Intrathecal Injection of Morphine for Obstetric Analgesia

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Abstract
Intrathecal injection of morphine was used to provide obstetric analgesia in 20 primiparous women in labor. When the cervix was at least 3 cm dilated, morphine, 1 or 2 mg, was injected intrathecally. In all parturients, labor pains were completely relieved after 15-60 min and analgesia lasted as long as 8-11 h. The analgesia was not assoicated with any alteration of pin-prick sensation or motor power and there was no change in the arterial blood pressure or heart rate. All infants were delivered vaginally by use of episiotomy and a low forceps, except 2 infants of mothers in the 2 mg morphine group who needed cesarean section. During the 2nd stage of labor, analgesia was supplemented by lidocaine, 2%, using local perineal infiltration in 14 parturients and pudendal block in 2 parturients, and by epidural block in 4 parturients. Of the 20 newborns 19 cried immediately at birth and had Apgar scores of 7-9 at 1 min and 8-10 at 5 min. During the first 24 h of life, the neurobehavioral responses of all newborns were scored as normal. Systemic maternal side effects i.e., somnolence, nausea, vomiting and itching occurred in a high proportion of the parturients. In the majority of cases, the side effects were mild. Only 2 parturients of the 2 mg morphine group complained of marked somnolence, itching and vomiting, which persisted postpartum; these were effectively reversed by the specific antagonist naloxone. The analgesic effect of intrathecal morphine is due to its action on the opiate receptors in the substantia gelatinosa of the dorsal horn of the spinal cord. Supraspinal effects of morphine cannot be excluded. The low lipid solubility of morphine can explain its slow onset and prolonged duration of action. This will result in minimal systemic absorption of morphine, which protects the fetus and results in selective maternal analgesia.