By what mechanism does
neuraxial (epidural and subarachnoid) opioid administration produce analgesia?
Neuraxial opioids were first used in humans in
1979. Since that time, they have been widely used for the treat-ment of
postoperative and chronic cancer pain. Neuraxial opioids interact with the
opiate receptors in the sub-stantia gelatinosa of the spinal cord’s dorsal
horn, causing inhibitory modulation of afferent pain messages entering the
cord, prior to their reaching the cerebral cortex. Thus, analgesia occurs in
the absence of motor loss or autonomic blockade and resultant hypotension seen
with neuraxially administered local anesthetics. Excellent analgesia can be
obtained with small doses of neuraxial morphine, resulting in serum
concentrations far below those required for analgesia.
Epidural-administered lipid-soluble opioids,
such as fentanyl, achieve higher serum opioid concentrations than similarly
administered water-soluble opioids, such as morphine. Multiple studies have
shown continuous intra-venous and continuous epidural fentanyl infusions to
yield similar plasma fentanyl concentrations after 18–24 hours of use. Thus,
some researchers have questioned whether the analgesia obtained from epidural
fentanyl is secondary to a spinal effect or whether it is due to high plasma
fentanyl levels producing analgesia at a supraspinal level. A study (Salomaki
et al., 1991) in patients undergoing thoracotomy showed a definite advantage to
epidural fentanyl compared with intravenous administration. Similar analgesia
was obtained with the use of smaller amounts of opioids, reduced plasma
fentanyl levels, and a lower incidence of side-effects. However, this question
is still being debated. To use epidural fentanyl appropriately, it is
imperative to place the catheter tip as close to the middle of the surgical
field as possible. It is the belief of these authors that when used in an
appropriate manner, lipid-soluble epidural opi-oids produce an effect at the
spinal cord level. Appropriate use means that the epidural catheter is in the
middle of the surgical field, a low concentration of fentanyl (i.e., 5 μg/ml) is utilized in conjunction with a low concentration of local
anesthetic (bupivacaine 0.1%), and careful titration is achieved. However, with
prolonged use plasma levels will rise. This point is still being debated.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.