To achieve effective epidural opioid analgesia, a critical mass or amount of opioid must be injected into the epidural space. A further increase in that dose does not materially shorten the onset time of analgesia. For example, the onset times of analgesia following epidural administration of 2, 4 or 8 mg morphine for management of pain after orthopedic surgery do not differ significantly. These doses also produce analgesia of equal quality. The quality of analgesia after epidural injection of 0.5 or 1 mg morphine, however, is significantly inferior (Martin et al. 1982).
KeywordsOpioid Receptor Respiratory Depression Epidural Analgesia Epidural Space Epidural Morphine
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