Maximal OxytOcin Dosage. The maximal efective dose of oxytocin to achieve adequate contractions in all women is diferent. Wen and colleagues (2001) studied 1151 consecutive nulliparas and found that the likelihood of progression to vaginal delivery decreased at and beyond an oxytocin dosage of 36 mU/min. Still, at a dosage of 72 mU/min, half of the nulliparas were delivered vaginally. Thus, if contractions are not adequate-less than 200 Montevideo units-and if the fetal status is reassuring and labor has arrested, an oxytocin infusion dose greater than 48 mU/min has no apparent risks.
Unless the uterus is scarred, uterine rupture associated with oxytocin infusion is rare, even in parous women. Flannelly and associates (1993) reported no cases of uterine ruptures, with or without oxytocin, in 27,829 nulliparas. here were eight instances of overt uterine rupture during labor in 48,718 parous women. Only one of these was associated with oxytocin use. A population-based retrospective review from Denmark reported a rupture rate of 3.3 per 100,000 women without prior cesarean, with the highest risk among multiparas (This ted, 2015). Our experiences from Parkland Hospital are that oxytocin induction and augmentation are associated with uterine rupture (Happe, 2017). During an 8-year period in which there were about 95,000 births, 15 women sufered a primary uterine rupture, and 14 of these cases were associated with oxytocin use. In half of these women, prostaglandins were also given before augmentation with oxytocin.
Oxytocin has amino-acid homology similar to arginine vasopressin and has signiicant antidiuretic action. When infused at doses of 20 mU/min or more, renal free water clearance drops markedly. If aqueous luids are infused in appreciable amounts along with oxytocin, water intoxication can lead to convulsions, coma, and even death. In general, if oxytocin is to be administered in high doses for a considerable period of time, its concentration should be increased rather than raising the flow rate of a more dilute solution. Consideration also should be given to use of crystalloids-either normal saline or lactated Ringer solution.
Contraction forces in spontaneously laboring women range from 90 to 390 Montevideo units (Chap. 24, p. 479). Caldeyro-Barcia (1950) and Seitchik (1984) with their coworkers found that the mean or median spontaneous uterine contraction pattern between 140 and 150 Montevideo units resulted in progression to vaginal delivery.
In the management of active-phase arrest, and with no contraindication to intravenous oxytocin, decisions must be made with knowledge of the safe upper range of uterine activity. Hauth and colleagues (1986) described an efective and safe protocol for oxytocin augmentation for active-phase arrest. With it, more than 90 percent of women achieved an average of at least 200 to 225 Montevideo units. They later reported that nearly all women in whom active-phase arrest persisted despite oxytocin generated more than 200 Montevideo units (Hauth, 1991). Importantly, despite no labor progression, no adverse maternal or perinatal efects were noted in those ultimately requiring cesarean delivery. here are no data regarding safety and eicacy of contraction patterns in women with a prior cesarean delivery, with twins, or with an overdistended uterus.