66: Epidural Analgesia and Anesthesia

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CHAPTER 66 Epidural Analgesia and Anesthesia

Joy L. Hawkins, MD

1 Where is the epidural space? Describe the relevant anatomy

The epidural space lies just outside the dural sac containing the spinal cord and cerebrospinal fluid (CSF). As the epidural needle enters the midline of the back over the bony spinous processes, it passes through:

image Skin
image Subcutaneous fat
image Supraspinous ligament
image Interspinous ligament
image Ligamentum flavum
image Epidural space

Beyond the epidural space lie 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. Superiorly the space extends to the foramen magnum, where dura is fused to the base of the skull. Caudally it ends at the sacral hiatus. 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 (see Question 3).

2 Differentiate between a spinal and an epidural anesthetic

When performing a spinal anesthetic, a small amount of local anesthetic drug is placed directly in the CSF, producing rapid, dense, predictable neural blockade. An epidural anesthetic requires a tenfold increase in dose of local anesthetic to fill the epidural space and penetrate the nerve coverings, and onset is slower. 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. 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 end point 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.

3 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 is limited by major problems:

image Highly variable sacral anatomy in adults
image Risk of injection into a venous plexus
image Difficulty in maintaining sterility if a catheter is used

Caudal anesthesia is primarily used 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.

4 What are the advantages of using epidural anesthesia vs. general anesthesia?

image Avoidance of airway manipulation; useful for asthmatics, known difficult airways, and patients with a full stomach
image Decreased stress response; less hypertension and tachycardia and cortisol release
image Less thrombogenesis and subsequent thromboembolism; a proven benefit in orthopedic hip surgery
image Improved bowel motility with less distention; sympathetic blockade provides relatively more parasympathetic tone
image The patient can be awake during the procedure; desirable for cesarean deliveries and certain arthroscopic procedures
image Less postoperative nausea and sedation
image Better postoperative pain control, especially for thoracic, upper abdominal, and orthopedic procedures
image Less pulmonary dysfunction, caused by both better pain control and absence of airway manipulation
image Faster turnover at the end of the case because there is no emergence time

5 What are the disadvantages of epidural compared with general anesthesia?

image Initiation is slower at the beginning of the case.
image It is less reliable, with higher failure rate.
image There are occasional contraindications, including coagulopathy, hemodynamic instability, spinal instrumentation, or patient refusal.

6 What are the advantages of epidural anesthesia over spinal anesthesia?

image Epidural anesthesia can produce a segmental block focused only on the area of surgery or pain (e.g., during labor or for thoracic procedures).
image The gradual onset of sympathetic block allows time to manage associated hypotension.
image Duration of anesthesia can be prolonged by redosing through an indwelling epidural catheter.
image There is more flexibility in the density of block; if less motor block is desired (for labor analgesia or postoperative pain management), a lower concentration of local anesthetic can be used.
image Theoretically with no hole in the dura there can be no spinal headache; however, an inadvertent dural puncture occurs 0.5% to 4% of the time with the large-bore epidural needle, and about 50% of such patients require treatment for headache. Because newer technology in spinal needles has decreased the incidence of headache requiring treatment to less than 1%, this advantage is probably no longer true.

7 What are the disadvantages of epidural compared with spinal anesthesia?

image The induction of epidural anesthesia is slower because of more complex placement, the necessity of incremental dosing of the local anesthetic, and the slower onset of anesthesia in the epidural space.
image Because a larger volume of local anesthetic is used, there is risk of local anesthetic toxicity if a vein is entered with the needle or catheter.
image Epidural anesthesia is less reliable; it is not as dense, the block can be patchy or one-sided, and there is no definite end point (comparable to seeing CSF in the needle) during placement.

8 What factors should the anesthesiologist address in the preoperative assessment before performing an epidural anesthetic? Should special laboratory tests be performed?

In addition to the general preoperative assessment of every patient before surgery, the following specific items should be assessed before performing epidural anesthesia:

image History

image Previous back injury or surgery
image Neurologic symptoms or history of neurologic disease (e.g., diabetic neuropathy, multiple sclerosis)
image Bleeding tendencies or disease associated with coagulopathy (e.g., preeclampsia)
image Prior regional anesthesia and any associated problems

image Physical examination

image Brief neurologic examination for strength and sensation
image Back examination for landmarks and potential anatomic abnormalities (scoliosis) or pathology (infection at the site of placement)

image Surgery

image Expected duration and blood loss
image Positioning required
image Amount of muscle relaxation necessary

image General information

image The patient should be given a detailed explanation of the procedure, risks, benefits, and options (including general anesthesia if the block fails).
image Discuss whether the patient desires sedation and how much.

image Laboratory tests

image None is specifically necessary except as based on the history and physical examination.

9 Describe the technique for performing a lumbar epidural anesthetic

image Resuscitation equipment must be immediately available: oxygen, equipment for positive-pressure ventilation and intubation, and pressors to treat hypotension.
image Place a well-running intravenous (IV) line and give an appropriate preload of fluid to protect against hypotension after sympathetic blockade.
image The patient may be sitting or in a lateral position. The spinous processes should be aligned in the same vertical or horizontal plane and maximally flexed. Administer sedation as deemed appropriate.
image 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 the widest or the closest to the desired anesthetic level. For abdominal or thoracic surgeries, know the appropriate level for catheter placement.
image Make a skin wheal after sterile preparation and draping of the field. The anesthesiologist must wear a hat, mask, and sterile gloves.
image The epidural needle is inserted in the midline through the skin wheal until increased 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.
image 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.
image 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.
image The syringe is removed; and, while the nondominant hand grasps the hub of the needle to brace it, the dominant hand threads the catheter about 5 cm into the space.
image 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 is given. The catheter is then taped in place.

10 Are there any contraindications to epidural anesthesia?

Absolute contraindications

image Patient refusal. Sometimes a more thorough explanation will allay the patient’s fears and make the technique acceptable. Some common concerns include:

image Having to watch surgery or remain wide awake
image Fear of a needle in the spinal cord
image Pain
image Reassure patients that a curtain will block their view and that the desired degree of sedation can be provided. For lumbar epidural placements, explain that the spinal cord ends at about L1 in adults and that the needle is placed below that level. Compare the procedure to the placement of the IV line, which also uses a 16- or 18-G needle, and explain that a local anesthetic will be used in the skin.

image Sepsis with hemodynamic instability. The induction of sympathetic blockade decreases systemic vascular resistance (SVR) even further. There is also a remote risk of epidural abscess if bacteremic blood is introduced into the epidural space without prior antibiotic coverage.
image Uncorrected hypovolemia. With ongoing hemorrhage the fall in SVR can produce severe refractory hypotension.
image Coagulopathy. If a vessel is injured within the epidural space, an epidural hematoma could form, causing neurologic damage by spinal cord compression.

Relative contraindications

image Elevated intracranial pressure
image Prior back injury with neurologic deficit
image Progressive neurologic disease such as multiple sclerosis
image Chronic back pain
image Localized infection at the injection site

11 What are the potential complications of epidural anesthesia? Can they be anticipated or prevented?

image Hypotension caused by sympathetic blockade, which may be prevented by fluid preload and patient positioning.
image Intravascular injection of local anesthetic, which can be prevented by aspirating the catheter for blood, injecting a marker such as epinephrine that will cause tachycardia if injected into a vessel, and using incremental dosing (no more than 5 ml at a time). If an intravascular injection occurs:

image Stop convulsions with an induction agent or rapid-acting anticonvulsant. Intubate the trachea, if necessary, for ventilation and airway protection.
image Treat cardiovascular collapse with pressors, inotropes, and advanced cardiac life support protocols.
image Recent work in animals and case reports in humans have supported use of intralipid infusion to treat local anesthetic toxicity from bupivacaine. The lipid may act as a sink to remove circulating bupivacaine from the circulation.

image Subarachnoid injection of a large volume of local anesthetic (“total spinal”). This can be prevented by aspirating the catheter for CSF and giving a small initial dose of local anesthetic to look for rapid onset of sensory block if the drug enters the CSF. (Remember: the onset of an epidural anesthetic is slow.) If a total spinal occurs, treat hypotension with pressors and support ventilation with positive pressure by mask or intubation.
image Postdural puncture headache caused by accidental dural puncture with the large-bore epidural needle. Although not completely preventable, this can be treated in various ways, depending on the preference of the patient and anesthesiologist. Common therapies include analgesics, caffeine, or an epidural blood patch. Factors determining choice of treatment include severity of the headache and how aggressively the patient wishes to be treated. To provide a blood patch, up to 20 ml of the patient’s blood is placed in the epidural space to seal the dural hole and elevate low CSF pressure.
image Epidural hematomas, which are extremely rare and usually occur spontaneously in clinical settings outside of the operating room rather than following neuraxial procedures. When they are associated with regional anesthesia, there is almost always a preexisting coagulopathy. Epidural hematomas present as back pain and leg weakness and must be diagnosed by computed tomography (CT) or magnetic resonance imaging. If the hematoma is not surgically decompressed in 6 to 8 hours, neurologic recovery is rare.

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

image Decrease in blood pressure: Afterload reduction can actually be useful for patients with hypertension or congestive heart failure if preload is maintained.
image Changes in heart rate: Tachycardia may occur as cardiac output increases to compensate for a drop in SVR. Bradycardia may occur if blockade above T4 disrupts the cardiac sympathetic accelerator fibers.
image 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 their accessory muscles of respiration may be impaired at lower levels of anesthesia. The ability to cough and protect the airway may be lost even if ventilation is adequate.
image Bladder distention: Sympathetic blockade and loss of sensation may require catheterization for urinary retention.
image Intestinal contraction: Sympathetic blockade with parasympathetic predominance contracts the bowel.
image Change in thermoregulation: Peripheral vasodilation lowers core body temperature if the patient is not covered. Shivering is common during epidural anesthesia.
image Neuroendocrine changes: Neural blockade above T8 blocks sympathetic afferents to the adrenal medulla, inhibiting the neural component of the stress response. Glucose control is better maintained.

13 How does one choose which local anesthetic to use?

The choice of local anesthetic is usually based on the onset, duration, safety profile, and the special clinical characteristics of the patient and surgical procedure (Table 66-1).

TABLE 66-1 Local Anesthetics used in Epidural Anesthesia for Surgical Procedures

image

14 Why is epinephrine sometimes combined with the local anesthetic? Should it be included in all cases?

Epinephrine is often added to local anesthetic solutions in a concentration of 5 mcg/ml (1:200,000) or less. There are several benefits to this practice:

image Vasoconstriction prolongs blockade by reducing uptake into the bloodstream and delaying metabolism
image Improves quality and reliability of blockade, either by increasing the available local anesthetic through decreased uptake or by an intrinsic anesthetic mechanism on central α-adrenergic receptors
image Reduces peak blood levels and risk of toxicity by slowing vascular absorption
image Helps to identify an intravascular injection when used as the marker in a test dose. If the epinephrine-containing solution is unintentionally injected into a blood vessel, tachycardia usually results

Epinephrine may be added to the local anesthetic solution for all blocks except those involving end arteries (digits, penis) or for patients in whom the tachycardia and hypertension may be detrimental (coronary artery disease, preeclampsia).

15 When should opioids be included in the epidural anesthetic?

Opioids may be combined with local anesthetic to improve the quality of the surgical block or to manage postoperative pain, either alone or with a dilute local anesthetic solution. Examples of epidural bolus doses are fentanyl 50 to 100 mcg, sufentanil 20 to 30 mcg, and morphine 2 to 5 mg. The opioids act at the mu receptors in the substantia gelatinosa of the spinal cord. The more lipophilic opioids such as fentanyl and sufentanil have fast onset (5 minutes), short duration (2 to 4 hours), and lower incidence of side effects. Morphine is hydrophilic and does not attach to the receptor as easily. It has a long onset (1 hour), long duration (up to 24 hours), and a high incidence of side effects such as itching and nausea. Respiratory depression, although rare, is the most serious concern and requires special monitoring for the duration of the drug.

KEY POINTS: Epidural Analgesia and Anesthesia image

1. Epidural anesthesia is segmental (i.e., it has an upper and a lower level). The block is most intense near the site of catheter or needle insertion and diminishes with distance.
2. Advantages include avoidance of airway manipulation, decreased stress response, less thrombogenesis, improved bowel motility, awake patient, less postoperative nausea and sedation, better postoperative pain control, and faster turnover.
3. Disadvantages include slower initiation and higher failure rate than general anesthesia.
4. Contraindications include coagulopathy, hemodynamic instability, some types of spinal instrumentation, and patient refusal.
5. Complications include hypotension caused by sympathetic blockade, intravascular injection of local anesthetic, subarachnoid injection of a large volume of local anesthetic (total spinal), postdural puncture headache, and neurologic injury.

16 Why can some patients with epidural blocks move around and even walk, whereas others have a dense motor block?

Preserving motor function is especially important in postoperative patients and laboring women. The degree of motor block can be decreased by lowering the concentration of local anesthetic and by choosing a local anesthetic with favorable sensory-motor dissociation. As local anesthetic concentration decreases, the intensity of the block decreases, and fewer motor nerves are affected. Sensory block can be augmented by the addition of epidural opioids if desired. Bupivacaine and ropivacaine provide relatively less motor block for a given amount of sensory block (so-called sensory-motor dissociation). This property accounts for much of their popularity in obstetric anesthesia. For example, a common epidural infusion for postoperative pain or labor analgesia is 0.1% bupivacaine with 2 to 5 mcg/ml fentanyl.

17 When is analgesia preferable to anesthesia?

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 or epidural opioids.

18 How do you determine the level of anesthesia needed for different types of surgeries? What is a segmental block? When is it used?

To provide adequate surgical blockade with an epidural anesthetic, it is necessary to know the innervation of the structures stimulated during the procedure. For example, a transurethral resection of the prostate requires a T8 level because the bladder is innervated by T8 through its embryologic origins. A laparotomy such as a cesarean delivery requires a T4 level to cover the innervation of the peritoneum.

Epidural anesthesia is segmental (i.e., it has an upper and lower level). The block is most intense near the site of catheter insertion and diminishes with distance. The needle and catheter should be placed as close to the site of surgery as possible (e.g, a thoracic injection is used for chest surgery, whereas a midlumbar injection is used for hip surgery). In labor the lower limit of block can be kept above the sacral nerve roots until the second stage of labor to preserve pelvic floor tone and the perineal reflex.

19 How do you determine the amount of local anesthetic solution used for different procedures? What factors affect spread in the epidural space?

The extent of epidural blockade is determined primarily by the volume of local anesthetic; more dermatomes are blocked by more milliliters of local anesthetic. To achieve a T4 level from a lumbar epidural catheter, 20 to 30 ml of solution is required. Other factors that may affect spread in the epidural space include:

image Age: older patients require less local anesthetic
image Pregnancy: requires less
image Obesity: may or may not require less
image Height: taller patients may require more

20 What is a combined spinal-epidural anesthetic? Why use both?

For a combined spinal-epidural anesthetic, a long spinal needle is passed through an epidural needle that has been placed in the epidural space, and the dura is punctured. When CSF is obtained from the spinal needle, a dose of local anesthetic is placed in the subarachnoid space, and the spinal needle is removed. The epidural catheter is then threaded into the epidural space, and the epidural needle is removed. This technique combines the advantages of both spinal and epidural anesthesia: fast onset of an intense spinal block so that the surgery can proceed quickly and an epidural catheter to extend the duration of the block if necessary for a long surgical procedure or for postoperative pain management.

21 What is a combined epidural-general anesthetic? Why give the patient two anesthetics?

In some surgical procedures controlled ventilation may be safer for the patient or necessary for the surgical procedure. Examples would be intrathoracic or upper abdominal operations. Because these procedures often result in moderate to severe postoperative pain, an epidural anesthetic can be an ideal way to provide pain relief and aid in postoperative mobilization to prevent pulmonary and thromboembolic complications. The epidural anesthetic is placed before induction of general anesthesia. By using the epidural catheter intraoperatively, smaller amounts of general anesthetic agents are required, which may result in fewer hemodynamic effects and faster awakening. At the same time the patient’s airway can be protected, ventilation controlled, and hypnosis and amnesia provided.

22 What should the anesthesiologist ask the patient postoperatively after an epidural anesthetic?

image Satisfaction with the anesthetic: Was there anything that the patient thought should have been done differently? Assess patient satisfaction and try to correct any misunderstandings.
image Regression of sensory and motor block: Is there any residual blockade? Can the patient ambulate? Does the patient have any problem with bowel or bladder function? Any of these complaints requires a thorough neurologic examination to localize the deficit. Although the complaint is usually caused by residual local anesthetic or nerve compression during the surgical procedure (which often resolves with time), in rare instances further evaluation may be needed. Depending on the pattern and severity of the neurologic dysfunction, a formal neurology consultation, electromyogram, or CT may be needed to rule out pathology in the epidural space (such as hematoma).
image Complaints of back pain: Examine the site for bruising, redness, or swelling.
image Complaints of headache: If an accidental dural puncture occurred, the patient should be followed for several days; such headaches can appear up to 1 week later.
image Adequacy of postoperative pain relief: Did any side effects of epidural narcotics (itching, nausea) require treatment?

Websites image

American Society of Regional Anesthesia and Pain Medicine

http://www.asra.com

New York Society of Regional Anesthesia

http://www.nysora.com

SUGGESTED READINGS

1. Brown D.L. Spinal, epidural and caudal anesthesia. In: Miller R.D., editor. Anesthesia. ed 6. Philadelphia: Elsevier Churchill Livingstone; 2005:1653-1683.

2. Weinberg G. Lipid infusion resuscitation for local anesthetic toxicity. Anesthesiology. 2006;105:7-8.

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