Oxytocin pharmacodynamics: effect of long infusions on uterine activity

Despite common use in obstetrics for almost 50 years, there is still disagreement concerning optimal clinical protocols for the use of oxytocin. This disagreement arises in part from inadequate data on oxytocin pharmacokinetics and pharmacodynamics. This report evaluates the uterine response to fixed doses of oxytocin (1-3 mIU/min) over long infusion times (up to 120 min) in 10 patients with secondary arrest of dilatation. The uterine activity increased from 132 +/- 61 Alexandria units during the control period to 199 +/- 64 Alexandria units at an infusion rate of 1 mIU/min. Increasing the infusion rate to 2 mIU/min increased the uterine activity to 240 +/- 64 Alexandria units. The uterine activity increased until a steady state was achieved between 40 and 70 min after the initiation of infusion. Continuing the infusion at a fixed rate for more than 90 min appears to result in a decreasing uterine activity. Patients requiring oxytocin for augmentation of labor developed adequate uterine activity with dose rates of 1-3 mIU/min. Clinical protocols for oxytocin augmentation with an interval of 40-60 min between increases in the dosing seem reasonable based on these pharmacodynamic data.