Caudal block Spinal block Subarachnoid space in spinal canal Base of spinal cord (Conus medullaris) Dura Epidural space T12 L1 L2 L3 L4 L5 S1 S2Epidural space in caudal canal Filum terminale Cauda equina Epidural block FIGURE 26–4 Schematic diagram of the typical sites of injection of local anesthetics in and around the spinal canal. When local anesthet-ics are injected extradurally, it is referred to as an epidural block. A caudal block is a specific type of epidural block in which a needle is inserted into the caudal canal via the sacral hiatus. Injections around peripheral nerves are known as perineural blocks (eg, paravertebral block). Finally, injection into cerebrospinal fluid in the subarachnoid (intrathecal) space is referred to as a spinal block.
In clinical practice, there is generally an orderly evolution of block components beginning with sympathetic transmission and progressing to temperature, pain, light touch, and finally motor block. This is most readily appreciated during onset of spinal anesthesia, where a spatial discrepancy can be detected in modalities, the most vulnerable components achieving greater dermatomal (cephalad) spread. Thus, loss of the sensation of cold (often assessed by a wet alcohol sponge) will be roughly two segments above the analgesic level for pinprick, which in turn will be roughly two segments rostral to loss of light touch recognition. However, because of the anatomic considerations noted earlier for peripheral nerve trunks, onset with peripheral blocks is more variable, and proximal motor weakness may precede onset of more distal sensory loss. Additionally, anesthetic solution is not generally deposited evenly around a nerve bundle, and longitudinal spread and radial penetration into the nerve trunk are far from uniform.
With respect to differential block, it is worth noting that “successful” surgical anesthesia may require loss of touch, not just ablation of pain, as some patients will find even the sensation of touch distressing during surgery, often fearing that the procedure may become painful. Further, while differences may exist in modalities, it is not possible with conventional techniques to produce surgical anesthesia without some loss of motor function.
A. Effect of Added Vasoconstrictors
Several benefits may be derived from addition of a vasoconstrictor to a local anesthetic. First, localized neuronal uptake is enhanced because of higher sustained local tissue concentrations that can translate clinically into a longer duration block. This may enable adequate anesthesia for more prolonged procedures, extended duration of postoperative pain control, and lower total anesthetic requirement. Second, peak blood levels will be lowered as absorption is more closely matched to metabolism and elimination, and the risk of systemic toxic effects is reduced. Moreover, when incorporated into a spinal anesthetic, epinephrine may not only contribute to prolongation of the local anesthetic effect via its vasoconstrictor properties, but also exert a direct analgesic effect mediated by postsynaptic α2 adrenoceptors within the spinal cord. Recognition of this potential has led to the clinical use of the α2 agonist clonidine as a local anesthetic adjuvant for spinal anesthesia.
Conversely, inclusion of epinephrine may also have untoward effects. The addition of epinephrine to anesthetic solutions can potentiate the neurotoxicity of local anesthetics used for peripheral nerve blocks or spinal anesthesia. Further, the use of a vasoconstrictor agent in an area that lacks adequate collateral flow (eg, digital block) is generally avoided, although some have questioned the validity of this proscription.
B. Intentional Use of Systemic Local Anesthetics