The cephalad aspect of the spinal cord is continuous with the brainstem through the foramen magnum. The spinal cord terminates as the conus medullaris, most often at the level of the lower border of the first lumbar vertebral body. The conus medullaris is attached to the coccyx by means of a neural-fibrous band called the filum terminale, which is surrounded by the nerves of the lower lumbar and sacral roots, known as the cauda equina. Within the bony vertebral column are three membranes: the pia mater, the arachnoid mater, and the dura mater. The pia mater is a highly vascular membrane that closely invests the spinal cord. The arachnoid mater is a delicate, nonvascular membrane closely adherent to the third and outermost layer, the dura. The subarachnoid space, located between the pia mater and arachnoid mater, contains (1) cerebrospinal fluid (CSF), (2) spinal nerves, (3) a trabecular network between the two membranes, (4) blood vessels that supply the spinal cord, and (5) lateral extensions of the pia mater—the dentate ligaments. The dura mater is a membrane, composed of collagen that encapsulates the spinal cord, the deeper meningeal layers, and the subarachnoid space. This layer forms a connective tissue sheath along the vertical axis of the central nervous system (CNS) that is contiguous with connective tissue covering the lateral extension of spinal nerve roots as they exit the intervertebral foramina. The interface between the dural and arachnoid layers has been described as a potential space capable of expansion after mechanical trauma. Unintentional injection of local anesthetic into this subdural space may explain some cases of failed spinal anesthesia. It may also explain the rare cases of high spinal anesthesia after the unintentional subdural injection of larger volumes of local anesthetic intended for epidural administration. Although the spinal cord ends at the level of the bodies of L1 and L2 in most adults, the subarachnoid space and cauda equina continue to the S2 level ( Fig. 12.1 ).

Distal Neuraxial Anatomy .

In pregnant women, the spinal cord usually ends at the lower border of the first lumbar vertebral body. The subarachnoid space continues to the second sacral vertebral level.

The epidural space is located external to the sac of the dura mater and contains loose connective tissue, adipose tissue, lymphatics, spinal nerve roots, and the internal vertebral venous plexus (Batson plexus) ( Fig. 12.2 ). This space is bound by the posterior longitudinal ligament anteriorly, the ligamentum flavum and the periosteum of the lamina posteriorly, the pedicles of the vertebrae, and the intervertebral foramina with their contents laterally. The epidural space is closed at the foramen magnum, where the spinal dura attaches to the dura of the cranium and at the sacral hiatus by the sacrococcygeal ligament. Frequently, anatomic references will illustrate neuraxial anatomy by way of sagittal and/or transverse cross-section. This may result in the erroneous impression that the epidural space is a continuous columnar entity that envelops the dural sac at all points about its perimeter. Investigations using cryomicrotome sections and three-dimensional reconstruction of radiologic data verify that the epidural space is in fact discontinuous along the vertical and lateral axes of the spinal canal. It varies in anteroposterior thickness according to dermatomal distribution, being widest at the level of lumbar vertebrae and thinnest in the cervical region. Epiduroscopy and epidurography at autopsy suggest the presence of a dorsal median connective tissue band in some individuals. Anatomic dissection and computed tomographic epidurography have also suggested the presence of epidural space septa. This band (or these septa) may provide an explanation for unilateral or incomplete epidural anesthesia. However, an in vivo evaluation of 106 patients with epiduroscopy could not identify the presence of the dorsal median band, but did identify a fibrotic septa in some patients, suggesting that it may be a cadaveric artifact.

(A) Sagittal section of the epidural space demonstrates that the contents of the epidural space depend on the level of the section. (B) Three-dimensional drawing of the epidural space shows the discontinuity of the epidural contents. However, this potential space can be dilated by the injection of fluid into the epidural space.

Redrawn from Stevens RA. Neuraxial blocks. In: Brown DL, ed. Regional Anesthesia and Analgesia . Saunders; 1976:323.

The ligamentum flavum lies posterior to the epidural space ( Fig. 12.3 ). The lamina, the spinous processes of the vertebral bodies, and the interspinous ligaments lie posterior to the ligamentum flavum. Posterior to these structures is the supraspinous ligament (which extends from the external occipital protuberance to the coccyx), subcutaneous tissue, and skin. Historically, some have described the ligamentum flavum as a single ligament. In actuality, however, it is composed of two curvilinear ligaments that join in the middle and form an acute angle with a ventral opening. Much like the epidural space, the ligamentum flavum is not uniform from skull to sacrum; indeed, it is not uniform even within a single intervertebral space. The thickness of the ligamentum flavum varies with vertebral level, body mass index (BMI), race, and age, as does the distance between the skin and epidural space. In a mixed-race population of parturients with a BMI of approximately 25 kg/m at the first prenatal visit, the mean (standard deviation [SD]) distance from the skin to the epidural space was 5.4 cm (1.1). The distance was greater among black parturients and white parturients than among Asian parturients at any given BMI ( Table 12.1 ).

Central Neuraxial Anatomy .

Note the variable thickness of the ligamentum flavum, which is greatest in the midline and decreases laterally.

Illustration by Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

The American Society of Anesthesiologists (ASA) has published guidelines for administration of neuraxial anesthesia in obstetric patients. Among other things, these recommendations address (1) the required presence of qualified anesthesia and obstetric care providers, (2) immediate availability of resuscitation medication and equipment ( Box 12.1 ), (3) mandatory preprocedural intravenous access, and (4) employment and documentation of maternal vital signs and fetal heart rate (FHR) monitoring.

During the initiation of neuraxial analgesia for labor, all patients are monitored with a blood pressure cuff and a pulse oximeter to facilitate continuous assessment of the maternal heart rate and oxygenation. Maternal blood pressure is measured every 1 to 2 minutes after the administration of the test and therapeutic doses of local anesthetic for approximately 15 to 20 minutes, or until the mother is hemodynamically stable. Subsequently (during maintenance of neuraxial analgesia), maternal blood pressure is measured every 15 to 30 minutes or more frequently if hypotension ensues. Continuous pulse oximetry during maintenance analgesia is used in selected patients (e.g., patients with obstructive sleep apnea or cardiovascular disease). Rarely, invasive hemodynamic monitoring is necessary. The sensory level of analgesia and the intensity of motor block are assessed after the administration of the test and therapeutic doses of local anesthetic and at regular intervals thereafter.

The ASA Task Force on Obstetric Anesthesia has made the following recommendation regarding FHR monitoring during the performance of neuraxial anesthesia procedures:

Fetal heart rate patterns should be monitored by a qualified individual before and after administration of neuraxial analgesia for labor. Continuous electronic recording of the fetal heart rate patterns may not be necessary in every clinical setting and may not be possible during placement of a neuraxial catheter .

The anesthesia provider cannot predict when hypotension will occur during the administration of neuraxial anesthesia. In addition, rapid onset of pain relief during the initiation of neuraxial labor analgesia is associated with nonreassuring FHR patterns (see Chapter 24 ). Thus we believe that continuous electronic FHR monitoring should be performed both during (if possible) and after the administration of neuraxial analgesia in all laboring women. In some cases, the mother’s position or maternal obesity precludes the use of an external Doppler device to monitor the FHR. In such cases, especially when there is concern regarding fetal well-being, it is helpful for the obstetric provider to place a fetal scalp electrode to monitor the FHR.

The American College of Obstetricians and Gynecologists states, “in the absence of a medical contraindication, maternal request is a sufficient medical indication for pain relief during labor and delivery,” including neuraxial labor analgesia. All patients should be informed of the risks, benefits of, and alternatives to neuraxial analgesia. The risk associated with neuraxial procedures relates to (1) the physical instrumentation of the spinal axis and (2) physiologic changes associated with medication administration via this anatomic route. Contraindications to needle or catheter placement include patient refusal or inability to cooperate, ongoing bleeding diathesis, infection either at the site of intended intervention or untreated systemic blood-borne illness, and increased intracranial pressure predisposing to cerebral herniation. Contraindications to injecting local anesthetics via the epidural or spinal route include severe hypovolemia and allergy to local anesthetics. It is unquestionable that patient refusal represents an absolute contraindication to an elective procedure. A thorough preoperative assessment of current fetal well-being, maternal volume status, intrapartum systemic opioid use, antibiotic administration for ongoing chorioamnionitis or other infectious process, and a brief reiteration of known maternal disease states, including allergies, will readily identify most of the major concerns that render neuraxial anesthesia potentially hazardous. The anesthesia provider should weigh the risks and benefits of neuraxial anesthesia for each patient.

Informed consent should include a frank discussion about anesthetic procedures and risks (see Chapter 32 ). Surveys of postpartum women have demonstrated that most parturients want to know the possible complications of epidural analgesia, even those that are rare. It is best to relay this information before the onset of labor (e.g., during antenatal classes), or early in the intrapartum period, although doing so is not always feasible. Some anesthesia providers fear that distressed, desperate, or sedated parturients may not understand the discussion of anesthetic procedures. A 2017 survey study of 206 postpartum women found no difference in recall of risks (discussed early after admission for labor) between women with and without pain at the time of the consent discussion. The preanesthetic evaluation allows the physician to communicate a sense of concern and to demonstrate a commitment to the patient’s care. Most laboring women understand the need for informed consent, and they appreciate the opportunity to participate in decisions about their care. Adequacy of consent can be demonstrated not only by documentation of information provided to the patient but also by the lack of patient objection to a procedure and the cooperation provided by the patient during the procedure. Before initiation of neuraxial anesthesia, a preprocedural verification process (i.e., “time-out”) is performed in compliance with national patient safety recommendations. The participation of the patient, the anesthesia care provider, and a third party such as a member of the nursing staff may lead to the discovery of concerns that should be addressed before the initiation of neuraxial anesthesia.

Management of the pregnant patient occurs in a unique clinical care environment in which the presence of the patient’s spouse or family members must be addressed. Most often, the dictates of local institutions will establish whether a partner’s presence during administration of neuraxial labor analgesia is acceptable. Intuition may suggest that a partner who remains present during the placement of neuraxial analgesia may help alleviate the patient’s ongoing anxiety regarding the procedure. Conversely, the partner may be so apprehensive or disruptive that the partner’s presence becomes counterproductive to the care of the patient. Orbach-Zinger et al. randomized 84 nulliparous women to either presence or absence of their partner during labor epidural catheter placement. Of interest, patient and partner anxiety, as measured by a validated anxiety questionnaire, was less when partners were absent during the procedure. Morell et al. randomized 143 nulliparous patients to either presence or absence of a companion during labor epidural catheter placement. While anxiety scores were not different between groups, maternal satisfaction was improved with the presence of a companion during epidural catheter placement, and 89% of patients randomized to the group without a companion would have preferred to have a companion present during the procedure.

Pregnant women have an exaggerated lumbar lordosis, making it potentially more difficult for them to flex the lumbar spine. However, most pregnant patients have sufficient flexibility to facilitate the insertion of a needle into the epidural or subarachnoid space. Whether the block is initiated in the lateral or sitting position is a matter of provider and patient preference. Notable advantages of the lateral position include (1) orthostatic hypotension is less likely and (2) the position often facilitates continuous FHR monitoring during placement of the epidural catheter. Vincent and Chestnut performed a study in which they observed that neither the sitting nor the lateral position was consistently superior with regard to patient comfort. The sitting position is likely associated with a higher incidence of orthostatic hypotension and syncope. However, the sitting position is preferred—and may be required—in obese parturients, in whom identification of the midline is usually significantly easier in the sitting position.

One study demonstrated a greater reduction in maternal cardiac output with maximal lumbar flexion in the lateral decubitus position than in the sitting position during identification of the epidural space in laboring women. The researchers speculated that maximal lumbar flexion in the lateral decubitus position results in concealed aortocaval compression. They recommended that “the tight fetal curl position be avoided,” especially when the patient assumes the lateral decubitus position.

Aortocaval compression must be avoided to the extent possible (see Chapter 2 ). If maternal hydration is inadequate and if aortocaval compression is not avoided, the onset of anesthesia-induced sympathetic blockade may result in decreased venous return, cardiac output, and uteroplacental perfusion.

Maternal position during placement of the epidural catheter does not seem to affect the incidence of unintentional dural puncture. However, adoption of the lateral recumbent head-down position for epidural catheter placement may reduce the incidence of unintentional epidural venous puncture.

When spinal or epidural anesthesia is performed with the patient in a lateral position, the patient’s back should lie at, and parallel to, the edge of the bed, for at least two reasons. First, the edge is the firmest section of the mattress. If the patient lies away from the edge of the bed, the patient’s weight will depress the mattress and the pelvis may rotate if the patient’s knees extend past the far edge of the bed. Second, this position allows anesthesia providers to keep their elbows flexed, facilitating control of fine hand and wrist muscle movements. The plane of the entire back should be perpendicular to the mattress. When asked to flex the lower back, patients typically roll the top shoulder forward, an action that rotates the spine (which is undesirable) but does not flex the lower back.

Similarly, patients positioned sitting should have their feet supported by a stool with the backs of their knees against the edge of the bed, a maneuver that helps position the patient’s back closer to the anesthesia provider. Alternatively, the cross-leg position may be a viable option in the sitting position. On preprocedural ultrasonography, the cross-leg position significantly increased the measured lengths of the posterior longitudinal ligament, ligamentum flavum, and interlaminar distance compared to standard positioning. The shoulders should be relaxed symmetrically over the hips and buttocks. Beds in obstetric units often break at the foot, and the split in the mattress encourages the patient’s seat to slope downhill if straddling the mattress split; this position will cause spine rotation and may make the procedure more difficult.

When spinal anesthesia is performed, the patient’s position relative to the baricity of the anesthetic solution should be considered, because it influences the extent and latency of blockade as well as the incidence of hypotension. The incidence, timing, and extent of hypotension in the period immediately after initiation of the block depend on the type of block (i.e., spinal, epidural, or combined spinal-epidural [CSE]), drug characteristics (i.e., baricity, concentration, dose), patient position during the procedure, and patient position in the period following the procedure. For example, when spinal anesthesia is initiated with a hyperbaric solution for operative vaginal delivery, it often makes sense for the patient to be sitting to ensure the rapid onset of sacral anesthesia. Conversely, spinal anesthesia for cervical cerclage can be initiated with the patient in the steep lateral Trendelenburg position with a hypobaric anesthetic solution.

Posture has less influence on the spread of epidural anesthesia. During epidural anesthesia, a unilateral block more likely results from the malposition of the catheter or an anatomic barrier within the epidural space than from patient position, particularly after a bolus injection. Norris and Dewan observed that gravity did not augment the spread of anesthesia in patients receiving epidural anesthesia for cesarean delivery, and they concluded that posture does not need to be manipulated to ensure adequate bilateral epidural anesthesia. In at least two studies, it was noted that the use of the sitting position is not necessary for the development of good sacral anesthesia when large volumes of epidural local anesthetic are given for cesarean delivery. However, Reid and Thorburn observed that use of the sitting position appeared to delay the spread of anesthesia to the midthoracic dermatomes. In comparison with the bolus administration of epidural local anesthetic, the extent of blockade may be more gravity dependent when the anesthetic is administered as a continuous infusion over a prolonged period.

Some anesthesiologists contend that maternal position after epidural catheter placement affects the efficacy of epidural analgesia, but this is a matter of some dispute. Beilin et al. observed that the placement of the laboring woman in the supine position with a 30-degree leftward tilt was associated with better epidural analgesia than maintenance of the left lateral decubitus position. In contrast, Preston et al. observed no difference in analgesia and a significantly higher incidence of fetal bradycardia with the supine wedged position than with the full lateral position.

An understanding of the importance of sterile technique and the gravity of infectious complications related to neuraxial anesthesia is crucial. The incidence of epidural abscess and spinal meningitis is generally so low that many of the available recommendations are based on evidence from other domains of infection control (e.g., surgical wound site and central venous catheter–related infection). Nonetheless, death and devastating neurologic compromise resulting from neuraxial infection can occur.

Infection of the epidural space tends to result in the formation of an abscess, most commonly formed by Staphylococcus aureus found in the epidermis of either the patient or the anesthesia provider. In contrast, meningitis associated with neuraxial procedures is most commonly caused by Streptococcus viridans . Viridans species of streptococcus may reside in the oronasopharyngeal tract of providers or patients or in the patient’s vagina. Potential routes of infection include the (1) epidural catheter track, (2) bloodstream, (3) equipment, and (4) injectate contamination. A more in-depth discussion of neuraxial infection is found in Chapter 31 .

Guidelines describing aseptic technique for regional and neuraxial anesthetic procedures have been published by professional anesthesiology organizations and the US Centers for Disease Control and Prevention (CDC). The following recommendations deserve emphasis:

• 1.

  Given that the oropharyngeal and skin flora of the anesthesia provider are implicated in many cases of neuraxial infection, the provider should don a surgical face mask and hat before initiation of spinal/epidural anesthesia. Microbial sampling in laminar-flow operating theaters has shown a 22-fold increase in bacterial counts when a face mask and hat are not worn.

• 2.

  Washing hands with an alcohol-based antiseptic solution is recommended because this has been shown to be superior to antimicrobial soap. Jewelry (e.g., rings, watches) should be removed before and sterile gloves worn after hand cleansing.

• 3.

  The patient’s skin should be decontaminated, preferably with a chlorhexidine-in-alcohol solution. Evidence suggests that chlorhexidine has superior bactericidal and bacteriostatic efficacy compared with povidone-iodine. If chlorhexidine is not available, then povidone-iodine with alcohol, rather than povidone-iodine alone, is preferred. Of importance, the anesthesia provider is encouraged to exercise patience in allowing the antiseptic to dry, because a major mechanism of antisepsis is the desiccating action of alcohol.

The first equipment decision involves determining whether to perform a single-shot or continuous spinal technique. Traditionally, a large-bore epidural needle and catheter are used for continuous spinal anesthesia because the US Food and Drug Administration (FDA) rescinded approval for the use of small-bore microcatheters in 1992. Therefore the risk for postdural puncture headache (PDPH) is significant. Continuous spinal analgesia or anesthesia using an “epidural” catheter sited in the subarachnoid space is useful after unintentional dural puncture with an epidural needle.

A 23-gauge, FDA-approved, spinal catheter is now available in the United States. The catheter is inserted using a “catheter-over-needle” technique. Initial observational study found that this catheter may have clinical utility ; however, a case series of five patients reported complications in all parturients, with three patients experiencing paresthesias and two PDPH. Further study is required to determine the true incidence of complications and its utility for continuous spinal anesthesia in obstetric patients. Several commercial spinal catheters are available in Europe and have been used successfully in parturients, but at the current time, a single-shot technique is preferred for spinal anesthesia for most obstetric patients.

The primary equipment choice for spinal anesthesia concerns the type and size of the spinal needle. Cutting-bevel needles (e.g., Quincke) should be avoided because of the unacceptably high incidence of PDPH associated with their use. Instead, noncutting (pencil-point) needles (e.g., Whitacre, Sprotte, Gertie Marx) should be used ( Fig. 12.6 ). Pencil-point needles cause more trauma to the dura than occurs with cutting-bevel needles. This likely results in a more intense inflammatory response. Presumably, the inflammation results in more rapid closure of the dural defect.

Spinal Needle Assortment Often Used in Parturients .

The Sprotte and Whitacre needles have cone-shaped bevels, whereas the Quincke and Atraucan needles have a cutting bevel. (Other sizes are available in some of these needle designs.)

Needle size must also be determined. Larger needles offer a greater fidelity of tactile feedback as the anesthesia provider traverses tissue planes of variable impedance when performing spinal anesthesia. Furthermore, larger needles are more likely to withstand the high resistance encountered when contacting bone without bending or shearing. In general, the “ease-of-use” advantages associated with larger needles must be balanced against a lower incidence of PDPH with smaller needles. Most anesthesia providers use 25- or 27-gauge noncutting needles for routine spinal anesthesia in obstetric patients. The urgency of the procedure may also influence the choice of needle size. For example, a 25-gauge needle might be chosen for spinal anesthesia for an elective procedure, and a larger (e.g., 24-gauge or 22-gauge) needle might be chosen when the subarachnoid space must be entered quickly because of severe fetal compromise.

With a small-gauge needle (i.e., 24-gauge or smaller), use of an introducer needle is preferable. The introducer needle engages the interspinous ligament and more accurately guides the trajectory of the smaller spinal needle than is possible with use of a small-gauge spinal needle alone. The introducer needle also aids with skin puncture as it is often difficult to puncture the skin with noncutting needles.

Either the midline or the paramedian approach can be used to enter the subarachnoid space. The midline approach requires the patient to reduce her lumbar lordosis to allow access to the subarachnoid space between adjacent spinous processes in the lumbar region. Traditionally, the anesthesia provider identifies the interspinous space using a landmark technique, although there is growing use of preprocedural ultrasonography to identify the space (see later). The interspinous space may be identified with one (usually the thumb or index finger) or two fingers (usually the index and middle fingers) of the anesthesia provider’s nondominant hand. The single finger “slides” along the skin in the midline from cephalad to caudad until it “settles” into an interspinous space. The two fingers identify the interspinous space by palpating the caudad border of the more cephalad spine. The fingers identify the midline by rolling in a medial-to-lateral direction ( Fig. 12.7 ).

The midline approach for spinal needle insertion requires accurate identification of a lumbar interspinous space. The palpating fingers are rolled in a medial-to-lateral and cephalad-to-caudad direction; an introducer is then inserted through the interspinous space almost perpendicular to the lumbar spinous process. Once the introducer is seated in the interspinous ligament, the spinal needle is inserted; the needle is stabilized in a tripod fashion during insertion (much like a dart being thrown).

Next, the anesthesia provider injects local anesthetic intradermally and subcutaneously. The introducer needle is inserted into the substance of the interspinous ligament. It is helpful if the introducer needle is embedded in the interspinous ligament; therefore obese patients may require a longer introducer needle. The introducer needle should lie in the sagittal midline plane. It is then grasped and steadied with the fingers of the nondominant hand while the dominant hand holds the spinal needle like a dart. The fifth finger may be used as a tripod against the patient’s back to prevent patient movement from causing unintentional needle insertion to a level deeper than intended, and to “brake” the needle. As the needle passes through the ligamentum flavum and the dura, characteristic changes in resistance are noted. A “pop” is often perceived as the needle tip traverses the ligamentum flavum. A subsequent and more pronounced pop is perceived as the needle tip exits the dura-arachnoid. The stylet is removed, and CSF should appear in the needle hub. If CSF does not appear, the stylet is replaced, and the needle is advanced until another change in resistance or “pop” is appreciated and again checked for CSF flow. This process continues until either bone is encountered or CSF returns through the needle. If neither occurs, the needle and introducer are withdrawn, and the process is repeated.

Although with time and practice the tactile feedback produced by advancing a needle through tissues of variable resistance will become utterly familiar to the anesthesia provider, the novice may be unsure of the anatomic position of the needle tip, especially if unexpected resistance (i.e., contact with bone) or an unexpected and premature “pop sensation” is encountered during needle advancement. A stepwise problem-solving approach is reasonable. First, the anesthesia provider should reconfirm that (1) the patient has normal anatomy (i.e., not scoliotic), (2) she is acceptably positioned without rotation of the spine (often recognized by asymmetric shoulder position), and (3) the chosen point of needle insertion is the true midline plane. If these assertions are true, and the needle tip encounters bone, it is highly likely that the osseous structure is either the inferior or superior spinous process. One of two maneuvers may overcome this barrier. After slight withdrawal of the needle, simple angulation in a cephalad or caudad direction may redirect the needle trajectory sufficiently to achieve access to the central neuraxial canal. One must appreciate the “toughness” of the interspinous ligament. Even a 17-gauge epidural needle can be bent if the angle is changed without some prior retraction of the needle. Furthermore, if a spinal needle/introducer complex is used, care must be exercised that angulation of the spinal needle does not occur without first withdrawing the needle into the lumen of the introducer. Thereafter the entire spinal needle/introducer complex is angulated before the spinal needle is readvanced. Angulation of the spinal needle without first withdrawing it into the introducer creates a fulcrum at the junction of the introducer tip where the spinal needle emerges and can potentially damage or even shear the delicate spinal needle ( Fig. 12.8 ).

Change of Needle Trajectory During Spinal Anesthesia .

Note that if both the spinal needle and its introducer needle are manipulated without prior retraction of the spinal needle into the lumen of the introducer (open arrow) , a fulcrum is created (dark arrow) where the risk for bending or shearing the delicate spinal needle may occur.

Illustration by Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

Alternatively, the introducer/needle complex may be withdrawn fully into subcutaneous tissue and either raised or lowered (cephalad or caudad) while still maintaining an angulation that is parallel to the original trajectory. Which approach is more effective may depend on the reason for the initial bone contact. If the patient has very narrow interspaces, then careful raising or lowering of the needle while maintaining a trajectory parallel to the floor may be appropriate. However, if the patient is overly flexed forward, it is possible that her lumbar spinous processes are projecting in a slightly upward angulation (relative to the perpendicular transverse plane). This may require that the needle be reangulated accordingly.

If bone is still encountered despite these considerations, it is likely that the needle tip is in fact not in the midline plane and is contacting the vertebral lamina. This may occur if the initial skin puncture is not in the midline, the needle tip deviates from the midline as it is advanced, or the patient’s spine is rotated (either from poor positioning or scoliosis). Clues that the needle tip is not midline include (1) the patient complaining of lateralizing pain; (2) lack of CSF flow despite appropriate needle depth; and (3) the perception of “soft” or “mushy” tissue (paraspinous tissue) during needle advancement rather than the more “rigid” ligamentous tissue, or even a false “pop” as the needle tip exits the interspinous ligament laterally into paraspinous tissue. Much like the progressive modifications described earlier for correct alignment in the superioinferior plane, so too can these approaches be employed for redirecting in the lateral plane ( Fig. 12.9 ). The novice is advised to make systematic changes in a stepwise fashion, rather than indiscriminately changing needle direction without first considering the anatomic problem.

Troubleshooting Contact With Bony Structures During Needle Placement .

The gray needle represents the initial needle trajectory; the blue needle represents the adjusted needle trajectory. (A) Assuming correct midline needle placement, the needle can be retracted slightly and angulated to overcome a spinous process. (B) Alternatively, the needle may be “lifted” after slight retraction while keeping the original trajectory constant. (C) Assuming the needle is deviating from the midline plane and contacting lamina, an action similar to that in A may be executed. (D) Alternatively, a stepwise lateral shift similar in concept to that shown in B may correctly achieve midline alignment.

Illustration by Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

Once CSF is freely dripping from the needle hub, the dorsum of the provider’s nondominant hand steadies the spinal needle against the patient’s back while the syringe with local anesthetic is attached to the needle. After aspirating to ensure the free flow of CSF, local anesthetic is injected at a rate of approximately 0.2 mL/s. After completion of the injection, some providers again aspirate approximately 0.2 mL of CSF and reinject it into the subarachnoid space. This last step reconfirms the needle location and clears the needle of the remaining local anesthetic. The patient is then repositioned as appropriate.

For most patients, the midline approach is faster and less painful than the paramedian approach. The midline approach is also easier to teach than the paramedian approach because it requires mental projection of the anatomy in only two planes, whereas the paramedian approach requires appreciation of a third plane and estimation of the depth of the subarachnoid space from the skin ( Fig. 12.10 ). Nevertheless, the paramedian approach is a useful technique that allows for the successful identification of the subarachnoid or epidural space in difficult cases. The paramedian approach does not require that the patient fully reduce her lumbar lordosis. This approach exploits the larger target that is available when the needle is inserted slightly off the midline.

Vertebral Anatomy of Midline and Paramedian Approaches for Spinal and Epidural Anesthesia .

The midline approach requires anatomic projection in only two planes: sagittal and horizontal. The paramedian approach also requires consideration of the oblique plane. However, the paramedian approach requires less patient cooperation in reducing lumbar lordosis to allow for successful needle insertion. The paramedian needle insertion site is made 1 cm lateral and 1 cm caudad to the caudad edge of the more cephalad spinous process. The paramedian needle is inserted 10–15 degrees off the sagittal plane (inset) .

A common error that is made with the paramedian approach is the insertion of the needle too far off the midline; the vertebral lamina then becomes a barrier to needle insertion. With the paramedian approach, the palpating fingers should again identify the caudad edge of the more cephalad spinous process. A skin wheal is raised 1 cm lateral and 1 cm caudad to this point; a longer needle is then used to infiltrate the deeper tissues in a cephalomedial plane. This step contrasts to the midline approach, in which the local anesthetic is not injected beyond the subcutaneous tissue. The spinal introducer is then inserted 10 to 15 degrees off the sagittal plane in a cephalomedial direction, and the spinal needle is advanced through the introducer needle toward the subarachnoid space. Another common error is to use an excessive cephalad angle with initial needle insertion. When the needle is inserted correctly and contacts bone, it is redirected slightly cephalad. If bone is again encountered, but at a deeper level, the slight stepwise increase in cephalad angulation is continued, and the needle is “walked” up and off the lamina. As with the midline approach, the characteristic feel of the ligamentum flavum and dura can be appreciated. The aim of the paramedian approach is to puncture the dura in the midline, even though the needle is inserted off the midline. Use of the paramedian approach requires insertion of a greater length of needle. Once CSF is obtained, the block is performed as it is with the midline approach.

During the performance of any nerve block technique, needle advancement should stop if the patient complains of pain. If pain is the result of inadequate soft tissue anesthesia, additional local anesthetic should be injected. Pain or paresthesias may also result from needle contact with central nerves or the spinal cord. Patient perception of paresthesias during the initiation of spinal anesthesia may indicate that the needle tip is in the subarachnoid space. The anesthesia provider should remove the stylet and check for CSF. If the paresthesia has resolved, the local anesthetic may be injected. If the paresthesia persists, however, the needle should be withdrawn and repositioned. In any case, the anesthesia provider should never inject the local anesthetic if the patient is complaining of paresthesias or lancinating pain, either of which may signal injection into a nerve or the spinal cord.

Special equipment for epidural analgesia or anesthesia includes an epidural needle, an epidural catheter (for a continuous technique), and a loss-of-resistance syringe (for the loss-of-resistance technique to identify the epidural space). Single-shot epidural anesthesia is rarely used in obstetric practice because the major advantage of epidural over spinal anesthesia is the ability to provide continuous anesthesia or analgesia without puncturing the dura with a large needle. An epidural needle with a lateral opening (e.g., Hustead, Tuohy, Weiss) is most commonly used because it allows a catheter to be threaded through its orifice ( Fig. 12.11 ).

Epidural Needle Assortment Often Used in Parturients .

Each needle is shown in an open-bevel view and an oblique orientation. The 18-gauge Hustead and 17-gauge Tuohy needles have lateral-facing openings, which direct epidural catheters to enter the epidural space more easily than if a single-shot Crawford needle design is used. (Other sizes and needle designs are available for obstetric epidural anesthesia.)

Epidural anesthesia is typically initiated by injecting local anesthetic and adjuvants into the lumber epidural space. Although a caudal approach was used in the early era of neuraxial labor analgesia, it is now rarely used. Two methods are used to identify the epidural space during needle advancement: (1) hanging drop method and (2) loss-of-resistance method. Most anesthesia providers use the loss-of-resistance method ( Fig. 12.12 ).

Loss-of-Resistance Technique for Identifying the Epidural Space .

The needle is first inserted into the interspinous ligament or ligamentum flavum, and a syringe containing an air bubble in saline is attached to the hub. After compression of the air bubble by pressure on the syringe-plunger, the needle is carefully advanced until a loss of resistance to syringe-plunger pressure is noted as the needle enters the epidural space.

The traditional loss-of-resistance syringe is a finely ground glass syringe with a Luer-lock connector. Plastic syringes are also available as are slip-tip syringes, and the choice is generally a matter of the anesthesia provider’s preference. The syringe is filled with 2 to 4 mL of saline, air, or saline with a small (0.25 to 0.5 mL) air bubble. There is some controversy regarding the use of air versus saline for detecting the point of loss of resistance. Saline causes some syringe plungers to stick and may be confused with CSF during initiation of CSE anesthesia. Conversely, injection of air into the epidural space may contribute to patchy anesthesia and unintentional pneumocephalus may increase the risk for headache.

Results of investigations comparing air to saline for loss of resistance are conflicting. A 2014 metaanalysis that included 852 patients (most were obstetric patients) found no differences in inability to locate the epidural space, unintentional intravascular or intrathecal catheter placement, block failure, unblocked segments, or pain between the two mediums. However, the confidence intervals of the relative risks were wide and the authors concluded that the evidence was of low quality. In a retrospective, single-institution study of loss of resistance to air versus saline by Segal and Arendt, no significant differences in block success were found in 929 patients. The authors intentionally chose a retrospective approach to the question; they stated that because “it is impossible to mask the anesthesiologist to the medium used for loss-of-resistance, [they] hypothesized that randomized controlled trials might overestimate the difference between air and saline by forcing the operator to use a less-preferred technique in half of the subjects.” Thus we recommend that anesthesia providers use the technique with which they are most comfortable. We use saline with a small bubble of air.

Regardless of the technique used, success depends on correct placement of the needle tip. The needle should be advanced sufficiently into the interspinous ligament before the syringe is attached or before the hanging drop of solution is placed into the needle hub. This approach has at least three advantages. First, it encourages the anesthesia provider to use proprioception while directing and advancing the needle. Second, it shortens the time required for successful identification of the epidural space. Third, it lowers the likelihood of a false-positive loss of resistance. Undoubtedly, this false-positive identification of the epidural space is responsible for many cases of unsuccessful epidural anesthesia; it is even possible to insert a catheter between the interspinous ligament and the ligamentum flavum.

During advancement of the needle-syringe assembly, the needle should be moved toward the epidural space by the provider’s nondominant hand while the thumb of the dominant hand applies constant pressure on the syringe plunger, thereby compressing the small air bubble. Alternatively, the intermittent, oscillating technique is employed when using the loss-of-resistance to air technique. When the needle enters the epidural space, the pressure applied to the syringe plunger causes the saline solution or air to flow easily into the epidural space (see Fig. 12.12 ).

In most obstetric cases, the anesthesia provider inserts a catheter and uses an intermittent bolus or continuous infusion technique to maintain analgesia (see Chapter 24 ). Most practitioners insert the catheter before injecting local anesthetic to allow for the slow, incremental injection of local anesthetic/opioid solution and the more controlled development of epidural anesthesia. If the principal reason for using an epidural technique is the provision of continuous analgesia, it seems most practical to insert the catheter before injecting the therapeutic dose of local anesthetic so that correct catheter placement can be verified promptly.

Several types of single-use, disposable epidural catheters are available. Catheters are made from plastic materials and differ as to the degree of “stiffness.” Wire-embedded catheters are more flexible and are associated with a lower incidence of paresthesias and intravascular placement during catheter insertion. The single-orifice catheter has one opening at its tip, whereas the multiorifice catheter has a closed “bullet” tip with three lateral orifices between 0.5 and 1.8 cm from the tip ( Fig. 12.13 ).

Epidural Catheters .

(A) Single-orifice catheter; (B) multiorifice catheter with bullet tip; (C) coiled wire reinforced catheter. Bottom , Epidural catheter with centimeter markings along distal end and Luer-lock connector at proximal end.

Illustration by Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

The proposed advantage of single-orifice, open-end catheters is that the injection of drugs is restricted to a single anatomic site. In theory, this arrangement should facilitate the detection of intravenous or subarachnoid placement of the catheter. Likewise, a theoretical disadvantage of multiorifice, closed-end catheters is that local anesthetic may be injected into more than one anatomic site (e.g., both the epidural and subarachnoid spaces). A catheter initially placed in the epidural space can migrate into a vein or the subdural or subarachnoid space. Fortunately, this does not seem to be a common clinical problem. Regardless of the choice of catheter, aspiration should be performed before each dose of local anesthetic is injected.

An advantage of the multiorifice catheter over the single-orifice catheter is the consistent ability to aspirate fluid (either blood or CSF) when the catheter is in a vessel or the subarachnoid space. Early studies suggested that multiorifice catheters may lead to more even distribution of local anesthetic and a lower incidence of “patchy” or unilateral anesthesia when the anesthetic is injected as a bolus. With low infusion rates into the epidural space, such as those used with continuous epidural infusions, the solution exits only the most proximal hole, and multiorifice catheters thus behave like single-orifice catheters. Use of higher infusion rates, such as those used with programmed intermittent epidural bolus (PIEB), will more consistently make use of all three orifices, depending on flow rate. A randomized trial comparing single- and multiorifice wire-reinforced catheters found no differences in block initiation success, analgesia maintenance with a continuous epidural infusion, or complications. In contrast, when labor analgesia was maintained with a PIEB technique, single-orifice catheters had improved analgesia and sacral coverage with fewer patient-controlled epidural analgesia boluses and less local anesthetic consumption compared with multiorifice catheters. Additional studies are needed with the PIEB technique to fully delineate if single-orifice catheters are advantageous.

After locating the epidural space but prior to threading the catheter, it may be helpful to inject 5 to 10 mL of saline because this may reduce the incidence of epidural vein cannulation, particularly when using stiffer epidural catheters. Six to eight centimeters of catheter are threaded into the epidural space before the epidural needle is removed. The catheter may then be pulled back until it is at the desired distance at the skin. Occasionally, the anesthesia provider will have difficulty advancing the catheter past the tip of the epidural needle. This difficulty may indicate that the epidural needle tip is not in the epidural space. However, if the provider is convinced that the needle is correctly placed, several maneuvers may facilitate catheter advancement. Having the patient take a deep breath often allows catheter advancement. Saline may be injected through the epidural needle if this has not been done. Although some providers rotate the epidural needle in an attempt to successfully advance the catheter, we do not recommend this maneuver because it may increase the risk for dural puncture. Instead, the epidural needle should be withdrawn 0.5 to 1 cm and again advanced into the epidural space. In a randomized trial comparing two different epidural kits from two different manufacturers with different epidural catheter and epidural needle types, difficulty in threading the epidural catheter into the epidural space was significantly higher with one kit compared with the other. The authors suggested that variations in the morphologic features of the epidural needles and catheters, and their combination, may influence the success of siting the epidural catheter.

Many techniques are available for securing the epidural catheter at the skin entry site. A transparent, sterile adhesive dressing applied over the catheter after application of skin adhesive generally works well, and the periphery of the dressing can be reinforced with tape. The position of the epidural catheter may change significantly with patient movement from the sitting-flexed to the sitting-upright or lateral decubitus position. Beilin et al. found 5 cm was the optimal distance to thread a multiorifice catheter. Less than 5 cm led to catheter dislodgement and more than 5 cm led to a greater incidence of unilateral analgesia. D’Angelo et al. found that the risk for catheter dislodgement was higher when single-orifice catheters were inserted 2 cm, but the risk for unilateral blockade was greater when catheters were inserted 6 to 8 cm. Therefore, if the catheter is to be used for a short period (e.g., during cesarean delivery), it should be left 2 to 4 cm into the epidural space. In contrast, if the catheter will be used for many hours (e.g., during labor), it should be left 4 to 6 cm into the space. To minimize catheter movement at the skin, the patient should be positioned sitting upright or in the lateral position before the catheter is secured, especially if the patient is obese. The epidural catheter should be labeled to decrease the risk of unintentional injection of a drug intended for intravascular injection.

The potential for the contamination of local anesthetic solutions has prompted the use of micropore filters during the administration of continuous epidural analgesia for labor. There is no evidence that filters decrease the rate of infection or of injection of undesirable foreign substances. Additionally, filters may reduce the reliability of aspiration and absorb local anesthetic solution, unless they are primed. We believe that micropore filters have little use in clinical obstetric anesthesia practice.

CSE anesthesia combines the advantages and mitigates the disadvantages of single-shot spinal anesthesia and continuous epidural anesthesia ( Box 12.2 ). Anesthesia is initiated with a subarachnoid injection of opioid with or without a local anesthetic and maintained via an epidural catheter. It is useful for both cesarean delivery anesthesia and labor analgesia. For cesarean delivery, the injection of the smaller dose of local anesthetic required for spinal (compared with epidural) anesthesia is inherently safer with regard to the possibility of unintentional intravascular injection. Additionally, the anesthesia provider can inject a local anesthetic dose that is lower than the ED ₉₅ (effective dose in 95% of cases) without fear of inadequate anesthesia. If surgical anesthesia is inadequate, the block can be “rescued” with epidural administration of local anesthetic. A randomized trial comparing 7, 8, and 9 mg of intrathecal bupivacaine administered as part of a CSE technique for cesarean delivery produced equivalent latencies to T4 sensory block with lower rates of maternal hypotension in the lowest-dose group. The shorter duration of action seen in the low-dose (7 mg) group was easily addressed through the administration of local anesthetic via the indwelling epidural catheter. Compared with conventional epidural anesthesia for cesarean delivery, CSE anesthesia is associated with a more rapid onset of surgical anesthesia, less intraoperative pain and discomfort (because of a more dense block), better muscle relaxation, and less shivering and vomiting.

The CSE technique is associated with a faster onset of labor analgesia, achieved with the subarachnoid injection of an opioid alone or an opioid combined with a small dose of local anesthetic. Studies differ as to whether CSE analgesia is associated with greater maternal satisfaction and fewer requests for supplemental analgesia. Goodman et al. randomized 100 parous women in early labor to receive either CSE or conventional epidural analgesia. There were no differences in requests for supplemental analgesia, although pain scores were lower in the CSE group within the first 30 minutes. A single-institution randomized trial in 800 women found better first-stage analgesia and fewer requirements for treatment of breakthrough pain in women randomized to receive CSE compared with epidural analgesia. A 2012 systematic review comparing CSE and epidural labor analgesia concluded that onset was faster with the CSE technique, but that there was no evidence for differences in maternal satisfaction, mode of delivery, ability to ambulate, or incidence of hypotension between the two techniques. Several studies have found a lower incidence of failed epidural analgesia as well as failed extension of epidural labor analgesia to cesarean delivery anesthesia after the initiation of labor analgesia with a CSE technique. Presumably, verification of the correct placement of the spinal needle by visualization of CSF increases the likelihood that the tip of the epidural needle is correctly placed in the epidural space.

A purported disadvantage of the CSE technique is that correct placement of the epidural catheter in the epidural space cannot be verified until spinal analgesia or anesthesia wanes. Therefore it has been suggested that if a functioning epidural catheter is important to the safe care of the mother and fetus (e.g., in the setting of a suspected difficult airway or nonreassuring fetal status), an epidural rather than CSE technique is indicated. However, in a retrospective study that included 2395 neuraxial labor analgesia procedures, the CSE technique did not result in delayed recognition of epidural catheter failure, and the overall catheter failure rate was lower when the catheter was sited as part of CSE analgesia. Therefore fear of unrecognized catheter malposition should not be a consideration when choosing a neuraxial technique.

There are several techniques for initiation of CSE anesthesia/analgesia. The most popular is the needle-through-needle technique, in which the epidural needle is sited in the epidural space and serves as an introducer for the spinal needle. The spinal needle passes through the epidural needle to puncture the dura. After injection of the subarachnoid dose, the spinal needle is removed, and the epidural catheter is threaded through the epidural needle. An alternative technique uses two skin punctures and two different interspaces: the spinal needle and epidural needle and catheter are introduced sequentially in two different interspaces.

The needle-through-needle technique requires a long spinal needle. Typically, a small (25-, 26-, or 27-gauge) noncutting needle is used to minimize the risk for PDPH. The tip of the spinal needle must protrude 12 to 17 mm beyond the tip of the epidural needle when the two needles are fully engaged ( Fig. 12.14 ). Failure to puncture the dura and visualize CSF occurred in 25% of patients when the spinal needle protruded 9 mm, compared with no patients when the needle protruded 17 mm. A 127-mm spinal needle is commonly used with a standard 9-cm epidural needle. However, because of differences in hub configurations among needles, the two hubs may not engage, and spinal needle protrusion may vary with specific needle combinations. Some manufacturers sell CSE needle “kits,” in which the spinal needle is designed for a specific epidural needle. An additional small syringe (1 to 3 mL) is required for the spinal dose.

Combined Spinal-Epidural Needle Configuration .

Top , Spinal needle exits the epidural needle through the normal epidural needle bevel. Because the epidural needle bevel opening faces sideways, the spinal needle exits the epidural needle at a slight angle to the long axis of the epidural needle. Bottom , Spinal needle exits the epidural needle through a special orifice. The axes of the spinal and epidural needles are aligned. The spinal needle must protrude 12–17 mm from the tip of the epidural needle when the hubs are engaged, or the ability to puncture the dura with the spinal needle is compromised.

Illustration by Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

CSE anesthesia is initiated much like epidural anesthesia. The epidural needle is sited in the epidural space ( Fig. 12.15 ). Before inserting the epidural catheter, the spinal needle is introduced through the epidural needle with the anesthesia provider’s dominant hand, while the nondominant hand is anchored against the patient’s back to serve as a brake for further advancement of the spinal needle. The provider usually perceives the tip of the spinal needle passing the tip of the epidural needle as a slight increase in resistance. Spinal needle advancement should stop immediately after the anesthesia provider perceives the dural puncture “pop.” Dural puncture is verified by visualization of CSF after removal of the spinal needle stylet. The provider’s nondominant hand is anchored on the patient’s back, and the spinal and epidural needle hubs are grasped together between the thumb and index finger of this hand. The dominant hand attaches the spinal syringe. Aspiration of CSF before injection of the anesthetic solution is not necessary. After injection of the spinal dose and removal of the spinal syringe and needle as a unit, the epidural catheter is threaded in the usual fashion.

Needle-Through-Needle Combined Spinal-Epidural Technique .

(A) The epidural needle is sited in the epidural space. (B) The long spinal needle is passed through the epidural needle and punctures the dura mater. The operator’s nondominant hand stabilizes the spinal and epidural needles, and the spinal needle stylet is withdrawn. Cerebrospinal fluid is seen spontaneously dripping from the spinal needle. (C) The syringe is attached to the spinal needle, and the intrathecal dose is injected. (D) The spinal needle is withdrawn, and the epidural catheter is threaded through the epidural needle into the epidural space.

Illustration by Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

Failure to puncture the dura with the spinal needle, or to observe free-flowing CSF in the spinal needle, may occur in several circumstances ( Fig. 12.16 ). The epidural needle tip may not be in the epidural space, or the needle tip may be correctly placed, but the spinal needle may fail to puncture the dura or may not reach the dura because of the depth of the posterior epidural space. Alternatively, the epidural needle may be angled away from the midline or in a sagittal plane off the midline, and the spinal needle may traverse the lateral epidural space without puncturing the dura. Although some clinicians elect to site the epidural catheter without observing free flow of CSF in the spinal needle, this practice is associated with a higher catheter replacement rate (29%) than if free-flowing CSF is observed (4%).

Reasons for Failure of the Combined Spinal-Epidural Technique .

(A) The spinal needle tents the dura but does not puncture it. (B) The spinal needle does not reach the dura. (C) The spinal needle passes to the side of the dural sac.

Redrawn with permission from Riley ET, Hamilton CL, Ratner EF, Cohen SE. A comparison of the 24-gauge Sprotte and Gertie Marx spinal needles for combined spinal-epidural analgesia during labor. Anesthesiology . 2002;97:574.

Dural puncture epidural (DPE) analgesia is another neuraxial technique that aims to exploit the advantages of CSE anesthesia while minimizing its shortcomings. It is procedurally identical to CSE anesthesia except for the omission of drug injection into the subarachnoid space. Presumably, excluding direct subarachnoid administration of opioid and local anesthetic at the time of dural puncture avoids the precipitous development of adverse effects such as hypotension and fetal bradycardia. However, the presence of the dural puncture allows for augmented transdural migration of epidurally injected drugs into the subarachnoid space. Hypothetically, this facilitates more reliable block of sacral dermatomes, which may be inadequately anesthetized with conventional epidural analgesia/anesthesia. The subarachnoid translocation of drugs may also act to compensate for an epidural catheter that would otherwise provide asymmetric block, leading to breakthrough pain and the requirement for manual top-up bolus injections of local anesthetic ( Fig. 12.17 ).

Variations of Approach to Neuraxial Analgesia .

In epidural analgesia, after siting an epidural needle within the epidural space, a catheter is placed through which local anesthetic is administered. In DPE analgesia, a small hole is created in the dura by puncturing the dura using a spinal needle with a needle-through-needle technique prior to threading the epidural catheter. No medication is injected into the intrathecal space. Subsequent medication administered into the epidural space will have increased access to the cerebrospinal fluid due to the presence of a dural puncture. In CSE analgesia, a dose of local anesthetic and/or opioid is administered into the intrathecal space with a spinal needle inserted through the epidural needle using a needle-through-needle technique. Thereafter an epidural catheter is placed.

Illustration byy Naveen Nathan, MD, NorthShore University HealthSystem, University of Chicago Pritzker School of Medicine, Evanston, IL.

Results of two early trials comparing traditional epidural to DPE analgesia were inconsistent. Thomas et al., using a 27-gauge spinal needle, found no differences in analgesia outcomes, including catheter manipulation and replacement rate, inadequate analgesia, unilateral block, sacral sparing, and the incidence of breakthrough pain. In contrast, Cappiello et al., using a 25-gauge spinal needle, found improved sacral analgesia, lower pain scores at 20 minutes, and a lower incidence of unilateral block in women randomized to receive DPE compared with epidural analgesia. A third trial compared epidural, CSE, and DPE analgesia. Onset of analgesia (the primary outcome) was faster in the CSE group than in the epidural and DPE groups, but there was no difference between the epidural and DPE techniques. Compared with the traditional epidural technique, women randomized to receive DPE analgesia had a greater incidence of bilateral sacral block at 10 minutes and a lower incidence of asymmetric block at 30 minutes. Compared with CSE analgesia, women in the DPE group had a lower incidence of pruritus and hypotension. The combined incidence of uterine tachysystole and hypertonus was greater in the CSE compared with the epidural and DPE groups. Finally, a fourth trial comparing epidural to DPE analgesia found no difference in adequate analgesia at 10 minutes, but those in the DPE group had a shorter time to adequate analgesia (median time 8 versus 10 minutes). There were no differences in complication rates, including maternal hypotension, nausea, and pruritus. Contreras et al. compared analgesia characteristics in women randomized to DPE analgesia with a 25- or 27-gauge spinal needle. While onset of analgesia was slightly faster with the 25-gauge spinal needle, no differences were found in sacral block, motor block, and the number of top-up doses.

In larger retrospective studies, DPE analgesia has been associated with fewer catheter failures and replacements compared to the traditional epidural analgesia technique, with failure rates of 6.5% and 9.8%, respectively. Thus DPE analgesia is frequently used clinically in high-risk obstetric populations such as complex cardiac patients and morbidly obese parturients. Tan et al. conducted a double-blind randomized control trial comparing DPE analgesia with a 25-gauge spinal needle to traditional epidural analgesia in obese parturients. Although the study found no difference in a composite outcome (asymmetric block, epidural top-up doses, catheter adjustments/replacements, failure to extend analgesia to anesthesia for cesarean delivery), the range of BMI was wide and only 20% of the cohort had a BMI ≥ 50 kg/m ² , a group of patients in whom landmark identification of the interspinous space may be particularly challenging.

Beyond quality of labor analgesia, there remains the question of whether differences exist between the DPE and epidural techniques when epidural labor analgesia is extended to epidural anesthesia for intrapartum cesarean delivery. Sharawi et al. hypothesized that the time for extension of labor epidural analgesia to surgical anesthesia is faster in women who had epidural analgesia initiated with a DPE compared with a traditional epidural technique. Nonlaboring women undergoing scheduled cesarean delivery were randomized to receive “mock” labor analgesia initiated with a DPE or traditional epidural technique. Approximately 1 hour later, the block was extended with epidural 3% chloroprocaine, mimicking clinical care for laboring women who undergo urgent cesarean delivery. The investigators found that women in the DPE group had faster surgical extension (422 versus 655 seconds) and improved quality of anesthesia compared with the standard epidural technique.

A 2019 systematic review of randomized controlled studies compared DPE and traditional labor epidural analgesia (5 studies, 581 patients). Although some studies found advantages in speed of onset and pain scores in women randomized to receive DPE analgesia, results were not consistent. Further, the quality of the block was not different among groups. The authors attributed the lack of consistent results to differences in study design (e.g., differences in the gauge of the spinal needle chosen for the dural puncture, epidural loading solution, epidural loading volume, the time over which the loading dose was administered, and method used to maintain epidural analgesia). Further investigation is needed to better define the role of the DPE technique in the routine management of labor pain.