Applied anatomy of the epidural space

The epidural space, a potential space surrounding the spinal meninges, contains fat, nerve roots, and vascular plexuses. The anatomy of the spine, ligaments, meninges, and blood flow throughout the spinal cord are described in detail in Chapter 57. Knowledge of surface anatomy (Figure 123-1) and key anatomic features of the cervical, thoracic, and lumbar spinal regions (Box 123-1) are critical to the performance of safe and reliable epidural needle placement.

All segments of the spinal canal from the base of the skull to the sacral hiatus are accessible to epidural injection. Epidural anesthesia, provided either alone or in combination with general anesthesia, may be adapted to almost any surgical procedure that takes place below the level of the patient’s chin. Ideally, needle and catheter placement should occur at the level of the surgical incision (e.g., lumbar placement for lower extremity operations and thoracic placement for thoracic/abdominal operations) to allow for block of only the parts of the body that fall within the surgical field. However, a lumbar technique may be used for even upper abdominal procedures, although it would result in a complete sympathectomy, including potentially blocking the cardiac accelerator fibers. Assessment of the dermatomal sensory level enables the anesthesiologist to determine approximate level of sympathectomy and anticipate the resulting hemodynamic effects (Table 123-1).

Identification of the epidural space

The epidural space may be approached using a midline or a paramedian needle insertion (Figure 123-2). The epidural space is identified by the passage of the needle from an area of high resistance (ligamentum flavum) to an area of low resistance (epidural space). After the needle is positioned in the ligamentum flavum, a syringe with a freely movable plunger is attached, and continuous pressure is applied to the plunger. If the needle is positioned correctly in the ligament, the syringe should not inject when pressure is applied to the plunger. As the needle passes into the epidural space, a sudden loss of resistance in the plunger will be felt, and the air or fluid will easily inject. At this point, a flexible nylon catheter may be advanced 3 to 4 cm through the needle into the epidural space to allow repeated and incremental injections. Preinsertion ultrasound imaging has been demonstrated to accurately identify the level of the vertebrae as well as to estimate the depth of the epidural space (Figure 123-3).

A test dose of 3 mL of local anesthetic solution (typically lidocaine, 1.5%) containing 1:200,000 epinephrine is then injected, and the patient is observed for signs of intravascular, subdural, or subarachnoid injection. An increase in systolic blood pressure of at least 15 mm Hg or an increase in heart rate of at least 10 beats/min represents intravascular injection, whereas a change in lower extremity sensation (with or without a decrease in blood pressure) denotes subdural or subarachnoid injection.

Selection and dose of local anesthetic agent

The major sites of action of epidurally injected local anesthetics are the spinal nerve roots, where the dura is relatively thin. Only a small amount of local anesthetic agent actually diffuses across the dura into the subarachnoid space. A local anesthetic agent should be selected on the basis of speed of onset, degree of motor blockade required, and duration of the surgical procedure (Table 123-2).

Local anesthetic dose may be calculated by the following formula: dose equals 1 to 1.5 mL of local anesthetic agent per segment blocked. The dose may need to be significantly reduced in parturients and in obese and elderly patients. In these situations, incremental doses are advised. A second dose of approximately 50% of the initial dose will maintain the original level of anesthesia if injected when the blockade has regressed 1 or 2 dermatomes (see Table 123-2).

The addition of epinephrine can prolong the duration of lidocaine nerve block by up to 50%. Less dramatic results are usually observed when bupivacaine or etidocaine is used. The addition of vasoconstricting agents reduces blood flow in the richly vascularized epidural space, reducing systemic absorption; because more of the drug remains in proximity to the nerve, the onset of block is quicker and the duration of action is longer. Confirmation of this concept comes from studies demonstrating that the peak plasma levels of various agents are lower when epinephrine is present. Epinephrine also acts on α-adrenergic receptors located in the central nervous system, modulating central pain processing at those sites.

Complications

Although severe or disabling neurologic complications are rare with the use of epidural anesthesia, the results of recently conducted surveys suggest that the frequency of some serious complications, including spinal hematoma and central nervous system infections, is increasing. An epidemiologic study evaluating severe neurologic complications after neuraxial block conducted in Sweden between 1990 and 1999 reported some disturbing trends. During the 10-year study period, approximately 1,260,000 spinal and 450,000 epidural (including 200,000 epidural blocks for labor analgesia) were performed. A total of 127 serious complications were noted, including spinal hematoma (33), cauda equina (32), meningitis (29), and epidural abscess (13). The nerve damage was permanent in 85 patients. Complications occurred more often after epidural than after spinal blockade and were different in character, with cauda equina syndrome, spinal hematoma, and epidural abscess more likely to occur after epidural block and meningitis more often associated with a spinal technique. Undiagnosed spinal stenosis (detected during evaluation of the new neurologic deficits) was a risk factor for cauda equina syndrome and paraparesis. The results of this large series suggest that the incidence of severe anesthesia-related complications is not as low as previously reported.