Approximately 25 percent of deliveries in the United States are induced or augmented by oxytocin, but there is little consensus regarding the optimal dosage regimen. Recommendations for the initial dose range from 0.5 to 6.0 mU per minute, with dosage adjustments ranging from 1 to 6 mU per minute. The recommended interval between adjustments also varies from 15 to 60 minutes. Typically, the end points in studies of augmented labor are time to delivery and need for cesarean delivery caused by fetal distress related to hyperstimulation of the uterus. However, these studies are technically challenging because of difficulties associated with blinding caretakers and addressing ethical concerns regarding the safety of mother or fetus. Merrill and Zlatnik compared the impact of high and low doses of oxytocin on duration of labor and need for cesarean delivery in women who required induction or augmentation of labor.
All women beyond 24 weeks' gestation who were admitted to a university obstetric unit for augmented labor and delivery were eligible for the study. The women had to have medical indications for augmented labor and no contraindications to oxytocin therapy or augmentation of delivery. Those who met the study criteria were randomized to receive high- or low-dose oxytocin. The solutions were prepared in the pharmacy, and obstetric staff had no knowledge of the patient's group assignment. Patients in the low-dose group received 5 U of oxytocin per 500 mL of solution, while those in the high-dose group received 15 U of oxytocin per 500 mL of solution. The initial infusion rate in both groups was 9 mL per hour. Patients in the low-dose group received 1.5 mU per minute initially, which was then increased by 1.5 mU per minute every 30 minutes until adequate labor was established. High-dose patients received 4.5 mU per minute, which was then increased by 4.5 mU per minute every 30 minutes. The infusion was discontinued if severe fetal heart rate abnormalities occurred or if more than seven contractions were documented in a 15-minute period. If no progress was documented after eight to 12 hours, the induction was regarded as “failed,” and alternative treatments could be used.
A total of 816 patients was included in the analysis: 412 patients in the low-dose group and 404 in the high-dose group. Patient age, race, gestation, parity and indication for induction were similar between groups. Despite randomization, more multiple births were noted in the low-dose group, but these accounted for less than 4 percent of patients. The mean time to achieving full dilation was significantly reduced in the high-dose group (7.8 hours) compared with the low-dose group (9.7 hours). The mean time to delivery was also significantly reduced (8.5 compared with 10.5 hours). Women in the high-dose group also had a lower rate of cesarean delivery (11.3 compared with 15 percent), but this difference was not statistically significant. Oxytocin was reduced or discontinued twice or more in 41 percent of the high-dose group and in 30 percent of the low-dose group. The side effect panel, including placental abruption, infection, postpartum hemorrhage and length of maternal hospital stay, was similar between groups. One woman in each group experienced uterine rupture; both cases were associated with previous cesarean delivery. Neonatal outcomes were also similar between groups, including Apgar score, birth weight, acidosis and length of hospital stay.
The authors conclude that high-dose oxytocin significantly shortens labor and tends to be associated with fewer cesarean deliveries. In addition, the use of a high dose does not appear to increase or intensify the side effects associated with oxytocin.