Mini-dose intrathecal morphine for the relief of post-cesarean section pain: safety, efficacy, and ventilatory responses to carbon dioxide.

To determine the safety, efficacy, and the ventilatory responses to carbon dioxide (CO2) of mini-dose intrathecal morphine, 33 healthy women who underwent cesarean section with spinal anesthesia using 0.75% bupivacaine in 8.25% dextrose were studied. Patients were randomly assigned to receive, in a double-blind fashion, either morphine 0.25 mg (group I, n = 11), morphine 0.1 mg (group II, n = 10), or saline (group III, placebo group, n = 12) in 0.5 ml volume mixed with the bupivacaine. In both groups I and II excellent postoperative analgesia with long duration was obtained (27.7 +/- 4.0 and 18.6 +/- 0.9 hours, respectively, X +/- SEM). All patients in group III required an analgesic (8 mg subcutaneous morphine) within 3 hours of spinal anesthesia. Seven patients in group I and four patients in group II developed mild pruritus that did not require treatment. Ventilatory responses to CO2 showed no evidence of depression attributable to either the 0.25 or 0.1 mg of morphine, but significant depression of the CO2 responses was observed in group III patients after administration of subcutaneous morphine. It is concluded that a dose as low as 0.1 mg of intrathecal morphine gives excellent analgesia with minimal to no side effects and that subcutaneous morphine is associated with marked depression of the ventilatory variables.