Describe the advantages and
disadvantages of subarachnoid and epidural opioids.
The advantages of subarachnoid opioids include
sim-plicity of administration and small dose requirement. Although they can be
administered through a subarach-noid catheter, it is more common to inject
subarachnoid opioids as a bolus dose through a needle. Thus the ability to
titrate small amounts of a medication to the desired effect is lost. In
addition, the one-shot dosing provides analgesia of only a specified duration,
with morphine last-ing from 8 to 24 hours. The incidence of side-effects is
greater with subarachnoid opioids compared with epidural opioids.
Epidural opioid via a continuous infusion is
the pre-ferred way to administer neuraxial opioids in the postoper-ative
period. Because access to the epidural space is often provided through an
indwelling catheter, multiple small doses or continuous infusions may be
administered. If placed in a sterile fashion, the catheter can be kept in place
for many days, allowing for a long duration of analgesia. The use of PCA via
the epidural route adds the ability for a patient to self-administer additional
boluses when pain increases, such as prior to mobilization. Combinations of
opioid and local anesthetic can be used to enhance analgesia.
Disadvantages of epidural opioids include
larger dose requirements, predisposing patients to higher plasma narcotic concentrations
with lipid-soluble opioids. Indwelling catheters may migrate into the CSF or a
blood vessel, placing patients at risk for complications. Any indwelling
catheter can become a source of infection. If a patient is to receive an
anticoagulant in the postoperative period, the catheter must be removed prior
to institution of the medication, or, if appropriate, within the safe window of
the drug’s administration. If a patient develops a coagulopathy in the
postoperative period, it must be decided on a risk–benefit basis whether or not
to remove the catheter, for fear of dislodging a clot and causing an epidural
hematoma. In addition, it is imperative to use tubing without ports or other
access, to prevent inappro-priate injections into the epidural space. Also, the
epidural catheter may be difficult to place in some patients, such as the obese
or those with a curvature of the spine.
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