- •
Neuraxial analgesia/anesthesia (spinal, epidural, combined spinal-epidural) are mainstays of safe anesthesia care of the obstetric patient. The anesthesia provider is caring for the mother, and by extension, the fetus.
- •
Hormonal and anatomic changes during pregnancy influence neuraxial technique and drug administration.
- •
Neuraxial analgesia (usually epidural or combined spinal-epidural) is the only analgesic technique that provides complete analgesia for labor and vaginal delivery.
- •
Neuraxial labor analgesia is usually initiated in a midlumbar interspace; a sensory block from T10 to S4 is necessary for complete labor analgesia.
- •
During cesarean delivery, a dense block to T6 is necessary to block afferent nerves innervating pelvic and abdominal organs. The addition of lipid-soluble opioids to neuraxial local anesthetics potentiates the density of the block.
Perspectives
The anesthesia care of obstetric patients is dominated by regional anesthesia and analgesia, primarily neuraxial techniques (spinal, epidural, combined spinal-epidural). The anesthetic care of the obstetric patient must also consider effects on the fetus/neonate. In general, neuraxial compared to systemic analgesia/anesthesia results in less drug transfer across the placenta to the fetus.
Neuraxial analgesia is the only analgesic technique that can provide complete analgesia for labor and vaginal delivery. Given that labor lasts a variable duration (less than an hour to several days), a continuous technique is optimal. Neuraxial labor analgesia is typically initiated with a bolus injection of local anesthetic combined with a lipid-soluble opioid into the subarachnoid or epidural space. An epidural catheter (rarely a spinal catheter) is sited and used to maintain analgesia throughout labor and delivery. Other regional techniques and nerve blocks may be performed, often by the obstetrician (bilateral paracervical blocks, bilateral pudendal nerve blocks), but these techniques are not continuous and do not provide complete analgesia. Paracervical blocks provide analgesia for the first stage of labor, when pain impulses originate from the cervix and lower uterine segment. Pudendal nerve blocks are useful for the second stage of labor as the fetus descends in the birth canal, when pain originates from the vagina and perineum. Only neuraxial analgesia can block these pain impulses simultaneously.
Neuraxial anesthesia is considered the optimal technique for cesarean delivery, both scheduled and intrapartum deliveries. Advantages compared with general anesthesia include: 1) it is safer for the mother (no need to manipulate the airway), 2) less drug(s) crosses the placenta and depresses the fetus/neonate, and 3) it allows the mother to be awake and father to be present for the delivery of their child. Single-shot spinal anesthesia is frequently used for elective procedures. Combined spinal-epidural anesthesia is used when long case duration is anticipated (e.g., repeat procedure, obese body habitus). Epidural anesthesia is a common technique for intrapartum cesarean delivery in women who have an indwelling epidural catheter sited for labor analgesia; epidural analgesia is transitioned to epidural anesthesia .
Postcesarean delivery analgesia is most often provided using multimodal analgesia—a common component of multimodal analgesia is single-shot neuraxial morphine analgesia. In women who are not able to receive neuraxial morphine (e.g., general anesthesia is used), the transversus abdominal plane block (see Chapter 36 ), and the quadratus lumborum block (see Chapter 38 ) have been shown to supplement systemic analgesia. However, these blocks do not improve the analgesia provided by neuraxial morphine and are therefore not indicated in women who receive spinal or epidural morphine.
Patient selection
Most obstetric patients are candidates for neuraxial analgesia/anesthesia. Contraindications mimic those for nonpregnant patients. Absolute contraindications include patient refusal, coagulopathy, infection at the site of needle placement, and uncorrected maternal hypotension (e.g., in the setting of hemorrhage). Systemic infection is a relative contraindication, although most clinicians will proceed with a neuraxial technique once antibiotics have been administered and the patient is not exhibiting signs of frank septicemia. Thromboembolic disease is a major cause of maternal morbidity and mortality; thus many pregnant women receive pharmacologic anticoagulation. Guidelines for the initiation of neuraxial procedures in women who have received pharmacologic anticoagulation generally mimic those for the nonobstetric population, although a thorough risk-benefit analysis should be individualized to each patient. Several obstetric conditions are associated with a frank consumptive coagulopathy, including placental abruption and amniotic fluid embolism. Coagulopathy should be ruled out in these women before proceeding with a neuraxial procedure.
Pharmacologic choice
In the United States, four local anesthetics are commonly used for obstetric neuraxial analgesia/anesthesia. Bupivacaine (0.0625%–0.125%) and ropivacaine (0.1%–0.2%) are commonly used for epidural labor analgesia. Spinal anesthesia for cesarean delivery is usually initiated with 0.75% bupivacaine with dextrose (hyperbaric bupivacaine), although some clinicians use plain bupivacaine. In most patients this formulation is slightly hypobaric. Epidural anesthesia is commonly initiated with 2% lidocaine plus epinephrine 1:200,000, or 3% 2-chloroprocaine (useful for emergency procedures when short latency to onset of anesthesia is critical). Typically, 18–25 mL of a local anesthetic solution is required for epidural anesthesia for cesarean delivery. Neuraxial anesthesia is also indicated for cervical cerclage placement (both elective and rescue procedures) and postpartum tubal ligation surgery, although these are usually short-duration procedures and use of short-acting local anesthetic agents may be appropriate.
In almost all cases, a lipid-soluble opioid (fentanyl or sufentanil) is added to the local anesthetic. The opioid and local anesthetic work synergistically, thus lower doses of both drugs are needed, contributing to decreased side effects from both the local anesthetic and opioid. Fentanyl 10–25 µg and sufentanil 1.5–5 µg are common spinal adjuvants for initiation of combined spinal-epidural labor analgesia and spinal anesthesia for cesarean delivery. Fentanyl 1.5–3 µg/mL or sufentanil 0.2–0.4 µg/mL is often combined with a low-concentration local anesthetic solution to maintain epidural labor analgesia. Neuraxial morphine is commonly used as part of a multimodal technique for postcesarean delivery analgesia (spinal dose 0.050–0.150 mg, epidural dose 2–4 mg).
Epinephrine (0.2 mg) may be added to spinal bupivacaine to prolong the duration of anesthesia. Spinal and epidural clonidine may enhance analgesia and anesthesia, although the use is off-label in the United States for obstetric patients, because it may cause hypotension and sedation.
Placement
Anatomy
During labor, painful impulses from the uterus and cervix are transmitted by visceral afferent nerve fibers that travel with sympathetic nerve fibers and enter the spinal cord at the low thoracic and high lumber (T10 through L1) spinal segments. As labor progresses and the fetus descends in the birth canal, distention of the vagina and stretching of the perineum results in pain impulses that are transmitted via the pudendal nerves to the S2 to S4 spinal segments. Epidural catheters are commonly sited in the midlumbar epidural space. Early in labor, the sensory block must extend cephalad to T10 to block labor pain. Later in labor, sensory blockade must also extend caudad to the sacral dermatomes to provide complete analgesia as the fetus descents in the birth canal.
During cesarean delivery, afferent nerves innervating the abdominal and pelvic organs must be blocked. These nerves travel with sympathetic nerves fibers from the T5 through L1 spinal segments. Therefore dense motor and sensory block, which extends from the sacral dermatomes to T6, is necessary to provide satisfactory cesarean delivery anesthesia. Because of differential sensory blockade (i.e., block to touch occurs at a lower dermatome than block to temperature), most clinicians aim for a sensory block to a cold stimulus at the T4 dermatome. Inadequate extent and density of anesthesia during cesarean delivery—which is more common with epidural than spinal anesthesia—will contribute to intraoperative nausea, vomiting, and pain.
Anatomic and hormonal changes during pregnancy contribute to progressively lower anesthetic requirements as gestation advances. Elevated progesterone levels, among other hormonal changes, contribute to altered pharmacodynamics during pregnancy. Progressive lumbar lordosis during pregnancy alters the relationship of the vertebral column to surface anatomy. The imaginary line joining the posterior superior iliac crests (Tuffier’s line) crosses the vertebral column at a higher (more cephalad) position in pregnancy. This may increase the risk of misidentifying the actual lumbar interspace (i.e., anesthesia providers using landmark techniques may identify an interspace that is one or two levels higher than the intended interspace). The space between adjacent spinous processes is narrower and it may be harder for pregnant women to assume the flexed position that facilitates needle access to the neuraxial canal. Ligaments become more “lax,” causing the ligamentum flavum to feel less “dense” during advancement of the spinal or epidural needle. Blood volume increases and by midgestation the enlarging uterus compresses the inferior vena cava and impedes venous return from the lower extremities, and along with an increase in intraabdominal pressure, this results in the shunting of blood from the inferior vena cava to the azygous system. The expanded blood volume in the lumbar neuraxial canal, along with the increasing fat volume that accompanies pregnancy, causes a decrease in lumbar cerebrospinal fluid (CSF) volume as the dural sac is compressed and lumbar CSF is translocated cephalad. Additionally, the apex of the normal lumbar lordosis shifts caudad and the normal thoracic kyphosis is reduced and shifts cephalad. Finally, CSF specific gravity decreases during pregnancy. These changes contribute to altered distribution of subarachnoid hyperbaric (or hypobaric) anesthetic solutions. At term, the local anesthetic dose required for neuraxial anesthesia is reduced by 25%–30% compared to nonpregnant patients.
The enlarged epidural veins may increase the risk of unintentional cannulation of epidural veins by a needle or catheter. Additionally, engorgement of the foraminal veins may block egress of anesthetic solution injected into the epidural space and contribute to the lower epidural anesthetic requirement observed during pregnancy.
Position
Patient positioning for the initiation of neuraxial analgesia does not differ in the pregnant patient (see Chapters 40 and 41 ). Laboring patients may prefer the lateral or sitting position, and clinicians may have a preference for one position. Occasionally, patient comorbidities may dictate patient position (for example, a patient with a dilated cervix and fetus with a footling breech presentation should be placed in the lateral position to decrease the risk of umbilical cord prolapse). Patient position should be considered when initiating spinal anesthesia with a hyperbaric (or less commonly, hypobaric) anesthetic solution. Monitoring during the initiation of anesthesia mimics that for nonpregnant patients, with the addition of fetal heart rate monitoring. The anesthesia provider may need to collaborate with the nurse or midwife to optimally position the patient for neuraxial anesthesia and fetal monitoring.
Increasingly more common, preprocedure ultrasonography is used to facilitate identification of lumbar vertebral anatomy (see Chapter 39 ). It is particularly useful for obese parturients in identifying the midline, the interspinous space, and the estimated depth to the epidural space. Two views are typically obtained with a low frequency (2–5 MHz) curved array probe, the parasagittal oblique (PSO) view, and the transverse median (TM) view. The acoustic window may be larger using the PSO view, especially in women with narrow interspaces. The depth to the epidural space is estimated by obtaining an image of the posterior complex (the ligamentum flavum, the epidural space, and the posterior dura-arachnoid) using the PSO or TM view. The estimation of the depth to the epidural space may underestimate the actual depth measured by the neuraxial needle by as much as 1 cm, especially in obese women. This underestimation is attributable to soft tissue compression by the ultrasound probe, which is often necessary to obtain a satisfactory view.
Spinal anesthesia is initiated in a midlumbar interspace (L2–L3 or lower). Epidural analgesia/anesthesia is also usually initiated in a midlumber interspace because of the need for both a thoracic and sacral anesthesia. Rarely, a two-epidural catheter technique is used in which one catheter is placed at a low-thoracic interspace to provide analgesia for the first stage of labor, and a second caudal catheter is placed to provide sacral analgesia. Like spinal anesthesia, combined spinal-epidural analgesia/anesthesia is initiated at the L2–L3 interspace or lower because of the need to avoid trauma to the conus medullaris.
Use of sterile technique during the initiation of neuraxial anesthesia is critical to the safety of the parturient. When in the sitting position, the mother should don a surgical hat to keep hair and scalp skin flakes from falling onto the sterile field. All individuals in the labor room should don a face mask, and commonly, only one support person is allowed to stay in the labor room during the procedure. The skin over the needle puncture site is usually decontaminated with chlorhexidine in alcohol. The proceduralist should wash their hands with an alcohol-based antiseptic solution and all hand and wrist jewelry and watches should be removed before donning sterile gloves.
Needle puncture
Young women of childbearing age are at increased risk for postdural puncture headache, therefore subarachnoid puncture should be performed with a small-gauge (usually 25- to 27-gauge) pencil-point needle.
Epidural analgesia/anesthesia is usually initiated with a 17-gauge Tuohy or other epidural needle appropriate for catheter insertion. In virtually all cases, a single or multiorifice epidural catheter (19- or 20-gauge) is passed through the epidural needle and secured for the duration of labor and delivery. A midline approach is common—obstetric patients are young and the interspinous ligament is rarely calcified. The depth to the epidural space is increased in pregnancy because of an increase in subcutaneous fat. Loss-of-residence to saline is a common technique to identify the epidural space; the hanging-drop method is less reliable during pregnancy because of increased intraabdominal, and therefore epidural-space pressure. A small bolus dose of anesthetic solution may be injected through the epidural needle before the epidural catheter is passed though the needle. More commonly, the epidural catheter is threaded into the epidural space and the anesthetic solution necessary to establish analgesia/anesthesia is injected incrementally through the catheter.
Combined spinal-epidural analgesia/anesthesia combines the advantages of both spinal and epidural anesthesia: rapid onset of analgesia with a low dose of drug(s), followed by the ability to provide continuous analgesia/anesthesia. It may be used for both labor analgesia and cesarean delivery anesthesia. It is most common to use a needle-through-needle technique in which the epidural needle is advanced into the epidural space ( Fig. 48.1 ). The epidural needle then acts as an “introducer” for a long spinal needle (25- to 27-gauge). The spinal needle is advanced through the epidural needle using the anesthesia provider’s dominant hand while the nondominant hand anchors the epidural needle by grasping the epidural needle hub between the thumb and first finger and placing the back of the hand against the patient’s back. This technique is similar to spinal anesthesia when the nondominant hand steadies the introducer needle as the spinal needle is advanced with the dominant hand. As the spinal needle tip passes the tip of the epidural needle, a small increase in resistance to advancement is usually noted. As soon as the anesthesia provider perceives the “pop” indicating puncture of the dura-arachnoid with the tip of the spinal needle, spinal needle advancement should stop. The hubs of the spinal needle and epidural needle are then firmly grasped together between the thumb and first finger of the nondominant hand and the spinal needle stylet is removed. After verifying backflow of CSF through the spinal needle, the syringe with the prepared spinal anesthetic solution is attached to the spinal needle hub and injected, and the empty syringe and spinal needle are removed together. The epidural catheter is then sited in the epidural space, exactly as if initiating continuous epidural anesthesia.
