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Patient-controlled analgesia (PCA) with morphine is a convenient method for providing postoperative analgesia. Despite the fact that it is used after cesarean delivery, data on transfer of morphine and of its active metabolite morphine-6 glucuronide (M6G) into maternal milk are scarce. It is not known whether breast-feeding during PCA with morphine has neonatal implications. We sought to measure morphine and M6G concentrations in colostrum during postpartum IV PCA and evaluate the potential for drug intake by neonates being breast-fed by these mothers. Seven informed and consenting mothers, given IV PCA with morphine, were investigated. Plasma and milk samples were obtained at titration, and at 12, 24, 36, and 48 h. Morphine and M6G were measured by high-performance liquid chromatography. In plasma, morphine concentrations ranged from <1 to 274 ng/mL, M6G ranged from <5 to 974 ng/mL. In milk, opioids were found in only 3 patients in whom morphine concentrations ranged from <1 to 48 ng/mL and M6G from <5 to 1084 ng/mL. The milk-to-plasma ratio was always <1 for morphine. In conclusion, we observed very small morphine and M6G concentrations in colostrum during PCA with morphine. Under these conditions, the amounts of drug likely to be transferred to the breast-fed neonate are negligible.Colostrum concentrations of morphine and its active metabolite morphine-6 glucuronide were measured in mothers receiving patient-controlled analgesia with morphine after cesarean delivery. The concentrations were found to be very small, thus supporting the safety of breast-feeding in mothers receiving IV patient-controlled analgesia with morphine.