As with spinal anesthesia, the key to carrying out successful epidural anesthesia is understanding the midline neuraxial anatomy. As illustrated in
, it is essential for the anesthesiologist to create a three-dimensional image of the neuraxial midline structures that underlie the palpating fingers. When a lumbar approach to the epidural space is used in adults, the depth from the skin to the ligamentum flavum is commonly about 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 2.0 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 (
). In the cervical region the depth to the ligamentum flavum is approximately the same as that of the lumbar region (i.e., 4–6 cm).
As with spinal anesthesia, the key to carrying out successful epidural anesthesia is understanding the midline neuraxial anatomy. As illustrated in Figure 49-1 , it is essential for the anesthesiologist to create a three-dimensional image of the neuraxial midline structures that underlie the palpating fingers. When a lumbar approach to the epidural space is used in adults, the depth from the skin to the ligamentum flavum is commonly about 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 2.0 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. 49-2 ). In the cervical region the depth to the ligamentum flavum is approximately the same as that of the lumbar region (i.e., 4–6 cm).
It is emphasized that if the needles are kept in the midline, the ligamentum flavum is perceived as a thicker ligament than if the needles are inserted off the midline and enter the lateral extension of the ligamentum flavum. Figure 49-3 illustrates how important it is to maintain the midline position of the epidural needle during lumbar epidural techniques. If an oblique approach is taken, a “false release” can be produced ( Fig. 49-3 , needle C), or a perception of a “thin” ligament can be reinforced (needle B).
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. 49-4 ). When choosing a needle for epidural anesthesia, a decision must be made about whether a continuous or a 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, either a Tuohy or other needle with a lateral-facing opening is chosen.
The midline approach is most often indicated when a lumbar epidural procedure is to be carried out. The needle is inserted into the midline in the same way as that used for spinal anesthesia. With the epidural technique, the needle is slowly advanced until a 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. As illustrated in Figure 49-5 , the syringe is attached to the needle; and if the needle tip is in the substance of the ligamentum flavum, the air bubble is compressible, as illustrated in Figure 49-5 B . If the ligamentum flavum has not yet been reached, pressure on the syringe plunger does not compress the air bubble (see Fig. 49-5 A ). 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 allows the solution to flow without resistance into the epidural space. An alternative technique, although one that has a less precise endpoint I believe, is the hanging-drop identification of entry into the epidural space. With this technique, when the needle is placed in the ligamentum flavum a drop of solution is placed in the hub of the needle. No syringe is attached; and, when the needle is advanced into the epidural space, the solution is “sucked into” the space ( Fig. 49-6 ).
No matter what method is chosen for needle insertion, when the epidural space is cannulated with a catheter, the chance of success may be increased by advancing the needle 1 to 2 mm farther once the space has been identified. Additionally, the incidence of unintentional intravenous cannulation with an epidural catheter may be decreased by injecting 5 to 10 mL of solution prior to 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. 49-7 ). With this technique, the paramedian approach is preferred because it allows easier access to the epidural space. This is true because the spines in the mid-thoracic region overlap each other from cephalad to caudad ( Fig. 49-8 ). 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 results in contact with the thoracic vertebral lamina by the epidural needle ( Fig. 49-9 ). When this occurs, the needle is withdrawn slightly and the tip is 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 method is an alternative method for identifying the thoracic epidural space, although the classic Bromage needle-syringe grip is my first choice for the thoracic epidural block ( Fig. 49-10 ).
For the cervical epidural technique, the patient is typically in a sitting position with the head bent forward and supported on a table ( Fig. 49-11 ). 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 (C7 and T1) are identified with the neck flexed. 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) ( Fig. 49-12 ). Abutment of the needle onto the ligamentum flavum is appreciated at a depth similar to that seen in the lumbar epidural block (3.5–5.5 cm), and needle placement is then performed using the same loss-of-resistance technique used with any of the epidural methods. Again, the hanging-drop method is also an option for identifying the cervical epidural space.
One of the most feared complications of epidural anesthesia is systemic toxicity resulting from intravenous injection of the intended epidural anesthetic ( Fig. 49-13 ). This can occur with either catheter or needle injection. One way to avoid intravenous injection of the pharmacologic doses of local anesthetic needed for epidural anesthesia is to administer a test dose prior to the epidural injection. The current recommendation is to use 3 mL of local anesthetic solution containing 1: 200,000 epinephrine (15 μg of epinephrine). Despite a negative test dose, the anesthesiologist should inject the solution incrementally, be vigilant in watching for unintentional intravascular injection, and have all necessary equipment and drugs available to treat local anesthetic-induced systemic toxicity.
Another problem that can occur with epidural anesthesia is the unintentional administration of an epidural dose into the spinal fluid. In this event, as with any neuraxial block that reaches high sensory levels, the blood pressure and heart rate should be supported pharmacologically, and ventilation is assisted as indicated. Usually atropine and ephedrine suffice to manage this situation or at least provide time to administer more potent catecholamines. If the entire dose (20–25 mL) of local anesthetic is administered into the cerebrospinal fluid (CSF), tracheal intubation and mechanical ventilation are indicated because it will be approximately 1 to 2 hours before the patient consistently maintains adequate spontaneous ventilation after such an event. When epidural anesthesia is performed and a higher than expected block develops after a delay only of 15 to 30 minutes, subdural placement of the local anesthetic must be considered. Treatment is symptomatic, the most difficult part of treatment involving recognition that a subdural injection is possible.
As with spinal anesthesia, if neurologic injury occurs following epidural anesthesia, a systematic approach to the problem is necessary. No particular local anesthetic, 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 use 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 epidural needles or catheters. This probably happens less frequently than severe neurologic injury following general anesthesia. Concern about epidural hematoma formation is increased in patients who have been taking antiplatelet drugs, such as aspirin, or who have been receiving anticoagulants preoperatively. The magnitude of an acceptable level of preoperative anticoagulation and the risk-benefit ratio of using epidural anesthesia remain indeterminate at this time. The use of epidural techniques in patients undergoing 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. One perioperative anticoagulant regimen that demands special consideration is the use of low-molecular-weight heparin (LMWH) concurrently with epidural block. LMWH products are used for prophylaxis of deep venous thrombosis and produce 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.
As with spinal anesthesia, postdural puncture headache can result from epidural anesthesia when unintentional subarachnoid puncture occurs. When using the larger-diameter epidural needles (18 and 19 gauge), it can be expected that at least 50% of patients who have had an unintentional dural puncture will have a headache postoperatively.