Interindividual variability makes it difficult to reliably predict the height and duration of central neuraxial block that will result from a particular local anesthetic dose (Table 34-3: Representative Surgical Procedures Appropriate for Spinal Anesthesia).

1. Spinal Anesthesia
   1. Block Height (Table 34-4: Factors that May Influence the Spread of Local Anesthetic Solutions in the Subarachnoid Space)
      1. Baricity and Patient Position. Of the factors that exert significant influence on local anesthetic spread, the baricity of the local anesthetic solution relative to patient position is probably the most important.
      2. Hyperbaric solutions (more dense than CSF) are typically prepared by mixing the local anesthetic solution with 5% to 8% dextrose. Gravity causes hyperbaric solutions to flow downward in the CSF to the most dependent regions of the spinal column. Spinal anesthesia can be restricted to the sacral and lower lumbar dermatomes (“saddle block”) by administering a hyperbaric local anesthetic solution with the patient in the sitting position.
      3. Hyperbaric solutions can be used to advantage for unilateral surgical procedures performed in the supine position if the operative site is dependent during drug injection and the patient is left in the lateral position for at least 6 minutes.
      4. When the patient is turned supine after hyperbaric drug injection in the lateral position, the normal spinal curvature influences subsequent movement of the injected solution. Hyperbaric solutions injected at the height of the lumbar lordosis tend to flow cephalad to pool in the thoracic kyphosis and caudad to pool in the sacrum (Fig. 34-6: In the supine position, hyperbaric local anesthetic solutions injected at the height of the lumbar lordosis (circle) flow down the lumbar lordosis to pool in the sacrum and in the thoracic kyphosis).
      5. Gravity influences the distribution of hyperbaric and hypobaric solutions only until they are sufficiently diluted in CSF so that they become isobaric (solution no longer moves in response to changes in position).
      6. Dose, Volume, and Concentration. Drug dose and volume appear to be relatively unimportant in predicting the spread of hyperbaric local anesthetic solutions injected in the horizontal plane (reflecting the predominate effect of baricity and patient position).
      7. Injection site is the same as for drug dose and volume.
      8. Patient Characteristics. The most important variable governing block height may be the patient's lumbosacral CSF volume. The patient's age, weight, and height have not been proven to be important predictors of block height.
   2. The onset of spinal anesthesia is within a few minutes regardless of the drug used, although time to reach peak block is different among drugs (lidocaine sooner than bupivacaine).
   3. The duration of spinal anesthesia is characterized by gradual waning of the block beginning with the most cephalad dermatome.
      1. When speaking about duration of block, it is necessary to distinguish between duration at the surgical site and the time required for anesthesia to completely resolve (which influences discharge time) (Table 34-5: Dose and Duration of Local Anesthetics Used for Spinal Anesthesia).
      2. A thorough understanding of the factors that govern the duration of anesthesia is necessary for the anesthesiologist to choose techniques that result in an appropriate duration (Table 34-6: Factors that May Influence the Duration of Sensory Blockade Produced by Spinal Anesthesia). Intrathecal epinephrine decreases blood flow in the dura mater without altering spinal cord blood flow, which is consistent with decreased drug clearance via the dural vasculature.
2. Epidural Anesthesia. Any procedure that can be performed under spinal anesthesia can also be performed under epidural anesthesia and requires the same block height (see Table 34-3: Representative Surgical Procedures Appropriate for Spinal Anesthesia). As with spinal anesthesia, there is great interindividual variability in the spread and duration of epidural anesthesia (Table 34-7: Local Anesthetics Used for Surgical Epidural Anesthesia)
   1. Block Spread. To choose the most appropriate local anesthetic and dose for a particular clinical situation, the anesthesiologist must be familiar with the variables that affect the spread and duration of epidural anesthesia (Table 34-8: Factors that May Influence the Spread of Local Anesthetic Solutions in the Epidural Space).
   2. The onset of epidural anesthesia can usually be detected within 5 minutes in the dermatomes immediately surrounding the injection site.
      1. The time to peak effect is 15 to 20 minutes with shorter acting drugs and 20 to 25 minutes with longer acting drugs.
      2. Increasing the dose of local anesthetic speeds the onset of both motor and sensory block.
   3. Duration (Table 34-9: Factors that Influence the Duration of Sensory Blockade Produced by Epidural Anesthesia)

Epidural and Spinal Anesthesia

1. Anatomy
2. Technique
3. Spinal Anesthesia
4. Continuous Spinal Anesthesia
5. Epidural Anesthesia
6. Continuous Epidural Anesthesia
7. Epidural Test Dose
8. Combined Spinal–Epidural Anesthesia
9. Pharmacology
10. Physiology
11. Complications
12. Contraindications
13. Choice of Spinal or Epidural Anesthesia