Classification by Mechanism of Action
In clinical practice, medications from various pharmacological groups are used to relax the uterus. They share a common goal — tocolysis, but their mechanisms of action differ radically:
- Beta-2 adrenomimetics: hexoprenaline, salbutamol, terbutaline, fenoterol.
- Progestogens (progesterone preparations): hydroxyprogesterone caproate, a combination of ethinylestradiol and acetomepregenol.
- Oxytocin receptor antagonists: atosiban.
- Myotropic antispasmodics: magnesium sulfate.
- CNS depressants: general anesthetics (enflurane) and anxiolytics (diazepam).
Beta-adrenomimetics — the mainstay of tocolysis
This is the primary group of drugs for stopping preterm labor. The main representative in obstetrics is hexoprenaline. It is a selective beta-2 adrenergic receptor agonist that markedly and prolongedly reduces the frequency and amplitude of uterine contractions.
Hexoprenaline is used for premature contractions, for uterine immobilization before cesarean section or the application of obstetric forceps, as well as in emergency situations (umbilical cord prolapse, acute intrapartum fetal asphyxia, version from transverse lie).
Other representatives of the group:
- Salbutamol: used for threatened abortion in the 3rd trimester. At therapeutic doses, it has almost no effect on cardiac beta-1 receptors and bladder tone.
- Terbutaline: used to stop premature contractions starting from the 16th week. Administered as an intravenous infusion (infusion duration — 8 hours); the solution requires protection from light.
- Fenoterol: stimulates both beta-2 and beta-1 receptors. Indicated for uterine dystocia and excessive uterine tension.
Side effects are associated with systemic receptor stimulation: muscle tremor, headache, vasodilation, palpitations, decreased diuresis, and transient hypokalemia in the first days of therapy. The drugs are strictly contraindicated in severe cardiovascular disorders, liver and kidney diseases, and closed-angle glaucoma.
Oxytocin antagonists and other groups
In addition to adrenomimetics, preparations with other sites of action are used to maintain pregnancy:
- Atosiban: a competitive oxytocin receptor antagonist. Effectively stops labor at 24–33 full weeks of gestation. Potential side effects include arterial hypotension, tachycardia, and uterine atony leading to hemorrhage.
- Magnesium sulfate: a myotropic antispasmodic for parenteral administration. In the context of obstetrics, its main objective is lowering blood pressure and stopping (as well as preventing) seizures in eclampsia.
- Progestogens (oxyprogesterone): hormonal agents that reduce overall uterine excitability.
Differential pharmacology: tocolytics and uterotonics
It is important to clearly distinguish tocolytics from drugs that stimulate the myometrium. Erroneous administration can lead to severe complications.
- Oxytocin induces physiological rhythmic contractions that promote delivery.
- Dinoprost (prostaglandin F2-alpha) also stimulates the uterus, with myometrial sensitivity to it increasing as pregnancy progresses.
- Ergometrine (an ergot alkaloid) induces a tonic (continuous) myometrial contraction. This mechanically compresses blood vessels, stopping uterine hemorrhage. The drug does not affect blood clotting factors.
- Methyloxytocin is a uterine stimulant and is not classified as a tocolytic agent.
Briefly: answers to topical questions
Through what cellular mechanism does magnesium sulfate relax the myometrium?
Magnesium sulfate relaxes the myometrium due to the functional antagonism of magnesium and calcium ions. At the cellular level, it blocks calcium channels, preventing calcium entry into the cell. This provides a myotropic antispasmodic effect and suppresses myometrial contractile activity.
Which specific pharmacological agents are used for the relaxation and ripening of the cervix?
Prostaglandins and antiprogestogens are used for the relaxation, dilation, and ripening of the cervix.
- Dinoprostone — promotes relaxation and dilation of the cervix.
- Dinoprost — promotes relaxation and dilation of the cervix.
- Mifepristone — an antiprogestogenic drug used for cervical dilation and ripening during labor preparation.
To remember the 5 main tocolytic groups, use the mnemonic: "Pregnancy Can Be Safely Maintained" (Beta-mimetics, Progestogens, Oxytocin antagonists, Myotropic antispasmodics, CNS depressants).
Class and exam questions
What is the difference between tocolytics and cervical-relaxing agents?
Tocolytics act on the myometrium of the uterine body, decreasing its tone to stop contractions. Drugs that relax the cervix are used for the opposite clinical purpose — to accelerate delivery or perform intrauterine interventions.
Why do beta-adrenomimetics cause tachycardia and tremor?
This is due to off-target stimulation of beta receptors in other organs and tissues. The drugs cause peripheral vasodilation, a feeling of skeletal muscle tension, and palpitations.
How does atosiban work?
Atosiban is a competitive oxytocin receptor antagonist. It blocks these receptors in the myometrium, which allows preterm labor to be halted at 24 to 33 weeks of gestation.
Can ergometrine be used to maintain pregnancy?
Categorically no. Ergometrine causes a strong tonic contraction of the myometrium. It is used exclusively to stop uterine hemorrhage by mechanically compressing the uterine blood vessels.
What else to study on this topic
- Detailed pharmacodynamics of hexoprenaline in emergency obstetric situations (fetal distress, umbilical cord prolapse).
- Specific features of intravenous terbutaline infusion and protecting the solution from light.
- The effect of magnesium sulfate on hemodynamics and the management of seizure syndrome in eclampsia.
- Mechanisms of development of transient hypokalemia during beta-adrenomimetic therapy.
- Differences in the effects of oxytocin and prostaglandins on the myometrium across different trimesters of pregnancy.
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