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Oxytocin |
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Oxytocin is a naturally-occurring hormone that stimulates the smooth muscle of
the uterus to contract. It is most effective at term when the uterine muscle
is most sensitive, but it can have stimulatory effects even early in
pregnancy. Oxytocin is used to induce labor, to stimulate pre-existing labor,
and as a uterotonic agent to reduce postpartum bleeding.
During labor, the usual effects of administering oxytocin are to make the contractions stronger, more frequent, and of longer duration. Of these, the most desirable for labor is the increased frequency. If too much oxytocin is given, the contractions may occur so frequently that there is too little time for utero-placental resupply of oxygen and removal of carbon dioxide. If needed to stimulate labor and not enough oxytocin is given, then the abnormal labor may continue. The dosage of oxytocin must be titrated to just the right amount...not too much and not too little.
To aid in achieving just the right dose, a dilute solution of oxytocin is often administered by a controlled infusion pump. The precise amount of oxytocin delivered is less important than the consistency of dosing. As some uteruses are very sensitive to even small doses, a small dose is initiated, and then gradually increased until the desired effect is achieved (contractions every 2 1/2 to 3 minutes, lasting close to 60 seconds, with peak strength of at least 60 mm Hg.). Although usually given intravenously, other forms of administration may be appropriate in some clinical circumstances.
If the contractions are too frequent (>10 contractions in 20 minutes), or the uterus fails to completely relax between contractions, then the oxytocian infusion rate is adjusted downward. As labor progresses, this is often the case, and many patients will receive oxytocin for much of their labor only to have it turned off at the end of labor because they no longer need it. If overstimulation of the uterus occurs, not only can this adversely affect fetal oxygen exchange through the placenta, but uterine rupture can occur.
Following delivery of the baby and placenta, oxytocin is commonly given in moderate doses to control uterine bleeding. In this case, overstimulation of the uterus is not a concern.
Oxytocin can have other, non-obstetrical effects. The most important of these is an anti-diuretic-hormone-like effect, sometimes seen after prolonged administration of relatively high doses of oxytocin and large volumes of crystalloid.
Oxytocin is indicated for the:
- Initiation (induction) of labor, whenever the benefits of delivery exceed the risks of continuing intrauterine existence.
- Stimulation (augmentation) of labor, whenever labor abnormalities such as prolonged latent phase or arrest of the active phase occur.
- Control of postpartum hemorrhage or prophylaxis for such control, following delivery of the fetus and placenta.
- Providing enough contractions to assess fetal well-being in the context of a contraction stress test.
- Assistance of milk let-down in postpartum, breastfeeding women (This is an infrequent use.)
- To help complete an incomplete abortion, or control bleeding following a complete abortion.
Oxytocin is usually not given in the presence of known cephalopelvic dysproportion, fetal distress, or other conditions in which the increase in frequency, strength and duration of contractions is ill-advised. It is also not usually given when:
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There is an unfavorable fetal position or presentation which is undeliverable without conversion prior to delivery
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Vaginal delivery is contraindicated (invasive cervical carcinoma, active genital herpes, total placenta previa or vasa previa)
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There is an obstetrical emergency where the risk-to-benefit ratio of maternal and fetal safety favors surgical intervention
Amniotomy is sometimes performed in conjunction with oxytocin administration to stimulate uterine contractions.
