Neuraxial Morphine: A review of Mechanism of Action and Respiratory Depression

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Irin Charuwattanapanit


Neuraxial morphine have been used as the mainstay for moderate to severe acute pain due to its postoperative analgesia efficacy and prolonged duration of action. However, the awareness remains about their potential adverse effects on ventilation. Opioid induced respiratory depression results from the interaction of opioid in the cerebrospinal fluid with ventral medullary opioid receptors and preBotzinger complex, which located in the medulla. Even though the incidence of opioid induced respiratory depression was identified as a rare event around 0-23% during a perioperative setting. Risks are often higher in patients with extreme age, obesity or having cardiopulmonary disease. Therefore, identification of patients at increased risk of respiratory depression is recommended by focused history and physical examination. The optimal neuraxial opioid dose should be used to achieve optimal analgesia while minimizing dose-related adverse effects. Doseresponse studies show that neuraxial morphine appears to have an analgesic efficacy “ceiling”. The optimal single shot intrathecal morphine appears to be 0.075-0.15 mg and the ideal single shot epidural morphine dose is 2.5-3.75 mg. Moreover, the detection of respiratory depression is crucial after giving neuraxial opioids in order to early detection and management of respiratory depression.

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