Intrathecal injection of morphine for obstetric analgesia

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 eight to 11 hours. The analgesia was not associated 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 annd a low forceps, except two infants of mothers in the 2 mg of morphine group who needed cesarean section. During the second stage of labor, analgesia was supplemented by lidocaine, 2 per cent, using local perineal infiltration in 14 parturients and pudendal block in two parturients, and by epidural block in four parturients. Nineteen of the 20 newborns cried immediately at birth, and had Apgar scores o 7-9 at 1 min and 8-10 at 5 min. During the first 24 hours of life, the neurobehavioral responses of all newborns were scored as normal. Systemic maternal side effects such as somnolence, nausea, vomiting, and itching occurred in a high proportion of the parturients. However, in the majority of cases, these side effects were mild. Only two parturients of the 2 mg morphine group complained of marked somnolence, itching, and vomiting, which persisted post partum; these were effectively reversed by the specific antagonist naloxone. The analgesic effect of intrathecal morphine can be attributed to its action on the opiate receptors in the substantia gelatinosa of the dorsal horn of the spinal cord. However, supraspinal effects of morphine cannot be excluded. The low lipid solubility of morphine can explain its slow onset and prolonged duration of action. Also, this will result in minimal systemic absorption of morphine, which protects the fetus and results in selective maternal analgesia.