concentrations47 and higher cerebral spinal fluid (CSF) progesterone levels.48
Nerves from pregnant animals (including humans) appear more susceptible to local anesthetic blockade. In intact rats49,50 and humans,51,52 pregnancy enhanced the effect of central and peripheral local anesthetics. Proposed mechanisms of enhanced neural blockade during pregnancy include hormone-related changes in the actions of spinal cord neurotransmitters, potentiation of the analgesic effect of endogenous analgesic systems, increased permeability of the neural sheath, and other pharmacodynamic or pharmacokinetic differences between pregnant and nonpregnant women.53 The spread of epidural local anesthetic blockade is increased during early pregnancy,51 a phenomenon explained by altered sensitivity to local anesthetics, as opposed to gross changes in spinal column anatomy.
However, anatomic changes in the mother may affect regional anesthesia techniques. The epidural and vertebral foraminal veins are enlarged, resulting in an increased risk of intravascular placement of an epidural catheter and a decreased egress of epidural anesthetic agents from the epidural space. Engorged epidural veins are associated with a decrease cerebral lumbosacral CSF volume.54 An increase in intra-abdominal pressure also is associated with decreased CSF volume.55 In volunteers, decreased lumbosacral CSF volume, as measured by magnetic resonance imaging, was associated with increased sensory blockade after spinal anesthesia.56
The central nervous system (CNS) and cardiac toxicity of lidocaine58 and ropivacaine59 are not altered by pregnancy. Although an earlier study concluded that the plasma bupivacaine concentration necessary for cardiac toxicity was markedly decreased in the pregnant sheep l,60 a more recent study does not support this conclusion.61
Just as peripheral nerves are more susceptible to local anesthetic during pregnancy, the parturient also is more susceptible to CNS depressants. The minimum alveolar concentration of volatile halogenated anesthetic agents (the median effective dose) is decreased 30% during pregnancy.62 Similarly, there is increased sensitivity to thiopental.63
Pregnancy alters disposition or drugs by several mechanisms. Volume of distribution may be altered. For example, the elimination half-life of thiopental is markedly prolonged secondary to a large increase in the volume of distribution.64 Plasma protein concentration decreases, leading to altered drug binding.65 For example, lidocaine is less protein bound during pregnancy, resulting in a higher free fraction in the blood.66 Increased renal blood flow and glomerular filtration and altered hepatic microsomal activity change renal and hepatic drug clearance.67
Pregnancy is a state of enhanced activation of the coagulation system. The levels of most clotting and fibrinolytic factors are elevated.68 However, in otherwise normal parturients, 0.9% have a platelet count of less than 100,000/mm3.69 The platelet count falls after delivery and during the first postpartum day.70
UTEROPLACENTAL BLOOD FLOW.
Uterine blood flow (UBF) increases markedly during pregnancy, from 50 to 100 mL/min before pregnancy to 700 to 900 mL/min at term. There is a generalized decrease in vascular sensitivity to endogenous and exogenous vasopressors; however, the diminution in response may vary between uterine and systemic vessels. The altered vascular reactivity in pregnancy may be secondary to (1) changes in receptor number or function, (2) changes in metabolism or clearance of drugs, (3) changes in release of endogenous vasoactive substances, or (4) changes in sensitivity to vasoactive substances.71 In general, however, UBF is related to uterine perfusion pressure and vascular resistance:
UBF is not autoregulated. Any decrease in uterine arterial pressure (systemic hypotension), increase in venous pressure (caval compression, uterine contraction, Valsalva maneuver), or increase in uterine vascular resistance (increase in uterine vasoconstriction relative to systemic vasoconstriction) decrease UBF. The net effect on UBF of any therapeutic intervention depends on relative changes in systemic and uterine vessels as they affect the determinants of UBF.
EFFECT OF REGIONAL ANALGESIA/ANESTHESIA ON UTEROPLACENTAL BLOOD FLOW.
Regional analgesia/anesthesia may effect UBF, in both directions, by a number of mechanisms.71 Pain causes activation of the sympathetic nervous system. In pregnant ewes, acute stress increases plasma norepinephrine levels by 25% and decreases UBF by 50%.72 Labor pain also is associated with a marked increase in epinephrine concentration,73,74 which has been associated with an increased incidence of abnormal fetal heart rate patterns.75 Labor analgesia is associated with a marked reduction in circulating catecholamines.73,74 Pain also causes maternal hyperventilation. Hyperventilation may cause a decrease in UBF.76
Regional anesthesia-induced sympathetic blockade does not appear to alter UBF in normal parturients in the absence of hypotension.77 In the setting of severe pre-eclampsia, epidural analgesia with local anesthetic actually may increase intervillous blood flow.78
Regional analgesia/anesthesia may adversely effect UBF primarily as a result of sympathetic blockade-induced maternal hypotension. Maternal hypotension decreases uterine arterial pressure and increases uterine vascular resistance secondary to reflex release of vasoconstrictors.71 Regional analgesia has been associated with uterine hypertonus resulting in a decrease in UBF (see later).
Unintentional intravenous injection of local anesthetic may decrease UBF by several mechanisms. A test dose containing epinephrine often is administered through the epidural catheter to rule out inadvertent intravenous placement of the catheter. If the catheter is in a vein, the intravenous epinephrine will cause transient maternal tachycardia and hypertension. In gravid ewes, intravenous epinephrine also caused a transient decrease in UBF.79 The epidural administration of local anesthetic solutions containing epinephrine does not appear to effect UBF in the healthy parturient.80 However, there is some concern that the epidural injection of epinephrine containing local anesthetics may adversely effect UBF in patients with pre-eclampsia.81
The amount of local anesthetic contained in a normal test dose does not effect uterine perfusion if injected into a vein,82 nor does the amount of local anesthetic absorbed from the epidural space after epidural anesthesia. However, higher concentrations of local anesthetics, achieved after inadvertent intravenous injection or after a paracervical block (see later), may cause uterine hypertonus, a decrease in UBF, and subsequent fetal bradycardia.71
EFFECT OF GENERAL ANESTHESIA ON UTEROPLACENTAL BLOOD FLOW.
Anesthetic agents and adjuvants, in general, have no direct effect on UBF. Hypotension associated with general anesthesia may adversely affect UBF. The typical rapid sequence induction of general anesthesia for cesarean delivery (induction agent, followed by neuromuscular blocking agent, followed by laryngoscopy) often is accompanied by a catecholamine surge that may cause a decrease in UBF.83 Mechanical hyperventilation decreases UBF.84,85 It is unclear whether this is a result of hypocarbia or a direct result of mechanical hyperventilation (resulting in decreased venous return and decreased cardiac output).84
The Placenta and Fetus
The anatomy and physiology of the placenta are discussed in detail elsewhere in this text. Whether anesthetic agents cross the placenta depends on a number of factors, including drug concentration and electrochemical gradients across the placenta, molecular weight, lipid solubility, degree of ionization, membrane surface area and thickness, maternal and fetal blood flow, placental binding and metabolism, and degree of maternal and fetal protein binding.86 Most drugs that parturients receive during the administration of analgesia/anesthesia cross the placenta. This includes volatile anesthetic gases, nitrous oxide, sedative/hypnotics, opioids, local anesthetics, antihypertensive agents, and vasopressors.86 Neuromuscular blocking agents and most anticholinesterases are charged molecules and have limited placental transfer. The effects of individual anesthetic agents on the fetus/neonate are discussed later.
|ANALGESIA FOR LABOR AND VAGINAL DELIVERY|
Labor pain, a form of acute pain, is perceived by many women as very severe or intolerable.87 The optimal analgesic for labor provides pain relief for first- and second-stage labor but otherwise has minimal effect on the mother or baby. Regional labor analgesia (epidural, spinal, or combined spinal-epidural [CSE] analgesia) currently is the most effective method of labor and delivery analgesia. The side effect profile of these techniques is acceptable to most women and obstetricians. Current research aims to decrease the incidence and severity of side effects while maintaining excellent analgesia.
The Pain of Parturition
Parturition pain has both a visceral and somatic component. Visceral stimulation occurs as the cervix and lower uterine segment dilate. Early labor pain (latent phase and early active phase) is primarily visceral and occurs during uterine contractions. Afferent impulses are transmitted via sensory nerves (myelinated Aδ and unmyelinated C fibers) that pass through the paracervical nerve plexus (Fig. 1). The nerves accompany sympathetic nerves to the inferior, middle, and superior hypogastric and then celiac plexus. They enter the lumbar sympathetic chain and then pass with T10, T11, T12, and L1 white rami communicates to the posterior spinal roots to synapse with neurons in the dorsal horn of the spinal cord. Pain often is referred to other areas, including skin, abdominal muscles, and the back.88
The somatic component of labor pain is caused by distension, inflammation, and tissue injury of the pelvic joints, vagina, pelvic floor, and perineum. This occurs as the fetus descends in the birth canal during the late first stage and second stage of labor. Afferent impulses are transmitted via the pudendal nerve through the sacral plexus to the spinal cord at S2, S3, and S4. The pudendal nerve also supplies motor innervation to the pelvic floor and perineum.
The visceral and somatic components of labor pain can be blocked at various levels by one or more nerve blocks (see Fig. 1). For example, epidural analgesia can block the visceral, somatic, or both components, depending on the spinal levels blocked. A lumbar sympathetic block alleviates visceral labor pain and a pudendal block alleviates somatic pain.
Analgesic requirements increase as labor progresses. The late active phase of labor (transitional stage) is the most painful period of labor. Minimum local analgesic concentration, or the median effective concentration, increases with advancing cervical dilation.89 A combined dose of intrathecal sufentanil and bupivacaine has a longer duration of action in early labor compared with late labor.90
Regional Labor Analgesia
In addition to being the most effective form of labor analgesia,91 regional analgesia has other salutary effects on maternal and fetal well-being. Effective analgesia prevents maternal hyperventilation during labor. Hyperventilation and hypocarbia may decrease UBF.76 In addition, hypocarbia causes a left shift in the maternal oxyhemoglobin dissociation curve, causing oxygen to be more tightly bound and decreasing the transfer of oxygen across the placenta.92 Finally, hyperventilation during contractions may be followed by periods of maternal hypoventilation between contractions, resulting in maternal and fetal hypoxemia.13,93 Regional analgesia results in a decrease in maternal oxygen consumption.94 Compared to meperidine analgesia, epidural analgesia is associated with higher maternal oxygen saturation.95 Most parturients and fetuses are not harmed by this hyperventilation/hypoventilation cycle and increase in oxygen consumption. However, regional analgesia may be advantageous for fetuses with marginal uteroplacental circulation and function.96
The increase in cardiac output associated with labor is attenuated by regional analgesia.97 The regional analgesia-associated decrease in systemic vascular resistance (SVR) may be advantageous to pre-eclamptic parturients and fetuses.78,98
Regional analgesia results in attenuation of the stress response during labor. The normal increase in maternal plasma beta-endorphin concentrations is attenuated by epidural analgesia.99 Circulating norepinephrine and epinephrine levels decrease after regional analgesia,73,74 and this may have a beneficial effect on fetal heart rate patterns.75 Fetal catecholamines probably facilitate adaptation to extrauterine life and are not affected by regional analgesia.100,101 Blockade of adrenergic-induced lipolysis results in lower free fatty acid levels in women who receive regional analgesia.102 Finally, regional analgesia is associated with lower maternal,103,104 fetal, and neonatal lactic acid levels.18,105
REGIONAL ANALGESIA AND THE PROGRESS OF LABOR.
There has been much controversy recently as to whether regional labor analgesia adversely affects the progress of labor and method of delivery. Early investigators noted that regional analgesia appeared to be an effective treatment for dysfunctional labor.106–108 However, more recently, there has been concern that regional labor analgesia may prolong labor and increase the rate of operative delivery. Indeed, many retrospective studies show an association between regional analgesia, prolonged labor, and operative delivery. However, this does not necessarily imply a cause-and-effect relationship. Early, painful labor may be a marker for longer, more difficult labors. Independent of regional analgesia, women with early, and more severe, pain have a higher incidence of instrumental deliveries and abnormal fetal heart rate patterns.109
Retrospective studies have other limitations. Management of labor is not controlled. Many studies include nulliparous and parous parturients. Oxytocin administration is not standardized. Cesarean delivery rates differ markedly among institutions, obstetricians,110 and insured versus uninsured patients.111
Randomly assigned, prospective studies addressing the issue of whether regional analgesia increases the risk of cesarean delivery are difficult to perform. Blinding is impossible; therefore, standard labor management protocols must be rigidly followed or the study results may be biased. Regional labor analgesia is superior to other forms of analgesia, and therefore, a significant number of patients may cross over study groups. Despite these limitations, several prospective, randomly assigned studies have been done in the past decade. Two meta-analyses of seven to 10 studies that met inclusion criteria have been performed.112,113 All studies compared epidural analgesia to systemic opioid analgesia. Both meta-analyses reached the same conclusion: epidural analgesia did not increase the risk of cesarean delivery or instrumental vaginal delivery (Fig. 2). Retrospective studies comparing the rate of cesarean delivery immediately before and after the institution of on-demand regional analgesia provide another method of studying this question. In four such studies, dramatic increases in the rate of epidural analgesia had no impact on the rate of cesarean delivery for dystocia.114–117
Another concern is whether regional analgesia adversely affects the first stage of labor, particularly when administered in the latent phase. Prospective, randomly assigned studies comparing regional analgesia to systemic opioid analgesia both support118 and refute119 this possibility. Individual studies of labor duration may be difficult to compare because of the definition of when labor begins. Additionally, many studies use an inappropriate statistical method of data analysis (duration of labor data usually are not normally distributed but are treated as such in many studies).112 In the only studies that specifically addressed this issue, Chestnut and colleagues120,121 randomly assigned nulliparous patients to early (cervical dilation greater than 3 cm, less than 5 cm) versus late (cervical dilation 5 cm or greater) epidural analgesia. The late group received systemic nalbuphine until cervical dilation was 5 cm. The investigators found no difference in the length of the first stage of labor in parturients with spontaneous, induced, or augmented labor.
Similarly, study results differ as to whether epidural analgesia increases the need for oxytocin augmentation.118–122 Whether active management of labor was used may explain some of the variability in results.119 Fluid management before initiation of epidural analgesia may influence uterine activity.123,124 In addition, most studies compare epidural analgesia to systemic opioids. Opioid125 and other forms of analgesia also may influence the progress of labor.
Two meta-analyses of randomly assigned, prospective studies, with the primary outcome variable of cesarean delivery rate, concluded that epidural analgesia was associated with prolonged first stage of labor and oxytocin augmentation.112,113 However, there is no evidence that this modest prolongation of the first stage of labor adversely affects the mother or neonate.
A final question is the effect of regional analgesia on the second stage of labor. Again, studies are difficult to do and analyze for several reasons. Indications for instrumental vaginal delivery vary and are difficult to control. All regional analgesia is not the same. Epidural administration of less-concentrated local anesthetic solutions (often containing opioid) decreases maternal motor block and results in less fetal malrotation and fewer instrumental deliveries compared to more concentrated solutions.126–129 Some parturients are instructed to begin pushing when the cervix is fully dilated; others are instructed to begin pushing when they feel the urge or when the fetal head is on the perineum. Studies have shown that delayed pushing with epidural analgesia is associated with a higher incidence of spontaneous delivery.130–132
Most studies support the conclusion that the second stage of labor is prolonged in the presence of regional analgesia. The meta-analyses comparing parturients with regional analgesia to those without did not find a statistically significant increase in the risk of instrumental delivery, but the second stage of labor was longer. Indeed, ACOG defines a prolonged second stage differently in parturients with labor analgesia.44 Again, recent studies suggest that a longer second stage is, in itself, not harmful to mother or neonate as long as the fetal heart rate pattern is reassuring and there is ongoing descent of the fetal head.133
Lumbar epidural analgesia has been the mainstay of regional labor analgesia. Placement of an epidural catheter allows analgesia to be maintained until after delivery. Randomly assigned studies consistently show that pain scores are lower and patients are more satisfied with epidural analgesia compared to other forms of analgesia.91,118,120,121,134,135 There are few absolute contraindications to epidural analgesia. They include patient refusal or inability to cooperate, frank coagulopathy (see later), uncorrected maternal hypovolemia, soft tissue or skin infection at site of needle placement, frank sepsis without prior antibiotic administration, and increased intracranial pressure secondary to a mass lesion. The specific epidural analgesic technique should be tailored to individual patient needs. The risks and benefits of epidural analgesia should be discussed with each parturient, preferably early in the labor process.
Lumbar epidural analgesia is initiated in either the sitting or lateral position. The epidural space is located with a 17- or 18-gauge needle. A flexible catheter is passed through the needle approximately 4 cm into the epidural space. The needle is removed and the catheter is secured. A test dose may be administered to rule out intrathecal or intravascular catheter placement. Analgesia is initiated by bolus injection through the needle, catheter, or both. Analgesia can be maintained with intermittent bolus injections or a continuous infusion. The catheter is removed after delivery when there is no further need for analgesia/anesthesia.
The ideal regional analgesic technique would provide rapid onset of analgesia, effective analgesia with minimal motor block, minimal risk of maternal toxicity, minimal placental transfer and effects on the fetus, and long duration of action. Local anesthetics have been the mainstay of epidural analgesia for many years. There are advantages and disadvantages to using different local anesthetic agents. Bupivacaine is the most common local anesthetic used for labor analgesia. Low concentrations (0.125% or less) provide excellent analgesia with minimal motor block. Bupivacaine is highly protein bound with minimal placental transfer,136 and duration of analgesia is approximately 2 hours.137 However, it may take 20 minutes to reach peak effect, and the plasma concentration required for cardiac toxicity may decrease during pregnancy.60 Lidocaine has a faster onset than bupivacaine, but duration of analgesia is shorter. It is less protein bound and therefore has a higher umbilical vein/maternal vein ratio than bupivacaine.138 However, controlled studies have found no difference in neonatal neurobehavioral scores between epidural lidocaine, bupivacaine, and 2-chloroprocaine.139,140 2-chloroprocaine, an ester local anesthetic, has a rapid onset, but duration of analgesia is less than 45 minutes, limiting its routine use for labor analgesia. The half-life in maternal and fetal blood is less than 1 minute.141 It is most useful for rapidly extending epidural anesthesia for urgent operative delivery.
Two new amide anesthetic agents have become available in the United States in the past several years. Ropivacaine, a single S-enantiomer local anesthetic agent (older agents are racemic mixtures), is similar to bupivacaine in structure and pharmacodynamics.142 Compared to an equivalent concentration of bupivacaine, ropivacaine may be associated with less motor block.143 Levobupivacaine (the S-enantiomer of bupivacaine) is similar to bupivacaine but has less cardiotoxicity.144
Opioids often are added to local anesthetics for epidural analgesia. This allows for a decreased concentration of local anesthetic, thus decreasing motor block. Opioids administered into the epidural or subarachnoid space block visceral pain signals by binding to presynaptic and postsynaptic receptors in the dorsal horn of the spinal cord.145 They provide analgesia without sympathetic or motor block. Neuraxial opioids provide excellent analgesia for early labor, when the pain stimuli are primarily visceral. The addition of local anesthetics is necessary to block somatic stimuli later in labor.
Epidural opioids and local anesthetics interact synergistically to provide analgesia.146 Epidural opioids alone can provide moderate analgesia for early labor, but the necessary dose is accompanied by bothersome side effects (e.g., pruritus, nausea, vomiting, neonatal respiratory depression). Combining local anesthetics with opioids allows for effective analgesia while minimizing the undesirable side effects.147 In addition, onset of analgesia is more rapid and duration of analgesia is longer with the addition of opioid to local anesthetics.148
The lipid soluble opioids, fentanyl and sufentanil, often are used for epidural labor analgesia. They have a relatively rapid onset (5 to 10 minutes).148,149 Their short duration of action (60 to 90 minutes)148 is overcome by maintaining analgesia with a continuous epidural infusion. Doses commonly used for epidural analgesia initiation and maintenance have been shown to be safe for both the mother and neonate.150,151 In contrast, water-soluble morphine has a much slower onset (30 to 60 minutes) and a longer duration of action (12 to 24 hours).152,153 However, epidural morphine does not result in satisfactory labor analgesia.154
Epidural analgesia may be maintained with intermittent bolus injection or continuous epidural infusion. Patient-controlled epidural analgesia (PCEA) is a modification of both techniques. Continuous epidural infusions result in less need for bolus injections155–159 and increased patient satisfaction.159,160 Continuous infusion usually is accompanied by the gradual onset of perineal analgesia, obviating the need for a bolus dose at delivery. In studies comparing bolus injection to continuous infusion of bupivacaine, continuous infusion results in a higher total bupivacaine dose.155,159–161 However, the infusion of lower concentration-bupivacaine at a higher rate may result in similar analgesia with less motor block and no increase in total dosage.157,160
COMBINED SPINAL-EPIDURAL ANALGESIA.
Combined spinal-epidural analgesia (CSE) has become increasingly popular in the past decade. CSE is a method of initiating neuraxial labor analgesia. After identifying the epidural space with an epidural needle, the anesthesiologist passes a small gauge spinal needle through the epidural needle and punctures the dura. A low dose of opioid, or opioid combined with local anesthetic, is injected into the subarachnoid space (spinal dose). The spinal needle is withdrawn and an epidural catheter is threaded through the epidural needle. Analgesia is maintained via the epidural catheter, as with traditional epidural analgesia.
There are advantages and disadvantages to CSE compared with those of traditional epidural analgesia (Table 2). Initiation of analgesia with an intrathecal opioid provides excellent analgesia for early labor without motor block or a sympathectomy. This may be ideal for patients who wish to ambulate or for those who might poorly tolerate an acute sympathectomy. The effective opioid dose is significantly less than for systemic or epidural administration. Therefore, systemic absorption is minimal as are direct fetal effects.166 Another advantage is the rapid onset of analgesia. Complete analgesia occurs within 3 to 5 minutes.167 Finally, the addition of bupivacaine to a lipid-soluble opioid results in sacral analgesia within several minutes. Sacral analgesia is difficult to accomplish after a single lumbar epidural dose of local anesthetic. Therefore, CSE provides more complete analgesia for women in advanced stages of labor.
PDPH, postdural puncture headache.
There are several undesirable side effects of CSE. The incidence of pruritus is higher with intrathecal versus epidural opioids.168–170 Gastric emptying may be delayed more in patients who receive intrathecal versus epidural or systemic opioids.30 CSE may be associated with a slightly higher risk of postdural puncture headache (estimated excess rate of 3 in 1000).171
Another potential drawback of CSE is that it will be unclear for 1 to 2 hours after initiation of analgesia as to whether the epidural catheter is properly located in the epidural space. Therefore, if it is important to have a functioning epidural catheter in place (e.g., in the presence of a worrisome fetal heart rate pattern or an anticipated difficult airway), then CSE may not be the technique of choice.
Several potentially serious side effects of CSE are of concern. Fetal bradycardia not associated with maternal hypotension occurs after initiation of both epidural and CSE analgesia. Circulating maternal epinephrine levels decrease acutely after the initiation of neuraxial analgesia.73,74 Epinephrine stimulation of uterine β2 -adrenergic receptors results in uterine tocolysis. It has been hypothesized that an acute decrease in maternal epinephrine levels results in temporary imbalance of uterine tocolytic/tocodynamic forces, resulting in uterine hypertonus and a decrease in uterine perfusion. A prospective study comparing CSE to epidural analgesia found no difference in the incidence of fetal bradycardia (17%).172 However, Riley and coworkers173 suggested that the incidence of fetal bradycardia may be higher after CSE because of patient selection. CSE is more likely to be used in patients with more pain or in more advanced labor, and therefore changes in maternal epinephrine levels may be more profound. A large, retrospective review found no increase in emergency cesarean delivery rate in patients who received intrathecal sufentanil compared to patients who received no or systemic analgesia.174 Nitroglycerin has been used successfully to treat uterine hypertonus associated with the initiation of neuraxial analgesia.175
Respiratory depression or arrest has been reported after the intrathecal injection of sufentanil in laboring women.176 The risk appears to be increased when intrathecal opioids are administered before or after systemic opioids, and the risk is dose dependent.177
Finally, there have been several case reports of meningitis after CSE.171 It is unclear whether this is a reporting phenomena of a new technique or the risk of meningitis is increased with CSE.
In general, single-shot spinal analgesia is not useful for laboring patients because of its limited duration of action. It may be used in parturients who require analgesia/analgesia shortly before anticipated delivery. Continuous spinal analgesia currently is not practical for most parturients. The available catheters (essentially epidural catheters) require a large-gauge introducer needle and are therefore associated with an unacceptably high incidence of postdural puncture headache. However, the placement of a continuous spinal catheter is a management option in patients with inadvertent dural puncture with an epidural needle.
Continuous caudal epidural analgesia is used infrequently in the practice of modern obstetric anesthesia. It is technically more difficult to place a caudal catheter. Large volumes of local anesthetic are required for first-stage analgesia and result in higher maternal plasma concentrations of drug. There is a risk of needle/catheter misplacement and direct injection into the fetus.
OTHER DRUGS FOR REGIONAL ANALGESIA.
Other neuraxial drugs have been investigated in the search for the perfect analgesic technique. Epinephrine potentiates analgesia when added to epidural bupivacaine178,179 or spinal bupivacaine/sufentanil.180 Epinephrine enhances local anesthetic-induced motor blockade. Stimulation of spinal cord postsynaptic α2 receptors produces analgesia by a different (cholinergic) mechanism than local anesthetics or opioids.181 This may be one mechanism by which epinephrine enhances analgesia. Recent studies have investigated the role of epidural or spinal clonidine (α2 agonist) and neostigmine (anticholinesterase that increases spinal acetylcholine levels) in labor analgesia.182
Other Nerve Blocks
Neuraxial regional analgesia is the most effective and flexible analgesic technique for labor and delivery. However, some parturients may not be candidates for regional analgesia or may not want it. Other nerve blocks may provide acceptable, albeit less-flexible, analgesia (Table 3).
A paracervical block stops transmission of visceral afferent nerve impulses from the uterus and cervix through the paracervical, or Frankenhäuser's, ganglion. Advantages include excellent analgesia for the first stage of labor, before fetal descent, without somatic sensory or motor block. However, the block is not continuous and it does not relieve somatic pain caused by distension of the pelvic floor, vagina, or perineum.
The block is performed with the patient in lithotomy position with left uterine displacement. The obstetrician injects 5 to 10 mL of dilute local anesthetic solution by introducing the needle through the vagina into the left and right lateral vaginal fornix to a depth of 2 to 3 mm. Aspiration and incremental injection are recommended.
The choice of local anesthetic is controversial. In the United States, the manufacturers of bupivacaine state that it is contraindicated for paracervical block because it may be associated with a higher incidence of adverse neonatal outcome (see later).183 2-chloroprocaine has been advocated for paracervical block because its short intravascular half-life may limit adverse maternal and neonatal effects. Studies addressing this issue have too few patients to show a statistically significant improvement in outcome with 2-chloroprocaine.184,185 Lidocaine 1% is the most commonly used local anesthetic for paracervical blocks.
Serious maternal complications are uncommon. They include systemic local anesthetic toxicity from inadvertent intravascular injection or rapid systemic absorption, postpartum neuropathy secondary to direct sacral plexus trauma or from hematoma formation,186 or retropsoal or subgluteal abscesses.187
Serious perinatal complications, including death, may result from paracervical block. Inadvertent direct fetal scalp injection can result in fetal systemic local anesthetic toxicity.188 This seems more likely to occur with advanced cervical dilation (greater than 8 cm).189
Fetal bradycardia is the most common fetal complication. It usually develops within 2 to 10 minutes after injection and resolves within 5 to 10 minutes. The reported incidence ranges from 0% to 70%.189 The etiology of fetal bradycardia is unclear. Most likely, fetal bradycardia results from a decrease in uteroplacental or fetoplacental perfusion secondary to local anesthetic-induced uterine hypertonus190 or uterine/umbilical artery vasoconstriction.191
Recommendations to reduce complications include the following:
LUMBAR SYMPATHETIC BLOCK.
A paravertebral lumbar sympathetic block interrupts transmission of visceral afferent nerve impulses from the uterus and cervix at the level of the L-2 to L-3 sympathetic chain. Similar to a paracervical block, it provides analgesia for the first stage but not the second stage of labor. Disadvantages include the following:
Advantages include the following:
The pudendal nerve, which includes somatic fibers from S-2, S-3, and S-4, is the primary source of sensory innervation of the lower vagina, vulva, and perineum. It also provides motor innervation to the perineal muscles and the external anal sphincter. Thus, a pudendal block alleviates pain during the second stage of labor resulting from vaginal, vulvar, and perineal distention. It provides -satisfactory analgesia for spontaneous vaginal and low- or outlet-forceps delivery, but not mid-forceps delivery or exploration of the upper vagina, cervix, or uterine cavities. Bilateral success rate may be as low as 50%.195
The pudendal nerve can be blocked via the transperineal or transvaginal route. Most obstetricians in the United States use the transvaginal route immediately before delivery. However, earlier pudendal nerve blocks (just before or after complete cervical dilation) provide better analgesia and do not increase the incidence of instrumental delivery.196 This also allows for a repeat block, should an initial block fail.
The transvaginal pudendal block is performed through a needle guide. The needle is introduced through the vaginal mucosa and sacrospinous ligament, just medial and posterior to the ischial spine. Usually, 7 to 10 mL of local anesthetic solution is injected on each side. The pudendal artery lies close to the nerve; therefore, aspiration before and during injection is imperative. Rapid maternal absorption of local anesthetic occurs, resulting in peak levels in maternal venous and fetal scalp capillary blood between 10 and 20 minutes after injection.196 Lidocaine and 2-chloroprocaine often are used, with the same advantages and disadvantages as for paracervical block. Dilute concentrations (lidocaine 1% or 2-chloroprocaine 2%) are preferable, thus minimizing the risk of toxic maternal concentrations of local anesthetic.
Maternal and fetal complications of pudendal nerve block are rare. Maternal complications include 1) local anesthetic toxicity from direct intravascular injection or absorption of an excessive dose of local anesthetic; 2) vaginal, ischiorectal, or retroperitoneal hematomas197; or 3) subgluteal or retropsoal abscesses.187 Fetal complications include fetal trauma, direct fetal injection, of local anesthetic, or both.198
Perineal infiltration often is used immediately before delivery to provide anesthesia for an episiotomy or repair. It provides no motor relaxation. Several milliliters of local anesthetic are injected into the posterior fourchette. In one study, after a mean dose of 79 plus or minus 3 mg lidocaine, rapid maternal absorption and fetal transfer resulted in significantly higher fetal/maternal lidocaine ratio than that reported after paracervical, pudendal, or epidural blocks.199 In contrast, 2-chloroprocaine was not detected in neonatal plasma at delivery.200 The complication associated with perineal infiltration is direct injection of local anesthetic into the fetal scalp, resulting in neonatal local anesthetic toxicity.201
INTRAMUSCULAR/INTRAVENOUS OPIOID ADMINISTRATION.
Systemic opioid analgesia is widely used, either as the sole analgesic agent or before administration of regional labor analgesia. Systemic opioids provide some analgesia, particularly in early labor. Analgesia is incomplete for active labor. There is little scientific evidence that any one opioid is better than another. All have similar maternal and fetal side effects that are dose related. Maternal side effects include nausea, vomiting, delayed gastric emptying, dysphoria, and respiratory depression. All opioids cross the placenta. In utero, opioids may result in a slower fetal heart rate and decreased beat-to-beat variability. The likelihood of neonatal respiratory depression depends on the dose and timing of administration. Opioids may be administered by the subcutaneous, intramuscular, or intravenous route.
PATIENT-CONTROLLED INTRAVENOUS ANALGESIA.
Patient-controlled intravenous analgesia (PCA) has been used for labor analgesia.202 Although PCA may result in higher patient satisfaction than nurse-administered opioid analgesia, studies have not shown reduced drug use or improved analgesia. Meperidine,203 nalbuphine,204 and fentanyl205 PCA analgesia have been reported. No studies have adequately addressed optimal dosing, lockout periods, and continuous infusion-PCA versus PCA alone. Several recent case reports document the use of a new opioid, remifentanil, PCA in parturients.206,207 Remifentanil has the theoretical advantage of rapid onset and offset.
The administration of inhalation analgesia for labor and vaginal delivery is unusual in the United States but is more common outside the United States. The only inhaled anesthetic agent still commonly used for labor analgesia is nitrous oxide; 50% nitrous oxide provides significant analgesia in approximately 50% of laboring parturients.208 The intermittent use of nitrous oxide during labor results in negligible fetal accumulation and does not depress neonatal respiration or neurobehavioral scores.209 Several studies suggest that the concomitant use of nitrous oxide and systemic opioids may increase the risk of maternal hypoxemia.210,211 Safe administration of nitrous oxide necessitates an apparatus that limits the concentration of nitrous oxide, so a hypoxic mixture cannot be delivered. In addition, a mask or mouthpiece with a one-way valve should be used to limit pollution from unscavenged gases. Even so, environmental pollution from unscavenged gas may be significant.212
Volatile halogenated anesthetic agents have been used for labor analgesia. They are not routinely used for labor analgesia because of incomplete analgesia, concern about environmental pollution, and potential loss of maternal protective airway reflexes.213
Other Forms of Analgesia
Nonpharmacologic analgesic techniques include childbirth education, emotional support, massage, therapeutic use of hot and cold, and hydrotherapy. Techniques that require specialized training or equipment include biofeedback, transcutaneous electrical nerve stimulation (TENS), acupuncture or acupressure, and hypnosis.
Childbirth education is widely practiced. Studies of childbirth education tend to lack scientific rigor.214,215 Some investigators have found decreased use of pharmacologic analgesia, shorter labor, decreased incidence of operative deliveries, and fetal distress, whereas other investigators found no change in these variables.216
Emotional support often is provided by the parturient's husband or a friend. Several studies have documented salutary effects on the length of labor, analgesic requirements, and the incidence of operative deliveries when doulas (an unfamiliar support person not part of the medical establishment) are present during labor and delivery.217–219 Studies of biofeedback, TENS, acupuncture, and hypnosis do not show consistent benefits beyond other childbirth education techniques.216
|ANESTHESIA FOR CESAREAN DELIVERY|
The anesthesia-related maternal mortality is 17 times higher for general anesthesia compared with that of regional anesthesia.32 Most obstetric anesthesiologists recommend that regional anesthesia should be administered whenever possible and that general anesthesia for cesarean delivery be used only when absolutely necessary.220,221 During the past several decades, the use of regional anesthesia has increased, and anesthesia-related maternal morbidity has decreased.32,222
Spinal anesthesia is indicated for most elective cesarean deliveries and for urgent cesarean deliveries in patients without pre-existing epidural analgesia. The advantages of spinal compared to epidural anesthesia include the following (Table 4):
PDPH, postdural puncture headache.
Disadvantages include the following:
With the advent of pencil-point spinal needles, the incidence of postdural puncture headache is approximately equal between spinal and epidural anesthesia.223
Spinal anesthesia may be initiated in the sitting or lateral decubitus position. A high thoracic (T4) sensory level is necessary to block visceral stimulation. Local anesthetic choice depends on the desired duration of action. Opioids often are added to the local anesthetic. Lipid-soluble opioids (e.g., fentanyl and sufentanil) may decrease the incidence of intraoperative nausea and vomiting and increase the duration of anesthesia.224 Nausea and vomiting during cesarean delivery frequently are associated with peritoneal traction and exteriorization of the uterus. Opioids may help block this visceral response. Morphine often is added to the spinal anesthetic solution for postoperative analgesia (see later).
Epidural anesthesia frequently is used for parturients with indwelling epidural catheters placed for labor analgesia. The absolute dose of local anesthetic used for epidural anesthesia is five to 10 times greater than for spinal anesthesia; therefore, the epidural dose must be injected incrementally to rule out intrathecal or intravascular placement of the catheter. Intrathecal injection of an epidural dose results in total spinal anesthesia, and intravascular injection of an epidural dose results in systemic local anesthetic toxicity.
The advantage of epidural anesthesia is that the local anesthetic dose can be titrated to the desired sensory level. Theoretically, it should be easier to extend a “low” block or prevent a “high” block. However, the incidence of “patchy” blocks is increased with epidural compared with that of spinal anesthesia.
In some situations, the slow onset of epidural anesthesia may be a benefit (e.g., patients with cardiac disease). Epidural anesthesia results in a less-dense motor block than spinal anesthesia. This may be advantageous in patients with pulmonary disease who rely on abdominal and intercostal muscle tone for ventilation and clearing of secretions. Block duration can be extended by reinjecting the epidural catheter. Finally, postoperative analgesia can be accomplished by continuous or incremental injection of the epidural catheter.
The choice of local anesthetic for epidural anesthesia depends on the desired onset time and duration of action. 2-chloroprocaine (3%) often is used for urgent cesarean deliveries because its onset time is quickest (5 to 10 minutes if epidural analgesia is already established). However, its duration of action is only 40 to 50 minutes, and it may interfere with subsequent morphine analgesia.225 Lidocaine 2% with epinephrine 1:200,000 is the workhorse of epidural anesthesia. It has an intermediate-onset time (15 minutes,226 faster in presence of labor analgesia) and intermediate duration of action (75 to 100 minutes).226 Sodium bicarbonate may be added to lidocaine to speed the onset time.227 Bupivacaine has a longer onset time and greater risk of cardiac toxicity. Although it has a longer duration of action, bupivacaine rarely is necessary for a routine cesarean delivery. The presence of the epidural catheter allows lidocaine to be reinjected should the procedure outlast the initial dose. Ropivacaine also has a long onset time and duration. Although local anesthetics cross the placenta during administration of epidural anesthesia, most studies show that there is no significant effect on neonatal neurobehavioral scores.140,228
COMBINED SPINAL-EPIDURAL ANESTHESIA.
Combined spinal-epidural anesthesia can be used several ways for cesarean delivery anesthesia. CSE anesthesia combines the advantages of spinal anesthesia (rapid onset of a dense block) with those of epidural anesthesia (ability to prolong the block). Anesthesia may be initiated with a standard spinal dose, followed by placement of the epidural catheter.231 The catheter is injected if the block needs to be prolonged. Alternatively, a “low” spinal dose can be injected, and the remaining initiation dose is injected through the epidural catheter. Proponents of this technique suggest that it is associated with a lower incidence of hypotension.232
General anesthesia for cesarean delivery is indicated any time regional anesthesia is contraindicated in the setting of dire fetal distress or failed regional anesthesia. There are no significant differences in umbilical cord pH and gas measurements between neonates exposed to general versus regional anesthesia for elective233 or emergency cesarean delivery.234 During regional anesthesia, as long as hypotension is prevented or treated, a prolonged incision to delivery time does not adversely affect neonatal acid-base status or Apgar scores. In contrast, during general anesthesia, a prolonged incision to delivery time may result in a greater incidence of neonatal acidosis and low 1-minute235 and 5-minute236 Apgar scores. However, despite a higher incidence of newborn neonatal depression, there is no difference in ultimate neonatal outcome.236
The general anesthetic technique for cesarean delivery is straightforward. The airway is examined and aspiration prophylaxis is administered. The parturient is placed in left-lateral tilt and oxygen is administered by face mask. If a difficult airway is anticipated, the parturient should be intubated awake before the induction of anesthesia. After preparations for surgery are completed, anesthesia is induced with a sedative-hypnotic agent, usually thiopental, followed by a neuromuscular blocking agent, usually succinylcholine. Because of the risk of aspiration, a rapid-sequence induction is performed (i.e., the patient is not ventilated until the endotracheal tube is in place). This prevents introduction of air into the stomach. Anesthesia is maintained with inhalation anesthetic (volatile anesthetic agent with or without nitrous oxide) until delivery of the neonate. After delivery, intravenous agents (e.g., opioids and benzodiazepines) usually are administered as the concentration of volatile agent is decreased. The volatile gases are smooth muscle relaxants and may cause uterine atony. The patient should have recovered protective airway reflexes before extubation, as patients also are at risk of aspiration on emergence from anesthesia.
Postcesarean Delivery Analgesia
The optimal postcesarean delivery analgesic regimen would provide profound analgesia, have no maternal or neonatal side effects, and have a low cost. Multiple studies have shown superior analgesia with neuraxial versus systemic (intramuscular237–240 or intravenous237,238,240–244) opioid administration. The total opioid dosage is significantly less, and mothers are significantly less sedated.238–240,242,244 In a study of nonobstetric, morbidly obese patients undergoing abdominal surgery, epidural morphine analgesia resulted in superior analgesia, less sedation, fewer pulmonary complications, and earlier return of bowel function compared with those of intramuscular morphine.245 In post-cesarean delivery patients, a single-shot neuraxial opioid technique was associated with lower costs compared with PCA.237
Neuraxial opioids may be administered by multiple techniques. Intrathecal morphine often is administered as a single-shot technique at initiation of spinal anesthesia. Epidural morphine may be administered as a single bolus after delivery. The duration of action of intrathecal or epidural morphine generally is 12 to 24 hours.246 Diamorphine has been used outside the United States.247,248 Single bolus techniques have the advantage of being less cumbersome (the patient is not connected to a continuous infusion) and cheaper (no infusion pump is necessary). The downside of single bolus techniques is that the dose is not titratable to the degree of pain. Administering a higher dose may provide more satisfactory analgesia to all patients but at the cost of a higher incidence of undesirable side effects. In addition, the risk of late respiratory depression is greater with single bolus injection of hydrophilic opioids (see later).246
Side effects of neuraxial opioids include pruritus, nausea and vomiting, sedation, and respiratory depression. Most studies have found that pruritus occurs more commonly with neuraxial compared to systemic opioids.237,238,240,244 The incidence of nausea and vomiting is similar between the two techniques.238,239,241–244
Respiratory depression occurs after intramuscular, intravenous, and neuraxial opioid administration.251 Early respiratory depression (within 30 to 90 minutes of administration) occurs after the neuraxial administration of all opioids secondary to systemic absorption and transport of the drug via the circulatory system to the brain-stem respiratory centers.252 Late respiratory depression (6 to 10 hours after administration) occurs after a neuraxial bolus dose of morphine.253 Morphine is hydrophilic and circulates in the CSF to the brain stem. Risk factors for respiratory depression include advanced age and morbid obesity.246,254
Neuraxial opioid analgesia may be supplemented with systemic agents. The current trend is toward multimodal analgesic therapy. This allows the use of lower neuraxial opioid doses, resulting in fewer side effects. Analgesia can be supplemented with nonsteroidal anti-inflammatory agents,255–259 acetaminophen,258 or low-dose intravenous PCA-opioid administration.
Neuraxial analgesia is not practical or desirable for some patients. Studies comparing intramuscular to intravenous-PCA opioid administration after cesarean delivery found a comparable degree of analgesia.238,240,260 However, PCA provided faster pain relief, fewer side effects, and greater patient satisfaction than intramuscular administration. In a study comparing intravenous-PCA meperidine to intravenous-PCA morphine after epidural morphine, PCA meperidine was associated with neonatal neurobehavioral depression.261
PCA can be administered with or without a continuous basal (background) infusion. In gynecologic surgery patients, a continuous background infusion did not improve pain relief or sleep but resulted in an increased total dosage and an increase in severity of side effects.262,263 The inherent safety of the PCA technique (patients only receive as much opioid as they need) is diminished if a continuous infusion is added.264 Therefore, a basal-PCA infusion is not recommended for patients recovering from cesarean delivery.265
|ANESTHESIA FOR SPECIAL CIRCUMSTANCES (TABLE 5)|
DIC, disseminated intravascular coagulation; HIV, human immunodeficiency virus; HSV-2, herpes simplex virus type 2; CNS, central nervous system; SVR, systemic vascular resistance.
Operative Vaginal Delivery
Dense sacral analgesia/anesthesia and perineal muscle relaxation facilitate operative delivery, particularly the application of forceps. This can be accomplished by extending segmental lumbar epidural analgesia or via spinal or CSE anesthesia, caudal epidural anesthesia, or a pudendal block.
Regional labor analgesia for planned breech vaginal delivery offers several benefits, including inhibition of early pushing, relaxation of the pelvic floor and perineum, and the option of extending anesthesia for emergency cesarean delivery. Early pushing may increase the risk of umbilical cord prolapse and delivery of a lower extremity before full cervical dilation. A relaxed pelvic floor at delivery facilitates delivery of the after-coming head. The challenge to the anesthesiologist is to provide enough first-stage analgesia to inhibit early pushing but minimize second-stage motor block to facilitate spontaneous delivery of the infant to the umbilicus and to rapidly extend the perineal block density at delivery to minimize head entrapment in the perineum and facilitate the application of forceps. There are no prospective, randomly assigned studies comparing maternal and neonatal outcome in parturients who receive regional analgesia compared to those who do not.
The fetal head may be entrapped in the perineum or more often in the cervix. Perineal skeletal muscle relaxation may be achieved with regional anesthesia or by the administration of a neuromuscular blocking agent (usually succinylcholine) during the induction of general anesthesia. Cervical smooth muscle relaxation traditionally has been accomplished with the rapid sequence induction of general anesthesia, followed by the administration of a high concentration of a volatile anesthetic agent. More recently, intravenous or sublingual nitroglycerin has been used for uterine relaxation.266–269
Regional analgesia has several benefits in multiple gestation. Effective regional analgesia facilitates internal podalic version and total breech extraction of Twin B. Anesthesia can be extended for an emergency cesarean delivery, thus avoiding the risk of general anesthesia.
Vaginal Trial of Labor After Cesarean Delivery
In the past, there has been concern that regional labor analgesia for vaginal trial of labor after cesarean delivery would mask the pain of uterine rupture. In addition, it was thought that the sympathectomy associated with neuraxial analgesia would prevent reflex hemodynamic compensation in the event of uterine rupture and hemorrhage. However, more recent data support the safety of neuraxial analgesia for vaginal trial of labor.
Several studies have found that abdominal pain or uterine tenderness has low sensitivity and specificity as a marker for uterine rupture or scar dehiscence.270,271 There are reports of women with effective continuous epidural analgesia who acutely had breakthrough pain develop secondary to uterine rupture.272–275 Most cases of lower uterine segment scar dehiscence do not result in severe maternal hemorrhage.276 Epidural analgesia did not affect the outcome of the trial of labor in several studies.277,278 Finally, many women may agree to a trial of labor only if guaranteed effective analgesia.
In conclusion, most obstetricians and anesthesiologists and the ACOG279 agree that neuraxial analgesia is appropriate for vaginal trial of labor after cesarean delivery. Most anesthesiologists elect to maintain analgesia with dilute solutions of local anesthetic and opioid so that the pain of uterine rupture is not masked.
Several aspects of pre-eclampsia/eclampsia are of particular concern to anesthesiologists. These include (1) coagulation deficits that might preclude regional anesthesia/analgesia, (2) airway edema, and (3) interaction of magnesium with anesthetic techniques and agents.
Approximately 10% of pre-eclampsia/eclamptic parturients have a platelet count of less than 100,000/mm3.280 Several investigators have described prolonged bleeding times in both thrombocytopenic and nonthrombocytopenic patients, presumably secondary to platelet dysfunction.281,282 There are no data to support the use of a specific platelet count, above which neuraxial analgesia/anesthesia is “safe.” Epidural hematoma is a rare complication of regional anesthesia. Almost all anesthesiologists initiate neuraxial anesthesia if the platelet count is above 100,000/mm3.283 A platelet count of 50,000/mm3 seems to be the absolute lower limit practiced by most anesthesiologists. A review of systems and physical examination are both necessary to rule out a bleeding diathesis.
Also of concern to anesthesiologists is the parturient with a rapidly falling platelet count. In this circumstance, an epidural catheter may be placed several hours before the patient requires analgesia, before the platelet count is unacceptably low. This also allows an analgesic-free period to observe for symptoms of epidural hematoma. Finally, in parturients with falling platelet counts, a platelet count should be determined immediately before removal of the epidural catheter, as epidural hematoma formation has been described on epidural catheter removal.284
A small percent of pre-eclampsia/eclamptic parturients have coagulation factor deficits. If the platelet count is greater than 100,000/mm3, determination of prothrombin time (PT) and activated partial thromboplastin time is not necessary.285
Most parturients with pre-eclampsia/eclampsia in the United States receive magnesium sulfate therapy. Magnesium sulfate results in more profound maternal hypotension after the initiation of regional anesthesia to the T10 level in gravid ewes.286 In addition, magnesium potentiates the action of neuromuscular blocking agents.287 Finally, magnesium blunts the response to vasoconstrictors.
Epidural analgesia may be the preferred method of labor analgesia for pre-eclampsia/eclamptic parturients for several reasons.288 First, pre-eclamptic parturients have an exaggerated hypertensive response to catecholamines and pain. This is blunted by effective epidural analgesia, thus blood pressure control is facilitated.289 Second, epidural analgesia may improve intervillous blood flow.78 Third and finally, pre-eclamptic parturients are at increased risk of urgent cesarean delivery compared with that of normotensive parturients. The early initiation of epidural analgesia allows for epidural anesthesia (thus avoiding general anesthesia) should an urgent/emergent cesarean delivery be necessary.
General anesthesia for cesarean delivery in the pre-eclamptic parturient is associated with significant increases in systemic and pulmonary artery pressures289 and a decrease in intervillous blood flow.83 Compared to general anesthesia, regional anesthesia is associated with a more stable hemodynamic profile and a decrease in the level of stress hormones. Epidural anesthesia also avoids transient neonatal depression associated with general anesthesia. Finally, epidural anesthesia avoids manipulation of the airway with the increased risk of difficult airway because of pharyngeal edema.290
Controversy exists as to whether spinal anesthesia is appropriate for the severely pre-eclamptic parturient. Opponents of spinal anesthesia argue that the rapid sympathectomy may result in more profound hypotension in the volume-constricted, severely pre-eclamptic patient and that hypotension is less likely to be tolerated by a fetus with chronic uteroplacental insufficiency. However, in a retrospective study of 103 severely pre-eclamptic patients undergoing cesarean delivery, Hood and Curry291 showed equal maternal and neonatal outcomes in severely pre-eclamptic parturients who received spinal versus epidural anesthesia.
The anesthetic implications of placenta previa revolve around the risk of maternal hemorrhage and regional anesthesia induced sympathectomy. Hemorrhage was associated with significantly worse maternal hypotension, UBF and fetal oxygenation in pregnant ewes with epidural anesthesia compared to control animals, unless the anesthetized ewes received prompt fluid resuscitation.292 Most obstetric anesthesiologists will consider regional anesthesia for primary cesarean delivery in parturients who are not actively bleeding or hypotensive.293 The risk of placenta accreta increases markedly in patients with placenta previa and a previous uterine scar.294 Controversy exists regarding the appropriate type of anesthesia for patients at high risk of placenta accreta and Cesarean hysterectomy. Arguments in favor of regional anesthesia include several series in the literature that support the safety of this technique.295–297 In a series of 25 patients scheduled for elective Cesarean hysterectomy, 7 patients required the induction of general anesthesia after initial regional anesthesia because of patient discomfort or inadequate operating conditions.295 Arguments against the use of regional anesthesia include the potential for more severe maternal hypotension associated with regional anesthesia, patient discomfort associated with intraperitoneal manipulation and traction, and patient discomfort associated with a prolonged surgical procedure. In addition, airway edema markedly worsens during volume resuscitation, thus increasing the risk of a difficult airway if urgent endotracheal intubation is necessary.298 In the circumstance of placenta previa and acute hemorrhage, general anesthesia is indicated.
The anesthetic considerations in patients with placental abruption are similar to those with placenta previa. An additional consideration is the presence of disseminated intravascular coagulation (DIC). Regional anesthesia is contraindicated in the presence of DIC. Patients who are hemodynamically stable without ongoing hemorrhage and without a coagulopathy are candidates for regional analgesia/anesthesia. General anesthesia is indicated for acute hemorrhage or in the presence of DIC.
The anesthetic considerations in postpartum hemorrhage are hemodynamic stability and maternal blood volume. For example, for repair of genital trauma, if the mother is hemodynamically stable and euvolemic, then extension of epidural analgesia or initiation of regional anesthesia is appropriate. However, in the face of hypovolemia and hemodynamic instability, general anesthesia is the anesthetic of choice.
Intrauterine Fetal Demise
Intrauterine fetal demise can be associated with coagulation deficits for several reasons. Thromboplastin released by the dead fetus may lead to a consumptive coagulopathy, although this is unusual unless the fetus has been dead for more than a month.299 Approximately 15% of fetal deaths can be attributed to placental abruption, and abruption can be associated with DIC.300 Therefore, before initiating regional analgesia in women with fetal demise, a disseminated coagulation profile may be indicated.301
Maternal Substance Abuse
Many drugs, both legal and illegal, have the potential to adversely impact the anesthetic care of a parturient.
There are increased risks to both general and regional anesthesia in cocaine-abusing parturients. General anesthesia is associated with a higher risk of severe hypertension and dysrhythmia.302 In an animal model, acute cocaine intoxication and general anesthesia also resulted in myocardial ischemia.303 Chronic cocaine use is associated with an increased sensitivity to inhalation anesthetic agents, whereas acute toxicity may increase the requirement for anesthetic agents.304
There are several considerations when choosing regional analgesia/anesthesia. Cocaine abuse is associated with thrombocytopenia.305 The platelet count should be determined before initiating neuraxial analgesia/anesthesia. After neuraxial blockade, the degree of hypotension is more profound.306 Because ephedrine is both an indirect and a direct acting vasopressor, it is less effective in treating hypotension in the cocaine-positive parturient.307 Finally, cocaine-abusing parturients often report inadequate analgesia despite apparent adequate regional anesthesia.304
Acute and/or chronic amphetamine abuse complicates the administration of both general and regional anesthesia. As with cocaine, general anesthesia is associated with hypertension and dysrhythmia. Acute ingestion increases anesthetic requirements, whereas chronic use decreases anesthetic requirements.308 Regional anesthesia may be associated with severe hypotension and unpredictable response to vasopressors. There are two reported cases of cardiac arrest after general309 and regional310 anesthesia for cesarean delivery.
Chronic opioid use affects the administration of analgesia/anesthesia in several ways. The parturient's baseline opioid use should be calculated and administered to prevent maternal and neonatal withdrawal. Treatment of labor pain or postoperative pain requires higher doses of opioid. Regional analgesic/anesthetic techniques may be advantageous for the opioid addict because analgesia can be provided without opioids. As with cocaine abuse, apparently adequate regional anesthesia may not provide acceptable pain relief in some opioid users.311
Alcohol abuse is associated with multiorgan dysfunction and with multiple anesthetic implications beyond the scope of this discussion. Liver disease may be associated with a coagulopathy; therefore, this must be assessed before initiation of regional analgesia/anesthesia.
There are few studies of the anesthetic management of diabetic parturients. In nonpregnant diabetics with autonomic neuropathy, general anesthesia is associated with higher vasopressor use.312 Gastroparesis, another manifestation of autonomic dysfunction, may increase the risk of pulmonary aspiration. Maternal catecholamine levels increase markedly during labor. Theoretically, these changes in catecholamine levels could influence the activity of insulin. No study has addressed this issue adequately.313
Although preterm infants probably are more vulnerable than the term infant to the effects of drugs used for analgesia and anesthesia, the direct drug effects are far less important than preventing neonatal asphyxia and providing for an atraumatic delivery.314 Good maternal anesthetic care generally provides for the best possible neonatal outcome.
There has been much debate about the use of regional analgesia/anesthesia in infected patients. The classic teaching has been that regional anesthesia is relatively contraindicated in patients at risk for bacteremia. However, epidural abscess and meningitis associated with regional anesthesia are rare events. Four reviews of more than 500,000 obstetric patients who received epidural analgesia/anesthesia found two cases of epidural abscess and no cases of meningitis.315–318 Given the frequency of fever and bacteremia (1% of laboring patients in one study319), it is likely that some of these patients were bacteremic.
Several retrospective studies have not found any cases of epidural abscess or meningitis in parturients with chorioamnionitis.320–322 However, given the low incidence of neuraxial infection, taken together, these studies are not large enough to rule out the possibility that the risk of neuraxial infection after regional anesthesia is increased in parturients with chorioamnionitis.
An additional issue is the difficulty in identifying which patients with chorioamnionitis are bacteremic. Fever and leukocytosis do not differentiate patients with positive and negative blood cultures.319–321 In an Escherichia coli bacteremic rat model, investigators found that administration of antibiotics before initiation of dural puncture protected the rat from the development of meningitis.323
The current general consensus of obstetric anesthesiologists is that regional analgesia/anesthesia may be administered to parturients with chorioamnionitis after the systemic administration of antibiotics, unless they appear overtly septic.324
HUMAN IMMUNODEFICIENCY VIRUS.
Human immunodeficiency viral (HIV) particles can be isolated from CSF at the time of initial infection.325 Therefore, there is no chance that regional analgesia/anesthesia will introduce CNS HIV infection, as this has already occurred. In a small group of 18 HIV-positive patients who received regional anesthesia, there was no evidence of accelerated disease progression or increased infectious or neurologic complications.326
Protease inhibitors (saquinavir, indinavir, nelfinavir, and ritonavir) are metabolized by the CYP3A isoenzyme of the liver microsomal cytochrome P-450 system. All the protease inhibitors, but particularly ritonavir, competitively inhibit this enzyme. Many CNS depressants also are metabolized by this isoenzyme. Hence, plasma levels are markedly elevated and the clinical effect and side effects are markedly pronounced. Drugs that fall in this category include fentanyl, meperidine, diazepam, midazolam, and methylergonovine.327
HERPES SIMPLEX VIRUS.
Patients with symptomatic herpes simplex virus type 2 (HSV-2) often are admitted for cesarean delivery. Primary HSV-2 infection is associated with transient viremia, but recurrent infections are not.328 Retrospective studies have found no sequelae after regional anesthesia in patients with symptomatic HSV-2.329–332 However, few patients in these studies had primary infections. There is a general consensus among anesthesiologists that regional anesthesia is safe for parturients with secondary HSV-2 infections. There is not enough data to support the safety of regional anesthesia in parturients with primary infections. Therefore, the theoretical risk of introducing virus into the CNS must be weighed against the risk of general anesthesia.324
The diagnosis and treatment of congenital heart disease have improved markedly over the past several decades, leading to an increase in the number of pregnant women with congenital heart disease. Regional analgesic/anesthetic techniques that induce an acute sympathectomy acutely decrease SVR and myocardial preload. This acute decrease in SVR or preload or both is not well-tolerated in women with stenotic valvular lesions, asymmetric septal hypertrophy, or right-to-left shunts. In contrast, patients with left-to-right shunts and regurgitant valvular lesions often benefit from a decrease in SVR and preload. Intrathecal opioid administration (for labor analgesia) or slow epidural administration of local anesthetic (for labor analgesia or cesarean delivery) is ideal for patients who will not tolerate acute decreases in SVR or preload. Single-shot spinal anesthesia is contraindicated in these patients.333
Regional labor analgesia is beneficial to patients with ischemic heart disease. Effective analgesia decreases maternal circulating catecholamine levels,73,74 thus preventing tachycardia, and increases in myocardial work.
Many parturients with cardiovascular disease remain at risk of having cardiovascular complications (e.g., congestive heart failure) in the first 12 to 24 hours after delivery. These patients may benefit from the sympathectomy and analgesia afforded by the continuous infusion of epidural local anesthetics in the immediate postpartum period.333
Regional anesthesia is contraindicated in the presence of a coagulopathy. Spinal hematoma may result in permanent neurologic damage if not diagnosed and decompressed within 6 to 12 hours after onset of symptoms.284 Unfortunately, it often is difficult to assess whether patients with abnormal laboratory coagulation parameters have a clinical coagulopathy. In general, an international normalized ratio greater than 1.5334 or a prolonged activated partial thromboplastin time is considered a contraindication to regional anesthesia.335 Platelet count and function are discussed in the section on pre-eclampsia.
The American Society of Regional Anesthesia recently published Consensus Statements on neuraxial anesthesia and anticoagulation.336 In summary:
These same parameters apply for removing an epidural catheter as epidural hematomas can occur immediately after catheter removal.284
Obesity accentuates many of the physiologic and anatomic changes of pregnancy. For example, there is less pulmonary reserve because FRC is smaller and oxygen consumption is higher than the nonobese parturient. Similarly, cardiac output and blood volume are increased in obesity. There is a higher incidence of difficult airway (i.e., inability to intubate the patient). Obesity increases the risk of anesthesia-related maternal mortality during cesarean delivery.337,338 Early labor epidural analgesia is recommended. Epidural analgesia did not effect the outcome of labor in obese parturients weighing more than 300 pounds.339 Finally, most anesthetic procedures, including simply measuring noninvasive blood pressures, are technically more challenging in the obese parturient.
Inability to intubate is a major cause of anesthesia morbidity and mortality. In the United Kingdom, the incidence of failed intubation was estimated at one in 2230 in the nonobstetric population6 but was one in 280 to 300 in the obstetric population.6,340 In the United States, the majority of anesthesia-related maternal mortality occurred during cesarean delivery and 49% were caused by a difficult airway.32 Overall, anesthesia-related mortality decreased in the triennium 1988 to 1990 compared to the triennium 1979 to 1982, presumably because the use of regional anesthesia increased. However, the risk of general anesthesia is unchanged.32
Several changes occur during pregnancy that may contribute to a difficult airway and its consequences. There is an increase in pharyngeal edema, made worse by pre-eclampsia and prolonged pushing efforts during the second stage. Weight gain also contributes to an increase in pharyngeal soft tissue. Enlarged breasts make the insertion of a standard laryngoscope handle difficult. Finally, a decrease in FRC and an increase in oxygen consumption lead to more rapid arterial oxygen desaturation during apnea.19
Risk factors for a difficult airway include emergency surgery, obesity, short neck, missing or protruding maxillary incisors, receding mandible, facial edema, and a swollen tongue.341 Ideally, the potentially difficult airway should be identified as early as possible. ACOG suggests that obstetricians be alert to the presence of risk factors and refer the patient for antenatal consultation by an anesthesiologist.342 Epidural analgesia is recommended for the laboring parturient with a potentially difficult airway, so that emergency general anesthesia can be avoided. If general anesthesia is necessary for a parturient with a recognized, potentially difficult airway, the airway should be secured before the induction of general anesthesia while the patient is awake and spontaneously breathing.
If failure to intubate occurs after the induction of anesthesia, a difficult airway algorithm should be followed.343 If mask ventilation is not possible, the ASA difficult airway algorithm should be followed and a surgical airway established if necessary. If mask ventilation is possible but there is no fetal distress, the patient should be allowed to awaken, and other options for anesthesia and airway management should be undertaken. If mask ventilation is possible but fetal distress is present, then delivery can proceed, albeit with an increased risk of maternal aspiration.
Malignant hyperthermia is a hypermetabolic disease of skeletal muscle that is triggered in genetically susceptible people by exposure to volatile anesthetic agents or succinylcholine. It is life threatening if not recognized and treated appropriately. A history of malignant hyperthermia susceptibility in the parturient or her family should prompt an antenatal interview with an anesthesiologist. Epidural analgesia for labor decreases the likelihood that a patient with malignant hyperthermia susceptibility will require general anesthesia for emergent cesarean delivery.
|ANESTHESIA FOR OTHER PROCEDURES|
External Cephalic Version
Elective external cephalic version is an uncomfortable procedure. Many obstetricians prefer to perform the procedure with a small amount of maternal sedation, believing that more profound analgesia/anesthesia may increase the likelihood that excessive force will be used, resulting in increased morbidity and mortality. Several studies have addressed the question of whether maternal analgesia/anesthesia increases the success rate of external version. Schorr and colleagues344 randomly assigned parturients to receive epidural anesthesia with lidocaine 2% with epinephrine or no anesthesia and found a higher success rate in the anesthesia group. Conversely, Dugoff and colleagues345 randomly assigned parturients to receive spinal analgesia (sufentanil 10 μg and bupivacaine 2.5 mg) or no analgesia and found no difference in the success rate between the two groups. Currently, the American College of Obstetricians and Gynecologists believes not enough information exists to support or oppose the use of regional analgesia/anesthesia for external cephalic version.346
Regional or general anesthesia is required for most women undergoing cervical cerclage. Regional anesthesia is an excellent choice for women undergoing prophylactic cerclage. Some obstetricians prefer general anesthesia when the cervix is dilated or the membranes are bulging. They believe general anesthesia provides a decrease in intra-abdominal pressure and uterine relaxation, thus decreasing the risk of membrane rupture and facilitating replacement of bulging membranes.347 However, coughing on the endotracheal tube and vomiting acutely increase intra-abdominal pressure. Outcome differences between regional and general anesthesia have not been well-studied, although one retrospective study found no outcome differences between the two techniques.348
Manual Exploration of the Uterus and Postpartum Curettage
Manual exploration of the uterus and postpartum curettage often are facilitated by anesthesia. Regional anesthesia may be extended or initiated in a hemodynamically, volume-resuscitated patient. Alternatively, nitrous oxide by face mask or small intravenous boluses of ketamine or opioid or both may provide sufficient analgesia. Uterine relaxation may be achieved by the administration of nitroglycerin349,350 or general endotracheal anesthesia with a volatile anesthetic agent.
The anesthetic considerations for cesarean hysterectomy or postpartum hysterectomy are discussed in the Placenta Previa section.
Postpartum Tubal Sterilization
Many anesthesiologists prefer to perform regional anesthesia for immediate postpartum tubal sterilization because parturients still may be at increased risk for pulmonary aspiration. In particular, parturients who have received opioids for labor analgesia have delayed gastric emptying compared to parturients who do not receive opioids.25,28–30 Both systemic25 and neuraxial opioids28,29 delay gastric emptying compared with that of control subjects. Candidates for postpartum sterilization procedures should be hemodynamically stable and not have additional anesthetic risk factors (e.g., morbid obesity or difficult airway).
Either epidural or spinal anesthesia is appropriate. An epidural catheter placed for labor analgesia may be reinjected to provide surgical anesthesia. One study of epidural catheter reinjection suggests that the failure rate of epidural anesthesia increases as the delivery to surgery interval increases,351 whereas others have documented an 87% to 92% success rate regardless of time from delivery.352,353
Nonobstetric Surgery During Pregnancy
The maternal physiologic and anatomic changes that effect intrapartum anesthesia also effect the conduction of anesthesia for nonobstetric surgery during pregnancy. It often is difficult to determine whether fetal adverse outcomes are a result of maternal disease, the operation, or the anesthetic. Severe maternal hypoxemia and hypotension pose the greatest risk to the fetus. During the first trimester, fetuses are at risk for teratogenicity. No anesthetic agent is a proven teratogen.354
|COMPLICATIONS OF REGIONAL ANALGESIA/ANESTHESIA|
Hypotension after regional analgesia/anesthesia (i.e., a 20% decrease in baseline blood pressure or a systolic blood pressure less than 100 mmHg) results primarily from a decrease in cardiac output secondary to decreased preload resulting from increased venous capacitance after sympathetic blockade. Because uteroplacental perfusion is not autoregulated, uncorrected maternal hypotension may result in decreased uteroplacental perfusion and the development of fetal hypoxia and acidosis.355 Hypotension is more likely to occur in women who are not in labor at the time of induction of regional anesthesia.356 Measures to decrease the incidence and severity of maternal hypotension include left uterine displacement, or the lateral position, and prophylactic intravenous fluid administration. Despite these measures, greater than 70% of parturients have hypotension develop after the induction of spinal anesthesia for elective cesarean delivery.357 Uncorrected hypotension results in fetal and neonatal acidosis.355 Small doses of a vasopressor usually readily correct the hypotension. Although laboratory studies of hypotension suggest that ephedrine maintains UBF better than pure α agonists,358 clinical studies suggest that small doses of phenylephrine are safe for the mother and fetus.359
Fetal bradycardia after initiation of neuraxial labor analgesia is discussed in the Combined Spinal-Epidural Analgesia section.
Postdural Puncture Headache
Postdural puncture headache (PDPH) is a complication of spinal or epidural anesthesia. It must be differentiated from other causes of postpartum headache. A PDPH is exacerbated when the patient moves from the horizontal to the upright position. The pain often is localized to the frontal and occipital regions. It may radiate from the neck, and some patients report a stiff neck. Symptoms are markedly worse in the upright position. The severity of the headache ranges from mild to incapacitating. The headache may be accompanied by nausea, vomiting, photophobia, diplopia, difficulty in accommodation, hearing loss, hyperacusis, or tinnitus.360 Symptoms may be secondary to cranial nerve palsies. Seizures have been reported.361 Postdural puncture headaches commonly occur within 48 hours of dural puncture, and may last 7 to 10 days if left untreated. The incidence of PDPH is approximately 1% after dural puncture with a 25-gauge pencil-point spinal needle. The incidence of inadvertent dural puncture during initiation of epidural analgesia/anesthesia also is approximately 1%.362 Seventy-five to 90% of these patients go on to have PDPH develop.363,364
Treatment of PDPH depends on the severity of the symptoms. Mild headaches that do not interfere with activities of daily living can be treated conservatively. Conservative treatment includes assuming the horizontal position, preventing dehydration, and administering caffeine (intravenous365 or oral366).
The standard treatment for moderate to severe headaches is an epidural blood patch. A single epidural blood patch results in complete cure in 33% to 68% of patients.364,367,368 The patch may be repeated if unsuccessful. The most common side effect of an epidural blood patch is backache.369
Several studies have documented low-grade maternal fever in parturients who receive epidural labor analgesia compared to those who do not.370–372 Although Lieberman and coworkers371 reported a much higher incidence of neonatal sepsis workups in newborns delivered of mothers with epidural analgesia, two thirds of the sepsis workups were performed on neonates without maternal fever. There is no evidence that epidural analgesia is associated with maternal or neonatal sepsis. Alteration in heat dissipation is a possible mechanism (e.g., lack of hyperventilation or inhibition of diaphoresis).
Back pain is a common complaint both antepartum and postpartum. Recent prospective studies have found no relationship between the use of regional analgesia and the development of long-term postpartum back pain.373–375
Inadvertent Intravascular Injection of Local Anesthetics
Inadvertent intravascular injection of local anesthetic agents may lead to systemic toxicity. At lower concentrations, local anesthetic toxicity involves the CNS. Higher concentrations result in cardiovascular toxicity. CNS toxicity should be readily treatable with treatment of seizures and ventilatory support. However, serious maternal or neonatal sequelae may result if maternal hypoxemia is not treated expeditiously.376 Pregnancy may enhance cardiovascular toxicity of bupivacaine,60 and resuscitation may be particularly difficult in parturients.377 In addition to the standard treatment of cardiac arrest (ventricular dysrhythmia should be treated with bretylium), left uterine displacement is absolutely necessary to ensure adequate maternal circulation. Prompt delivery of the fetus may be necessary.378
Two new local anesthetic agents (ropivacaine and levo-bupivacaine) recently have been approved by the US Food and Drug Administration. In pregnant sheep, ropivacaine and levobupivacaine exhibit less cardiovascular toxicity compared with that of bupivacaine.61,144
Anesthetic agents may be inadvertently injected into the subdural space. The characteristics of a subdural block are “atypical” for both epidural and spinal injections.379 The onset time of the block typically is slower than for subarachnoid injection, but the extent of the block may be extensive. Treatment is supportive until the block resolves.
High/Total Spinal Anesthesia
High or total spinal anesthesia may result from the inadvertent subarachnoid injection of a large “epidural” dose of local anesthetic, inadvertent subdural injection, or as a result of “overdosing” an epidural anesthetic. This results in hypotension, dyspnea, inability to speak, and loss of consciousness. Treatment is support of ventilation and circulation until the block dissipates.
Space-Occupying Lesions of the Spinal Canal
Epidural/spinal abscesses or hematomas are rare complications of regional anesthesia. Hematomas are more likely to occur in patients with abnormal hemostasis.284 Although these hematomas are rare, timely diagnosis and treatment are essential to prevent permanent neurologic deficits. Signs and symptoms include backache, lower extremity sensory and motor deficits, bowel and bladder dysfunction, fever, and malaise. Separately, these signs and symptoms are not unusual after labor and delivery; therefore, a high index of suspicion must be maintained.
Meningitis is rare after dural puncture.380 The source of infection may be exogenous or endogenous. Meningitis has been reported after labor analgesia CSE.171 Currently, it is not clear whether the risk of meningitis is higher after CSE or the recent case reports are a reporting phenomena of a new technique.
Postpartum lower extremity nerve injury may be secondary to intrinsic maternal palsy, arising from the birth process, or secondary to neuraxial analgesia. Nerve injury secondary to epidural or spinal anesthesia is rare and often associated with paresthesia during initiation of the block.381 A recent prospective study found a 1% incidence of intrinsic maternal palsy after labor and delivery.382 The most common injuries were to the lateral femoral cutaneous nerve (lateral thigh numbness) and femoral nerve (anterior thigh numbness and weakness of knee extension and hip flexion). Fortunately, both intrinsic maternal palsy and iatrogenic injuries usually resolve spontaneously.
Childbirth is one of the most painful experiences a woman is likely to encounter in her lifetime. Fortunately, analgesic techniques exist that reduce or ameliorate labor pain. Neuraxial labor analgesia offers the most complete analgesia and is safe and reliable.
Anesthesia-related maternal morbidity and mortality have decreased steadily in recent years. This may be a result of the more widespread use of regional analgesia and anesthesia in obstetric patients.
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