41 Epidural Block
Perspective
Epidural anesthesia is the second primary method of neuraxial block. In contrast to spinal anesthesia, epidural block requires pharmacologic doses of local anesthetics, making systemic toxicity a concern. In skilled hands, the incidence of post–dural puncture headache should be lower with epidural anesthesia than with spinal anesthesia. Nevertheless, as outlined in Chapter 40, Spinal Block, I do not believe this should be the major differentiating point between the two techniques. Spinal anesthesia is typically a single-shot technique, whereas frequently intermittent injections are given through an epidural catheter, thus allowing reinjection and prolongation of epidural block. Another difference is that epidural block allows production of segmental anesthesia. Thus, if a thoracic injection is made and an appropriate amount of local anesthetic is injected, a band of anesthesia that does not block the lower extremities can be produced.
Placement
Anatomy
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.
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).
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.