Terbutaline is a bronchodilator primarily approved for managing respiratory conditions such as asthma. In obstetrics, it is used off-label as a tocolytic agent to suppress uterine contractions and temporarily halt preterm labor. This application is generally for short-term use in a hospital setting, as the medication acts quickly to relax the uterine muscle. Terbutaline is a short-acting drug, which allows for rapid control of its effects once administered.
How Terbutaline Stops Contractions
Terbutaline belongs to a class of drugs known as Beta-2 adrenergic receptor agonists. These receptors are found throughout the body, including the smooth muscle of the uterus, known as the myometrium. The drug works by selectively binding to and stimulating these Beta-2 receptors.
When terbutaline activates the receptor, it triggers a chain of events inside the cell. This involves activating an enzyme called adenylate cyclase, which increases the concentration of cyclic adenosine monophosphate (cAMP). The rise in cAMP levels ultimately causes a decrease in the amount of free calcium ions within the myometrial cell.
Calcium ions are the necessary trigger for muscle contraction. By reducing the intracellular calcium concentration, terbutaline effectively causes the uterine smooth muscle to relax. This relaxation decreases both the frequency and the intensity of the uterine contractions associated with preterm labor.
Clinical Context for Use in Preterm Labor
The goal of using Terbutaline is not to stop labor permanently, but to delay delivery for a brief, defined period, typically 48 hours. This temporary suspension of contractions is a window for two main clinical interventions.
The delay allows for the administration of corticosteroid medications, such as betamethasone, to the mother. These steroids cross the placenta and accelerate the maturation of the fetal lungs, significantly reducing the risk of severe respiratory distress syndrome in the newborn.
The 48-hour window also provides time to safely transfer the mother to a hospital with a specialized neonatal intensive care unit (NICU) if she is in a facility without one. Terbutaline is most commonly used in cases of threatened preterm labor occurring between 24 and 34 weeks of gestation, when the benefits of lung maturation are greatest.
In the clinical setting, Terbutaline is usually administered via a single, short-acting subcutaneous injection. This route allows for a rapid onset of action to quickly suppress contractions. Current medical practice focuses on short-term injectable use within the hospital setting, rather than continuous intravenous infusion or oral administration.
Maternal and Fetal Safety Profile
Terbutaline’s mechanism of action stimulates Beta-2 receptors throughout the body, leading to potential side effects in other organs. The most common adverse effects in the mother are cardiovascular and metabolic, including increased heart rate (tachycardia) and heart palpitations. Other reported maternal side effects include tremor, nervousness, headache, and transient hyperglycemia.
More serious complications, though less frequent, include pulmonary edema (fluid build-up in the lungs) and cardiac arrhythmias (irregular heart rhythms). Since the drug crosses the placenta, the fetus can also experience side effects, such as fetal tachycardia (increased heart rate) and neonatal hypoglycemia (low blood sugar).
Safety concerns led to a significant regulatory action by the U.S. Food and Drug Administration (FDA) in 2011, mandating a Boxed Warning for Terbutaline. This warning advises against using the injectable form for prolonged treatment of preterm labor beyond 48 to 72 hours, or for use outside of the hospital setting.
The FDA concluded that the risk of serious maternal heart problems, including myocardial ischemia and death, outweighs any potential benefit when Terbutaline is used long-term. Oral Terbutaline is also strongly discouraged for treating preterm labor due to similar safety risks and a lack of proven efficacy.