Neuraxial anesthesia and analgesia

1

What are the different kinds of neuraxial anesthesia?

• Epidural anesthesia refers to injection of anesthetics or analgesics into the epidural space at either the lumbar or thoracic level.

• Caudal anesthesia refers to injection into the sacral hiatus (see Question 11).

• Spinal anesthesia, spinal block, subarachnoid block, and intrathecal block are all terms used interchangeably to describe injection into the intrathecal space.

2

Differentiate neuraxial analgesia and anesthesia? When might analgesia be preferred?

Anesthesia implies an intense sensory and motor blockade, which is necessary to perform a surgical procedure. It is usually obtained by using the highest available concentration of local anesthetic (e.g., 2% lidocaine or 3% chloroprocaine). Analgesia implies sensory blockade only, usually for postoperative pain management or labor analgesia, and may be achieved with dilute local anesthetic, epidural opioids, or a combination of the two.

3

List the advantages of neuraxial (epidural or intrathecal) anesthesia over general anesthesia?

• The metabolic stress response to surgery and anesthesia is reduced.

• Significant blood loss reduction by comparison with general anesthesia with certain surgeries. Cesarean section and major lower extremity orthopedic procedures are the best examples.

• Decreased incidence of venous thromboembolic complications.

• Pulmonary compromise is likely reduced.

• Airway manipulation and loss of airway protection is usually avoided, adding benefit in patients with potentially difficult airways, severe reactive airway disease, and patients at risk for aspiration.

• Mental status remains intact, unless the patient is otherwise sedated.

• Total systemic dose of drug(s) is markedly decreased.

• Motor blockade from spinal avoids the need for muscle relaxant medications for surgical manipulation. Subsequent reversal medications are also unnecessary.

• Markedly lower incidence of anesthesia-induced postoperative nausea and vomiting (presuming hypotension is treated effectively).

• Markedly improved pain control with minimal or no systemic opioids using either low-dose intrathecal morphine and/or continuous epidural anesthesia in either the lumbar or thoracic region, as indicated.

4

Differentiate between a spinal and an epidural anesthetic.

A spinal anesthetic is performed by puncturing the dura and injecting a small amount of local anesthetic directly into the cerebrospinal fluid (CSF), producing a rapid, dense, and predictable neural blockade. An epidural anesthetic requires a 10-fold increase in dose of local anesthetic to fill the epidural space and penetrate the nerve coverings. The block onset is slower and often less dense, and the anesthesia produced tends to be segmental (i.e., a band of anesthesia is produced, extending upward and downward from the injection site). The degree of segmental spread depends largely on the volume of local anesthetic injected. For example, a 5-mL volume may produce only a narrow band of anesthesia covering three to five dermatomes, whereas a 20-mL volume may produce anesthesia from the upper thoracic to sacral dermatomes. Placement of an epidural anesthetic requires a larger needle, often includes a continuous catheter technique, and has a subtle endpoint for locating the space. The epidural space is located by following the feel of the ligaments, as they are passed through until there is loss of resistance, whereas the subarachnoid space is definitively identified by CSF flow from the needle following dural puncture.

5

List the advantages of spinal anesthesia over epidural anesthesia?

• Often quicker onset time from injection to adequate surgical blockade.

• More predictable block density, particularly regarding motor blockade.

• Less risk for block “patchiness” and/or unilateral block.

• Epidural anesthesia has no definitive confirmatory endpoint during placement, as opposed to observing CSF in the spinal needle during subarachnoid block.

• Lower volume of local anesthesia required essentially eliminates the risk of local anesthetic systemic toxicity (LAST).

6

What are some disadvantages of (single-shot) spinal anesthesia over catheter-based epidural anesthesia?

• More immediate and dramatic hemodynamic changes often observed.

• Not easily titratable.

• With rare exception, no ability to provide continued neuraxial analgesia beyond initial dose. (Very rarely, intrathecal catheters are used.)

• No ability to augment intensity or increase duration of blockade if inadequate.

• Block level or dermatomal window cannot be augmented or modified once set, as it can be using a continuous epidural technique.

7

Describe how one would assess a patient before performing a neuraxial anesthetic.

In addition to the normal preoperative evaluation, the following specific items should be assessed before performing neuraxial anesthesia:

• History

  • Previous back injury or surgery

  • Neurological symptoms or history of neurological disease (e.g., diabetic neuropathy, multiple sclerosis)

  • Bleeding tendencies or diseases associated with coagulopathy (e.g., preeclampsia)

  • Anticoagulation or antiplatelet medication (current or recent, including aspirin)

  • Prior regional anesthesia and any associated problems

• Physical Examination

  • Brief neurological examination for strength and sensation

  • Back examination for landmarks and potential anatomic abnormalities (scoliosis) or pathology (infection at the site of placement)

  • Cardiovascular examination, specifically for murmurs, indications of right or left heart failure, etc.

• Surgery Specific

  • Expected duration

  • Expected blood loss

  • Positioning required

  • Need for muscle relaxation

  • Surgeon's preferences (and potential discussion of risks and benefits with surgeon)

• General Information

  • The patient should be given a detailed explanation of the procedure, risks, benefits, and options (including block failure and possible conversion to other modes of anesthesia/analgesia)

  • Discuss if the patient desires sedation and how much

• Laboratory Tests

  • Not globally required unless medical history or medications indicate possible coagulation disorder

• Imaging

  • Magnetic resonance imaging and/or computed tomography (CT) are not necessary except in case-by-case instances, in patients with specific intracranial or spinal abnormalities

8

Where is the epidural space? Describe the relevant anatomy.

• The epidural space lies just outside and envelops the dural sac containing the spinal cord and CSF. As the epidural needle enters the midline of the back between the bony spinous processes, it passes through:

• From superficial to deep: skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space.

• Deep to the epidural space lies the spinal meninges and CSF. The epidural space has its widest point (5 mm) at L2. In addition to the traversing nerve roots, it contains fat, lymphatics, and an extensive venous plexus (Batson plexus). The epidural space wraps 360 degrees around the dural membrane. Superiorly, the space extends to the foramen magnum, where the dura is fused to the base of the skull. Caudally, it ends at the sacral hiatus. The most anterior boundary of the epidural space is the posterior longitudinal ligament, situated along the posterior surface of the vertebral bodies. The epidural space can be entered in the cervical, thoracic, lumbar, or sacral regions to provide anesthesia. In pediatric patients, the caudal epidural approach is commonly used.

9

Describe the technique for performing a lumbar epidural anesthetic.

• Ensure that resuscitation equipment is immediately available: oxygen, equipment for positive-pressure ventilation and intubation, and vasoactive medications to treat blood pressure changes.

• Monitor the patient with at least a pulse oximeter and blood pressure cuff.

• Place a well-running intravenous (IV) line and consider an appropriate coload of fluid to counteract hypotension after sympathetic blockade.

• The patient may be sitting or lying laterally. The spinous processes should be aligned in the same vertical or horizontal plane and maximally flexed. Administer sedation as appropriate.

• For lumbar epidural placement, visualize a line between the iliac crests to locate the L4 spinous process. Palpate the L2–L3, L3–L4, and L4–L5 interspaces and choose either the widest or the closest to the desired anesthetic level. For abdominal or thoracic surgeries, catheter insertion site should reflect the desired dermatomal coverage.

• The anesthesiologist or anesthesia provider must wear a hat, mask, and sterile gloves and remove all jewelry, including watches. The patient should wear a hat. Any additional providers or family members should wear a hat and mask. The site should be prepped/draped.

• A skin wheal is made with local anesthetic at the desired insertion site. The epidural needle is most often inserted in the midline. Once initial resistance from ligaments is felt, remove the needle stylet and attach a syringe with 3 to 4 mL of air or saline. When the barrel of the syringe is tapped, it should feel firm and bounce back, while the tip of the needle is in the ligament.

• Advance several millimeters at a time, tapping the syringe intermittently. The ability to recognize the feel of various layers of ligament comes with experience. Ligamentum flavum is often described as leathery, gritty, or producing a marked increase in resistance. This is the last layer before the epidural space.

• As the needle passes through ligamentum flavum and enters the epidural space, there is often a pop or give, and the air or fluid in the syringe injects easily; this marks the loss of resistance.

• While one hand grasps the hub of the epidural needle to brace it, the other threads the catheter about 5 cm into the epidural space (5 cm past the tip of the needle).

• The epidural needle is withdrawn carefully so as not to dislodge the catheter. After attaching the injector port to the catheter, it is aspirated for blood or CSF; if negative, a test dose may be given. The catheter is then secured in place.

10

What is the paramedian technique for epidural catheter placement? When is it useful?

• Paramedian epidural placement starts by identifying the spinous process at the desired interspace level and using local anesthetic to create a skin wheal 1 to 2 fingerbreadths lateral to this point.

• Insert the epidural needle through the wheal perpendicular to the patient's skin, until the transverse process of the vertebra is encountered.

• Walk the epidural needle off the transverse process, moving superiorly and medially (about 15–30 degrees, angulation—do not cross midline), until the needle is engaged in the ligamentum flavum.

• Attach a friction-free syringe and advance until loss of resistance is achieved.

• This approach is useful at high thoracic levels (above ≈ T9–T10), where the spinous processes are acutely angled downward. It can also be advantageous when the spaces between spinous processes are small, such as in patients with limited mobility who cannot “open” their spaces (i.e., provide relative lumbar or thoracic kyphotic bend).

11

How is caudal anesthesia related to epidural anesthesia? When is it used?

Caudal anesthesia is a form of epidural anesthesia in which the injection is made at the sacral hiatus (S5). Because the dural sac normally ends at S2, accidental spinal injection is rare. Although the caudal approach to the epidural space provides dense sacral and lower lumbar levels of block, its use in adults is limited by the following:

• Highly variable sacral anatomy

• Calcification/ossification of the sacral ligaments

• Risk of injection into a venous plexus

• Difficulty in maintaining sterility if a catheter is used

Caudal anesthesia is used primarily in children (whose anatomy is predictable) to provide postoperative analgesia after herniorrhaphy or perineal procedures. A catheter can be inserted for long-term use if desired.

12

What is a combined spinal-epidural anesthetic or dural-puncture epidural? What are the advantages and disadvantages of these?

A dural-puncture epidural (DPE) is performed by passing a long spinal needle through an epidural needle that has been placed into the epidural space. In this way the dura is punctured and CSF in the spinal needle acts as a confirmation of placement of the epidural needle adjacent to the dura (i.e., “epidural”) and in the midline of the space. A combined spinal-epidural (CSE) is the same technique, but additionally when CSF is confirmed in the spinal needle, a dose of local anesthetic with or without adjuvants (e.g., small dose of opioid) is deposited in the subarachnoid space. In this case, onset of block is markedly faster. In either case, the spinal needle is removed after confirmation/injection and the epidural catheter is then placed. Either technique combines the advantages of both spinal and epidural anesthesia and both techniques result in a more reliable epidural catheter. Note that these techniques can only be performed in the lumbar area (below L1–L2 in adults, below L3 in children) because of risk of spinal cord entry at higher levels.

13

What physiological changes should be expected after successful initiation of an epidural anesthetic?

• Decrease in blood pressure: Detailed discussion in question 22.

• Changes in heart rate: Tachycardia may occur as cardiac output increases to compensate for the drop in systemic vascular resistance. Bradycardia may also occur and is discussed in question 23.

• Ventilatory changes: In normal patients, ventilation is maintained as long as the diaphragm is not impaired (phrenic nerve: C3–C5), but patients may become subjectively dyspneic, as they become unable to feel their intercostal muscles. Patients who are dependent on the accessory muscles of respiration may experience significant respiratory distress with sedation. Note that the ability to cough and protect the airway may be lost even if ventilation is adequate. Upper extremity weakness or speech changes are signs of an evolving high block and the possibility of impending ventilatory failure.

• Bladder distention: Sympathetic blockade and loss of sensation may result in bladder atony and require catheterization. Typically, minimal with epidural placement above T9, but patients should be monitored for urinary retention.

• Change in thermoregulation: See question 28.

• Neuroendocrine changes: Neural blockade above T8 blocks sympathetic afferents to the adrenal medulla, inhibiting the neural component of the stress response. Note that this may result in better glucose regulation in some diabetic patients.

14

What are the usual doses of common local anesthetics used in epidural anesthesia?

• See Table 71.1 .

  Table 71.1

  Local Anesthetic Dosing for Spinal Anesthesia

  	Suggested Doses (mg)			Duration of Effect (Minutes)
  Usual concentration	Lower extremities and perineum	Lower abdomen	Upper abdomen	Without epinephrine	With epinephrine
  Lidocaine 5% in dextrose	25–50	50–75	75–100	60–75	75–90
  Bupivacaine 0.75% in dextrose	5–10	12–14	12–18	90–120	100–150
  Ropivacaine 0.25%–1%	8–12	12–16	16–18	90–120	90–120
  Tetracaine 1% in dextrose	4–8	8–12	10–16	90–120	120–240
  Ropivacaine 0.5% in dextrose	8–12	12–16	16–18	80–110	—
  Levobupivacaine 0.5%	8–10	10–15	12–20	90–120	100–150

15

What anatomic structures are trespassed when attempting to enter the intrathecal space?

From superficial to deep, the structures include: skin, subcutaneous/adipose tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, dura mater, and arachnoid membrane. Note that the dura mater and arachnoid are not trespassed in successful epidural placement.