CHAPTER 66 Epidural Analgesia and Anesthesia
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:
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:
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?
Avoidance of airway manipulation; useful for asthmatics, known difficult airways, and patients with a full stomach
Improved bowel motility with less distention; sympathetic blockade provides relatively more parasympathetic tone
The patient can be awake during the procedure; desirable for cesarean deliveries and certain arthroscopic procedures
Better postoperative pain control, especially for thoracic, upper abdominal, and orthopedic procedures
6 What are the advantages of epidural anesthesia over spinal anesthesia?
Epidural anesthesia can produce a segmental block focused only on the area of surgery or pain (e.g., during labor or for thoracic procedures).
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.
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?
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.
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.
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:
History
Neurologic symptoms or history of neurologic disease (e.g., diabetic neuropathy, multiple sclerosis)
General information
The patient should be given a detailed explanation of the procedure, risks, benefits, and options (including general anesthesia if the block fails).
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