The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems. Preganglionic nerves of the sympathetic nervous system arise from the thoracic and lumbar segments of the spinal cord; in the parasympathetic
nervous system, they arise from the brainstem and sacral spinal cord. Most visceral structures (except blood vessels and sweat glands) receive both sympathetic and parasympathetic innervation. The sympathetic chain lies along the anterolateral surface of the lumbar vertebral bodies. The psoas muscle and fascia separate the lumbar sympathetic chain from the somatic nerves. Anatomic studies have shown marked variability in the distribution of the ganglia, but suggest optimum blockade of the ganglia will result from sympathetic blocks at the level of the L3 vertebral body (90). However, caution should be exercised at the level of the middle one-third, where the segmental vessels pass under the dense fascia toward the epidural space, thus creating a potential passageway to the epidural space (90,91).

White rami communicantes are aggregates of preganglionic fibers that leave the spinal cord from the intermediolateral cell column and lateral horn (T1–L2). They leave the spinal canal with the corresponding anterior spinal nerves and join the sympathetic chain. They are white in color, as they are only slightly myelinated. The postganglionic fibers (gray rami communicantes) leave the sympathetic chain in association with the spinal nerves. They are unmyelinated and appear gray in color. The preganglionic fibers (white rami communicantes) may synapse in the sympathetic chain and then travel with the spinal nerve. Some may ascend or descend to another level of the spinal cord and synapse at this level before traveling with the spinal nerve. Some white rami communicantes transverse the sympathetic chain without synapsing. These preganglionic sympathetic fibers form the greater, lesser, and least splanchnic nerves (T6–T12), which synapse in the celiac and superior mesenteric ganglia within the abdomen. The postganglionic sympathetic fibers from these ganglia are distributed to the abdominal organs. The sympathetic nerves are usually accompanied by afferent fibers conducting sensory information from the appropriate viscera (see Figs. 39-1, 39-2, 39-5).

Early studies have suggested that the lower uterine and cervical afferent sensory fibers join the sympathetic chain at L2–L3. There were some reports of effective blockade of afferent sensory transmission from these structures to the spinal cord using lumbar sympathetic blocks (92,93). Lumbar sympathetic blocks provided effective analgesia of long duration (94). In addition, lumbar sympathetic blocks in nulliparous parturients with induced labor were associated with effective analgesia and rapid cervical dilatation during the first 2 hours of analgesia, and were also associated with a shorter duration of the second stage when compared to epidural analgesia (95). These findings, however, have not widely convinced clinicians to embrace this technique, which remains unpopular.

Blockade of the paravertebral sympathetic chain interrupts visceral afferents and sympathetic efferent transmission. The specificity and sensitivity of this procedure and the role of the sympathetic nervous system in the pain of parturition is unclear (96). The precise mechanism of action of lumbar sympathetic blocks in labor is also unclear, given the close anatomic relationship between the sensory afferents and the sympathetic chain (see Fig. 39-4).

The lumbar sympathetic nerves at the thoracolumbar level are reasonably amenable to non–image guided regional anesthesia techniques. However, the afferent nerves from the distal one-third of colon and all the pelvic viscera are less so. These fibers travel with the sympathetic fibers of the superior hypogastric plexus, which is located retroperitoneally in the pelvis at the level of L5–S1. Needle placement is technically difficult and thus impractical to use during labor (97,98). In addition, use of fluoroscopic guidance to facilitate needle placement in the parturient would be exceptionally rare because of risks of fetal exposure to radiation (see Figs. 45-15, 45-16).

Lumbar sympathetic blockade requires placement of a 22-gauge 10-cm needle approximately 2 cm deep to the transverse process of L2, bilaterally. Incremental local anesthetic injections can be administered. Bilateral doses of 10 mL of bupivacaine 0.5% with 25 μg of fentanyl and 50 μg epinephrine have been used to good effect (95). Complications include bleeding, infection, hypotension, genitofemoral neuralgia, unintentional epidural/spinal injection, and perforation of viscera
or blood vessels (91,99). As previously mentioned, these complications, combined with the potential for block failure, have limited the clinical usefulness of these blocks.

The paravertebral space is a narrow triangular space lateral to the vertebral column. The boundaries of the paravertebral space are formed by the iliopsoas anterolaterally; vertebral body, intervertebral disc, and foramen medially; and by the costotransverse ligament posteriorly. The space contains the sympathetic chain, dorsal and ventral nerve roots, and rami communicantes. Analgesia is achieved by deposition of local anesthesia in the distribution of the lateral aspect of the intervertebral foramen, within the paravertebral space, resulting in blockade of the ventral nerve roots and accompanying sympathetic nerves (see Figs. 16-4 and 16-15,16-16,16-17,16-18).

In the sitting position, the iliac crests (corresponding to L3–L4 ± 1 interspace) and spinous processes (midline) are identified and marked (see Chapter 16, Fig. 16-20). A line is drawn cephalad/caudad 2.5 cm lateral to midline, parallel to the spinous process of the thoracolumbar spine. Needle placement is along this line at the level of those nerve roots requiring blockade (T9–L1). Contact is made with the transverse process at each level required, and the needle is then angled 10 degrees cephalad/caudad and advanced 1 cm beyond the transverse process. Some anesthesiologists use a loss-of-resistance technique to identify the paravertebral space, although this can be quite subjective. This technique results in dermatomal analgesia at the level of the nerve roots blocked. The block is volume-dependent, and 20 mL of local anesthetic administered via a single needle should provide analgesia to approximately five somatic dermatomes and eight sympathetic levels. This volume of anesthetic agent may pose risks to the parturient in the event of accidental intravascular or intrathecal injection. Catheter techniques may be used to facilitate prolonged analgesia through the use of local anesthetic infusions and bolus doses as required. The absorption of local anaesthetic is difficult to predict, with analgesia of between 1 and 10 hours reported following a single injection (100,101) (see also Chapter 16).

The failure rate for paravertebral blocks performed in surgical patients is approximately 10%, with side effects including hypotension (4.6%), vascular puncture (3.8%), pleural puncture (1.1%), and pneumothorax (0.5%). The incidence of urinary complications and hypotension was found to be lower than with epidural analgesia (102,103). Total spinal anesthesia has also been reported as a complication (104). Paravertebral hematoma formation may also occur, although cadaveric studies suggest that fluid spread in the paravertebral space is caudal and lateral, thus away from the central neuraxis (105).

Bilateral lumbar sympathetic and paravertebral blocks have been described as effective techniques that can be used as an alternative to epidural labor analgesia (95,106). These, however, are non–image guided techniques (in the parturient), and the needle entry point is chosen on the basis of anatomic landmarks. The needle end-point is based on varying distances beyond contact with the transverse processes. This presents quite a large margin for error, and the distinction between the benefits of sympathectomy and nerve root block may be difficult to make for either procedure. These techniques have been suggested as suitable alternatives for epidural analgesia when central neuraxial blockade is deemed inappropriate for anatomic or medical reasons, but further investigation in the high-risk parturient is necessary. Use of short-acting IV opioid infusions may prove to be a safer and more flexible option.

The technique of epidural insertion in labor differs in some ways from standard epidural placement in the nonpregnant patient. Pregnancy-related anatomic changes can make positioning and identification of landmarks problematic. In addition, ongoing acute pain and associated distress during contractions can lead to difficulty in positioning the patient and can lead to ongoing physical and emotional stress to both patient and medical staff. Epidural insertion can be performed with the patient in the lateral decubitus or sitting position. Some evidence suggests that uteroplacental perfusion may be optimum in the lateral decubitus position (113). Likewise, the potential for the reduction of venous congestion by adopting the lateral recumbent head-down position may be associated with a reduction in the incidence of lumbar epidural venous puncture (114). However, the incidence of successful epidural placement may be higher in the sitting position, especially in the obese parturient. In the management of obese and morbidly obese patients, the sitting position may be associated with easier identification
of the midline and possibly improved respiratory parameters (115). To complicate the issue further, a decreased incidence of aortocaval compression during the identification of the epidural space was demonstrated in the sitting position compared to the left lateral decubitus position (116). Taking all these factors into account, we believe that positioning of the patient for epidural placement should be done taking user experience and preference into account. In addition, patient factors and preferences should also be considered. One report even suggested that use of the prone (knee-chest) position had a role in epidural placement (117). There is no doubt that the dynamic nature of acute pain medicine requires that each practitioner should be familiar with a broad range of procedures and positions in the performance of epidural, spinal, and combined spinal–epidural techniques.

Strict asepsis is essential in the performance of neuraxial blocks, especially those involving epidural catheter insertion. Several case reports have identified iatrogenic causes of meningitis during central neuraxial procedures (118,119,120,121). The routine use of face masks in the prevention of iatrogenic contamination by anesthesiologists has shown wide user variation. Surveys of United Kingdom obstetric anesthesiologists and of fellows of the Australia and New Zealand College of Anaesthetists with a special interest in obstetric practice showed a marked variation in practice standards. In the United Kingdom, more than 50% of anesthesiologists did not wear face masks for central neuraxial procedures, and this precaution was not seen to be essential in 29% of the latter group (122,123). The infectious disease community however, is united in their view that aseptic techniques, including use of face masks, is essential.

Disinfection of the skin with chlorhexidine, povidone iodine, or similar agent is strongly recommended. Standard aseptic precautions, including use of sealed bottles or single-use packets of povidone iodine, have proved to be more effective than multiuse bottles (124,125). In vitro studies have shown the effectiveness of iodine products for asepsis, although recent clinical evidence suggests that chlorhexidine in alcohol solution is more efficient as an antimicrobial agent (126). This combination agent has been extensively evaluated for use in placing central lines; however, its use in epidural or spinal placement has not been fully established. Although there is some worry about neurotoxicity based on preliminary animal research, clinical evidence in Europe suggests that chlorhexidine is both efficacious and safe for use as a disinfectant for neuraxial blocks (127,128).

Following appropriate antiseptic preparation of the lumbosacral spine, the skin is draped and local anaesthesia is administered to the skin and interspinal ligament. Use of clear plastic drapes offers the advantage of providing a sterile field while allowing better visualization of landmarks. Placement of the epidural needle at the L3–L4 ±1 intervertebral level should provide appropriate coverage of the lumbar and sacral nerve roots required for analgesia during labor and delivery. Use of loss-of-resistance techniques to both saline and air have been extensively described. Studies have assessed the quality of analgesia in women randomized to either technique (129,130): Beilin and co-workers found that patients who had epidural placement using a loss-of-resistance technique to air had a higher requirement for rescue medications following initial analgesia (129). The merits of a saline technique include avoidance of pneumocephalus-induced headache, nonuniform spread of local anesthetic, and nerve root irritation, all of which have been described following injection of epidural air (131,132) (see Figs. 11-12,11-13,11-14,11-15).

However, the judicious use of air during the loss-of-resistance technique should avoid many of these side effects. In fact, the use of air when performing a combined spinal–epidural (CSE) technique can be advantageous, as it allows clear identification of cerebrospinal fluid (CSF) without introducing any confusion caused by the concomitant use of saline. Because of the unpredictable duration of labor and the ever-present need to facilitate cesarean section delivery should it be necessary, placement of the epidural catheter is performed more commonly than performing a single-injection technique.

The use of both single-port (uniport) and multiport (multiple-orifice) epidural catheters has been widely described. The proposed advantage of the single-port (open-end) catheter is the delivery of medication to a single anatomic site, which may have safety implications. However, single-port catheters may also be associated with reduced spread of medication, leading to incomplete and/or unilateral blocks. This problem may be reduced through the use of the newly developed flexible-tip single-port catheters, which may also potentially offer the advantage of decreased incidence of paresthesias and intravascular placement (133).

Comparisons between multi- and single-port catheters showed that significantly fewer catheters needed replacement in the multiport group because of inadequate analgesia (defined as unblocked segments or unilateral block) and that paresthesias were less common in this same group (134,135). A comparison of multiport, firm-tipped, close-ended epidural catheters with uniport, open-ended, soft-tipped, wire-reinforced catheters showed the softer uniport to have a lower incidence of paresthesias and vascular puncture (136).

Multiport (closed-end) catheters have the theoretical disadvantage of potentially delivering anesthetic agent to more than one anatomic site, such as to the epidural and subarachnoid spaces at the same time. However, studies have not shown multiport catheters to be associated with multicompartment placement, and the incidence of vascular puncture and dislodgement requiring replacement were similar for both catheter types (single- and multiport). Multiport catheters have consistently been shown to be associated with a reduced incidence of inadequate analgesia and thus require less manipulation, presumably because of a more even distribution of medication (133,135).

A further technique suggested in the prevention of accidental vascular puncture during epidural catheter placement includes injection of 3 to 5 mL of saline through the epidural needle prior to advancing the catheter. This is thought to expand the epidural space, possibly decreasing the likelihood of unintentional IV cannulation (137).

Technical factors may play a major role in determining the effectiveness of labor analgesia. In a study of 100 laboring women, insertion of multiport catheters to a depth of 5 cm was shown to be associated with the highest incidence of satisfactory analgesia and minimal complications, compared to a higher rate of insertion complications at a 7-cm depth (138). Cephalad orientation of the epidural needle at catheter placement has also been shown to be associated with increased effectiveness and reduced complications. Epidural catheters are not fixed in the epidural space however, and their position can vary according to the posture of the parturient. Change of position from sitting to lateral recumbent can be associated with
movement of the catheter of between 1 and 2.5 cm, which can lead to inadequate analgesia (139). To minimize the risk of catheter displacement, especially in obese patients, it has been suggested that multiorifice catheters should be inserted to a depth of more than 4 cm into the epidural space and secured only upon assuming the lateral position (140).

Confirmation of successful catheter placement within the epidural space is often best measured by effective analgesia. Prior to administration of appropriate medications to achieve this degree of analgesia, unanticipated intrathecal or IV catheter placement must be excluded. Although this may not ensure appropriate epidural placement, it does however reduce the cardiorespiratory and neurologic risks associated with administration of a dose intended for epidural use into the CSF or systemic circulation. The use of test doses in labor has also been the subject of much controversy. Use of an appropriate test dose is considered important by some anesthesiologists, especially as many patients may subsequently require larger doses of concentrated local anesthetic administered epidurally in the event of emergency cesarean delivery (141). The sensitivity and specificity of test dosing is critically important in determining the appropriate placement of an epidural catheter. Given the extent of patient movement during labor and the potential for the epidural catheter to move according to patient position, it could be argued that frequent test dosing might be required. In addition, the impact of this epidural test dose on motor function in ambulating patients, and in preeclamptic patients and patients with cardiac disease, should also be considered (142,143,144,145,146). However, in clinical practice, if an epidural catheter has been placed and local anesthesia/opioid solution is infusing uneventfully into a comfortable parturient with minimal motor deficit, the catheter can be considered to be appropriately sited in the epidural space.

Intrathecal injection of 3 mL of lidocaine 1.5% (45 mg) with epinephrine 1:200,000 (5 μg/mL) will result in dense motor blockade within 2 to 4 minutes. Intravascular injection of 15 μg of epinephrine will result in tachycardia. The administration of this dose of local anesthetic systemically can result in lightheadedness and circumoral numbness or tingling within 1 minute of injection (141,142,143). Administration of the test dose should not be performed immediately before or during a uterine contraction, as labor pain itself is associated with increased heart rate variability. Aspiration of epidural catheters should be performed, but does not adequately identify all cases of intravascular or intrathecal catheter placement, especially if a single-port catheter is used (147,148).

Use of the “air test” combined with epidural catheter aspiration has also been described as a method to out rule accidental intravascular catheter placement (149). Injection of 1 mL of epidural air with concomitant precordial Doppler detection has been used successfully to identify intravascular placement. However, the sensitivity for this test appears to be less with use of the newer multiport epidural catheters (150).

The use of the amide local anesthetic bupivacaine is well established in obstetric anesthesia. Its prolonged duration of action, differential sensory blockade, and relative lack of tachyphylaxis make it an ideal agent for use in epidural and spinal anesthesia (151). The degree of drug ionization at physiologic pH and the extent of protein binding determine the degree of placental transfer. Bupivacaine is highly ionized at physiologic pH (pKa of 8.05) and is 95% protein bound, thus placental transfer is limited. The ratio at delivery of the concentration of local anesthetic in blood or plasma from the umbilical vein to the concentration of local anesthetic in maternal blood (UV:MA ratio) for bupivacaine ranges from 0.31 to 0.44 and is much lower than that of lidocaine (152). Bupivacaine has been the subject of concern in relation to its systemic cardiovascular toxicity (153,154), and the use of bupivacaine 0.75% concentration solution in the epidural space has been prohibited in obstetric practice by the U.S. Food and Drug Administration (FDA). Bupivacaine consists of two stereoisomers S(–) and R(+), and is marketed as a racemic mixture of these. TheR enantiomer was found to contribute to bupivacaine’s unwanted toxicity (155,156). Levobupivacaine and ropivacaine are the pure S(–) enantiomers of N-butyl- and N-propyl-2′,6′-pipercoloxylidide, which were developed as less cardiotoxic alternatives to bupivacaine (157) (see also Chapter 3).

Ropivacaine is a homologue of bupivacaine and mepivacaine formulated as a single levo rotary enantiomer. The putative advantages of ropivacaine over bupivacaine are the suggested lower risks of cardiovascular depression and neurologic toxicity (158) (see also Chapter 3).

In addition, sheep studies have shown faster clearance associated with ropivacaine compared to bupivacaine following intravascular administration, which suggests a greater margin of safety after unintentional intravascular injection (159).

Use of the minimum local anesthetic concentration (MLAC) in determining local anesthetic potency has shown that ropivacaine is 40% less potent than bupivacaine when used as a bolus for initiation of epidural analgesia in early labor (158,160). However, dilute ropivacaine (0.08%) with fentanyl (2 μg/mL) produced effective labor analgesia with preservation of spontaneous voiding and ambulation in parturients (161). Comparison between bupivacaine 0.075% and ropivacaine 0.075% using a patient-controlled epidural analgesia (PCEA) technique during labor showed both agents to be equally effective (162). Thus, there appears to be no clear clinical advantage of ropivacaine over bupivacaine for epidural labor analgesia in terms of either obstetric or neonatal outcome. However, cost may become a factor in the selection of any anesthetic technique, and ropivacaine is currently more expensive than racemic bupivacaine (163,164,165).

Levobupivacaine is a long-acting local anesthetic with a similar clinical profile to bupivacaine. Studies have shown that the cardiotoxicity of bupivacaine was more pronounced with the R(+) enantiomer, and use of levobupivacaine—the S(-) enantiomer—has gained acceptance in clinical practice (166). The safety of levobupivacaine has been compared to racemic bupivacaine in both animal and human volunteer studies (167,168,169). The lethal dose of levobupivacaine was 1.3- to 1.6-fold higher than that of bupivacaine in several animal studies, supporting the added safety of levobupivacaine should inadvertent intravascular injection occur (170) (see also Chapters 3 and 5).

Levobupivacaine crosses the placenta, with UV:MA ratios for levobupivacaine demonstrated to be 0.3 in women undergoing elective cesarean delivery following the epidural administration of 30 mL 0.5% levobupivacaine (171).
Levobupivacaine has been shown to compare favorably to bupivacaine for epidural use for cesarean delivery (172). Assessment of speed of onset and quality of sensory block between 0.5% bupivacaine and 0.5% levobupivacaine has shown similar results. There appears to be no difference between speed of onset and duration of sensory block when patients received either levobupivacaine 0.5% or bupivacaine 0.5% for cesarean delivery. In addition, the onset and reversal of sensory/motor blockade, quality of anesthesia, and muscle relaxation were comparable between the two groups. Levobupivacaine has also been used extensively to provide epidural analgesia for labor. A recent multicenter study comparing levobupivacaine and bupivacaine reported these drugs as having equivalent analgesic efficacy (173,174). Concomitant use of fentanyl has been shown to have a dose-sparing effect on levobupivacaine requirements. Levobupivacaine is not currently marketed in the United States, but is available in many other countries worldwide.

Lidocaine has been widely used in obstetric anesthesia. The rapid onset of action associated with lidocaine and its intermediate duration of action are particularly useful for many obstetric procedures. The UV:MA ratios reported are within the range of 0.4 to 0.6 (175). Epidurally administered lidocaine 2% solution combined with epinephrine is widely used for operative delivery; however, its duration of action and degree of motor blockade make it less ideal for use in facilitation of prolonged labor analgesia. Although early reports suggested that lidocaine compromised neonatal neurobehavioral function, this has not been borne out by further investigation or clinical experience (176,177).