The dose of oxytocin required to generate satisfactory uterine tone after delivery is lower than previously thought (see
Chapter 26 ). In a study of nonlaboring women undergoing elective cesarean delivery,
the ED 90 of bolus dose oxytocin for satisfactory uterine tone within 3 minutes of delivery was 0.35 international units (IU) ; The ED 90 was approximately 3 IU in laboring women undergoing cesarean delivery for labor arrest after labor augmentation with oxytocin. The ED 90 of oxytocin administered via infusion without a bolus dose in nonlaboring women was approximately 0.3 IU/min for 1 hour. Munn et al. randomized women undergoing a cesarean delivery during labor to receive a prophylactic infusion of oxytocin at 2.67 IU/min or 0.33 IU/min for 30 minutes after delivery; the higher dose was associated with less need for secondary uterotonics (19% versus 39%, respectively;
P < .001); however, the high dose may be associated with clinically significant tachycardia and hypotension (see later discussion).
Oxytocin is rapidly metabolized by hepatic oxytocinases and cleared in the urine and bile, resulting in a half-life of less than 6 minutes. Consequently, a prolonged intravenous infusion may be more effective than bolus administration in preventing uterine atony. In an international randomized, controlled trial, Sheehan et al. found that the addition of a 4-hour maintenance infusion of 0.17 IU/min (after an initial 5-IU bolus dose) decreased the need for secondary uterotonics compared with a 5-IU bolus dose alone. King et al. studied women at high risk for postcesarean uterine atony and demonstrated that administering a 5-IU bolus of oxytocin before a 1.3-IU/min infusion did not provide benefit compared with an infusion without a bolus. Administration of phenylephrine with oxytocin can mitigate the adverse hemodynamic consequences of oxytocin, but phenylephrine may not be necessary as long as an oxytocin bolus dose is avoided and the infusion rate is maintained below 1 IU/min, the threshold at which hemodynamic consequences become apparent
Data demonstrating lower oxytocin dose requirements than previously assumed and awareness of the dangers of high-dose administration call into question the common practice of injecting 10 to 40 IU of oxytocin into a 1-liter crystalloid solution and infusing the solution at an unspecified rate, often “wide open” (i.e., gravity-dependent flow). The doses administered with this method may approach those achieved with bolus administration. At my institution, my colleagues and I administer prophylactic oxytocin at a rate of 0.3 IU/min (the ED 90 ) and increase the rate to 0.6 IU/min (twice the ED 90 ) if there is inadequate response. The maximum beneficial oxytocin infusion rate to treat persistent uterine atony is unknown..
