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Oxytocin

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Last Update: October 27, 2018.

Indications

Oxytocin is indicated and approved by the FDA for 2 specific time frames in the obstetric world: antepartum and postpartum. In the antepartum period, exogenous oxytocin is FDA-approved for strengthening uterine contractions with the aim of successful vaginal delivery of the fetus. There are 3 situations during the antepartum period in which oxytocin is indicated:

  • For mothers who have preeclampsia, maternal diabetes, premature rupture of the membranes
  • For mothers with inactive uteri that need to be stimulated into labor
  • For mothers with inevitable or incomplete abortions in their second trimester

In regards to the postpartum period, oxytocin is FDA-approved when it is time to deliver the placenta during the third stage of labor and also to control postpartum hemorrhage. A former usage of oxytocin in the United States included an intranasal formula that was used to encourage postpartum milk ejection. Other non-FDA-approved indications for exogenous oxytocin include treatment of delayed orgasm, inducing sexual arousal, and treatment of autism. Oxytocin has long been known as a hormone that plays a role in social behaviors and bonding. Because oxytocin is released during sexual intercourse, it is thought to play a role in bonding. Autism is not known to be caused by lower levels of oxytocin when compared to non-autistic people; however, previous studies have shown that giving oxytocin to children with autism seems to spark social skills. Further studies and larger sample sizes are needed.

Mechanism of Action

Oxytocin is an oligopeptide hormone that contains 9 amino acyl residues, or in other words, nonapeptide hormone. It is 1 of the 2 hormones stored and released from the posterior pituitary gland but created in the hypothalamus. It is specifically released from the paraventricular nucleus of the hypothalamus into the posterior pituitary gland for later use. This specific part of the posterior pituitary gland that stores oxytocin is called the pars nervosa, also known as the neural or posterior lobe. Most hormones create negative feedback loops after they are released, but oxytocin is one of the few that exhibit positive feedback loops. This means that the release of oxytocin leads to actions that stimulate even more of a release of oxytocin. This can be compared to antidiuretic hormone (ADH) also known as vasopressin (the second and only other hormone stored and released from the posterior pituitary), which exhibits a negative feedback loop after release. Less of this hormone will be released after it exhibits its effect on the body.

Exogenous oxytocin causes the same response in the female reproductive system as that of endogenous oxytocin. Both types of oxytocin stimulate uterine contractions in the myometrium by causing G-protein coupled receptors to stimulate a rise in intracellular calcium in uterine myofibrils. Oxytocin receptor activation is what causes many signals that then stimulate uterine contraction by increasing levels of intracellular calcium. This is where positive feedback comes into play. When oxytocin is released, uterine contractions are stimulated, and these uterine contractions, in turn, cause more oxytocin to be released. This is what causes the increase in both intensity and frequency of contractions and enables a mother to carry out vaginal delivery completely. The head of the fetus pushes against the cervix, the nerve impulses from this action are sent to the mother’s brain, which activates the posterior pituitary to secrete oxytocin. This oxytocin is then carried through the blood to the uterus to increase uterine contractions further, and the cycle continues until birth is achieved.

Not only does oxytocin stimulate uterine contractions, but it also causes contractions of the myoepithelial cells in the female breasts. This occurs in the alveolar ducts. Such contractions are what force milk from these ducts into even larger sinuses which enable milk expulsion. Positive feedback is also relevant to this milk-ejection reflex. A baby that is attempting to latch on to his mother’s breast signals oxytocin secretion into the blood in the same manner as vaginal delivery; except, instead of uterine contractions, milk is ejected from the breast. The oxytocin makes its way to the brain at the same time to increase more oxytocin secretion.

Lastly, oxytocin also has both antidiuretic and vasodilatory effects, increasing cerebral, coronary, and even renal blood flow.

Administration

An injected form of oxytocin is administered intravenously using the drip method in the setting of delayed and potentially complicated labor. The same route of administration is indicated for both incomplete and inevitable abortions as well. Lastly, in the case of persistent uterine bleeding after giving birth, oxytocin may be given either intramuscularly or intravenously.

Adverse Effects

Common side effects of oxytocin administration include the following: erythema at the site of injection, intensified contractions, more frequent contractions, nausea, vomiting, stomach pain, and loss of appetite. Serious adverse effects that should be monitored for after oxytocin administration include cardiac arrhythmias, seizures, anaphylaxis, confusion, hallucinations, extreme increase in blood pressure, and blurred vision.

Contraindications

Specific contraindications to oxytocin include hypersensitivity to the hormone itself or any part of its synthetic version and vaginal deliveries that are in themselves contraindicated. These include the patient having an active genital herpes infection, vasa previa, complete placenta previa, invasive cervical cancer, and prolapse or presentation of the umbilical cord). Other contraindications to administering oxytocin include the fetus in an abnormal position (most notably including a transverse lie) and the fetus exhibiting distress when delivery is not about to happen. Antepartum usage of oxytocin is also contraindicated for women with pelvises not large enough to handle an infant passing through her birth canal, and for when the woman's uterus is either hyperactive or hypertonic.

Monitoring

It is important to monitor patient fluids (both intake and outtake) while administering oxytocin, as well as the frequency of uterine contractions, patient blood pressure, and heart rate of the unborn fetus.

Toxicity

An inappropriate dosage of oxytocin can lead to dangerous tachycardia, arrhythmias, and myocardial ischemia. High dosages of oxytocin can cause uterine rupture, hypertonicity, and spasms. When oxytocin is given to women who are in the first or second stages of labor, or to women to cause induction of labor, uterine rupture, as well as maternal subarachnoid hemorrhages, maternal death, and even fetal death, can result. If oxytocin is given in dosages too large or even slowly during a 24 hour period, the medication can exhibit an antidiuretic effect resulting in extreme water intoxication. This can result in coma, seizures, and even death in the mother. Note that patients who receive fluids orally are at higher risk for water intoxication and antidiuretic effects when given exogenous oxytocin.

Questions

To access free multiple choice questions on this topic, click here.

References

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Yang HP, Wang L, Han L, Wang SC. Nonsocial functions of hypothalamic oxytocin. ISRN Neurosci. 2013;2013:179272. [PMC free article: PMC4045544] [PubMed: 24967304]
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Steinbach X, Maasen S. [Oxytocin: From a Hormone for Birth to a Social Hormone : The Hormonal Governance of Sociability aka Society]. NTM. 2018 Mar;26(1):1-30. [PubMed: 29404640]
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Bookshelf ID: NBK507848PMID: 29939625

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