Tocolytics

Nifedipine in the Management of Preterm Labor

March 11th, 2011

I am always excited to see new medications and treatments for the treatment of preterm labor and management of high risk pregnancy complications. It is my ever present hope that one day there will be no need to put mamas on bed rest.  (Yes, I do hope to put myself out of business in this capacity!) But until that day comes, we need all the help that we can get.

Yet I get a bit nervous when physicians and researchers start shifting and changing medications and using a medication indicated for one ailment for something else-“off label”.  Recently, researchers at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, Perinatology Research Branch, National Institutes of Health, Department of Health and Human Services published a paper stating that Nifedipine, a calcium channel blocker used to lower blood pressure, is a useful tocolytic (anti-contraction) medication for the management of preterm labor. Nifedipine has been used to prevent preterm labor for many years and these researchers sought to determine its efficacy and how it stacked up against other routinely prescribed tocolytics.

Nifedipine is a cardiac medication. I first became acquainted with Nifedipine when I was a newbie physician assistant working with an interventional cardiology group. Many of our patients were on Nifedipine for high blood pressure as well as for aginal chest pain. Nifedipine works by relaxing the smooth muscle of the heart so that it is not pumping so hard. This relaxation allows for more blood to flow through the heart and to the rest of the body, providing more blood, nutrients and oxygen to the body. Because the muscles of the heart are relaxed and there is more oxygen available and delivered to (cardiac) cells, chest pain is relieved.

Nifedipine has a similar mechanism of action on the uterus. Preterm labor is premature contractions of the uterus that can lead to premature delivery. Just as Nifedipine relaxes smooth muscles in the heart, it also relax smooth muscles of the uterus slowing or halting contractions and avoiding preterm labor and delivery. Researchers found that Nifedipine was more efficacious in halting preterm labor contractions than many other tocolytics used, most notably Magnesium Sulfate, a mainstay of preterm labor therapy. Additionally, it was better tolerated by women who used it and women reported having fewer adverse reactions compared to the other medications. So it sounds like Nifedipine is a win win all around.

How will side effects routinely experienced by cardiac patients affect pregnant women? In the cardiac practice, the most common side effects that I saw were headaches, dizziness and flushing. Translating that to pregnant women, who may or not have high blood pressure, are we at risk of “bottoming out” their blood pressure? (Meaning, if they don’t have high blood pressure yet take the medication, will their blood pressures drop too low causing dizziness or fainting?) Women on bed rest, who are already de-conditioned and wobbly, will they be at increased risk for falls?, How about constipation and heartburn, already problematic during pregnancy, yet increased with administration of nifedipine?

Many would readily say that the risk of delivering a premature infant far outweighs the risks of developing so called “nuisance” side effects. However calcium channel blockers, the class to which Nifedipine belongs, are also known to slow heart rate, and are frequently used for patients experiencing atrial (top of the heart) arrhythmias. Now most mamas on bed rest are not experiencing arrhythmias, yet may still experience a slowing in their heart rates. Is Nifedipine transferred to the developing fetus? If so, what effect will it have on fetal heart rate? According to the article, many of these side effects were not experienced by pregnant women taking Nifedipine for preterm labor.

When Nifedipine was given to pregnant women experiencing preterm labor, the women (actually their infants) had significantly reduced incidences of respiratory distress syndrome,  necrotizing enterocolitis, intraventricular hemorrhage, neonatal jaundice, and admission to the neonatal intensive care units. Nifedipine was  also associated with a significant reduction in the risk of delivery within 7 days of initiation of treatment and before 34 weeks’ gestation. Mamas also experienced fewer adverse side effects compared with Beta Adrenergic Receptor Agents and Magnesium Sulfate.

So is Nifedipine the next great thing for the prevention of preterm labor? Perhaps. At the very least, it affords physicians and pregnant women another option for treatment while sparing infants potentially serious side effects.