Indications
Oxytocin administration is indicated primarily during the antepartum period to initiate or enhance uterine contractions, facilitating vaginal delivery when there is a medical concern for the fetus or mother. It is particularly useful for inducing labor in cases of Rh sensitization, maternal diabetes, preeclampsia near term, or when membranes have prematurely ruptured. However, it is crucial to note that oxytocin is not approved for elective induction of labor. Additionally, it is used to strengthen labor in certain instances of uterine inertia and can be a supportive treatment in managing incomplete or inevitable abortion. Postpartum, oxytocin is employed to induce contractions during the third stage of labor and to manage postpartum bleeding or hemorrhage.
Pharmacodynamics
Oxytocin is a nonapeptide hormone with multiple significant physiological roles. It is most recognized for its ability to stimulate labor and lactation but also influences metabolic and cardiovascular functions, sexual and maternal behaviors, social bonding, cognition, and fear response. Oxytocin receptors are not restricted to the reproductive system; they are prevalent in various peripheral tissues and central nervous system structures, including the brain stem and amygdala.
Absorption
Oxytocin is administered parenterally, ensuring full bioavailability. It achieves steady-state concentrations in the plasma within approximately 40 minutes following administration, highlighting its effective and rapid absorption into the bloodstream.
Metabolism
Oxytocin is swiftly eliminated from the plasma by the liver and kidneys, with the enzyme oxytocinase playing a pivotal role in its metabolism and regulation during pregnancy. Only a minor proportion of oxytocin is excreted unchanged in the urine. Oxytocinase activity increases throughout pregnancy, reaching its peak in the plasma, placenta, and uterus near term. The placenta serves as a primary source of oxytocinase, producing more of the enzyme in response to rising oxytocin levels from the mother. Furthermore, oxytocinase activity is present in the mammary glands, heart, kidneys, and small intestine, with lower activity levels in the brain, spleen, liver, skeletal muscle, testes, and colon. In non-pregnant women, men, and cord blood, oxytocin degradation is minimal.
Mechanism of Action
Oxytocin plays a crucial role in labor and delivery as it is synthesized in the hypothalamus and stored in the posterior pituitary after being secreted from the paraventricular nucleus. During childbirth, oxytocin is released in pulses to stimulate uterine contractions. The density of oxytocin receptors on the myometrium significantly increases throughout pregnancy, reaching its highest concentration during early labor. The activation of these receptors initiates a sequence of events that elevate intracellular calcium levels in uterine myofibrils, thereby enhancing both the strength and frequency of contractions. Unlike most hormones, which are regulated via negative feedback, oxytocin operates through a positive feedback mechanism. This process is initiated when the fetus's head presses against the cervix, prompting the release of oxytocin from the posterior pituitary. The hormone then travels to the uterus, where it further stimulates uterine contractions, which in turn leads to the release of more oxytocin, perpetuating the cycle until childbirth is completed. Due to the comparable effects of naturally secreted and synthetically administered oxytocin on the female reproductive system, synthetic oxytocin can be utilized in certain clinical situations during the antepartum and postpartum periods to induce or enhance uterine contractions.
