Oxytocin
Oxytocin is a hormone secreted by the posterior pituitary gland that has an effect on uterine contractions and breast milk excretion. It can also be created synthetically and is commonly used in obstetrics. It has dramatically decrease maternal morbidity and mortality from postpartum hemorrhage. For this reason, it is consider one of the drugs in the World Health Organization’s (WHO) list of essential drugs.
There are several uses for oxytocin in obstretic medicine:
Contraction Stress Test - evaluate fetal well being in ante-atrium period
Induction of Labor
Treatment of Labor Arrest
Active Management of Labor
Treatment of Uterine Atony
Failure of the uterus to contract (uterine atony) is more common after cesarean delivery vs vaginal delivery. If untreated, uterine atony can quickly develop into hemorrhagic shock. Risk factors for uterine atony include:
high parity (> 3 pregnancies that were over 20 weeks gestation)
An over distended uterus (multiple gestation, macrosomia, polyhydramnios)
Prolonged labor (augmented by oxytocin)
Chorioamnionitis
Abnormalities in placentation (placenta accrete, increased, or percreta)
Retained placental tissue
Mechanical barrier to effective contraction (fibroids, uterine anomalies)
Poor perfusion to the uterine muscle (myometrium) from hypotension
The primary use of oxytocin in obstretic anesthesia is to increase uterine tone to prevent post-partum bleeding. It is commonly given as a 30unit/500cc of normal saline bolus during cesarean section. Continued high-dose oxytocin exposure in the postpartum stage can lead to desensitization, making the muscles of the uterus (myometrium) less response to additional oxytocin. However it is still responsive to other uterotonic agents.
There are 3 additional uterotonic agents that can be used if uterine atony persists after high doses of oxytocin: methylergonovine (Methergine), carboprost (Hemabate), and misoprostol (Cytotec).
Methylergonovine is a ergot alkaloid derivative that is given intramuscularly (0.2mg). It has an onset time of 10 minutes and lasts about 3 to 6 hours. Intravenous use should be done with extreme caution as it can create intensive vasoconstriction leading to acute hypertension, seizures, cerebrovascular accidents, retinal detachment, and myocardial arrest.
Carboprost is a prostaglandin analog that has a dose dependent increase in the force and frequency of uterine contractions. It is also given intramuscularly with a starting dose of 250 micrograms with repeated doses every 15-90 minutes (up to 8 doses).
Lastly, misoprostol is a prostaglandin E1 analog typically used as a uterotonic adjunct with other medication trialed first. It is given sublingually or rectally and has a rapid onset of action. It’s dose ranges from 400 to 800 micrograms.
There are a few scenarios prior to delivery when oxytocin is contraindicated, such as fetal distress or abnormal fetal presentation (cephalopelvic disproportion). A few of the maternal factors that are contraindications include previous classical cesarean section, two previous cesarean sections, previous uterine rupture, or previous vesicovaginal or rectovaginal fistulae repair. These would make the mother a high risk for uterine rupture with oxytocin.
Although oxytocin is commonly used, its important to recognize the complications that come with it. As mentioned before with the contraindications, high doses can lead to excess stimulation of uterine contractions before delivery leading to fetal distress, abruptio placenta, and uterine rupture. It can also inadvertently activate the vasopressin receptors leading to excessive fluid retention, hyponatremia, heart failure, seizures, and potentially death. Boluses of high dose oxytocin can cause hypotension from vasculature smooth muscle relaxation, which is why is it mostly given as an infusion of a diluted solution. Other less serious side effects include headache, nausea, vomiting, tachycardia, bradycardia, and premature ventricular contractions.
