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Oxytocin and analogues
General information
Oxytocin is a hypothalamic nonapeptide that selectively stimulates the smooth muscle of the uterus and mammary glands. It is used in the induction or augmentation of labor and to prevent postpartum hemorrhage, and is well tolerated and effective in a wide range of infusion rates and concentrations. Contraindications to its use include placenta previa or vasa previa, a previous classical uterine incision, pelvic structural deformities, and an abnormal fetal presentation. Large fetal size and high maternal parity are relative contraindications. Prior non-classical cesarean delivery should not preclude oxytocin therapy.
Uterine contractions and fetal heart rate should be monitored during oxytocin administration [ , ]. There is no significant increase in uterine complications or in fetal morbidity or mortality in women with a previous cesarean section, although oxytocin-treated patients had a higher rate of failed trial of labor for reasons that are unclear [ ]. Oxytocin is structurally similar to vasopressin, and like the latter has water-retaining properties when used in pharmacological doses.
Oxytocin is in common use during induction of labor and in the third stage of labor to prevent uterine atony and postpartum hemorrhage. Carbetocin, a synthetic analogue with a half-life 4–10 times longer than the native hormone, has been studied in trials; both agents cause a small transient fall in blood pressure (less than 4 mmHg) [ ]. Common mild adverse effects of both drugs include headache, flushing, a feeling of warmth, a metallic taste, and abdominal pain.
Drug studies
Comparative studies
Carbetocin, a long-acting oxytocin analogue, and ergometrine maleate (Syntometrine) have been compared in a double-blind, randomized controlled trial in 329 women with primary postpartum hemorrhage [ ]. There were no differences in the fall in hemoglobin concentration in the first 48 hours, in the need for additional oxytocin, or the incidences of postpartum hemorrhage or retained placenta. Carbetocin was associated with significantly less nausea (RR = 0.18; 95% CI = 0.04, 0.78), vomiting (RR = 0.1; CI = 0.01, 0.74), hypertension at 30 minutes (0 versus 8 cases) and 60 minutes (0 versus 6 cases) after delivery; however there was a higher incidence of maternal tachycardia (RR = 1.68; CI = 1.03, 3.57).
Oral misoprostol and intramuscular ergometrine have been compared in the management of the third stage of labor in a double-blind, randomized, controlled trial in 355 women [ ]. There were no significant differences in hemoglobin concentrations and mean blood loss. Shivering was significantly more common with misoprostol and vomiting with ergometrine, but there were no differences in the incidences of nausea, headache, diarrhea, or pyrexia.
Organs and systems
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