Tocolytics are medications used to delay or stop preterm labor. Preterm labor is defined as labor that begins before 37 weeks of pregnancy. Giving birth before 37 weeks is considered a preterm birth. Preterm birth can lead to serious health problems for the baby. Tocolytics work by relaxing the uterine muscles and postponing delivery allowing more time for the baby’s organs to mature. They are used to buy time to administer corticosteroids which help accelerate fetal lung maturation or to transport the mother to a hospital with a neonatal intensive care unit. Tocolytics may be given orally, intravenously, or intramuscularly.
How Do Tocolytics Work?
There are several different classes of tocolytic medications that work in different ways:
- Beta-adrenergic receptor agonists – These medications, such as terbutaline and ritodrine, stimulate beta-adrenergic receptors which increases levels of cyclic AMP leading to relaxation of uterine smooth muscle. This inhibits uterine contractions.
- Calcium channel blockers – Medications like nifedipine block the influx of calcium ions into cells, which inhibits muscle cell contraction. With less calcium, uterine contractions are weakened.
- NSAIDs – Nonsteroidal anti-inflammatory drugs like indomethacin prevent the synthesis of prostaglandins which are involved in triggering uterine contractions.
- Magnesium sulfate – Magnesium competes with calcium for entry into cells. Higher magnesium levels prevent calcium influx required for muscle contraction.
- Oxytocin receptor antagonists – These medications like atosiban bind to oxytocin receptors on uterine muscle. This blocks the action of the hormone oxytocin which stimulates contractions.
- Nitric oxide donors – Nitric oxide causes smooth muscle relaxation which suppresses uterine contractility.
- The net effect of all tocolytics is to decrease the strength and frequency of uterine contractions. This provides a temporary delay to preterm birth.
Who is Prescribed Tocolytics?
Tocolytics are generally prescribed for women experiencing preterm labor between 24-34 weeks of pregnancy. Candidates for tocolytic therapy include:
- Women with regular uterine contractions occurring every 5-10 minutes.
- Evidence of cervical changes, like dilation and effacement.
- Positive fetal fibronectin test suggesting risk of preterm delivery.
- Women with preterm premature rupture of membranes.
Tocolytics are sometimes used as a temporary measure in women up to 37 weeks showing signs of preterm labor. The medications can delay delivery for up to 48 hours to allow time for corticosteroid administration or transportation to a hospital with advanced neonatal care facilities.
How are Tocolytics Administered?
There are different methods of administering tocolytic medications:
- Oral – Tocolytics like calcium channel blockers or NSAIDs may be taken orally in pill or liquid form. Oral administration is simple but can take some time before effects are seen.
- Intravenous – IV administration provides faster acting, controlled doses of tocolytics like magnesium sulfate or beta-agonists. This method is used for rapid effect in emergency situations.
- Intramuscular – Some tocolytics like NSAIDs may be given as an intramuscular injection when oral dosing is not feasible. Effects occur faster than oral but slower than IV.
- Subcutaneous pump – Terbutaline can be delivered continuously via an under the skin pump. This allows stable blood concentrations to be maintained.
The method chosen depends on the urgency of the situation, type of medication, and duration of treatment required. IV administration provides the fastest benefits in acute cases.
What are the Side Effects of Tocolytics?
Tocolytics can cause both mild and serious maternal and fetal side effects:
Maternal side effects
- – Nausea, vomiting, dizziness, headaches
- – Heart palpitations, chest pain
- – Tremors, anxiety
- – Hypokalemia – low potassium
- – Pulmonary edema
- – Cardiac arrhythmias
Fetal side effects
- – Tachycardia – fast heart rate
- – Hypoglycemia – low blood sugar
- – Neonatal respiratory distress
- – Hyperbilirubinemia – high bilirubin levels
The most common minor side effects are nausea, jitteriness, and headaches. More serious effects like pulmonary edema, cardiac arrhythmias, neonatal hypoglycemia rarely occur but require prompt attention. The risks of tocolytics must be weighed against the benefits of delaying preterm birth. Doctors monitor both maternal and fetal well-being closely during treatment.
What Factors Affect Tocolytics Success?
Several factors influence how effective tocolytics will be at postponing preterm delivery:
- Gestational age – Tocolytics work better earlier in pregnancy between 24-34 weeks when the risks of prematurity are highest. Delaying delivery as long as possible maximizes benefits.
- Cervical dilation – If the cervix is already significantly dilated (>3 cm) it is less likely tocolytics will successfully stop labor. Effect is best when cervix is closed or only slightly dilated.
- Contraction frequency – More frequent contractions usually indicate more advanced labor resulting in lower tocolytic success. Infrequent contractions are more easily suppressed.
- Cause of preterm labor – Tocolytics are less effective when preterm labor is caused by preterm premature rupture of membranes, placental abruption, fetal demise or infection.
- Prior preterm birth – Women with previous preterm deliveries have lower tocolytic success rates compared to those without prior preterm birth history.
- Medication choice – Some tocolytics like calcium channel blockers have higher success rates than others. Using the most effective medication for each woman improves results.
The key factor determining tocolytic efficacy is how far along labor has progressed. Tocolytics work best when started in early labor before advanced cervical changes occur.
Frequently Asked Questions about Tocolytics
What are the most common tocolytic medications?
The most frequently used tocolytics are calcium channel blockers like nifedipine, beta-agonists such as terbutaline, magnesium sulfate, NSAIDs like indomethacin, and atosiban. The choice depends on availability, side effect profile, and patient factors.
When should tocolytics be avoided?
Tocolytics should be avoided in women with severe preeclampsia, severe cardiovascular disease, pulmonary edema, and eclampsia. They should also be avoided late in pregnancy and when delivery would be safer for the fetus than postponing labor.
How long can tocolytics postponed delivery?
Most tocolytics can delay delivery for 24-48 hours but sometimes up to 7 days. Long enough for corticosteroid administration or to transfer to a facility with NICU capabilities. After stopping tocolytics, 60% of women will deliver within 1 week.
How much time do corticosteroids take to help fetal lung development?
Steroid administration at least 24 hours before delivery has significant benefits on reducing neonatal respiratory complications. The full effect on lung maturation occurs after 48 hours. Tocolytics aim to allow this 2-day window for maximal steroid effects.
What happens after a tocolytic drug is given?
After tocolytics, the fetus and any uterine contractions are closely monitored. If labor continues to progress, the medication may be changed or delivery will be planned. If contractions stop, monitoring continues until 37 weeks or until preterm labor recurs. Additional steroid doses may be given.
In summary, tocolytic medications can temporarily stop preterm labor to allow important interventions that improve prematurity outcomes. Careful patient selection and monitoring during use are key to achieving optimal results while minimizing adverse effects. Using tocolytics appropriately can make a significant difference for babies at highest risk of complications from preterm birth.
