Intrathecal Morphine

A single intrathecal morphine injection can produce analgesia lasting 24 to 48 hours due to prolonged retention in cerebrospinal fluid. The protocol selects 0.25 mg morphine based on evidence that 0.3 mg and 0.25 mg regimens were effective with acceptable safety in abdominal and cardiac surgery. Previous studies have used intrathecal morphine doses ranging from 0.1 mg to 4 mg, and doses of 0.2 to 0.4 mg have provided postoperative analgesia in major abdominal surgery without respiratory depression. Intrathecal morphine was first reported in 1979. Intrathecal morphine acts directly on central mu-opioid receptors through cerebrospinal fluid circulation. Morphine's hydrophilic nature limits spinal capillary absorption, enabling effective low-dose intrathecal analgesia. Intrathecal morphine's analgesic effects are dose-dependent, but adverse effects such as respiratory depression and pruritus are clinically important. A key limitation of intrathecal morphine alone is that its peak effect is delayed by about 6 hours, which may not satisfy intraoperative analgesic needs.