Failed Epidural Blocks
Unlike spinal anesthesia, in which the endpoint is usually very clear (free flowing CSF) and the tech-nique is associated with a very high success rate, epi-dural anesthesia is dependent on detection of a more subjective loss of resistance (or hanging drop). Also, the more variable anatomy of the epidural space and less predictable spread of local anesthetic make epi-dural anesthesia inherently less predictable than spi-nal anesthesia.
Misplaced injections of local anesthetic can occur in a number of situations. In some patients, the spinal ligaments are soft, and either good resis-tance is never appreciated or a false loss of resistance is encountered. Similarly, entry into the paraspinous muscles during an off-center midline approach may cause a false loss of resistance. Other causes of failed epidural anesthesia (such as intrathecal, subdural, and intravenous injection) are discussed in the sec-tion on complications.Even if an adequate concentration and volume of an anesthetic were delivered into the epidural space, and sufficient time was allowed for the block to take effect, some epidural blocks are not success-ful. A unilateral block can occur if the medication is delivered through a catheter that has either exited the epidural space or coursed laterally. The chance of this occurring increases as longer lengths of cath-eter are threaded into the epidural space. When uni-lateral block occurs, the problem may be overcome by withdrawing the catheter 1–2 cm and reinject-ing it with the patient turned with the unblocked side down. Segmental sparing, which may be due to septations within the epidural space, may also be corrected by injecting additional local anesthetic with the unblocked segment down. The large size of the L5, S1, and S2 nerve roots may delay adequate penetration of local anesthetic and is thought to be responsible for sacral sparing. The latter is particu-larly a problem for surgery on the lower leg; in such cases, elevating the head of the bed and reinjecting the catheter with additional anesthetic solution can sometimes achieve a more intense block of these large nerve roots. Patients may complain of visceral pain, despite a seemingly good epidural block. In some cases (eg, traction on the inguinal ligament and spermatic cord), a high thoracic sensory level may alleviate the pain; in other cases (traction on the peritoneum), intravenous supplementation with opioids or other agents may be necessary. Visceral afferent fibers that travel with the vagus nerve may be responsible.