As with spinal anesthesia, the key to carrying out successful epidural anesthesia is understanding the three-dimensional midline neuraxial anatomy that underlies the palpating fingers (Fig. 41-1). When a lumbar approach to the epidural space is used in adults, the depth from the skin to the ligamentum flavum is commonly near 4 cm; in 80% of patients the epidural space is cannulated at a distance of 3.5 to 6 cm from the skin. In a small number of patients the lumbar epidural space is as near as 2 cm from the skin. In the lumbar region, the ligamentum flavum is 5 to 6 mm thick in the midline, whereas in the thoracic region it is 3 to 5 mm thick. In the thoracic region, the depth from the skin to the epidural space depends on the degree of cephalad angulation used for the paramedian approach as well as the body habitus of the patient (Fig. 41-2). In the cervical region the depth to the ligamentum flavum is approximately the same as that in the lumbar region, 4 to 6 cm.

Figure 41-1. Epidural block: cross-sectional anatomy.

Figure 41-2. Thoracic epidural block anatomy: overlapping of mid-thoracic spinous processes requires a paramedian technique. A, Cross-section, superior view. B, Lateral view, paramedian section.

The ligamentum flavum will be perceived as a thicker ligament if the needle is kept in the midline than if the needle is inserted off the midline and enters the lateral extension of the ligamentum flavum. Figure 41-3 illustrates how important it is to maintain the midline position of the epidural needle (needle A) during lumbar epidural techniques. If an oblique approach is taken, a “false release” can be produced (needle C) or the perception of a thin ligament can be reinforced (needle B).

Figure 41-3. Epidural block: functional anatomy of ligamentum flavum.

Position

Patient positioning for epidural anesthesia is similar to that for spinal anesthesia, with lateral decubitus, sitting, and prone jackknife positions all applicable. The lateral decubitus position is applicable for both lumbar and thoracic epidural techniques, and the sitting position allows the administration of lumbar, thoracic, and cervical epidural anesthetics. The prone jackknife position allows access to the caudal epidural space.

Needle Puncture: Lumbar Epidural

A technique similar to that used for spinal anesthesia should be carried out to identify the midline structures, and the bony landmarks should be used to determine the vertebral level appropriate for needle insertion (Fig. 41-4). When choosing a needle for epidural anesthesia, one must decide whether a continuous or single-shot technique is desired. This is the principal determinant of needle selection. If a single-shot epidural technique is chosen, a Crawford needle is appropriate; if a continuous catheter technique is indicated, a Tuohy or other needle with a lateral-facing opening is chosen.

Figure 41-4. Neuraxial anatomy: surface relationships.

The midline approach is most often indicated for a lumbar epidural procedure. The needle is inserted into the midline in the same way as for spinal anesthesia. In the epidural technique, the needle is slowly advanced until the change in tissue resistance is noted as the needle abuts the ligamentum flavum. At this point, a 3- to 5-mL glass syringe is filled with 2 mL of saline solution, and a small (0.25 mL) air bubble is added. The syringe is attached to the needle, and if the needle tip is in the substance of the ligamentum flavum, the air bubble will be compressible (Fig. 41-5A). If the ligamentum flavum has not yet been reached, pressure on the syringe plunger will not compress the air bubble (Fig. 41-5B). Once compression of the air bubble has been achieved, the needle is grasped with the nondominant hand and pulled toward the epidural space, while the dominant hand (thumb) applies constant steady pressure on the syringe plunger, thus compressing the air bubble. When the epidural space is entered, the pressure applied to the syringe plunger will allow the solution to flow without resistance into the epidural space. An alternative technique, although one that I believe has a less precise end point, is the hanging-drop technique for identifying entry into the epidural space. In this technique, when the needle is placed in the ligamentum flavum a drop of solution is introduced into the hub of the needle (Fig. 41-6A). No syringe is attached, and when the needle is advanced into the epidural space, the solution should be “sucked into” the space (Fig. 41-6B).

Figure 41-5. A and B, Epidural block: loss-of-resistance technique showing bubble compression (A) and noncompression (B).

Figure 41-6. Epidural block: hanging-drop technique.

No matter what method is chosen for needle insertion, when the epidural space is cannulated with a catheter, success may be increased by advancing the needle 1 to 2 mm farther once the space has been identified. In addition, the incidence of unintentional intravenous cannulation with an epidural catheter may be decreased by injecting 5 to 10 mL of solution before threading the catheter. If a catheter is inserted, it should be inserted only 2 to 3 cm into the epidural space because threading it farther may increase the likelihood of catheter malposition. Obstetric patients require catheters to be inserted to 3 to 5 cm into the epidural space to minimize dislodgement during labor analgesia.

Needle Puncture: Thoracic Epidural

As with lumbar epidural anesthesia, patients are usually placed into a lateral decubitus position for needle insertion into the thoracic epidural space (Fig. 41-7). In this technique, the paramedian approach is preferred because it allows easier access to the epidural space. This is because the spinous processes in the mid-thoracic region overlap each other from cephalad to caudad (Fig. 41-8). The paramedian approach is carried out in a manner similar to that used for the lumbar epidural space, although in almost every instance the initial needle insertion will result in contact with the thoracic vertebral lamina by the epidural needle (Fig. 41-9). When this occurs, the needle is withdrawn slightly and the tip redirected cephalad in small, incremental steps until the needle is firmly seated in the ligamentum flavum. At this point, the loss-of-resistance technique and insertion of the catheter are carried out in a manner identical to that used for lumbar epidural block. Again, the hanging-drop technique is an alternative method of identifying the thoracic epidural space, although the classic Bromage needle–syringe grip is my first choice for the thoracic epidural block (Fig. 41-10).

Figure 41-7. Thoracic epidural block anatomy: mid-thoracic spine. A, Posteroanterior view. B, Oblique view. C, Lateral view. D, Lateral view after removal of right vertebral arch. E, Patient in left lateral decubitus position for thoracic epidural anesthesia.

Figure 41-8. Thoracic vertebral anatomy: degree of spinous process overlap changes from high thoracic to mid-thoracic to low thoracic. A, Oblique view. B, Lateral view and paramedian section.

Figure 41-9. Thoracic epidural block technique. A, Using the paramedian approach, needle insertion site is 1 cm caudad and 1 cm lateral to the tip of the more cephalad spinous process, similar to the needle insertion used in the lumbar paramedian technique. B, Parasagittal view of needle insertion and initial contact with lamina (blue shading).

Figure 41-10. Thoracic epidural block technique: Bromage grip for loss-of-resistance technique in thoracic block.

Needle Puncture: Cervical Epidural

In the cervical epidural technique, the patient is typically in a sitting position with the head bent forward and supported on a table (Fig. 41-11). A comparison of the cervical epidural block with the lumbar epidural block reveals many similarities. The spinous processes of the cervical vertebrae are nearly perpendicular to the long axis of the vertebral column; thus, a midline technique is applicable for the cervical epidural block. The most prominent vertebral spinous processes, those of C7 and T1, are identified with the neck flexed (Fig. 41-12). The second (index) and third fingers of the palpating hand straddle the space between C7 and T1, and the epidural needle is slowly inserted in a plane approximately parallel to the floor (or parallel to the long axis of the cervical vertebral spinous processes). Abutment of the needle onto the ligamentum flavum will be appreciated at a depth similar to that seen in the lumbar epidural block (i.e., 3.5 to 5.5 cm), and needle placement is then performed using the loss-of-resistance technique as in the other epidural methods. The hanging-drop method is also an option for identification of the cervical epidural space.

Figure 41-11. Cervical epidural anatomy. A, Patient sitting with head supported by table, and plane of vertebral cross-section. B, Posterior view. C, Vertebral cross-section at C7-T1.

Figure 41-12. Cervical epidural technique. A, Patient sitting with head supported by table with needle oriented parallel to floor. B, Application of fingers to posterior neck to facilitate cervical epidural block. C, Insertion of needle into ligamentum flavum. D, Insertion of needle during palpation. E, Bromage grip during needle advancement.

Potential Problems

One of the most feared complications of epidural anesthesia is systemic toxicity resulting from intravenous injection of the intended epidural anesthetic (Fig. 41-13). This can occur with either catheter or needle injection. One way to minimize intravenous injection of the pharmacologic doses of local anesthetic needed for epidural anesthesia is to verify needle or catheter placement by administering a test dose before the definitive epidural anesthetic injection. The current recommendation for the test dose is 3 mL of local anesthetic solution containing 1:200,000 epinephrine (15 µg of epinephrine). Even if the test dose is negative, the anesthesiologist should inject the epidural solution incrementally, be vigilant for unintentional intravascular injection, and have all necessary equipment and drugs available to treat local anesthetic–induced systemic toxicity.

Figure 41-13. Epidural block: cross-sectional anatomy, showing potential incorrect injection sites. IV, intravenous.

Another problem that can occur with epidural anesthesia is the unintentional administration of an epidural dose into the spinal fluid. In this event, as when any neuraxial block reaches high sensory levels, blood pressure and heart rate should be supported pharmacologically and ventilation should be assisted as indicated. Usually atropine and ephedrine will suffice to manage this situation, or at least will provide time to administer more potent catecholamines. If the entire dose (20 to 25 mL) of local anesthetic is administered into the cerebrospinal fluid, tracheal intubation and mechanical ventilation are indicated because it will be approximately 1 to 2 hours before the patient can consistently maintain adequate spontaneous ventilation. When epidural anesthesia is performed and a higher-than-expected block develops after a delay of only 15 to 30 minutes, subdural placement of the local anesthetic must be considered. Treatment is symptomatic, with the most difficult part involving recognition that a subdural injection is possible.

As with spinal anesthesia, if neurologic injury occurs after epidural anesthesia, a systematic approach to the problem is necessary. No particular local anesthetic, use of needle versus catheter technique, addition or omission of epinephrine, or location of epidural puncture seems to be associated with an increased incidence of neurologic injury. Despite this observation, the performance of cervical or thoracic epidural techniques demands special care with hand and needle control because the spinal cord is immediately deep to the site of both these epidural blocks.

An additional problem with epidural anesthesia is the fear of creating an epidural hematoma with the needles or catheters. This probably happens less frequently than severe neurologic injury after general anesthesia. Concern about epidural hematoma formation is greater in patients who have been taking antiplatelet drugs such as aspirin or who have been receiving preoperative anticoagulants. The magnitude of an acceptable level of preoperative anticoagulation and the risk–benefit calculation of performing epidural anesthesia in the anticoagulated patient remain indeterminate at this time. The use of epidural techniques in patients receiving subcutaneous heparin therapy is probably acceptable if the block can be performed atraumatically, although the risk–benefit ratio of the technique must be weighed for each patient. Perioperative anticoagulant regimens that demand special consideration are the use of low–molecular-weight heparin (LMWH) or potent antiplatelet drugs concurrently with epidural block. LMWH is used for prophylaxis of deep venous thrombosis and produces more profound effects than other intermittently dosed heparin products. It is currently recommended that no procedure, including withdrawal or manipulation of an epidural catheter, should occur within 12 hours after a dose of LMWH, and the next dose of LMWH should be delayed for at least 2 hours after atraumatic epidural needle or catheter insertion or manipulation. The antiplatelet drugs (e.g., ticlopidine, clopidogrel, and platelet glycoprotein IIb/IIIa receptor antagonists) are sometimes combined with aspirin and other anticoagulants. Expert guidelines need to be consulted when using regional blocks in the increasing number of patients on antiplatelet compounds.

As in spinal anesthesia, post–dural puncture headache can result from epidural anesthesia when unintentional subarachnoid puncture accompanies the technique. When using the larger-diameter epidural needles (18 and 19 gauge), it can be expected that at least 50% of patients experiencing unintentional dural puncture will have a postoperative headache.

Pearls

Avoiding catheters during epidural anesthesia—that is, by selecting an appropriate local anesthetic—can avoid a potential source of difficulty with the technique. Epidural catheters can be malpositioned in a number of ways. If a catheter is inserted too far into the epidural space, it can be routed out of foramina, resulting in patchy epidural block. The catheter can also be inserted into the subdural or subarachnoid space or into an epidural vein. Similarly, the use of epidural catheters may be complicated by a prominent dorsomedian connective tissue band (epidural septum or fat pad), which is found in some patients.

Another means of facilitating the success of epidural anesthesia is to allow the block enough “soak time” before beginning the surgical procedure. This is most effectively accomplished if the block is carried out in an induction room separate from the operating room. There appears to be a plateau effect in the doses of epidural local anesthetics; that is, once a certain quantity of local anesthetic has been injected, more of the same agent does not significantly increase the block height but rather may make the block denser, perhaps improving quality.

One observation about epidural anesthesia through a catheter that needs to be emphasized is the often faulty clinical logic that, by giving incremental doses through a catheter, the level of sensory anesthesia can be slowly developed, thereby allowing frail and physiologically compromised patients to undergo epidural anesthesia. However, when this approach is taken, anesthesiologists usually do not allow enough time between injections because of the reality of time pressures in the normal operating room. They inject small doses through the catheter but then do not allow sufficient time to pass before performing the next incremental injection. Often the clinical result is high block levels in just those patients in whom lower levels were the goal. Furthermore, this approach to epidural anesthesia unnecessarily delays preparing the patient for the operation and makes surgical and nursing colleagues less accepting of the technique.

Epidural catheters are indicated in many situations, especially when the technique is used for postoperative analgesia. To place a known length of catheter into the epidural space, either the catheter and needle must have distance markers, or a way must be found to maintain the catheter position once the needle has been withdrawn over the catheter. Because some epidural needles do not have distance markers, a method of maintaining catheter position while the needle is withdrawn over the catheter is required. One technique of positioning the catheter is illustrated in Figure 41-14. An object of known length, such as a syringe or the anesthesiologist’s finger, is selected, and that object is placed next to the needle–catheter assembly after the catheter has been inserted 3 cm (or other known distance) into the epidural space. Because the catheter is marked, a known point on the catheter can be related to a known point on either the finger or the syringe. As shown in Figure 41-14A, the 15-cm mark is opposite the plunger on the syringe or the anesthesiologist’s knuckle. Once this relationship has been noted, the needle is removed while the catheter position is maintained. The measurement object is then placed next to the catheter, as illustrated in Figure 41-14B, and the catheter is withdrawn to the point at which the distance marker on the catheter relates to the previously identified point. In this example, the 15-cm mark on the catheter is placed opposite the plunger of the syringe or the anesthesiologist’s knuckle. By using this technique, the epidural catheter can be accurately placed without the need for either a marked needle or a ruler.