40 Spinal Block

Perspective

Spinal anesthesia is unparalleled in that a small mass of drug, virtually devoid of systemic pharmacologic effect, can produce profound, reproducible surgical anesthesia. Further, by altering the small mass of drug, very different types of spinal anesthesia can be produced. Low spinal anesthesia, a block below T10, has a different physiologic impact than does a block performed to produce higher spinal anesthesia (above T5). The block is unexcelled for lower abdominal or lower extremity surgical procedures. However, for operations in the mid- to upper abdomen, light general anesthesia may have to supplement the spinal block because stimulation of the diaphragm during upper abdominal procedures often causes some discomfort. This area is difficult to block completely through high spinal anesthesia because to do so requires blockade of the phrenic nerve.

Patient Selection

Patient selection for spinal anesthesia often places too much emphasis on a side effect of the technique—namely, spinal headache—than on the applicability of the technique in a given patient. It is clear that the incidence of spinal headache increases with decreasing age and female sex; however, with proper technique and selection of needle size and tip configuration, the incidence of headache should not preclude the use of spinal anesthesia in young, healthy patients if the block has advantages over epidural anesthesia. Almost any patient who is to have a lower extremity operation is a candidate for spinal anesthesia, as are most patients scheduled for lower abdominal surgery, such as inguinal herniorrhaphy and gynecologic, urologic, and obstetric procedures.

Pharmacologic Choice

In the United States, three local anesthetics are commonly used to produce spinal anesthesia: lidocaine, tetracaine, and bupivacaine. Lidocaine is a short- to intermediate-acting spinal drug; tetracaine and bupivacaine provide intermediate- to long-acting block. Lidocaine, without epinephrine, is often chosen for procedures that can be completed in 1 hour or less. It is likely that the lidocaine mixture most commonly used is still a 5% solution in 7.5% dextrose, although increasingly anesthesiologists are using 1.5% to 2% concentrations of lidocaine without dextrose as alternatives. When epinephrine (0.2 mg) is added to lidocaine, the useful length of clinical anesthesia in the lower abdomen and lower extremities is approximately 90 minutes. Tetracaine is packaged both as niphanoid crystals (20 mg) and as a 1% solution (2 mL total). When dextrose is added to make tetracaine hyperbaric, the drug generally produces effective clinical anesthesia for procedures of up to 1.5 to 2 hours in the plain form, for up to 2 to 3 hours when epinephrine (0.2 mg) is added, and for up to 5 hours for lower extremity procedures when phenylephrine (5 mg) is added as a vasoconstrictor. Bupivacaine spinal anesthesia is commonly carried out with 0.5% or 0.75% solution, either plain or in 8.25% dextrose. My impression is that the clinical difference between 0.5% tetracaine and 0.75% bupivacaine as hyperbaric solutions is minimal. Bupivacaine is appropriate for procedures lasting up to 2 or 3 hours.

In addition to hyperbaric technique, local anesthetics can be mixed to produce hypobaric spinal anesthesia. A common method of formulating a hypobaric solution is to mix tetracaine in a 0.1% to 0.33% solution with sterile water. Also, lidocaine can be mixed to provide useful hypobaric spinal anesthesia. This drug is diluted from a 2% solution with sterile water to make a 0.5% solution, using a total of 30 to 40 mg.

Many anesthesiologists avoid vasoconstrictors for fear of somehow increasing the risk in spinal anesthesia. These anesthesiologists believe that phenylephrine or epinephrine has such potent vasoconstrictive action that it puts the blood supply of the spinal cord at risk. There are no human data supporting this theory. In fact, because most local anesthetics are vasodilators, the addition of these vasoconstrictors does little more than maintain spinal cord blood flow at a basal level. Commonly used doses of vasoconstrictors are 0.2 to 0.3 mg of epinephrine and 5 mg of phenylephrine added to the spinal anesthetic.

Placement

As outlined in Chapter 39, Neuraxial Block Anatomy, the spinous processes of the lumbar vertebrae have an almost horizontal orientation in relation to the long axis of their respective vertebral bodies (Fig. 40-1). When a midline needle is inserted between the lumbar vertebral spinous processes, it is most effective if it is placed almost perpendicularly in relation 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 40-1, as a midline needle is inserted into the cerebrospinal fluid (CSF), it logically must puncture the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, and finally the dura mater and arachnoid mater to reach the CSF.

Figure 40-1. Spinal block: functional lumbar anatomy.

Position

Spinal anesthesia is carried out in three principal positions: lateral decubitus (Fig. 40-2), sitting (Fig. 40-3), and prone jackknife (Fig. 40-4). In both the lateral decubitus and sitting positions, a well-trained assistant is essential if the block is to be easily and efficiently administered by the anesthesiologist. As illustrated in Figure 40-2, 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 having the assistant pull the head toward the chest, place an arm behind the patient’s knees, and push the head and knees together. The position can also be facilitated by using an appropriate amount of sedation that allows the patient to be relaxed yet cooperative.