A concept important to lumbar spinal anesthesia is illustrated in
. As outlined in
, the spinous processes of the lumbar vertebrae have an almost horizontal orientation in relation to the long axis of their respective vertebral bodies. When a midline needle is inserted between the lumbar vertebral spines, it is most effective if it is placed almost perpendicular relative to the long axis of the back. To facilitate spinal anesthesia, the anesthesiologist must constantly keep in mind the midline of the patient’s body and the neuraxis in relation to the needle. As illustrated in
, when the midline needle is inserted into the cerebrospinal fluid (CSF), from the surface inward it logically first punctures the skin, the subcutaneous tissue, the supraspinous ligament, the interspinous ligament, the ligamentum flavum, the epidural space, and finally the dura and arachnoid mater to reach the CSF.
A concept important to lumbar spinal anesthesia is illustrated in Figure 48-1 . As outlined in Chapter 47 , the spinous processes of the lumbar vertebrae have an almost horizontal orientation in relation to the long axis of their respective vertebral bodies. When a midline needle is inserted between the lumbar vertebral spines, it is most effective if it is placed almost perpendicular relative to the long axis of the back. To facilitate spinal anesthesia, the anesthesiologist must constantly keep in mind the midline of the patient’s body and the neuraxis in relation to the needle. As illustrated in Figure 48-1 , when the midline needle is inserted into the cerebrospinal fluid (CSF), from the surface inward it logically first punctures the skin, the subcutaneous tissue, the supraspinous ligament, the interspinous ligament, the ligamentum flavum, the epidural space, and finally the dura and arachnoid mater to reach the CSF.
Spinal anesthesia is carried out in three principal positions: lateral decubitus ( Fig. 48-2 ), sitting ( Fig. 48-3 ), and prone jackknife ( Fig. 48-4 ). In both the lateral decubitus and sitting positions, the use of a well trained assistant is essential if the block is to be easily administered by the anesthesiologist in a time-efficient manner. As illustrated in Figure 48-2 , the assistant can help the patient assume the position of legs flexed on the abdomen and chin flexed on the chest. This is most easily accomplished by the assistant pulling the patient’s head toward the chest, placing an arm behind the patient’s knees, and “pushing” the head and knees together. The position can also be facilitated using the amount of sedation that allows the patient to be relaxed yet cooperative.
In some patients, the sitting position can facilitate location of the midline, especially in obese patients or in those with some scoliosis, which makes midline identification more difficult. As illustrated in Figure 48-3 A , the patient should assume a comfortable sitting position, with the legs placed over the edge of the operating table and the feet supported by a stool. A pillow is placed in the patient’s lap and the patient’s arms are allowed to drape over the pillow, resting on the flexed lower extremities. The assistant is positioned immediately in front of the patient, supporting the shoulders and allowing the patient to minimize lumbar lordosis while ensuring that the vertebral midline remains in a vertical position (see Fig. 48-3 B ).
Sometimes it is more time-efficient to place the patient in a prone jackknife position before administering the spinal anesthetic (see Fig. 48-4 ). An assistant is not as essential for this technique as with the lateral decubitus and sitting positions, although to make the operating room block time-efficient it is often helpful for the assistant to position the patient in the prone jackknife position while the anesthesiologist readies the spinal anesthesia tray and drugs.
In all three positions, the goal is to place the patient so the midline is readily identifiable and lumbar lordosis is reduced. Figure 48-5 shows what the lumbar anatomy looks like when the patient’s lumbar lordosis has been ineffectively reduced by poor positioning. As illustrated, the intralaminal space is small and difficult to enter with a needle in the midline. In contrast, Figure 48-6 illustrates how effective positioning can open the intralaminal space to allow easy access for subarachnoid puncture.
One of the first decisions to be made when considering spinal anesthesia is the type of needle to use. Although there are many eponyms for spinal needles, they fall into two main categories: those that cut the dura sharply and those that disrupt the dural fibers by spreading it with the cone-shaped tip. The former include the traditional disposable spinal needle, the Quincke-Babcock needle; and the latter category includes the Greene, Whitacre, and Sprotte needles. If a continuous spinal technique is chosen, the use of a Tuohy or other thin-walled, curve-tipped needle facilitates passage of the catheter. To choose a spinal needle logically, the risks and benefits of each must be understood. The use of small needles reduces the incidence of postdural puncture headache; the use of larger needles improves the tactile “sense” of needle placement, thereby increasing operator confidence.
It is probable that the risk-benefit equation is not as simple as this. For example, the use of a small needle, such as a 27-gauge needle, does not decrease the incidence of headache in younger patients if a number of “passes” through the dura are required before CSF flow is recognized. Likewise, a larger needle, such as a 22-gauge Whitacre needle, may result in a lower incidence of postdural puncture headache if the subarachnoid needle location is recognized on the first pass. Different needle tip designs result in differences in the incidence of postdural puncture headache even when needle sizes are comparable.
With the patient in the proper position, the anesthesiologist uses the palpating hand to identify clearly the patient’s intervertebral space and midline. As illustrated in Figure 48-7 , the palpating hand can effectively carry out this important locating maneuver by moving the fingers in an alternating cephalocaudad direction and rolling them from side to side. When the appropriate intervertebral space has been clearly identified, a skin wheal is raised over the space. Next, an introducer is inserted into the substance of the interspinous ligament, taking care to seat it firmly in the midline (see Fig. 48-7 , step 2). Then the introducer is grasped with the palpating fingers and steadied while the other hand holds the spinal needle, somewhat like a dart, as illustrated in Figure 48-7 , step 3. With the fifth finger of the needle-holding hand used as a tripod against the patient’s back, the needle, with the bevel (if present) parallel to the long axis of the spine, is advanced slowly to heighten the sense of tissue planes traversed as well as to avoid skewing the nerve roots, until a characteristic change in resistance is noted as the needle passes through the ligamentum flavum and dura. The stylet is then removed, and CSF should appear at the needle hub. If it does not, the needle is rotated in 90-degree increments until CSF appears. If CSF does not appear in any quadrant, the needle is advanced a few millimeters and rechecked in all four quadrants. If CSF still has not appeared and the needle is at a depth appropriate for the patient, the needle and introducer should be withdrawn and the insertion steps repeated. The most common reason for lack of CSF return is that the needle was inserted off the midline. Another common error preventing subarachnoid placement is insertion of the needle with too great a cephalad angle during the initial insertion ( Fig. 48-8 ).
Once CSF is freely obtained, the dorsum of the anesthesiologist’s nondominant hand steadies the spinal needle against the patient’s back while the syringe containing the therapeutic dose is attached to the needle. CSF is again freely aspirated into the syringe, and the dose is injected. Sometimes when the syringe has been attached to a needle from which CSF was clearly previously dripping, aspiration of additional CSF becomes impossible. As illustrated in Figure 48-9 , a technique that can be utilized to facilitate CSF aspiration is to “unscrew” the syringe plunger rather than providing constant steady pressure as an aid.
After the local anesthetic has been injected, the patient and the operating table should be placed in the position appropriate for the surgical procedure and drugs chosen. The midline approach to subarachnoid block is the technique of choice because it requires anatomic projection in only two planes, and the plane is a relatively avascular one. When difficulties with needle insertion are encountered with the midline approach, another option is to use the paramedian route, which does not require the same level of patient cooperation or reversal of lumbar lordosis to be successful. As illustrated in Figure 48-10 , the paramedian approach exploits the larger “subarachnoid target” that exists if a needle is inserted slightly lateral to the midline. With the paramedian approach, the palpating fingers should identify the caudal edge of the cephalad spinous process of the intervertebral space chosen, and a skin wheal is raised 1 cm lateral and 1 cm caudal to this point. A longer needle, such as a 4-cm, 22-gauge, short-beveled needle, is then used to infiltrate the deeper tissues in a cephalomedial plane. The spinal introducer and needle are then inserted 10 to 15 degrees off the sagittal plane in a cephalomedial plane, as noted in Figure 48-10 . As with the midline approach, the most common error made with this technique is to angle the needle too far cephalad during its initial insertion. Once the needle contacts bone with this approach, it is redirected in a slightly cephalad direction. If bone is again contacted after the needle has been redirected, but at a deeper level, this needle redirection is continued because it is likely that the needle is being “walked up” the lamina toward the intervertebral space. After CSF is obtained, the block continues in the same way as that described for the midline approach.
A variation of the paramedian approach is the lumbosacral approach of Taylor. The technique is carried out at the L5–S1 interspace, the largest interlaminal interspace of the vertebral column. As illustrated in Figure 48-11 , the skin insertion site is 1 cm medial and 1 cm caudal to the ipsilateral posterosuperior iliac spine. Through this point, a 12- to 15-cm spinal needle is inserted in a cephalomedial direction toward the midline. If bone is encountered on the first needle insertion, the needle is walked off the sacrum into the subarachnoid space, as in the method used for a lumbar paramedian approach. Once CSF is obtained, the steps are similar to those previously outlined.
The complication most feared by patients and many anesthesiologists after spinal anesthesia is neurologic injury. Admittedly, neurologic injury can occur after spinal anesthesia; however, to use spinal anesthesia appropriately, the risk-benefit equation of overall neurologic injury occurring after anesthesia must include the neurologic injuries that are possible after general anesthesia. If these comparisons are made, it is likely that the incidence of neurologic injury after spinal anesthesia is in fact lower than that after general anesthesia. However, this statement must remain speculative.
Additionally, in patients in whom the spinal block level has to be precisely controlled, or in those in whom the operation is expected to outlast the usual duration of the anesthetic drugs, a continuous spinal catheter may be used. However, when using a continuous spinal technique one should be cautious about repeating local anesthetic injections if the block height does not reach the predicted levels. It has been hypothesized that neurotoxicity (cauda equina syndrome) is possible when the spinal catheter position allows local anesthetic concentrations to reach higher than expected levels.
A more common complication of spinal anesthesia is postoperative headache. Factors that influence the incidence of postdural headache are age (more frequent in younger patients), sex (female patients more likely than male patients), needle size (more frequent with larger needles), needle bevel orientation (increased incidence when dural fibers are cut transversely), pregnancy (incidence increased), and the number of dural punctures necessary to obtain CSF (more with multiple punctures). Perhaps more important than knowing the variables resulting in an increased incidence of postdural puncture headache is the knowledge of how and when to carry out definitive therapy (i.e., an epidural blood patch). To use spinal anesthesia effectively, epidural blood patching, when indicated, must be used early. The success rate with a single epidural blood patch should be 90% to 95%; and a similar percentage should be obtained if a second patch is required.
One other common side effect of spinal anesthesia is the appearance of a backache in approximately 25% of patients. Patients often blame “the spinal” for backache, but, when looked at systematically it appears that just as many patients have backaches after general anesthesia as after spinal anesthesia. Thus, backache after neuraxial block should not be attributed immediately to “needling” of the back.