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.
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