Abstract

MUGABURE, B.; ECHANIZ, E.  and  MARIN, M.. Physiology and clinical pharmacology of epidural and intrathecal opioids. Rev. Soc. Esp. Dolor [online]. 2005, vol.12, n.1, pp.33-45. ISSN 1134-8046.

The history of intrathecal and epidural anaesthesia is in parallel with the development of general anaesthesia. The first published report on opioids for intrathecal anaesthesia belongs to a Romanian surgeon, who presented his experience at Paris in 1901. It was almost a century before the opioids were used for epidural analgesia. Epidural and intrathecal opioids are today part of a routine regimen for intra and postoperative analgesia. Over the last 30 years, the use of epidural opioids has became a standard for analgesia in labor and delivery, and for the management of acute and chronic pain. It has been widely asumed that any opioid placed in the epidural or intrathecal spaces will produce highly selective spinally mediated analgesia that is superior to that produced by other analgesic techniques. Unfortunately, this is simply not true. In fact, multiples opioids are currently employed for spinal use despite the fact that clinical evidence has shown that spinal administration does not produce analgesia with a selective spinal mechanism or the analgesia produced is not superior to that produced by intravenous administration. Appropriate use of spinal opioids necessitates under-standing the physiology and clinical pharmacology of these drugs and which opioids produce selective spinal analgesia and which do not. In short, spinal selectivity is greatest for hidrophilic opioids and least for lipophilic opioids. This differences result from inherent differences in the bioavility of opioids at spinal cord opioid receptors. Bioavility differs because lipophilic drugs are more rapidly cleared into plasma than hidrophilic drugs, consequently they produce more early supraspinal side-effects and have a considerably shorter duration of analgesic action. Morphine is probably the most spinally selective opioid currently used in the intrathecal and epidural spaces. Methadone is another opioid that has been shown to have moderate spinal selectivity after epidural administration. However, the long plasma half-life of this opioid results in its acumulation in plasma and greater supraspinal effects over time. Epidural administration of fentanyl offers little or no benefit over the intravenous route except in obstetrics where it does appear to produce modestly selective spinal analgesia. Finally, epidurally administered sufentanil and alfentanil appear to produce analgesia by systemic uptake and redistribution to brainstem opioid receptors.

Keywords : Epidural; Intrathecal; Opioids; Spinal; Analgesia.

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