Tocolytics: Nursing pharmacology

Tocolytics are a group of medications that suppress uterine contractions. For that reason, tocolytics are typically used to prolong pregnancy and delay birth after preterm labor starts before 34 weeks of gestation. Delaying labor is usually done to transfer the client to a higher health care facility if needed, or to administer medications that improve fetal outcomes, such as corticosteroids, which promote fetal lung maturation.

Now, the most commonly used tocolytics are magnesium sulfate, calcium channel blockers like nifedipine, beta2-agonists like terbutaline, and NSAIDs like indomethacin.

When tocolytics are administered, they cause smooth muscle relaxation in the uterus via various mechanisms. Both magnesium sulfate and calcium channel blockers like nifedipine block calcium channels, which inhibits the entry of calcium ions into uterine smooth muscles and thus decreases their contractility. In addition to its tocolytic effect, magnesium sulfate also has a neuroprotective effect on the preterm brain, which is more susceptible to injury. On the other hand, beta2-agonists bind to beta2-receptors located on the surface of smooth muscle cells, ultimately leading to a decrease in the level of intracellular calcium and decreasing their contractility. Finally, NSAIDs inhibit the enzyme cyclooxygenase, which normally helps to produce prostaglandins. As a result, there’s a decrease in prostaglandin levels, which ultimately results in relaxation of the uterine smooth muscle.

Now, tocolytics can cause several maternal and fetal side effects. Side effects of magnesium sulfate include nausea, flushing, and headache. In addition, magnesium sulfate toxicity can lead to respiratory depression, cardiac arrest, as well as neurological side effects like altered mental status, reduced deep tendon reflexes, and muscle weakness. Now, magnesium sulfate may have side effects on the fetus. As it relaxes the muscles, some babies who are exposed to magnesium can present with hypotonia or low muscle tone. Fortunately, this side effect is not permanent, and usually improves as magnesium sulfate clears from the baby.