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Chapter: Clinical Cases in Anesthesia : Acute Postoperative Pain

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.

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.


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Clinical Cases in Anesthesia : Acute Postoperative Pain : By what mechanism does neuraxial (epidural and subarachnoid) opioid administration produce analgesia? |


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